Download 8.1 Why Do Cells Divide?

8.1 Why Do Cells Divide?
 Cells reproduce by cell division.
• One cell gives rise to two or more cells, called
daughter cells.
• Each daughter cell receives a complete set of
heredity information—identical to the information in
the parent cell—and about half of the cytoplasm.
 Cell division transmits hereditary information to
each daughter cell.
• The hereditary information in each cell is
deoxyribonucleic acid (DNA).
• DNA is contained in chromosomes.
• A molecule of DNA consists of smaller subunits called
nucleotides.
Copyright © 2009 Pearson Education Inc.
8.1 Nucleotide Structure
 A nucleotide consists of a phosphate, a
sugar (deoxyribose), and one of four bases.
•
•
•
•
Adenine (A)
Thymine (T)
Guanine (G)
Cytosine (C).
 The nucleotides are held together by
hydrogen bonding between the bases in two
strands forming a double helix.
Copyright © 2009 Pearson Education Inc.
8.1 Why Do Cells Divide?
 The structure of DNA
phosphate
nucleotide
base
T
A
sugar
C
G
G
C
C
C
G
A
A
A
T
C
G
A
T
T
A
T
(a) A single strand of DNA
(b) The double helix
Fig. 8-1
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8.1 Why Do Cells Divide?
 Genes, segments DNA, are the units of
inheritance.
 Each gene spells out the instructions for
making one or more proteins.
 When a cell divides, it first replicates its
DNA, and a copy is transferred into each
daughter cell.
Copyright © 2009 Pearson Education Inc.
8.1 Why Do Cells Divide?
 Cell division is required for growth and
development.
• Cell division in which the daughter cells are
genetically identical to the parent cell is called
mitotic cell division.
• After cell division, the daughter cells may grow
and divide again, or may differentiate,
becoming specialized for specific functions.
• The repeating pattern of division, growth, and
differentiation followed again by division is
called the cell cycle.
Copyright © 2009 Pearson Education Inc.
8.1 Why Do Cells Divide?
 Most multicellular organisms have three
categories of cells.
• Stem cells: retain the ability to divide and can
differentiate into a variety of cell types
• Other cells capable of dividing: typically
differentiate only into one or two different cell
types (progenitor cells)
• Permanently differentiated cells: differentiated
cells that can never divide again
Copyright © 2009 Pearson Education Inc.
8.1 Why Do Cells Divide?
 Cell division is required for sexual and
asexual reproduction.
• Sexual reproduction in eukaryotic organisms
occurs when offspring are produced by the
fusion of gametes (sperm and eggs) from two
adults.
• Gametes are produced by meiotic cell
division, which results in daughter cells with
exactly half of the genetic information of their
parent cells.
• Fertilization of an egg by a sperm results in
the restoration of the full complement of
hereditary information in the offspring.
Copyright © 2009 Pearson Education Inc.
8.1 Why Do Cells Divide?
 Reproduction in which offspring are formed
from a single parent, without having a sperm
fertilize an egg, is called asexual
reproduction.
• Asexual reproduction produces offspring that
are genetically identical to the parent.
• Examples of asexual reproduction occur in
bacteria, single-celled eukaryotic organisms,
multicellular organisms such as Hydra, and
many trees, plants, and fungi.
Copyright © 2009 Pearson Education Inc.
8.2 What Occurs During The Prokaryotic
Cell Cycle?
 The prokaryotic cell cycle consists of a long
period of growth, during which the cell
duplicates its DNA.
cell division
by binary
fission
cell growth and
DNA replication
The prokaryotic cell cycle
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8.2 What Occurs During the Prokaryotic
Cell Cycle?
 Cell division in prokaryotes occurs by binary
fission, which means “splitting in two.”
 The prokaryotic chromosome is attached at
one point to the plasma membrane of the
cell.
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8.2 The Prokaryotic Cell Cycle?
attachment
site
cell
wall
plasma
membrane
circular
DNA
The circular DNA double helix is
attached to the plasma membrane at
one point.
The DNA replicates and the two DNA
double helices attach to the plasma
membrane at nearby points.
New plasma membrane is added
between the attachment points, pushing
them further apart.
The plasma membrane grows inward
at the middle of the cell.
Fig. 8-3b(1)
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8.2 The Prokaryotic Cell Cycle
 The prokaryotic cell cycle (continued)
The parent cell divides into two daughter
cells.
Fig. 8-3b(5)
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8.3 How Is The DNA In Eukaryotic Cells
Organized?
 Unlike prokaryotic chromosomes, eukaryotic
chromosomes are separated from the
cytoplasm by a membrane-bound nucleus.
 Eukaryotic cells always have multiple
chromosomes.
 Eukaryotic chromosomes contain more DNA
than prokaryotic chromosomes.
 The eukaryotic chromosome consists of
DNA bound to protein.
Copyright © 2009 Pearson Education Inc.
8.3 How Is The DNA In Eukaryotic Cells
Organized?
 Duplicated chromosomes separate during cell
division.
• Prior to cell division, the DNA within each
chromosome is replicated.
• The duplicated chromosomes then consist of two
DNA double helixes and associated proteins that are
attached to each other at the centromere. Each of the
duplicated chromosomes attached at the centromere
is called a sister chromatid.
• During mitotic cell division, the sister chromatids
separate and each becomes a separate chromosome
that is delivered to one of the two resulting daughter
cells.
Copyright © 2009 Pearson Education Inc.
8.3 How Is the DNA In Eukaryotic Cells
Organized?
 Eukaryotic chromosomes during cell division
centromere
genes
duplicated
sister
chromosome
chromatids
(2 DNA double
helices)
(a) A replicated chromosome consists of two sister chromatids
independent
daughter
chromosomes,
each with one
identical DNA
double helix
(b) Sister chromatids separate during cell division
Copyright © 2009 Pearson Education Inc.
Fig. 8-5
8.3 How Is The DNA In Eukaryotic Cells
Organized?
 Chromosomes with the same genes are
called homologous chromosomes, or
homologues.
 Cells with pairs of homologous
chromosomes are called diploid.
 Homologous chromosomes are usually not
identical.
• The same genes on homologous
chromosomes may be different due to
mutations, changes in the sequence of
nucleotides in the DNA.
Copyright © 2009 Pearson Education Inc.
8.3 How Is The DNA In Eukaryotic Cells
Organized?
 Not all cells have paired chromosomes.
 The ovaries and testes undergo a special kind
of cell division, called meiotic cell division, to
produce gametes (eggs and sperm).
• Gametes contain only one member of each pair of
autosomes, plus one of the two sex
chromosomes.
• Cells with half the number of each type of
chromosome are called haploid cells.
• Fusion of two haploid cells at fertilization
produces a diploid cell with the full complement of
chromosomes.
Copyright © 2009 Pearson Education Inc.
8.3 How Is The DNA In Eukaryotic Cells
Organized?
 The number of different types of
chromosomes in a species is called the
haploid number and is designated n.
• In humans, n = 23.
 Diploid cells contain 2n chromosomes.
• Humans body cells contain 2n = 46 (2 x 23)
chromosomes.
Copyright © 2009 Pearson Education Inc.
8.4 What Occurs During The Eukaryotic
Cell Cycle?
 The eukaryotic cell cycle is divided into two
major phases: interphase and cell division.
• During interphase, the cell acquires nutrients
from its environment, grows, and duplicates its
chromosomes.
• During cell division, one copy of each
chromosome and half of the cytoplasm are
parceled out into each of two daughter cells.
Copyright © 2009 Pearson Education Inc.
8.4 What Occurs During The Eukaryotic
Cell Cycle?
 The eukaryotic cell cycle
telopha
se and
cytokinesis
se
ha
se
ha
ase
anaph
tap
me
op
pr
ell
cc n
i
t
o
o
mit ivisi
d
cell growth and
differentiation
cell
growth
interphase
synthesis
of DNA;
chromosomes
are duplicated
Fig. 8-7
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8.4 What Occurs During The Eukaryotic
Cell Cycle?
 There are two types of division in eukarytic
cells: mitotic cell division and meiotic cell
division.
• Mitotic cell division may be thought of as
ordinary cell division, such as occurs during
development from a fertilized egg, during
asexual reproduction, and in skin, liver, and
the digestive tract every day.
• Meiotic cell division is a specialized type of
cell division required for sexual reproduction.
Copyright © 2009 Pearson Education Inc.
8.4 What Occurs During The Eukaryotic
Cell Cycle?
 Mitotic cell division
• Mitotic cell division consists of nuclear division
(called mitosis) followed by cytoplasmic
division (called cytokinesis) and the formation
of two daughter cells.
Copyright © 2009 Pearson Education Inc.
8.4 What Occurs During The Eukaryotic
Cell Cycle?
 Meiotic cell division
• Is a prerequisite for sexual reproduction in all
eukaryotic organisms.
• Meiotic cell division involves a specialized
nuclear division called meiosis.
• It involves two rounds of cytokinesis,
producing four daughter cells that can become
gametes.
Copyright © 2009 Pearson Education Inc.
8.4 The Eukaryotic Cell Cycle
 The life cycle of eukaryotic organisms
include both mitotic and meiotic cell division.
mitotic cell division,
differentiation, and growth
mitotic cell division,
differentiation,
and growth
baby
adults
embryo
meiotic cell
division in
meiotic cell
ovaries
division in
testes
mitotic
cell division,
differentiation,
and growth
haploid
diploid
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egg
fertilized
egg
sperm
Flash
fusion of gametes
Fig. 8-8
8.5 How Does Mitotic Cell Division Produce
Genetically Identical Daughter Cells?
 Mitosis is divided into four phases.
•
•
•
•
Prophase
Metaphase
Anaphase
Telophase
Flash
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8.5 How Does Mitotic Cell Division Produce
Genetically Identical Daughter Cells?
 Interphase, prophase, and metaphase
nuclear
envelope
chromatin
nucleolus
centriole
pairs
(a) Late Interphase
Duplicated chromosomes
are in the relaxed
uncondensed state;
duplicated centrioles
remain clustered.
spindle pole
condensing
chromosomes
beginning of
spindle formation
(b) Early Prophase
Chromosomes condense
and shorten; spindle
microtubules begin to
form between separating
centriole pairs.
spindle
microtubules
kinetochore
spindle pole
(c) Late Prophase The
nucleolus disappears; the
nuclear envelope breaks
down; spindle microtubules
attach to the kinetochore
of each sister chromatid.
(d) Metaphase
Kinetochores interact;
spindle microtubules
line up the
chromosomes
at the cell’s equator.
Fig. 8-9a–d
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8.5 How Does Mitotic Cell Division Produce
Genetically Identical Daughter Cells?
 Anaphase, telophase, cytokinesis, and
interphase
unattached spindle
microtubules
(e) Anaphase Sister
chromatids separate
and move to opposite
poles of the cell; spindle
microtubules that are
not attached to the
chromosomes push the
poles apart.
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chromosomes
extending
nuclear envelope
re-forming
(f) Telophase One set of
chromosomes reaches
each pole and relaxes
into the extended state;
nuclear envelopes start
to form around each set;
spindle microtubles
begin to disappear.
(g) Cytokinesis
The cell divides in
two; each daughter
cell receives one
nucleus and about
half of the cytoplasm.
(h) Interphase of
daughter cells Spindles
disappear, intact nuclear
envelopes form,
chromosomes extend
completely, and the
nucleolus reappears.
Fig. 8-9e–h
8.5 How Does Mitotic Cell Division Produce
Genetically Identical Daughter Cells?
 Three major events happen in prophase:
• The duplicated chromosomes condense.
• The spindle microtubules form and attach to the
kinetochore of the chromatids.
• The chromosomes migrate with the spindle poles to
opposite sides of the nucleus.
Fig. 8-9b–c
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8.5 How Does Mitotic Cell Division Produce
Genetically Identical Daughter Cells?
 During metaphase, the chromosomes line
up along the equator of the cell.
• At this phase, the spindle apparatus lines up
the sister chromatids at the equator, with one
kinetochore facing each cell pole.
Fig. 8-9d
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8.5 How Does Mitotic Cell Division Produce
Genetically Identical Daughter Cells?
 During anaphase, sister
chromatids separate and
move to opposite poles of
the cell.
• Sister chromatids
separate, becoming
independent daughter
chromosomes.
• The kinetochores pull
the chromosomes
poleward along the
spindle microtubules.
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8.5 How Does Mitotic Cell Division Produce
Genetically Identical Daughter Cells?
 During telophase, nuclear
envelopes form around
both groups of
chromosomes.
• Telophase begins when
the chromosomes reach
the poles.
• The spindle
microtubules
disintegrate and the
nuclear envelop forms
around each group of
chromosomes.
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8.5 How Does Mitotic Cell Division Produce
Genetically Identical Daughter Cells?
 During Telophase cytokinesis occurs splitting the
cytoplasm between the two daughter cells ending
mitosis and beginning Interphase.
Microfilaments form
a ring around the cell’s
equator.
The microfilament
ring contracts, pinching
in the cell’s “waist.”
(a) Microfilaments contract, pinching the cell in two
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The waist
completely
pinches off,
forming two
daughter cells
(b) Scanning electron micrograph
of cytokinesis.
Fig. 8-10
8.5 How Does Mitotic Cell Division Produce
Genetically Identical Daughter Cells?
 Cytokinesis in plant cells has an additional
step.
Fig. 8-11
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8.6 How Does Meiotic Cell Division
Produce Haploid Cells?
 Meiosis is the production of haploid cells
with unpaired chromosomes derived from
diploid parent cells with paired
chromosomes.
 Meiosis includes two nuclear divisions,
known as meiosis I and meiosis II.
• In meiosis I, homologous chromosomes pair
up, but sister chromatids remain connected to
each other.
• In meiosis II, chromosomes behave as they do
in mitosis—sister chromatids separate and are
pulled to opposite poles of the cell.
Flash
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8.6 How Does Meiotic Cell Division
Produce Haploid Cells?
paired homologous
chromosomes
chiasma
recombined
chromatids
spindle
microtubule
(a) Prophase I
Duplicated chromosomes
condense. Homologous
chromosomes pair up
and chiasmata occur as
chromatids of homologues
exchange parts by crossing
over. The nuclear envelope
disintegrates, and spindle
microtubules form.
kinetochores
(b) Metaphase I
Paired homologous
chromosomes line up along
the equator of the cell. One
homologue of each pair
faces each pole of the cell
and attaches to the spindle
microtubules via the
kinetochore (blue).
(c) Anaphase I
Homologues separate,
one member of each
pair going to each
pole of the cell. Sister
chromatids do not
separate.
(d) Telophase I
Spindle microtubules disappear.
Two clusters of chromosomes
have formed, each containing
one member of each pair of
homologues. The daughter
nuclei are therefore haploid.
Cytokinesis commonly occurs
at this stage. There is little
or no interphase between
meiosis I and meiosis II.
Fig. 8-12a–d
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8.6 How Does Meiotic Cell Division
Produce Haploid Cells?
(e) Prophase II
If the chromosomes
have relaxed after
telophase I, they
recondense. Spindle
microtubules re-form
and attach to the
sister chromatids.
(f) Metaphase II
The chromosomes line
up along the equator,
with sister chromatids
of each chromosome
attached to spindle
microtubules that lead
to opposite poles.
(g) Anaphase II
The chromatids separate
into independent
daughter chromosomes,
one former chromatid
moving toward each
pole.
(h) Telophase II
The chromosomes
finish moving to
opposite poles.
Nuclear envelopes
re-form, and the
chromosomes
become extended
again (not shown
here).
(i) Four haploid
cells
Cytokinesis results
in four haploid cells,
each containing one
member of each
pair of homologous
chromosomes
(shown here in the
condensed state).
Fig. 8-12e–i
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8.6 How Does Meiotic Cell Division
Produce Haploid Cells?
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Flash
8.7 How Do Meiotic Cell Division And
Sexual Reproduction Produce Genetic
Variability?
 Ways to produce genetic variability from
meiotic cell division and sexual reproduction
• Shuffling of homologues
• Crossing over
• Fusion of gametes
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8.7 Meiotic Cell Division And Sexual
Reproduction Produce Genetic Variability
 Shuffling of homologues creates novel
combinations of chromosomes during
meiosis producing genetic variability.
(a) The four possible chromosome arrangements at metaphase
of meiosis I
(b) The eight possible sets of chromosomes after meiosis I
Fig. 8-13
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8.7 Meiotic Cell Division And Sexual
Reproduction Produce Genetic Variability
 Crossing over creates chromosomes with
novel combinations of genetic material.
sister
chromatids of
one duplicated
homologue
pair of
homologous
duplicated
chromosomes
chiasmata
(sites of
crossing over)
parts of chromosomes
that have been
exchanged between
homologues
Fig. 8-14
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8.7 How Do Meiotic Cell Division And
Sexual Reproduction Produce Genetic
Variability?
 Fusion of gametes creates genetically
variable offspring.
• Because every egg and sperm are genetically
unique, and it is random as to which sperm
fertilizes which egg, every fertilized egg is also
genetically unique.
Copyright © 2009 Pearson Education Inc.
9.1 What Is The Physical Basis Of
Inheritance?
 Inheritance occurs when genes are
transmitted from parent to offspring.
• The units of inheritance are genes.
 Genes are segments of DNA at specific
locations on chromosomes.
• A gene’s physical location on a chromosome is
called its locus.
• Each member of a pair of homologous
chromosomes carries the same genes, located
at the same loci.
• Different versions of a gene at a given locus are
called alleles.
Copyright © 2009 Pearson Education Inc.
9.1 What Is The Physical Basis Of
Inheritance?
 The relationship among genes, alleles, and
chromosomes
a pair of
homologous
chromosomes
Both chromosomes carry the same allele
of the gene at this locus; the organism is
homozygous at this locus
gene loci
This locus contains another gene for which
the organism is homozygous
Each chromosome carries a different allele
of this gene, so the organism is
heterozygous at this locus
the chromosome
from the male
parent
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the chromosome
from the female
parent
Fig. 9-1
9.1 What Is The Physical Basis Of
Inheritance?
 Mutations are the source of alleles.
• Differences in alleles at a given locus are due
to mutations at that gene.
• If a mutation occurs in gametes, it can be
passed on from parent to offspring.
 An organism’s two alleles may be the same
or different.
• A diploid organism has pairs of homologous
chromosomes with two copies of each gene at
a given locus.
Copyright © 2009 Pearson Education Inc.
9.1 What Is The Physical Basis Of
Inheritance?
 An organism’s two alleles may be the same
or different (continued).
• If both homologous chromosomes have the
same allele at a locus, the organism is said to
be homozygous.
• If two homologous chromosomes have
different alleles at a locus, the organism is
heterozygous at that locus.
• The gametes of a homozygous individual are
all the same at a particular locus, while
gametes of a heterozygous individual would
contain half one allele and half the other allele.
Copyright © 2009 Pearson Education Inc.
9.3 How Are Single Traits Inherited?
 True-breeding traits of organisms, such as
purple flower color, are always inherited by
all of their offspring that result from selffertilization.
• In one experiment, Mendel cross-fertilized
white-flowered plants with purple-flowered
plants.
• When he grew the resulting seeds, he found
all the first-generation offspring, or the F1
generation, produced purple flowers.
• What happened to the white color?
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9.3 How Are Single Traits Inherited?
 The F2 generation
• Next, Mendel allowed the F1 flowers to selffertilize, collected the seeds, and grew the
second generation, called the F2 generation.
• Flowers in the F2 generation were threefourths purple and one-fourth white, in a ratio
of 3 purple to 1 white.
• This showed that the gene for white flowers
was “hidden” in the F1 generation, but
appeared again in the F2 generation.
Copyright © 2009 Pearson Education Inc.
9.3 How Are Single Traits Inherited?
 Cross of F1 plants with purple flowers
Firstgeneration
offspring (F1)
self-fertilize
Secondgeneration
offspring (F2)
3/4 purple
Copyright © 2009 Pearson Education Inc.
1/4 white
Fig. 9-5
9.3 How Are Single Traits Inherited?
 All the white-flowered plants in the F2
generation only produced additional whiteflowered plants.
 Purple-flowered plants were of two types:
• About ⅔ were true-breeding for purple, while
⅔ produced both purple- and white-flowered
offspring (ratio 3 purple/1 white).
• Therefore, the F2 generation included ¼ truebreeding purple plants, ½ hybrid purple, and
¼ true-breeding white plants.
Copyright © 2009 Pearson Education Inc.
9.3 How Are Single Traits Inherited?

This allows us to develop a five-part hypothesis
to explain the inheritance of single traits.
1. Each trait is determined by pairs of distinct
physical units called genes.
• There are two alleles for each gene, one on
each homologous chromosome.
2. When two different alleles are present in an
organism, the dominant allele may mask the
expression of the recessive allele; but the
recessive allele is still present.
3. The two alleles of a gene segregate (separate)
from one another during meiosis
(Mendel’s law of segregation).
Copyright © 2009 Pearson Education Inc.
9.3 How Are Single Traits Inherited?
4. Which allele ends up in any given gamete is
determined by chance.
5. True-breeding (homozygous) organisms
have two copies of the same allele for a
given gene; hybrid (heterozygous)
organisms have two different alleles for a
given gene.
Copyright © 2009 Pearson Education Inc.
9.3 How Are Single Traits Inherited?
 The distribution of alleles in gametes
homozygous parent
A
A
gametes
A
A
(a) Gametes produced by a homozygous parent
heterozygous parent
A
a
gametes
A
(b) Gametes produced by a heterozygous parent
Copyright © 2009 Pearson Education Inc.
a
Fig. 9-6
9.3 How Are Single Traits Inherited?
 Mendel’s hypothesis was that two plants
may look alike (phenotype) but have a
different allele composition (genotype).
 Purple pea plants had PP or Pp genotypes,
but their phenotype (purple color) was the
same.
 The F2 generation could be described as
having three genotypes (¼ PP, ½ Pp, and ¼
pp) and two phenotypes (¾ purple and ¼
white).
Copyright © 2009 Pearson Education Inc.
9.3 How Are Single Traits Inherited?
 The Punnett square
method
Pp
self-fertilize
P
1
2
1
2
1
2
p
P
sperm
1
2
eggs
1
4
PP
1
4
Pp
1
4
pP
1
4
pp
p
Fig. 9-8
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9.3 How Are Single Traits Inherited?
PP or Pp
pp
all eggs
sperm unknown
if PP
p
all
sperm
p
if Pp
P
p
eggs
1
2
all Pp
1
2
eggs
P
1
2 Pp
sperm
 Mendel predicted
the outcome of
cross-fertilizing Pp
plants with
homozygous
recessive plants
(pp)—there should
be equal numbers of
Pp (purple) and pp
(white) offspring.
pollen
p
1
2 pp
Fig. 9-9
Copyright © 2009 Pearson Education Inc.
9.4 How Are Multiple Traits Inherited?
 Mendel next crossed pea plants that differed
in two traits, such as seed color (yellow or
green) and seed shape (smooth or
wrinkled).
• He knew from previous crosses that smooth
and yellow were both dominant traits in peas.
• His first cross was a true-breeding plant with
smooth, yellow seeds (SSYY) to a truebreeding plant with wrinkled, green seeds
(ssyy).
Copyright © 2009 Pearson Education Inc.
9.4 How Are Multiple Traits Inherited?
 All the offspring of this cross (F1 generation)
were SsYy and had smooth, yellow seeds
(both dominant traits).
 F1 plants were allowed to self-fertilize and
produced F2 offspring in the phenotypic ratio
9:3:3:1.
Copyright © 2009 Pearson Education Inc.
9.4 How Are Multiple Traits Inherited?
 Mendel concluded that multiple traits are
inherited independently.
• Mendel realized that these results could be
explained if the genes for seed color and seed
shape were inherited independently.
• The independent inheritance of two or more
distinct traits is called the law of independent
assortment.
• Multiple traits are inherited independently
because the alleles of one gene are
distributed to gametes independently of the
alleles of other genes.
Copyright © 2009 Pearson Education Inc.
9.4 How Are Multiple Traits Inherited?
 Predicting
genotypes and
phenotypes
SsYy
self-fertilize
eggs
1 SY
4
sperm
1 Sy
4
1 sY
4
1 sy
4
1 SY
4
1 Sy
4
1 sY
4
1
16 SSYY
1
16 SSYy
1
16 SsYY
1
16 SsYy
1
16 SSyY
1
16 SSyy
1
16 SsyY
1
16 Ssyy
1
16 sSYY
1
16 sSYy
1
16 ssYY
1
16 ssYy
1
16 sSyY
1
16 sSyy
1
16 ssyY
1
16 ssyy
1
4
sy
Fig. 9-11
Copyright © 2009 Pearson Education Inc.
9.4 How Are Multiple Traits Inherited?
 Independent
assortment of
alleles
S
pairs of alleles on
homologous chromosomes
in diploid cells
s
Y
y
chromosomes replicate
S
Y
s
y
replicated homologues
pair during metaphase
of meiosis I,
orienting like this
or like this
y
S
s
Y
meiosis I
S
Y
s
y
S
y
s
Y
S
Y
s
y
S
y
s
Y
meiosis II
S
S
s
Y
Y
SY
S
s
y
y
sy
s
S
y
y
s
Y
Y
Sy
sY
independent assortment produces four equally
likely allele combinations during meiosis
Fig. 9-12
Copyright © 2009 Pearson Education Inc.
9.5 How Are Genes Located on the Same
Chromosome Inherited?
 Genetic linkage is the inheritance of genes as a
group because they are on the same
chromosome.
• Genes that are located on the same chromosome are
inherited together, rather than sorted independently.
• In peas, the gene for flower color and the gene for
pollen shape occur on the same chromosome and are
inherited together.
• Because the two genes are located on the same
chromosomes, they tend to end up in gametes
together, and are then expressed in the plants.
Copyright © 2009 Pearson Education Inc.
9.5 How Are Genes Located on the Same
Chromosome Inherited?
 Crossing over can create new combinations of
linked alleles.
• Genes on the same chromosome do not always stay
together.
• During prophase I of meiosis, homologous
chromosomes sometimes exchange parts in the
process, called crossing over.
• Crossing over produces a new allele combination on
both homologous chromosomes.
• Therefore, the chromosomes of each haploid
daughter cell receives different combinations of
alleles from those of the parent cell.
Copyright © 2009 Pearson Education Inc.
9.6 How Is Sex Determined?
 Offspring sex is determined
sex chromosomes.
female parent
• In mammals, females have two X
chromosomes and males have an X
chromosome and a Y chromosome.
• Y chromosomes are much smaller
than the X chromosomes.
X1
eggs
Y
Xm
X2
female offspring
X1
Y
X2
Y
male offspring
Copyright © 2009 Pearson Education Inc.
Xm
Xm
sperm
Xm
X2
X1
X1
male parent
X2
Y
9.7 How Are Sex-Linked Genes Inherited?
 Genes that are found on one sex
chromosome but not on the other are called
sex-linked.
• Because females have two X chromosomes,
they can be either homozygous or
heterozygous for genes on the X
chromosome.
• Males only have one X chromosome, and
therefore express all the alleles they have on
their X chromosome.
Copyright © 2009 Pearson Education Inc.
9.8 Do Mendelian Rules Of Inheritance
Apply To All Traits?
 When a heterozygous phenotype is
intermediate between the two homozygous
phenotypes, the pattern of inheritance is
called incomplete dominance.
• Human hair texture is influenced by a gene
with two incompletely dominant alleles, C1 and
C2.
• A person with two copies of the C1 allele has
curly hair; two copies of the C2 allele produces
straight hair; heterozygotes with C1C2
genotype have wavy hair.
Copyright © 2009 Pearson Education Inc.
9.8 Do Mendelian Rules Of Inheritance
mother
Apply To All Traits?
 Two wavy-haired
people could have
CC
the following
children: ¼ curly
C
C
eggs
(C1C1), ½ wavy
(C1C2), and ¼
father
C
straight (C2C2).
CC
CC
1
2
1
2
sperm
1
C1C2
1
1
1
2
C2
C1C2
C2C2
Fig. 9-17
Copyright © 2009 Pearson Education Inc.
9.8 Do Mendelian Rules Of Inheritance
Apply To All Traits?
 A single gene may have multiple alleles.
• A single individual can have only two alleles
for any gene, one on each homologous
chromosomes.
• However, within all the members of a species
there could be dozens of alleles for every
gene.
Copyright © 2009 Pearson Education Inc.
9.8 Do Mendelian Rules Of Inheritance
Apply To All Traits?
Copyright © 2009 Pearson Education Inc.
9.8 Do Mendelian Rules Of Inheritance
Apply To All Traits?
 A single trait may be influenced by several
genes.
• Many physical traits are governed not by
single genes, but by interactions among two or
more genes, a phenomenon called polygenic
inheritance.
• The more genes that contribute to a single
trait, the greater the number of phenotypes
and the finer the distinctions among them.
Copyright © 2009 Pearson Education Inc.
9.10 How Are Single-Gene Disorders
Inherited?
 Some human genetic disorders are caused
by recessive alleles.
• Many genes encode information to synthesize
enzymes or structural proteins in cells, and a
defective allele in such a gene may cause
damaged or inactive protein.
• In some cases, a defective gene may be
masked when one normal allele is also
present and makes enough functional protein.
Copyright © 2009 Pearson Education Inc.
9.10 How Are Single-Gene Disorders
Inherited?
 Some human genetic disorders are caused
by dominant alleles.
• Many genetic diseases are caused by
dominant alleles, in which a single defective
allele is enough to cause the disorder.
• For dominant diseases to be inherited, at least
one parent must suffer from the disease but
live long enough to have children.
• Some diseases, like Huntington disease, do
not appear until after the affected person has
reproduced.
Copyright © 2009 Pearson Education Inc.
10.1 What Is The Structure Of DNA?
 Individual traits of an organism are
transmitted from parent to offspring in
discrete units of DNA called genes.
 Genes are located on chromosomes found
within the nucleus of cells.
 What makes all organisms different from
each other is the arrangement and
molecular composition of its genes.
Copyright © 2009 Pearson Education Inc.
10.1 What Is The Structure Of DNA?
 DNA is composed of four different subunits,
called nucleotides.
• Each nucleotide has three parts:
• A phosphate group
• Deoxyribose, a 5 Carbon sugar
• One of four different nitrogen-containing
bases
• Thymine
• Cytosine
• Adenine
• Guanine
Copyright © 2009 Pearson Education Inc.
10.1 What Is The Structure Of DNA?
 A DNA molecule contains two nucleotide
strands.
• A DNA molecule consists of two DNA strands
of linked nucleotides.
• Within each strand, the phosphate group of
one nucleotide binds to the sugar group of the
next nucleotide.
• The sugar-phosphate bonding produces a
sugar-phosphate backbone to the DNA
molecule.
Copyright © 2009 Pearson Education Inc.
10.1 What Is The Structure Of DNA?
 The Watson-Crick model of DNA structure
nucleotide
nucleotide
free
phosphate
phosphate
base
(cytosine)
sugar
free sugar
(a) Hydrogen bonds hold complementary base pairs
together in DNA
(b) Two DNA strands form
a double helix
(c) Four turns of a
DNA double helix
Fig. 10-2
Copyright © 2009 Pearson Education Inc.
10.1 What Is The Structure Of DNA?
 Nucleotide rungs only result in specific pair
combinations.
• Adenine only pairs with Thymine.
• Guanine only pairs with Cytosine.
• This A–T and G–C coupling is called
complementary base pairing.
Copyright © 2009 Pearson Education Inc.
10.3 How Is DNA Copied?
 Cells reproduce themselves by making two
daughter cells from each parental cell, each
with a complete copy of all the parental
cell’s genetic information.
 During cell reproduction, the parental cell
synthesizes two exact copies of its DNA
through a process called DNA replication.
 One copy goes into each daughter cell.
Copyright © 2009 Pearson Education Inc.
10.3 How Is DNA Copied?
 DNA replication produces two DNA double
helices, each with one original strand and
one new strand.
• DNA replication requires three ingredients:
• The parental DNA strands
• Free nucleotides that were synthesized in
the cytoplasm and then imported to the
nucleus
• Enzymes that unwind the parental DNA
double helix and synthesize the new DNA
strands
Copyright © 2009 Pearson Education Inc.
10.3 How Is DNA Copied?
 DNA replication produces two DNA double
helices, each with one original strand and one new
strand (continued).
• The first step involves enzymes called DNA helicases,
which pull apart the parental DNA double helix.
• Next, enzymes called DNA polymerases move along
each separated parental DNA strand, matching each
base on the strand with free nucleotides.
 DNA replication keeps, or conserves, one parental
DNA strand and produces one new daughter
strand (semiconservative replication).
Copyright © 2009 Pearson Education Inc.
10.3 How Is DNA Copied?
 The basic features
of DNA replication
1 Parental DNA
double helix
2 The parental DNA
is unwound
3 New DNA strands
are synthesized with
bases complementary
to the parental
strands
free nucleotides
4 Each new double helix is composed
of one parental strand (blue) and one
new strand (red)
Fig. 10-3
Copyright © 2009 Pearson Education Inc.
10.4 What Are The Mechanisms Of DNA
Replication?
 DNA helicase separates the parental DNA strands
by breaking the hydrogen bonds between
complementary bases.
• This activity separates the two strands and forms a
replication bubble where the parental strands are no
longer paired.
• Replication then proceeds.
• There is a replication fork on each end of the bubble,
where replication is taking place and the original DNA
strand is unzipping.
• The unzipping and replication continues in both
directions until the new strands are completely
formed.
Copyright © 2009 Pearson Education Inc.
10.4 What Are The Mechanisms Of DNA
Replication?
 The mechanism of DNA replication,
step (2)
DNA helicase
DNA helicase
replication forks
Fig. 10-5(2)
Copyright © 2009 Pearson Education Inc.
10.4 What Are The Mechanisms Of DNA
Replication?
 DNA polymerase synthesizes new DNA
strands.
• At the replication forks, DNA polymerase
recognizes unpaired nucleotide bases in the
parental strand and matches them up with free
nucleotides.
• It then links up the phosphate of the incoming
nucleotide with the sugar of the previously
added nucleotide, thereby contributing to the
growing molecule backbone.
Copyright © 2009 Pearson Education Inc.
10.4 What Are The Mechanisms Of DNA
Replication?
 DNA helicase and DNA polymerase work
together to copy each strand of separated
parental DNA.
• Polymerase # 1 lands on one strand of DNA
and follows behind the helicase toward the
free phosphate end of the DNA, making a
continuous new DNA strand.
• DNA polymerase # 2 on the other parental
strand moves away from the helicase and
makes only part of the new DNA strand.
Copyright © 2009 Pearson Education Inc.
10.4 What Are The Mechanisms Of DNA
Replication?
 As the helicase continues to unwind more of
the double helix, additional DNA polymerase
(# 3, # 4, etc.) must land on this strand to
synthesize more pieces of DNA.
 Therefore, DNA synthesis on the second
parental strand is discontinuous.
Copyright © 2009 Pearson Education Inc.
10.4 What Are The Mechanisms Of DNA
Replication?
 The mechanism of DNA replication,
step (4)
DNA polymerase #1
continues along the
parental DNA strand
DNA
polymerase #2
leaves
DNA
polymerase #3
Fig. 10-5(4)
Copyright © 2009 Pearson Education Inc.
10.4 What Are The Mechanisms Of DNA
Replication?
 Multiple DNA polymerases make many
pieces of DNA of varying lengths that need
to be tied together to form a single
continuous DNA polymer.
 DNA ligase joins together the separate
segments of DNA.
Copyright © 2009 Pearson Education Inc.
10.4 What Are The Mechanisms Of DNA
Replication?
 The mechanism of DNA replication,
step (5)
DNA polymerase #3
leaves
DNA
polymerase #4
DNA ligase joins the daughter
DNA strands together
Fig. 10-5(5)
Copyright © 2009 Pearson Education Inc.
10.4 What Are The Mechanisms Of DNA
Replication?
 Proofreading produces almost error-free replication
of DNA.
• DNA polymerase is almost 100% perfect in
matching free nucleotides with those on the
original parental strands.
• Once in every 10,000 base pairs, there is an
error in replication.
• Some types of DNA polymerase recognize errors
when they are made and correct them.
• This keeps the total errors in a complete DNA
molecule to one mistake in every billion base
pairs.
 Mistakes that remain in the DNA nucleotide
sequence are called mutations.
Copyright © 2009 Pearson Education Inc.
Types of Mutations





Point mutation - individual nucleotide in the
DNA sequence is changed
Insertion mutation - one or more nucleotide
pairs are inserted into the DNA double helix
Deletion mutation - one or more nucleotide
pairs are removed from the double helix
Inversion - piece of DNA is cut out of a
chromosome, turned around, and re-inserted
into the gap
Translocation - chunk of DNA (often very
large) is removed from one chromosome and
attached to another
Copyright © 2009 Pearson Education Inc.
Copyright © 2009 Pearson Education Inc.
Copyright © 2009 Pearson Education Inc.
Copyright © 2009 Pearson Education Inc.
11.1 How Is The Information In DNA Used
In A Cell?
 Most genes contain information for the
synthesis of a single protein.
• A gene is a stretch of DNA encoding the
instructions for the synthesis of a single
protein.
• Proteins form cellular structures and the
enzymes that catalyze cellular chemical
reactions.
Copyright © 2009 Pearson Education Inc.
11.1 How Is The Information In DNA Used
In A Cell?
Copyright © 2009 Pearson Education Inc.
11.1 How Is The Information In DNA Used
In A Cell?
 Protein synthesis
occurs in two steps,
called transcription
and translation.
gene
DNA
(nucleus)
(a) Transcription
messenger RNA
(cytoplasm)
Transcription of the
gene produces an
mRNA with a
nucleotide sequence
complementary to one
of the DNA strands
Translation of the mRNA
produces a protein molecule
with an amino acid sequence
determined by the nucleotide
sequence in the mRNA
(b) Translation
ribosome
protein
Copyright © 2009 Pearson Education Inc.
11.1 How Is The Information In DNA Used
In A Cell?
 Transcription: the information contained in
the DNA of a specific gene is copied into
one of three types of RNA
• Messenger RNA (mRNA)
• Transfer RNA (tRNA)
• Ribosomal RNA (rRNA)
 In eukaryotic cells, transcription occurs in
the nucleus.
Copyright © 2009 Pearson Education Inc.
Flash
11.1 How Is The Information In DNA Used
In A Cell?
 Translation: ribosomes convert the base
sequence in mRNA to the amino acid
sequence of a protein
• In eukaryotic cells, translation occurs in the
cytoplasm.
Copyright © 2009 Pearson Education Inc.
Flash
11.2 What Are The Functions Of RNA?
 Messenger RNA carries the code for a
protein from the nucleus to the cytoplasm.
• All RNA is produced by transcription from
DNA, but only mRNA carries the code for
amino acid sequence of a protein.
• mRNA is synthesized in the nucleus and
enters the cytoplasm through nuclear
envelope pores.
• In the cytoplasm, mRNA binds to ribosomes,
which synthesize a protein specified by the
mRNA base sequence; DNA remains in the
nucleus.
Copyright © 2009 Pearson Education Inc.
11.2 What Are The Functions Of RNA?
 Ribosomal RNA and proteins form ribosomes.
• Each ribosome consists of two subunits—one small
and one large.
• The small subunit has binding sites for mRNA, a
“start” tRNA, and other proteins that cooperate to
read mRNA to start protein synthesis.
• The large subunit has two binding sites for tRNA
molecules, and one catalytic site where peptide
bonds join amino acids together into a protein.
• During protein synthesis, the two subunits come
together, clasping an mRNA molecule between them.
Copyright © 2009 Pearson Education Inc.
11.2 What Are The Functions Of RNA?
 Transfer RNA molecules carry amino acids
to the ribosomes.
• Each cell synthesizes many different kinds of
transfer RNA, one or more for each amino
acid.
• Twenty different kinds of enzymes in the
cytoplasm, one for each amino acid, recognize
the rRNA and attach the correct amino acid.
• These “loaded” tRNA molecules deliver their
amino acids to the ribosome, where they are
incorporated into the growing protein chain.
Copyright © 2009 Pearson Education Inc.
11.3 What Is The Genetic Code?
 The genetic code translates the sequence of
bases in nucleic acids into the sequence of
amino acids in proteins.
• A sequence of three bases codes for an
amino acid; the triplet is called a codon.
• There are 64 possible combinations of
codons, which is more than enough to code
for the 20 amino acids in proteins.
Copyright © 2009 Pearson Education Inc.
11.3 What Is The Genetic Code?
Copyright © 2009 Pearson Education Inc.
11.3 What Is The Genetic Code?
 How does a cell recognize where codons
start and stop, and where the code for an
entire proteins starts and stops?
• Most codons specify a specific amino acid in a
protein sequence, but others are punctuation
marks that indicate the end of one protein
sequence and the start of another.
• All proteins begin with the start codon AUG
(methionine), and all end with UAG, UAA, or
UGA, called stop codons.
• Almost all amino acids are coded for by more
than one codon (e.g., six codons code for
leucine).
Copyright © 2009 Pearson Education Inc.
11.4 How Is The Information In A Gene
Transcribed Into RNA?
 Transcription copies the genetic information
of DNA into RNA in the nucleus of
eukaryotic cells.
• Transcription is made up of three different
processes:
• Initiation: the promotor region at the
beginning of a gene starts transcription
• Elongation: the main body of a gene is
where the RNA strand is elongated
• Termination: the termination signal at end of
a gene is where RNA synthesis stops
Copyright © 2009 Pearson Education Inc.
11.4 How Is The Information In A Gene
Transcribed Into RNA?
 Transcription begins when RNA polymerase
binds to the promotor of a gene.
• RNA polymerase catalyzes the transcription of
DNA to RNA.
• RNA polymerase first finds the promoter
region (a non-transcribed sequence of DNA
bases) that marks the start of a gene, and
then binds to it, opening up the DNA as it
does.
• Transcription of the gene begins after the
promoter is bound to RNA polymerase.
Copyright © 2009 Pearson Education Inc.
11.4 How Is The Information In A Gene
Transcribed Into RNA?
 Elongation generates a growing strand of
RNA.
• RNA polymerase adds complementary bases
to those in the DNA template strand, to make
a growing RNA strand that has uracil rather
than thymine complementary to adenine.
• The two strands of DNA re-form the original
double helix.
• One end of the growing RNA strand drifts
away from the DNA molecule, while the other
remains attached to the DNA template strand
by the RNA polymerase.
Copyright © 2009 Pearson Education Inc.
11.4 How Is The Information In A Gene
Transcribed Into RNA?
 Transcription stops when RNA polymerase
reaches the termination signal.
• RNA polymerase continues along the DNA
template strand until it comes to the
termination signal (a specific sequence of
DNA bases).
• At the termination signal, RNA polymerase
drops off the DNA and releases the completed
RNA molecule.
• The enzyme is ready to bind to another
promoter, to start the process over.
Copyright © 2009 Pearson Education Inc.
11.4 How Is The Information In A Gene
Transcribed Into RNA?
 Transcription is selective.
• Some genes are transcribed in all cells
because they encode essential proteins, like
the electron transport chain of mitochondria.
• Other genes are transcribed only in specific
types of cells.
• Proteins bind to “control regions” near gene
promotors and block or enhance the binding of
RNA polymerase.
• By this means, the amount of a specific
protein encoded by a specific gene in a cell
can be controlled.
Copyright © 2009 Pearson Education Inc.
11.5 How Is The Information In Messenger
RNA Translated Into Protein?
 mRNA, with a specific base sequence, is
used during translation to direct the
synthesis of a protein with the amino acid
sequence encoded by the mRNA.
• Decoding the base sequence of mRNA is the
job of tRNA and ribosomes in the cytoplasm.
• The ability of tRNA to deliver the correct
amino acid to the ribosomes depends on base
pairing between each codon of mRNA and a
set of three complementary bases in tRNA,
called the anticodon.
Copyright © 2009 Pearson Education Inc.
11.5 How Is The Information In Messenger
RNA Translated Into Protein?
 Like transcription, translation has three
steps:
• Initiation of protein synthesis
• Elongation of the protein chain
• Termination of translation
Copyright © 2009 Pearson Education Inc.
11.5 How Is The Information In Messenger
RNA Translated Into Protein?
 Initiation
Initiation:
second tRNA binding site
amino acid
met
met
tRNA
initiation
complex
methionine
tRNA
U A C
small
ribosomal
subunit
A tRNA with an attached
methionine amino acid binds
to a small ribosomal subunit,
forming an initiation complex.
catalytic site
anticodon
U A C
first tRNA
binding
site
mRNA
GC A U G GU U C A
me
t
U A C
large
ribosomal
subunit
GC A U G G U U C A
start codon
The initiation complex binds to an
mRNA molecule. The methionine (met)
tRNA anticodon (UAC) base-pairs with
the start codon (AUG) of the mRNA.
The large ribosomal subunit
binds to the small subunit. The
methionine tRNA binds to the first
tRNA site on the large subunit.
Fig. 11-5(1,2,3)
Copyright © 2009 Pearson Education Inc.
11.5 How Is The Information In Messenger
RNA Translated Into Protein?
 Elongation
Elongation:
catalytic site
met
val
U A C C A A
G C A U G G U U C A
The second codon of mRNA
(GUU) base-pairs with the
anticodon (CAA) of a second
tRNA carrying the amino acid
valine (val). This tRNA binds to
the second tRNA site on the
large subunit.
met
val peptide
bond
U A C C A A
G C A U G G U U C A
The catalytic site on the
large subunit catalyzes the
formation of a peptide bond
linking the amino acids
methionine and valine. The
two amino acids are now
attached to the tRNA in the
second binding site.
initiator
met
tRNA detaches
val
C
A A
G C A U G G U U C A U A G
ribosome moves one codon to the right
The “empty” tRNA is released
and the ribosome moves down the
mRNA, one codon to the right. The
tRNA that is attached to the two
amino acids is now in the first tRNA
binding site and the second tRNA
binding site is empty.
Fig. 11-5(4,5,6)
Copyright © 2009 Pearson Education Inc.
11.5 How Is The Information In Messenger
RNA Translated Into Protein?
 Elongation (continued)
met
val
his
C A A G U A
G C A U G G U U C A U A G
The third codon of mRNA (CAU)
base-pairs with the anticodon (GUA) of
a tRNA carrying the amino acid histidine
(his). This tRNA enters the second tRNA
binding site on the large subunit.
met
val
his
C A A G U A
G C A U G G U U C A U A G
The catalytic site forms a peptide bond
between valine and histidine, leaving the peptide
attached to the tRNA in the second binding site.
The tRNA in the first site leaves, and the
ribosome moves one codon over on the mRNA.
Fig. 11-5(7,8)
Copyright © 2009 Pearson Education Inc.
11.5 How Is The Information In Messenger
RNA Translated Into Protein?
 Termination
met
va
l
Termination:
his
arg
completed
arg
peptide
ile
stop codon
CGA A UC U AGUAA
This process repeats until
a stop codon is reached; the
mRNA and the completed
peptide are released from the
ribosome, and the subunits
separate.
Fig. 11-5(9)
Copyright © 2009 Pearson Education Inc.
11.5 How Is The Information In Messenger
RNA Translated Into Protein?
 Summing up: transcription and translation
• With a few exceptions, each gene codes for a
single protein.
• Transcription of a protein-coding gene
produces an mRNA that is complementary to
the template strand of the DNA for the gene.
• Enzymes in the cytoplasm attach the
appropriate amino acid to each tRNA.
• The mRNA moves from the nucleus to the
cytoplasm.
• tRNAs carry their attached amino acids to the
ribosome.
Copyright © 2009 Pearson Education Inc.
11.5 How Is The Information In Messenger
RNA Translated Into Protein?
 Summing up: transcription and translation
(continued)
• At the ribosome, the bases in tRNA
anticodons bind to the complementary bases
in mRNA codons.
• The amino acids attached to the tRNAs line up
in the sequence specified by the codons.
• The ribosome joins the amino acids together
with peptide bonds to form a protein.
• When a stop codon is reached, the finished
protein is released from the ribosome.
Copyright © 2009 Pearson Education Inc.
11.5 How Is The Information In Messenger
RNA Translated Into Protein?
 Complementary
base pairing is
critical to decoding
genetic information.
gene
(a) DNA
A
T
G G
G
A
G
T
T
T
A
C C
C
T
C
A
A
etc.
G
U
U
etc.
complementary
DNA strand
template DNA
strand
etc.
codons
(b) mRNA
A U
G G
G
A
anticodons
(c) tRNA
U
A
C
C C
U
C
A
A etc.
amino acids
(d) protein
methionine
glycine
valine
etc.
Fig. 11-6
Copyright © 2009 Pearson Education Inc.
11.6 How Do Mutations Affect Gene
Function?
 Mutations are the raw material for evolution.
• Mutations are the ultimate source of all genetic
differences among individuals.
• Without mutations, individuals would share the same
DNA sequence.
• Most mutations are harmful; some improve the
individual’s ability to survive and reproduce.
• The mutation may be passed from generation to
generation and become more common over time.
• This process is known as natural selection, and is the
major cause of evolutionary change.
Copyright © 2009 Pearson Education Inc.
11.7 Are All Genes Expressed?
 All of the genes in the human genome are
present in each body cell, but individual
cells express only a small fraction of them.
• The particular set of genes that is expressed
depends on the type of cell and the needs of
the organism.
• This regulation of gene expression is crucial
for proper functioning of individual cells and
entire organisms.
Copyright © 2009 Pearson Education Inc.
11.7 Are All Genes Expressed?
 Gene expression differs from cell to cell and
over time.
• The set of genes that are expressed depends
on the function of a particular cell.
• Hair cells synthesize the protein keratin, while
muscle cells make the proteins actin and
myosin but do not make keratin.
• A human male does not express a casein
gene, the protein in human milk, but will pass
on the gene for casein synthesis to his
daughter, who will express it if she bears
children.
Copyright © 2009 Pearson Education Inc.
11.7 Are All Genes Expressed?
 Environmental cues influence gene
expression.
• Changes in an organism’s environment help
determine which genes are transcribed.
• Longer spring days stimulate the sex organs
of birds to enlarge and produce sex
hormones.
• These hormones cause the birds to produce
eggs and sperm, to mate, and to build nests.
• All these changes result directly or indirectly
from alterations in gene expression.
Copyright © 2009 Pearson Education Inc.
11.8 How Is Gene Expression Regulated?
 A cell may regulate gene expression in
many different ways.
• It may alter the rate of transcription of mRNA.
• It may affect how long a given mRNA
molecule lasts before being broken down.
• It may affect how fast the mRNA is translated
into protein.
• It may affect how long the protein lasts, or how
fast a protein enzyme catalyzes a reaction.
Copyright © 2009 Pearson Education Inc.
11.8 How Is Gene Expression Regulated?
 Regulatory proteins that bind to promoters
alter the transcription of genes.
• Many steroid hormones act in this way.
• In birds, estrogen enters cells of the female
reproductive system and binds to a receptor
protein during the breeding season.
• The estrogen–receptor combination then
binds to the DNA in a region near the
promotor of an albumen gene.
Copyright © 2009 Pearson Education Inc.
11.8 How Is Gene Expression Regulated?
 Regulatory proteins that bind to promoters
alter the transcription of genes (continued).
• This attachment makes it easier for RNA
polymerase to bind to the promotor and to
transcribe large amounts of albumen mRNA,
which is translated into the albumin protein
needed to make eggs.
Copyright © 2009 Pearson Education Inc.
11.8 How Is Gene Expression Regulated?
 Some regions of chromosomes are
condensed and not normally transcribed.
• Certain parts of eukaryotic chromosomes are
in a highly condensed, compact state in which
most of the DNA is inaccessible to RNA
polymerase.
• Some of these tightly condensed regions may
contain genes that are not currently being
transcribed, but when those genes are
needed, the portion of the chromosome
containing those genes becomes
“decondensed” so that transcription can occur.
Copyright © 2009 Pearson Education Inc.
11.8 How Is Gene Expression Regulated?
 Entire chromosomes may be inactivated
and not transcribed.
• In some cases, almost an entire chromosome
may be condensed, making it largely
inaccessible to RNA polymerase.
• In human females, one of their two X
chromosomes may become inactivated by a
special coating of RNA called Xist, which
condenses the chromosome and prevents
gene transcription.
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