Download Chromosome

Chapter 02
Reproduction and
Chromosome
Transmission
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Reproduction and Chromosome Transmission
are Key to Genetics
• Reproduction – the process
by which new cells or
organisms are produced
• Requires the transmission of
chromosomes from parent to
offspring
• When eukaryotic cells divide,
they must sort their
chromosomes so each cell
receives the correct number
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2.1 General Features
of Chromosomes




Definition of the term chromosome
Key differences between prokaryotic and
eukaryotic cells
Procedure for making a karyotype
Similarities and differences between homologous
chromosomes
3
Chromosomes
• Chromosomes are structures within living cells that
contain the genetic material - the genes
• Chromosomes are composed of
o DNA, the genetic material
o Proteins, to provide an organized structure
• In eukaryotes the DNA-protein complex is called
chromatin
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4
Two Types of Cells
• Cells are classified as one of two types:
o Prokaryotes - Bacteria and archaea
o Eukaryotes - Protists, fungi, plants and animals
• Protists and fungi may be single-celled, but are still
eukaryotes because they have a membrane-bound
nucleus
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• Prokaryotes
1 mm
o No nucleus
o No membrane-bound organelles
o Usually contain a single type of circular chromosome
• Found in the nucleoid
o Outside the membrane is a rigid cell wall
o May contain other structures
• Outer membrane
• Flagellum
Ribosomes
in cytoplasm
Outer
Cell wall Plasma
membrane
membrane
(also known
as inner
membrane)
Flagellum
Nucleoid
(where bacterial
chromosome is
found)
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• Eukaryotes
o
o
o
o
o
Have a nucleus – DNA surrounded by membrane
Two or more linear chromosomes
May have flagella and other structures
May have cell wall, different than prokaryotes
Membrane-bounded organelles such as:
• Mitochondria – have DNA
• Chloroplasts – have DNA
Microfilament Golgi
Nuclear Nucleolus Chromosomal
• Lysosomes
body
envelope
DNA
• Golgi apparatus
Nucleus
Polyribosomes
Ribosome
Rough endoplasmic
reticulum
Cytoplasm
Membrane protein
Plasma membrane
Smooth endoplasmic
reticulum
Lysosome
Mitochondrial DNA Mitochondrion Centriole Microtubule
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Chromosomes are Examined Cytogenetically
to Produce a Karyotype
• Cytogenetics – The field of genetics that involves the
microscopic examination of chromosomes
• A cytogeneticist typically examines the chromosomal
composition of a particular cell or organism
o This allows the detection of individuals with abnormal chromosome number
or structure
o Provides a way to distinguish between two closely-related species
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• Animal cells are of two types
o Somatic cells
• Body cells, other than gametes
• Ex: Blood cells
o Gametes (germ cells)
• Sperm and egg cells
• Precursor cells that give rise to sperm and egg
• To get a complete karyotype, the cytogeneticist
examines somatic cells
o Usually blood cells
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• During cell division chromosomes can
be seen with
a light microscope.
• Each chromosome has a unique size,
shape and banding pattern (light and
dark areas)
• A karyotype is a set of images of the
chromosomes
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Eukaryotic Chromosomes Are Inherited in Sets
• Most eukaryotic species are diploid
o Two sets of chromosomes
• For example:
o
o
o
o
Humans – 46 total chromosomes (23 per set)
Dogs – 78 total chromosomes (39 per set)
Fruit fly – 8 total chromosomes (4 per set)
Tomato – 24 total chromosomes (12 per set)
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11
• Members of a pair of chromosomes are called
homologs
o The two homologs form a homologous pair
• The two chromosomes in a homologous pair
o
o
o
o
Are nearly identical in size
Have the same banding pattern
Have the same centromere location
Have the same genes
• But not necessarily the same alleles
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12
• The DNA sequences on homologous chromosomes
are also very similar
o There is usually less than 1% difference between homologs
• Nevertheless, these slight differences in DNA
sequence provide the allelic differences in genes
o Eye color gene
• Blue allele vs. brown allele
Homologous
pair of
chromosomes
Genotype:
The physical location of a
gene on a chromosome is
called its locus ( plural:
loci ).
Gene loci (location)
A
b
c
A
B
c
AA
Bb
cc
Homozygous Heterozygous Homozygous
for the
for the
dominant
recessive
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allele
allele
• The sex chromosomes (X and Y) are not homologous
o They differ in size and genetic composition
• They do have short regions of homology, though
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2.2 Cell Division

The process of binary fission in bacteria

Phases of the eukaryotic cell cycle
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15
Cell Division
• One purpose of cell division is asexual reproduction
o How some unicellular organisms make new individuals
o Examples:
• Bacteria
• Amoeba
• Baker’s Yeast (Saccharomyces cerevisiae)
• Cell division also allows multicellularity
o Plants, animals and some fungi derive from a single cell that has undergone
repeated cell divisions
o Ex: Humans
• Start as a fertilized egg, grow to several trillion cells
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16
Bacteria Reproduce Asexually
by Binary Fission
• The capacity of bacteria to divide is
really quite astounding
o Escherichia coli can divide every 20-30
minutes
• Prior to division, the bacterial cell
replicates its chromosome
• Then the cell divides into two
daughter cells by a process called
binary fission
• Binary fission is asexual – does not
involve genetic contributions from two
different gametes
Mother cell
Bacterial
chromosome
Replication of bacterial
chromosome
FtsZ protein
Septum
Two daughter
cells
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Eukaryotic Cell Cycle
• Eukaryotic cells must use a more complex process
o Each daughter cell must receive the right number of each type of
chromosome
o Series of phases is called the cell cycle
• Cell cycle:
o
o
o
o
G1 phase – Gap 1
S phase – Synthesis
G2 phase – Gap 2
M phase – Mitosis
Interphase
Mother cell
Interphase
Restriction
point
S
G1
G0
(Nondividing cell)
M
Mitosis
G2
Chromosome Nucleolus
Formation
of two
daughter
cells
18
• G1 phase
o The cell prepares to divide
o Restriction point – the point at which molecular changes have accumulated
to commit the cell to proceed through cell division
• S phase
o
o
o
o
Chromosomes are replicated
After replication the copies are called chromatids
The two sister chromatids are joined at the centromere to form a dyad
Kinetochore proteins on the centromere help
with sorting
• G2 phase
o The cell accumulates the material for nuclear and
cell division
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19
• M phase
o When mitosis occurs
o Distributes replicated chromosomes to produce two identical daughter
cells
o Cytokinesis – the process that divides the cell into two daughters
A pair of sister chromatids (a dyad)
Centromere
(DNA that is
hidden beneath
the kinetochore
proteins)
One
chromatid
Kinetochore
(proteins attached
to the centromere)
One
chromatid (a monad)
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• A cell may remain for long periods of time in the G0
phase
• A cell in G0 phase has either
o Postponed making a decision to divide
o Or made the decision to never divide again
• Ex: Terminally differentiated cells like neurons
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21
2.3 Mitosis and Cytokinesis

Structure and function of the mitotic spindle

Phases of mitosis

Key differences in cytokinesis between plants and
animals
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22
The Mitotic Spindle Apparatus
• Mitotic spindle apparatus
o Organizes and sorts eukaryotic chromosomes
o Forms from microtubule-organizing centers
(MTOCs)
• In animals, the two MTOCs are
called centrosomes
o Centrosomes lie at each spindle pole
o A pair of centrioles is within each
centrosome
• Plants do not have centrosomes
o The nuclear envelope functions as an MTOC
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23
• The mitotic spindle has three types of microtubules
o Aster microtubules
• From the centrosome to the plasma membrane
• Help position the spindle
o Polar microtubules
• Project from the centrosomes to the middle
• Help to push the spindle poles away from each other
o Kinetochore microtubules
• Attach to the kinetochore on the centromere of the chromosomes
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24
Mitosis
• Mitosis is the process of organizing and sorting the
chromosomes into two daughter cells during the cell
cycle
• Mitosis takes place in five phases
o
o
o
o
o
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
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25
• In these diagrams, the original mother cell is diploid
(2n)
o It contains a total of 6 chromosomes
o Three per set (n = 3)
• Before mitosis begins, the cell is in Interphase
• Refer to Figure 2.8 for the phases of mitosis
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27
28
29
30
31
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• Cytokinesis
o In most cases, mitosis is quickly followed by cytokinesis
to produce the separate daughter cells
S
o In animals
• Formation of a cleavage furrow
G1
G2
o In plants
• Formation of a cell plate
Cleavage
furrow
150 mm
33
• Mitosis and cytokinesis ultimately produce two
daughter cells with the same number of
chromosomes as the mother cell
• The two daughter cells are genetically identical
o Except for rare mutations
• Thus, mitosis ensures genetic consistency from
one cell to the next
• The development of multicellularity relies on the
repeated processes of mitosis and cytokinesis
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2.4 Meiosis

Phases of meiosis

Key differences between mitosis and meiosis
35
Meiosis
• Meiosis produces haploid cells from a cell that was
originally diploid
• Like mitosis, the cell has progressed through G1, S, and
G2
• Unlike mitosis, meiosis involves two successive divisions
called Meiosis I and Meiosis II, each subdivided into
•
•
•
•
•
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
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37
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Prophase of Meiosis I
• Prophase I is further subdivided into periods known as
o
o
o
o
o
Leptotene
Zygotene
Pachytene
Diplotene
Diakinesis
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39
Prophase of Meiosis I
• Leptotene
o Replicated chromosomes begin to
condense
• Zygotene
o Via the process of synapsis,
homologous chromosomes
recognize each other and align,
forming the synaptonemal
complex
• Pachytene
o Homologs are aligned
o Pairs of sister chromatids – four
total! – are called bivalents or
tetrads
o Crossing over occurs
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40
Crossing Over
• During pachytene of Prophase I
• Crossing over involves a physical exchange of
chromosome pieces that result in exchange of
genetic information
• On each chromosome, may occur a couple times or
a couple dozen times, depending on size and species
-- About twice per chromosome in human sperm
• During crossing over, a chiasma forms (plural:
chiasmata)
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41
Prophase of Meiosis I
• Diplotene
o The synaptonemal complex starts to dissociate, allowing the bivalent to
separate slightly
• Diakinesis
o The synaptonemal complex has disappeared
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42
43
Prometaphase of Meiosis I
o Pairing and crossing over are
completed at the end of
prophase
o In prometaphase I, the spindle
apparatus is formed
o Chromatids are attached to the
spindle via kinteochore
microtubules
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44
45
Metaphase of Meiosis I
o At metaphase, the bivalents (or tetrads) are organizaed along the
metaphase plate
o Pairs of sister chromatids form a double row
(not a single row as in mitosis)
o Sister chromatids may be aligned in a very large number of possible ways
• 2n possible alignments
• Humans would have 223 – more than 8 million possible arrangements!
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Anaphase of Meiosis I and Cytokinesis
o The two pairs of sister chromatids within a bivalent separate from one
another
o However, the connection between the sister chromatids does not break
• The joined pair of sister chromatids moves to the pole
• In other words, the two dyads within a tetrad separate and move to
opposite poles
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49
Telophase of Meiosis I and Cytokinesis
o The dyads have separated to opposite poles
• The chromatids may decondense and the nuclear membrane may reform at this point
o Meiosis I ends with two cells, each with three pairs
(in this example) of sister chromatids
o This is a reduction division, and the cells are considered haploid, because
they only carry one set of homologous chromosomes
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Meiosis II
• The sorting events that occur during meiosis II are
similar to those that occur during mitosis, however the
starting point is different
o For a diploid organism with six chromosomes
• Mitosis begins with 12 chromatids joined as six pairs of sister
chromatids
• Meiosis II begins with 6 chromatids joined as three pairs of
sister chromatids
• There is no chromosomal replication prior to meiosis II
• Meiosis II proceeds through prophase, prometaphase,
metaphase, anaphase, and telophase
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51
52
Mitosis vs. Meiosis
o Mitosis produces two diploid daughter cells
o Meiosis produce four haploid daughter cells
o Mitosis produces cells that are genetically identical
o Meiosis produces cells that are not genetically identical
• The daughter cells contain only one homologous chromosome from
each pair
• The daughter cells contain many different combinations of the single
homologs
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2.5 Sexual Reproduction



Definition of sexual reproduction
How animals make sperm and egg cells
How plants alternate between haploid and diploid
generations
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54
Sexual Reproduction
• The most common way for eukaryotic organisms
to produce offspring
o Parents make gametes with half the amount of genetic material
• These gametes fuse with each other during fertilization to begin
the life of a new organism
• The process of forming gametes is called gametogenesis
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55
• Some simple eukaryotic species are isogamous
o They produce gametes that are morphologically similar
• Example: Many species of fungi and algae
• Most eukaryotic species are heterogamous
o These produce gametes that are morphologically different
• Sperm cells
o Relatively small and mobile
• Egg cell or ovum
o Usually large and nonmobile
o Stores a large amount of nutrients, in animal species
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• Gametes are typically haploid
o They contain a single set of chromosomes
• Gametes are 1n, while diploid cells are 2n
o A diploid human cell contains 46 chromosomes
o A human gamete only contains 23 chromosomes
• During meiosis, haploid cells are produced from diploid
cells
o Chromosomes must be correctly distributed
• Each gamete must receive one chromosome from each pair
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57
Spermatogenesis
• The production of sperm
• In male animals, it occurs in the testes
• A diploid spermatogonial cell divides mitotically to
produce two cells
o One remains a spermatogonial cell
o The other becomes a primary spermatocyte
• The primary spermatocyte progresses through meiosis I
and II
MEIOSIS I
MEIOSIS II
Secondary spermatocyte
Primary
spermatocyte
(diploid)
Spermatids
Sperm cells
(haploid)
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Meiois I yields
two haploid
secondary
spermatocytes
Meiois II yields four
haploid spermatids
Each spermatid
matures into a
haploid sperm
cell
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MEIOSIS I
MEIOSIS II
Secondary spermatocyte
Primary
spermatocyte
(diploid)
Spermatids
(a) Spermatogenesis
Sperm cells
(haploid)
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• The structure of a sperm includes
o A long flagellum
o A head
• The head contains a haploid nucleus
o Capped by the acrosome
• In human males, spermatogenesis is a continuous
process
o A mature human male produces several hundred million sperm per day
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60
Oogenesis
• The production of egg cells
• In female animals, it occurs in the ovaries
• Early in development, diploid oogonia produce
diploid primary oocytes
o In humans, for example, about 1 million primary oocytes per ovary are
produced before birth
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61
• The primary oocytes initiate meiosis I
• However, they enter into a dormant phase
o They are arrested in prophase I until sexual maturity
• At puberty, primary oocytes are periodically activated
to progress through meiosis I
o In humans, one oocyte per month is activated
• The division in meiosis I is asymmetric producing two
haploid cells of unequal size
o A large secondary oocyte and a small polar body
Secondary oocyte
Primary
oocyte
(diploid)
62
First polar body
• The secondary oocyte enters meiosis II but is quickly
arrested in it
• It is released into the oviduct
o An event called ovulation
• If the secondary oocyte is fertilized
o Meiosis II is completed
o A haploid egg and a second polar body are produced
Second polar body
Egg cell
(haploid)
63
• The haploid egg and sperm nuclei then fuse to
create the diploid nucleus of a new individual
• Note that only one of the four cells produced in this
meiosis becomes an egg
Secondary oocyte
Primary
oocyte
(diploid)
Second polar body
Egg cell
(haploid)
First polar body
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Plants Alternate Between Haploid
and Diploid Generations
• The life cycles of plant species alternate between two
generations
o Haploid generation is called the gametophyte
o Diploid generation is called the sporophyte
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65
• Meiosis produces haploid cells called spores
o Spores divide by mitosis to produce the gametophyte
• In simpler plants, like mosses:
o Spores develop into gametophytes that have large numbers of cells
• In flowering plants:
o Spores develop into gametophytes that have only
a few cells – they are tiny
o What we see as “the plant” is the sporophyte
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• Meiosis occurs within two different
structures of the sporophyte
o Anthers
• Produce the male gametophyte – the
pollen grain
o Ovaries
• Produce the female gametophyte – the
embryo sac
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Animals vs. Plants
• Animals produce gametes by meiosis
• Plants produce spores by meiosis
o The spores develop into gametophytes
o The haploid gametophyte becomes multicellular by mitotic cell divisions
o The mutlicellular gametophyte then goes on to produce certain
specialized cells as gametes
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