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CHAPTER 12
The
Reproduction
of Cells
CELL DIVISION: terminology
 genome:
total genetic
information in a cell


prokaryotic genome = often a single DNA
molecule
eukaryotic genome = usually includes many
DNA molecules
(each DNA molecule contains 100-1000s of
genes)
CELL DIVISION: terminology
replication
and distribution
of DNA is manageable
becase it is packed into
chromosomes
chromo- = color
 some- = body

CELL DIVISION: terminology

every eukaryotic species has
characteristic chromosome number in
nucleus
SOMATIC CELLS = body cells (2n)
 GAMETES = sperm, egg (1n)

CELL DIVISION: terminology

DNA is associated with various
proteins that maintain structure

CHROMATIN = DNA-protein complex
--normally, a long thin strand
--however, it condenses after
duplication to prepare for division
CELL DIVISION: terminology
 each
duplicated chromosome
has 2 sister CHROMATIDS
 each
chromatid goes to separate
cell during division
CELL DIVISION: terminology
 MITOSIS-division of nucleus
 CYTOKINESIS-division of cytoplasm
 MEIOSIS—production of gametes
(occurs only in ovaries & testes)
 FERTILIZATION-fusion of gametes
CELL CYCLE
 MITOTIC
(M) PHASE
usually shortest part of cell
cycle
 INTERPHASE
much longer (90% of cell cycle)
CELL CYCLE
 G1
= Gap 1…growth
 S = synthesis of DNA
 G2 = Gap 2…growth, preparation for cell
division
M
= mitosis (or meiosis)
***cell grows by producing protein and
forming organelles***
CELL CYCLE: interphase
 1.
2 centrosomes present, each with
a pair of centrioles
 2. microtubules extend from
centrosomes, forming radial array
(asters)
CELL CYCLE: prophase
 1.
chromatin more tightly coiled
 discrete chromosomes
 2. nucleoli disappear
 3. mitotic spindle forms
 4. centrosomes migrate away
INTERPHASE
CELL CYCLE: prometaphase
 1.
nuclear envelope fragments
 2. microtubules “invade” to interact
with chromosomes
 3. each chromatid has a kinetochore
 4. some microtubules attach to
kinetochores
CELL CYCLE: metaphase
 1.
centrosomes now at opposite
poles
 2. chromosomes align at metaphase
plate
 3. all kinetochores attached to
microtubules
CELL CYCLE: anaphase
 1.
paired centromeres of
chromosomes separate
 2. chromosomes move toward
opposite poles
 3. poles of cell move further apart
CELL CYCLE: telophase
 1.
nonkinetochore microtubules
lengthen cell further
 2. daughter nuclei form at poles
 3. nuclear envelope arises from
existing fragments
 4. chromatin becomes less coiled
 5. cleavage furrow develops
CELL CYCLE: finer points
 each
of the 2 joined chromatids has a
kinetochore
 kinetochore = structure of proteins
and chromosomal DNA at
centromere
CELL CYCLE: finer points
 What
is the function of
nonkinetochore microtubules?
in animal cell, elongate cell during
anaphase
(motor proteins drive microtubules
past each other, using ATP)
CYTOKINESIS
 in
animal cells, occurs by process
known as cleavage
 first
sign is appearance of cleavage
furrow (begins near old metaphase
plate)
PLANT CELL DIVISION

no cleavage furrow

during telophase, vesicles derived from the Golgi
apparatus move along microtubules to the middle of
the cell where they coalesce, producing a cell plate
cell plate enlarges until its surrounding membrane
fuses with the plasma membrane along the perimeter
of the cell
2 daughter cells result…each with its own plasma
membrane


checkpoints




a checkpoint in the cell cycle is a critical control point
where stop and go-ahead signals can regulate the
cycle
generally built-in stop signals halt the cycle until
overridden by go-ahead signals
major checkpoints are found in G1, G2, and M phases
for many cells, G1 checkpoint seems to be most
important
(if no G1 signal is received, cell exits cycle into
nondividing state known as G0)
cancer cells escape cell cycle control




tumor: mass of abnormal cells within
otherwise normal tissue
benign tumor: abnormal cells remain at
original site
malignant tumor: becomes invasive enough
to to impair the functions of one or more
organs
metastasis: spread of cancer cells to locations
distant from their original site
Chapter 13
Meiosis and
Sexual Life
Cycles
heredity
 transmission
of traits from one
generation to the next
--offspring resemble parents
--however, there is variation
genome
 whole
complement of genes
gene
 passed
as hereditary units
 coded information
 segments of DNA
--polymer of 4 different kinds of monomer
(nucleotides)
--information is passed in form of sequences of
nucleotides
--most genes are programs for the synthesis of
enzymes or other proteins
 one
chromosome =
100s – 1000s of genes
 LOCUS =
specific location on
chromosome
ASEXUAL
REPRODUCTION
 single
individual is sole parent
 all genes are passed to offspring
(changes can occur due to
mutation)
 FORMS:
mitosis, budding
budding
SEXUAL
REPRODUCTION
 two
parents give rise to offspring
 unique combination of genes in
offspring inherited from parents
SOMATIC CELL



any cell other than sperm or ovum
in humans, each somatic cell has 46
chromosomes
chromosomes can be separated based on:
(1.) length
(2.) position of centromere
(3.) banding patterns
KARYOTYPE
 micrograph
of organized
chromosomes
 homologous
paired
chromosomes are
GAMETES



in humans, each has single set of 22
autosomes + 1 sex chromosome
union of gametes called
fertilization or syngamy  zygote
as zygote develops, genes are passed on
precisely to all somatic cells thanks to mitosis
MEIOSIS I-INTERPHASE
chromosomes replicate 
sister chromatids
 (b.) centrosomes replicate
 (a.)
MEIOSIS—
Overview
MEIOSIS I—
PROPHASE
 (a.)
chromosomes condense
 (b.) homologous chromosomes pair
 (c.) SYNAPSIS = homologous
chromosomes attached to one
another
--appears as tetrad (cluster of 4
chromatids)
--crossing-over occurs (chiasmata)
MEIOSIS I—
PROPHASE
 (d.)
centrosomes migrate to poles
 (e.) nuclear membrane & nucleolus
disperse
 (f.) spindle fibers capture
kinetochores
 (g.) chromosomes move toward
metaphase plate
MEIOSIS I—
METAPHASE
 chromosomes
line up at
metaphase plate (in homologous
pairs)
MEIOSIS I—
ANAPHASE
 (a.)
sister chromatids remain
connected and move toward
poles
 (b.) homologous chromosomes
move toward opposite poles
MEIOSIS I—
TELOPHASE
 cytokinesis
and telophase occur
simultaneously
MEIOSIS I
MEIOSIS II
THREE EVENTS
UNIQUE TO MEIOSIS
 1.
SYNAPSIS
 2. homologous chromosomes
(not sister chromatids) pair at
metaphase plate
 3. sister chromatids do not
separate at anaphase I.
SOURCES OF GENETIC
VARIATION
 A.
independent assortment
--alignment of chromosomes at
metaphase plate is random
--# of possible combinations at
metaphase = 2n (n = haploid #)
--223 (~8 million) possible
combinations in humans
independent assortment
SOURCES OF GENETIC
VARIATION
 B.
crossing over
--produces recombinant chromosomes, which
combine genes inherited from mother and
father
--begins early in prophase I
--in humans, 2-3 crossovers occur per
chromosome pair
--at metaphase II, chromosomes can be
oriented in nonequivalent ways
crossing over
SOURCES OF GENETIC
VARIATION
 C. random
--ovum
(~8 million)
+
fusion of gametes
sperm
(~8 million)

zygote
(1 of ~64 million)
CHAPTER 14
Mendel and
the Gene Idea
Mendel



brought experimental and quantitative
approach
CHARACTER: heritable feature (i.e. flower
color, height, etc.)
TRAIT: variant of color (i.e. purple vs. white
flower, tall vs. short plant)
Mendel and the pea

probably chose garden peas because they come
in many varieties

use of peas gave Mendel strict control (they
usually self-fertilize)
--each flower has male and female
organs
Mendel’s process


only tracked “either-or” traits
(no traits with a continuum)
started experiments with true-breeding (pure)
plants
pure = all offspring are the same when the
plants self-pollinate
Mendel’s process

HYBRIDIZATION = mating (crossing) of two
true-breeding varieties
P generation  F1 generation  F2 generation
P = parental
F = filial (son)
Mendel’s process



Mendel used very large sample sizes
he also kept very accurate records
observed patterns in flower color, along with 6
other characters
Ex.
P:
F1:
F2:
purple flower x white flower
purple
3 purple : 1 white
Modern explanation
of F2 ratio




(1.) alternative versions of genes (alleles) account for
variations in inherited characters
--DNA within locus can vary in sequence of
nucleotides
(2.) for each character, an organism inherits 2 alleles,
one from each parent
(Mendel didn’t know about chromosomes)
(3.) if 2 alleles differ, the dominant allele is fully
expressed; the recessive allele has no noticeable effect
(4.) the two alleles for each character segregate during
gamete production = LAW OF SEGREGATION
Terms





homozygous: pair of identical alleles
heterozygous: 2 different alleles
(not true-breeding)
phenotype: organism’s traits (appearance)
genotype: genetic makeup
testcross:
recessive homozygote x dominant phenotype
w/ unknown genotype
Terms



Mendel developed Law of Segregation by
following a single character
monohybrid: F1 organism produced by cross
dihybrid cross: looks at 2 traits
(are traits independent, or are they inherited as
a package?)
--results in 9:3:3:1 ratio
Terms

Law of Independent Assortment

independent segregation of each pair of alleles
during gamete formation
Probability
Scale from 0 to 1
(1 = certain to occur, 0 = certain not to occur)




Ex. coin… ½ heads; ½ tails
probabilities of all possible outcomes must add
up to 1
each coin toss is an independent event
(compare coin toss to gamete formation)
Rule of Multiplication

probability x each
=
of each
probability
independent
event
probability
of series of
events
Ex. probability of 2 heads = ½ x ½ = ¼
Ex. Probability of a white flower
Rule of Addition

probability of an event that can occur in 2 or
more different ways is the sum of the separate
probabilities

Ex. probability of having a heterozygous
individual in the F2
Mendel’s impact



since Mendel’s study of peas, his principles
have been extended to diverse organisms (most
with patterns of inheritance far more complex
than peas)
for Mendel, each character was determined by
one gene (with complete dominance)
the relationship between genotype and
phenotype is rarely simple
Using probability
PpYyRr
x Ppyyrr
Incomplete dominance


F1 hybrids have intermediate phenotype
Ex. CRCR x CWCW
(red)
(white)
Incomplete dominance
Codominance


two alleles affect the phenotype in separate,
distinguishable ways
Ex. human blood groups
(surface proteins on RBCs)
M = one type N = another type
MN = produces both proteins
Codominance
Codominance
Tay-Sachs


brain cells of baby are unable to metabolize
gangliosides because crucial enzyme doesn’t
work properly
as lipids accumulate in brain, brain cells
gradually cease to function, leading to death
Tay-Sachs



organismal level: allele is recessive
(need 2 copies to have disease)
biochemical level: intermediate phenotype
with incomplete dominance
(heterozygotes lack symptoms)
molecular level: produces equal amounts of
normal and dysfunctional enzyme molecules
(illustrates codominance)
Dominant alleles
do not subdue recessive alleles
(they don’t interact at all)
 Ex. round vs. wrinkled pea seeds
R = codes for synthesis of enzyme that helps
convert sugar to starch in seed
r = codes for defective form of enzyme
***in recessive homozygote, sugar accumulates
in the seed; as seed develops, high sugar
concentration causes osmotic uptake of H2O

Multiple alleles


most genes in populations have more than 2
allele types
Ex. ABO blood groups (A, B, AB, O)
--letters refer to carbohydrates on surface
AB
A
B
O
Other patterns

pleiotrophy—ability of a gene to effect an
organism in many ways
Ex. gene for sickle cell anemia  multiple symptoms


epistasis—gene at one locus alters the
phenotypic expression of a gene at a second
locus
polygenic inheritance
--quantitative characters; usually vary along
continuum
--2 or more genes have additive effect
--Ex. skin pigment, height
epistasis
polygenic inheritance
Nature vs. Nurture


plants—sun, wind, and water can affect the
expression of genes
humans—nutrition  height
exercise  body shape
tanning  skin color
experience  intelligence



twins have phenotypic differences due to
different experiences
product of genotype is not a rigidly defined
phenotype
most traits are multifactorial
(both genetic and environmental influences)
Human disorders

humans are not convenient subjects for
genetic research
--long generation times
--few offspring
--breeding experiment unacceptable

to study humans, the results of previous
matings must be studied
--family histories are used to assemble pedigrees
--Ex. widow’s peak, attached earlobes

pedigrees not only help us understand the
past, they help us predict the future
Human disorders

1000s with simple recessive traits
--Ex. albinism, CF

genes code for proteins of a specific
function
--allele that codes for genetic disorder codes for
malfunctional form of protein, or no protein at all

disorder only in homozygous recessive
individuals
--heterozygotes are carriers
Human disorders


most genetic disorders are unevenly
distributed in populations
Ex. cystic fibrosis
--1/2500 of European white descent have disease


--1/25 whites are carriers
normal allele codes for membrane protein
that functions in Cl- transport between cells
and ECF
defective allele results in defective protein
which allows abnormally high extracellular
[Cl-]  thick coating of mucus
Human disorders


most genetic disorders are unevenly
distributed in populations
Ex. Tay-Sachs
--1/3500 births of Ashkenazic Jewish population


dysfunctional enzyme fails to breakdown
class of brain lipid
symptoms: seizure, blindness, decreased
motor and mental performance…death
within years
Human disorders


most genetic disorders are unevenly
distributed in populations
Ex. sickle-cell disease
--1/400 births among African-Americans




caused by single amino acid substitution in
Hb protein of RBCs
If O2 content is low, sickle-cell HB crystallizes
into long rods
(crystals deform RBCs into sickle shape)
example of pleiotrophy
treated with regular transfusions
Human disorders




DOMINANT ALLELES
Ex. Achondroplasia (Dd)
--rest of population (dd)
lethal dominants less common than lethal recessives
--arise by mutation in sperm or egg
--if kills offspring before reproduction, won’t be
passed on
lethal recessives can escape elimination if they are
late-acting
--Ex. Huntington’s disease
--no phenotypic evidence until age 35-45
genetic testing
Multifactorial disorders






genetic + environmental components
heart disease
diabetes
cancer
alcoholism
manic-depression
CHAPTER 15
The
Chromosomal
Basis of
Inheritance
SEX-LINKED GENES


genes located on sex chromosome
for males, single copy of mutant allele yields
mutant phenotype
sex-linked
inheritance
LINKED GENES


genes located on the same chromosome seem
to be inherited together
the number of genes in a cell are far greater
than the number of chromosomes
linked genes
in Drosophila
GENETIC RECOMBINATION




general term for production of offspring with
new combinations of traits
Ex. YyRr x yyrr
Ex. if 50% of all offspring are recombinants,
said to be 50% frequency of recombination
50% frequency of recombination is observed
for genes that are located on different
chromosomes
LINKED GENES




do not assort independently because they are
located on the same chromosome
(they move together through meiosis and
fertilization)
if genes are linked, should see 1:1:0:0 ratio
if ratio varies, indicates recombination
occurred
crossing over = mechanism for breaking
linkage
(non-sister chromatids break at
corresponding points and swap fragments)
production of recombinant
gametes
production of recombinant
offspring
Linked genes led Alfred Sturtevant, one
of Mendel’s students, to construct a
genetic map.




map = ordered list of loci along a particular
chromosome
hypothesis: calculated recombination frequencies
from experiments reflects actual distances between
genes on chromosomes
(farther apart increased chance of crossing over)
1 map unit = 1% recombination frequency
max % = 50% (cannot distinguish from independent
assortment)
genetic map
genetic map
Linkage maps, etc.


a linkage map cannot be used to put together a
scaled model of the chromosome
(provides the order of the genes, but not
precise locations)
CYTOLOGICAL MAPS =locate genes with
reference to markers on chromosomes
Sex chromosomes



although they behave as homologs, there is
very little crossing over
In humans, anatomical signs of sex begin at ~2
months for embryo
(before that point, gonads are generic)
1990—SRY gene discovered
--if expressed  testes
--if no SRY  ovaries
--codes for protein that regulates many other
genes
sex determination
Sex chromosomes

also carry genes unrelated to sex
(mostly applies to X chromosome)

Fathers  daughters
Mothers  sons and daughters



if sex-linked trait is recessive, female will only
express it if she is homozygous
since males only have one X,
homo/heterozygous does not apply
Sex-linked disorders



color blindness
Duchenne muscular dystrophy
--1/3500 males in U.S.
--progressive weakening of muscles and loss of
coordination (rarely survive early 20s)
--lack key muscle protein = dystrophin
hemophilia
--absence of one or more proteins required for
blood clotting
--history…
X-INACTIVATION
in females, one X chromosome in each cell
becomes inactivated early in embryonic
development
 inactive X compacts into a small object known
as a Barr body
--a few genes remain active; most do not
 selection of which X becomes inactive is
totally random
--thus, females have a mosaic of two types of
cells
(½ with active X from mother, other ½ from

X-inactivation
ERRORS AND EXCEPTIONS IN
CHROMOSOMAL
INHERITANCE
NONDISJUNCTION
 members of a pair of homologous
chromosomes
(a.) do not move apart properly during
meiosis I, or
(b.) sister chromatids fail to separate during
meiosis II
 as a result, one gamete receives 2 copies of
the same chromosome, while the other
gamete receives no copy
(other chromosomes usually distribute normally)

nondisjunction
ERRORS AND EXCEPTIONS IN
CHROMOSOMAL
INHERITANCE
NONDISJUNCTION
 if one of these gametes combines with
another gamete, result is ANEUPLOIDY (an
abnormal chromosome number)
 3x (2n + 1) = trisomic
 1x (2n – 1) = monosomic
(mitosis then transmits the anomaly to all
embryonic cells)
 if organism survives, usually has a set of
symptoms due to an abnormal dose of genes

ERRORS AND EXCEPTIONS IN
CHROMOSOMAL
INHERITANCE





POLYPLOIDY
organism has more than 2 complete
chromosome sets
3n = triploidy
--due to nondisjunction of all chromosomes +
normal gamete
4n = tetraploidy
--2n zygote fails to divide after replicating its
chromosomes; mitosis  4n
polyploidy is fairly common in
plants…extremely rare in animals
ALTERATIONS OF CHROMOSOME
STRUCTURE





breakage of chromosome can lead to 4
possible changes:
1. deletion = chromosomal fragment lacking
a centromere is lost
(chromosome will be missing genes)
2. duplication = chromosome fragment may
become attached as an extra segment to a
sister chromatid
3. inversion = chromosome fragments
reattach in reverse orientation
4. translocation = fragment joins
nonhomologous chromosome
alterations of chromosome
structure
ALTERATIONS OF CHROMOSOME
STRUCTURE

a 2n embryo that is homozygous for a large
deletion (or a single X with a large deletion in
males) is usually missing many essential
genes and is therefore lethal

HUMAN DISORDERS
aneuploidy—results are usually so
devastating that spontaneous early abortion
occurs

ALTERATIONS OF CHROMOSOME
STRUCTURE


HUMAN DISORDERS (cont.)
Down syndrome—1/700 children born in U.S.
--extra chromosome 21 (trisomy 21)
--severely alters phenotype
(characteristic facial features, short stature,
heart defects, mental retardation)
--prone to leukemia and Alzheimer’s
--most are sexually undeveloped and sterile
--frequency correlates with age of mother
Down syndrome
ALTERATIONS OF CHROMOSOME
STRUCTURE
HUMAN DISORDERS (cont.)
 nondisjunction of sex chromosomes seems to
upset genetic balance less
(Y has relatively few genes, females can
deactivate faulty X)

ALTERATIONS OF CHROMOSOME
STRUCTURE


XXY (male) – Klinefelter’s Syndrome (1/2000
births)
--male sex organs
--breast enlargement
--normal intelligence
--abnormally small testes, sterile
XYY (male) – no well defined syndrome
(usually taller than average)
ALTERATIONS OF CHROMOSOME
STRUCTURE


XXX (female) – (1/1000 births)
--healthy
--cannot be distinguished from other females
except by karyotype
XO (female) – Turner Syndrome
(1/5000 births)
--only known viable monosomy in humans
--sex organs do not mature; sterile
--hormones can produce normal secondary
sexual characteristics
--normal intelligence
ALTERATIONS OF CHROMOSOME
STRUCTURE


HUMAN DISORDERS (cont.)
chromosome 5 deletion = “cri du chat”
--mentally retarded
--small head
--unusual facial features
--unusual mewing
--die in infancy or early childhood
ALTERATIONS OF CHROMOSOME
STRUCTURE


GENOMIC IMPRINTING
expression of trait depends on which
parent passed along the allele
--Prader-Willi: mental retardation, obesity,
short, small hands and feet
(chromosome from father)
--Angelman: laughter, jerky movements,
motor / mental symptoms
(chromosome from mother)
genomic imprinting
Chapter 16
The Molecular Basis
of Inheritance
DNA structure: history


Griffiths—studying Strep. pneumo. in 1928
--2 strains: (a) pathogenic (b) harmless variant
--process: (1.) heat-killed pathogenic strain
(2.) mixed remains with living
harmless variant
(3.) some of the cells were converted to
the pathogenic strain
(4.) pathogenicity was inherited by all
offspring of transformed bacteria
TRANSFORMATION: change in genotype and
phenotype due to assimilation of external DNA
Griffith’s experiment
DNA structure: history


Avery—sought to identify pathogenic substance
--process: (1.) purified chemicals from heatkilled pathogenic bacteria
(2.)tested each to see if it was the
“transforming” substance
(3.) only DNA worked
still much doubt in scientific community:
--bacterial DNA
--little was known about DNA
--most still believed that protein was the source
DNA structure: history


viruses that could infect bacteria were studied
these viruses are called bacteriophages, or just
phages
--to infect, a virus must take over the cell’s
reproductive machinery
--viruses that infect bacteria (bacteriophages) are
widely used in molecular genetics
DNA structure: history

bacteriophages
protein coat
DNA
DNA structure: history

Hershey & Chase =
process:
(1.) used different radioactive isotopes to tag phage
DNA and protein
(2.) T2 with E. coli in radioactive S
(radioactive atoms incorporated into protein)
(3.) T2 with E. coli in radioactive P
(radioactive atoms incorporated into DNA)
(4.) each set of labeled samples allowed to infect cells
(5.) after infection, samples whirled in a blender to
shake off loose phage parts
(6.) centrifuged

Hershey & Chase experiment
DNA structure: history


other circumstantial evidence:
--before mitosis, cell doubles DNA
--during mitosis, amount of DNA divided equally
Chargaff = DNA is polymer of nucleotides
--composition: (1) nitrogenous base (A,T,G,C)
(2) pentose sugar
(3) phosphate group
--DNA composition varies between species, but
bases are always present in a characteristic ratio
DNA structure
DNA structure: history

Watson

--helical
--width of helix = 2 strands (double helix)
--spacing of N-bases
vision of rope ladder with rigid rings
helix makes full turn every 3.4 nm
(10 layers of base pairs / turn of helix)
at first, Watson thought “liked paired with like”


DNA structure: history





purines = adenine, guanine
(2 organic rings)
pyrimidines = cytosine, thymine
(1 organic ring)
Since width of helix is always uniform, must be
formed by PURINE + PYRIMIDINE
A = T 2 H-bonds
C = G 3 H-bonds
DNA structure
DNA structure: history

REPLICATION MODELS

conservative— parental double helix remains intact
and an all-new copy is made

semi-conservative—two strands of parental
molecule separate; each functions as a template for
synthesis of a new complimentary strand

dispersive—each strand of both daughter
molecules contains a mixture of old and newly
synthesized parts
DNA structure: history

REPLICATION MODELS
DNA structure: history

REPLICATION MODELS

Meselson & Stahl (late 1950s)
DNA replication: basics

takes just a few hours to copy human DNA

6 billion bases in one cell
~ compares to letters in 900 books as thick as
the AP Biology text
extremely low error rate
~ 1 per billion nucleotides

DNA replication: step 1


begins at origins of replication
(specific nucleotide sequence)
--circular bacterial chromosome has 1 origin
--eukaryotic cells have 1000s
once sequence is recognized, replication “bubble”
opens
DNA replication: step 1


replication proceeds in both directions
replication fork at each end of “bubble”
DNA replication: step 2


DNA polymerases catalyze elongation of new
DNA strand
--rate: 500 bases / sec in bacteria
50 bases / sec in humans
energy for process derived from nucleotide
triphosphates
--hydrolysis of pyrophosphate drives polymerization
--structure of nucleotide triphosphates nearly
identical to ATP (use deoxyribose instead of ribose)
DNA replication: step 3

2 DNA strands are anti-parallel
--sugar-phosphate backbones run in opposite
directions
3’ OH– o o o o o o o o o o o o o o o o—P 5’
--new DNA strands elongate only in the
5’  3’ direction
(nucleotides are only added to the 3’ end)
DNA replication: step 3
DNA replication: step 3
DNA replication: step 3 (cont.)
2 approaches:
strand 1: LEADING STRAND
synthesizes 5’  3’ (continuous)
strand 2: LAGGING STRAND
synthesizes short fragments of DNA
--known as Okazaki fragments
--100-200 nucleotides long
--joined by DNA ligase

DNA replication: step 3 (cont.)
DNA replication: step 4


DNA polymerases cannot initiate synthesis
(can only add nucleotides)
start of a new chain (primer) is a short stretch of
RNA
--primase joins RNA nucleotides to make primer
(~10 nucleotides long)
--DNA polymerase eventually replaces the RNA
--leading strand only needs one primer; each
Okazaki fragment needs its own primer
(primers are converted to DNA before ligase joins
the fragments)
DNA replication: step 4
DNA replication: step 5

other proteins involved in replication:

helicase—untwists double helix at replication fork
single-strand binding protein—lines up unpaired
strands, holding them apart while they serve as
templates
(most models suggest that replication proteins are
stationary and “reel in” DNA

DNA replication: step 5
DNA replication: summary
DNA replication: step 6


proofreading enzymes
initial error rate is 1 in 10,000 base pairs
(a.) DNA polymerase proofreads as it adds each
base
--if an incorrect nucleotide is found, it is
immediately replaced
(b.) mismatch repair—used if error evades DNA
polymerase or occurs after replication is complete
--special proteins used to make corrections
--error in correction protein leads to one form of
colon cancer
DNA replication: step 6 (cont.)


proofreading enzymes
(c.) maintenance requires frequent repair due
to environmental damage
nucleotide excision repair
--usually, damaged segment is cut out by nuclease
--resulting gap is filled with proper bases
DNA replication: step 6 (cont.)
DNA replication: step 7

ends of DNA molecules
--usual replication machinery provides no way to
complete the 5’ ends of daughter DNA strands
--as a result, each round of replication produces
shorter and shorter DNA molecules
--eventually, essential genes would be deleted

telomeres = multiple short, repeated sequences
DNA replication: step 7
Chapter 17
From Gene to Protein
Genes provide the instructions for
making specific proteins.


nucleic acids and proteins can be seen as two
different languages
TRANSCRIPTION = synthesis of RNA under
direction of DNA (use same language)
--just as DNA provides template for replication, it
provides a template for transcription
--messenger RNA carries genetic message from
DNA to protein-synthesizing machinery of the cell
Genes provide the instructions for
making specific proteins.


nucleic acids and proteins can be seen as two
different languages
TRANSLATION = synthesis of protein under
direction of mRNA
--change in language: nucleic acid sequence must be
converted to amino acid sequence of polypeptide
--ribosomes are sites of translation (they facilitate
linking of amino acids)
transcription / translation
in prokaryotes
in eukaryotes
Potential problem:





only 4 nucleotides (A, C, G, T) available to code for
20 amino acids
1 base code  4 amino acids
2 base code  (4)2  16 amino acids
3 base code  (4)3  64 amino acids
triplet code = genetic instructions for a polypeptide
chain are written in the DNA as a series of 3nucleotide words
triplet code
TRANSCRIPTION

DNA  mRNA

transcription is an intermediate step: --only one of
the 2 DNA strands is transcribed = TEMPLATE
STRAND
--mRNA molecule is complimentary to its DNA
template (not identical)
TRANSLATION



mRNA  tRNA  protein
CODON = mRNA base triplets
--Ex. UGG  tryptophan
--codons are read in 5’  3’ direction along mRNA
--takes 300 nucleotides along an RNA strand to
code for polypeptide that is 100 amino acids long
1960s: Nirenberg deciphered first codon
(UUU  phenylalanine)…AAA, CCC, GGG followed
--all codons deciphered by mid 1960s
--AUG = start
--redundancy (but no ambiguity)
table of
mRNA codons
READING FRAME
 Ex.
5’--the red dog ate the cat—3’
 Ex.
5’—t her edd oga tet hec at—3’
The genetic code is nearly universal


shared by range of organisms from simple bacteria
to complex plants and animals
--genes can be transplanted between species and still
successfully undergo transcription and translation
--a few species, such as Paramecium, have a few
codons that differ
near universality provides evidence that the
common language has been present since very early
in the history of life
TRANSCRIPTION

SYNTHESIS & PROCESSING OF RNA
--RNA polymerase pries apart DNA strands
--combines RNA nucleotides as they are added
along the DNA template
--just like DNA, RNA elongates in a 5’  3’
direction
TRANSCRIPTION—overview
TRANSCRIPTION



1. INITIATION
promoter (upstream)
--DNA sequence where RNA polymerase attaches
and initiates transcription
--determines (a) actual start point and (b) which
strand will serve as template
terminator (downstream)
--sequence that signals the end of transcription
TRANSCRIPTION

1. INITIATION
TRANSCRIPTION



prokaryotes = RNA polymerase recognizes and
binds to promoter
eukaryotes = require RNA polymerase +
transcription factors
(called transcription initiation complex)
2. ELONGATION
--as RNA polymerase moves along DNA template,
~10-20 bases are exposed
--enzyme adds nucleotides to the growing 3’ end of
the RNA strand
--~60 nucleotides are added per sec. in eukaryotes
--many RNA polymerases can work simultaneously
TRANSCRIPTION

3. TERMINATION
--transcription proceeds until after RNA polymerase
transcribes a terminator sequence
--prokaryotes = transcription stops right at end of
termination signal
--eukaryotes = transcription carries on 100s of
nucleotides past terminator
TRANSCRIPTION



4. MODIFICATION OF RNA AFTER
TRANSCRIPTION
--enzymes in the nucleus modify pre-mRNA before
releasing it
5’ = immediately capped with a modified form of a
guanine nucleotide
--protects mRNA from degradation by hydrolytic
enzymes
--once in cytoplasm, acts as attachment sign for
ribosome
3’ = enzyme makes poly (A) tail with 50-250
adenine nucleotides
--inhibits degradation; helps ribosome attach
TRANSCRIPTION

4. MODIFICATION OF RNA AFTER
TRANSCRIPTION
TRANSCRIPTION


5. RNA SPLICING
--average length of transcription unit in eukaryotic
DNA = 8000 nucleotides
--takes only ~1200 nucleotides to code for average
protein
(means that there are long non-coding sequences)
--non-coding sequences (INTRONS) are mostly
interspersed between coding sequences (EXONS)
Signals for RNA splicing are short nucleotide
sequences at the end of introns
--particles called small nuclear nibonucleoproteins
(snRNPs or snurps) recognize these sequences
--several snRNPs join with additional proteins to
form a spliceosome
TRANSCRIPTION

5. RNA SPLICING
TRANSCRIPTION

snRNPs
TRANSCRIPTION

WHY HAVE INTRONS?
--introns may play a regulatory role in cell
--alternative RNA splicing
+ depending on which segments are
spliced, several different proteins can
result
+ may be reason why humans get by on
relatively few genes (only 2x more than flies)
--may also facilitate evolution of new proteins
+ proteins often have discrete regions =
domains
+ introns increase the probability of useful
crossover
exons (expressed genes) 
proteins
TRANSLATION



the message in codons along mRNA is interpreted
by tRNA
--tRNA functions to transfer amino acids to a
ribosome
cell keeps cytoplasm stocked with all 20 amino acids
by
(a) synthesizing them
(b) taking them up from the surrounding solution
molecules of tRNA are not identical
--each type of tRNA links a particular mRNA codon
with a particular amino acid
TRANSLATION
TRANSLATION




as a tRNA molecule arrives at a ribosome, it bears a
specific amino acid at one end
at other end is a nucleotide triplet called
ANTICODON
--anticodon base pairs with complementary codon
on mRNA
codon by codon, the genetic message is translated as
tRNAs deposit amino acids in the order prescribed
ribosome joins the amino acids into a chain
TRANSLATION


PROKARYOTE = tRNA transcribed from DNA
template
EUKARYOTE = tRNA produced in nucleus,
transported to cytoplasm
--in both types of cells, tRNA molecules are used
repeatedly
(pick up amino acid…deliver to ribosome…repeat)
TRANSLATION


tRNA molecle is a single RNA strand, only ~80
nucleotides long
(most mRNA are 100s of nucleotides long)
molecule folds to form clover-shaped structure
(when flattened…it’s actually more L-shaped in 3D)
--held together by H-bonds between complimentary
bases
TRANSLATION
TRANSLATION
TRANSLATION



if there was one tRNA for each codon, there would
be ~61 tRNA
(in reality there are about 45)
some tRNA have anticodons that can recognize 2 or
more different codons
versatility is possible because rules for base-pairing
are not as strict as those for DNA and mRNA
codons (called wobble)
TRANSLATION



Ex. U on tRNA anticodon can pair with A or G in
3rd position of mRNA codon
most versatile tRNA are those with inosine (I) in
3rd position
--I can associate with U, C, or A
--CCI can bind to GGU, GGC, or GGA
wobble explains why codons for the same base can
differ in the 3rd base, but not in another position
TRANSLATION
aminoacyl-tRNA synthetases
--correct match between tRNA and amino acid must
occur before codon and anticodon can match up
--each amino acid is joined to correct tRNA by
aminoacyl-tRNA synthetase
--20 different enzymes…one for each amino acid
--enzyme active site fits only a specific combination
of tRNA and amino acid
--enzyme catalyzes covalent bonding of amino acid
to tRNA (driven by hydrolysis of ATP)
TRANSLATION
aminoacyl-tRNA
synthetases
RIBOSOMES




facilitate coupling of mRNA and tRNA
have 2 subunits: large and small
constructed of protein and rRNA
(in eukaryotes, subunits are made in nucleolus)
rRNA gene on chromosomal DNA transcribed
--RNA processed, then assembled with proteins
imported from cytoplasm
--ribosomal subunits are exported via nuclear pores
RIBOSOMES



large and small subunits combine to form functional
ribosomes only when they attach to mRNA
prokaryote and eukaryote subunits differ in size and
molecular compositions
--useful in medicine…drugs like tetracycline and
streptomycin can paralyze prokaryotic ribosomes
without effecting eukaryotic ribosomes
rRNA is most abundant type of RNA
RIBOSOMES

P-site = holds tRNA carrying growing polypeptide
chain

A-site = holds tRNA carrying next amino acid to be
added to chain

E-site = discharged tRNAs leave ribosome
RIBOSOMES
BUILDING A POLYPEPTIDE



3 stages analogous to transcription:
(1) initiation
(2) elongation
(3) termination
all 3 require protein “factors” to aid process
energy required to elongate chain provided by
hydrolysis of GTP
BUILDING A POLYPEPTIDE





INITIATION
--brings together mRNA, tRNA with first amino
acid, and 2 ribosomal subunits
1. small subunit binds to mRNA + special initiators
tRNA
2. downstream from leader is initiation codon,
AUG, which signals start of translation
3. large subunit joins mRNA, initiator tRNA, small
ribosomal subunit
4. initiator tRNA sits in P site; vacant A site is ready
for next aminoacyl-tRNA
BUILDING A POLYPEPTIDE

INITIATION
BUILDING A POLYPEPTIDE




ELONGATION
--amino acids are added one by one to the preceding
amino acid
--each addition involves several elongation factors
(a.) codon recognition
(b.) peptide bond formation
(c.) translocation
BUILDING
A
POLYPEPTIDE
 ELONGATION
BUILDING A POLYPEPTIDE

TERMINATION
--translation continues until mRNA stop codon
reaches A site
--release factor binds to codon in A site
--causes addition of water instead of amino acid
--hydrolyzes completed polypeptide from tRNA
BUILDING A POLYPEPTIDE

TERMINATION
POINT MUTATIONS



mutation—change in genetic material of a cell (or
virus)
point mutation—chemical change in one base pair
if a point mutation occurs in a gamete or in a cell
that gives rise to gametes, it may be transmitted to
offspring and to a succession of future generations
–
–
if the mutation has an adverse effect on the phenotype of
a human or other animal, the mutant condition is referred
to as a genetic disorder
Ex. sickle-cell anemia
POINT MUTATIONS
TYPES OF POINT MUTATIONS

SUBSTITUTION
–
–
–
–
replacement of one nucleotide and its partner in the
complimentary DNA strand with another pair of
nucleotides
silent mutations—due to redundancy of genetic code,
have no effect on the encoded protein
Ex. CCG (mRNA GGC) changes to CCA (mRNA
GGU)…glycine results in both cases
substitutions of interest are those that cause a detectible
change in the protein



occasionally leads to an improved product or one with new,
unique capabilities
usually fit into category of missense mutations—the altered
codon still codes for an amino acid, but not the right one
nonsense mutations can also result—where the alteration
changes the codon to a stop codon
TYPES OF POINT MUTATIONS

INSERTION and DELETION
–
–
–
additions or loses of nucleotide pairs in a gene
effects are usually more significant than substitutions
frameshift mutation results—reading frame is altered,
causing a number of incorrect amino acids to be
produced
TYPES OF POINT MUTATIONS
MUTAGENS


interact with DNA to cause mutations
Ex. X-rays, radiation, chemicals
summary—
transcription and translation