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Reproduction and Chromosome Transmission are Key to Genetics •  Reproduction – the process
by which new cells or
organisms are produced
Chapter 02
Reproduction and
Chromosome
Transmission
•  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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Chromosomes
2.1 General Features of Chromosomes
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•  Chromosomes are structures within living cells that
contain the genetic material - the genes
Definition of the term chromosome
•  Chromosomes are composed of
Key differences between prokaryotic and
eukaryotic cells
o  DNA, the genetic material
o  Proteins, to provide an organized structure
Procedure for making a karyotype
•  In eukaryotes the DNA-protein complex is called
chromatin
Similarities and differences between homologous
chromosomes
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•  Prokaryotes
Two Types of Cells
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
•  Cells are classified as one of two types:
o  Prokaryotes - Bacteria and archaea
•  Outer membrane
•  Flagellum
•  Protists and fungi may be single-celled, but are still
eukaryotes because they have a membrane-bound
nucleus
1 mm
o  Eukaryotes - Protists, fungi, plants and animals
Ribosomes
in cytoplasm
Outer
Cell wall Plasma
membrane
membrane
(also known
as inner
membrane)
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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
•  Lysosomes
body
•  Golgi apparatus
Chromosomes are Examined Cytogenetically to Produce a Karyotype
•  Cytogenetics – The field of genetics that involves the
microscopic examination of chromosomes
Nuclear Nucleolus Chromosomal Nucleus
envelope
DNA
•  A cytogeneticist typically examines the chromosomal
composition of a particular cell or organism
Polyribosomes
Ribosome
o  This allows the detection of individuals with abnormal chromosome number
or structure
o  Provides a way to distinguish between two closely-related species
Rough endoplasmic
reticulum
Cytoplasm
Membrane protein
Plasma membrane
Smooth endoplasmic
reticulum
Lysosome
Mitochondrial DNA Mitochondrion Centriole Microtubule
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•  Animal cells are of two types
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•  During cell division chromosomes can
be seen with
a light microscope.
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
•  Each chromosome has a unique size,
shape and banding pattern (light and
dark areas)
•  To get a complete karyotype, the cytogeneticist
examines somatic cells
o  Usually blood cells
•  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
•  Members of a pair of chromosomes are called
homologs
o  The two homologs form a homologous pair
o  Two sets of chromosomes
•  The two chromosomes in a homologous pair
•  For example:
o
o
o
o
o  Humans – 46 total chromosomes (23 per set)
o  Dogs – 78 total chromosomes (39 per set)
o  Fruit fly – 8 total chromosomes (4 per set)
o  Tomato – 24 total chromosomes (12 per set)
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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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•  The DNA sequences on homologous chromosomes
are also very similar
•  The sex chromosomes (X and Y) are not homologous
o  There is usually less than 1% difference between homologs
o  They differ in size and genetic composition
•  Nevertheless, these slight differences in DNA
sequence provide the allelic differences in genes
o  Eye color gene
•  Blue allele vs. brown allele
Gene loci (location)
A
Homologous
pair of
chromosomes
Genotype:
The physical location of a
gene on a chromosome is
called its locus ( plural:
loci ).
•  They do have short regions of homology, though
b
c
A
B
c
AA
Bb
cc
Homozygous Heterozygous Homozygous
for the
for the
dominant
recessive
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allele
allele
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Cell Division
•  One purpose of cell division is asexual reproduction
2.2 Cell Division
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The process of binary fission in bacteria
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Phases of the eukaryotic cell cycle
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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Bacteria Reproduce Asexually by Binary Fission
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
•  Eukaryotic cells must use a more complex process
Mother cell
Bacterial
chromosome
Replication of bacterial
chromosome
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
FtsZ protein
Septum
G1 phase – Gap 1
S phase – Synthesis
G2 phase – Gap 2
M phase – Mitosis
Interphase
Mother cell
Interphase
Restriction
point
S
G1
Two daughter
cells
C
G0
(Nondividing cell)
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Chromosome Nucleolus
Formation
of two
daughter
cells
kin
yto
G2
is M
es Mitosis Pr
se
ha
ap
et
om
Pr
se
Metapha
•  Binary fission is asexual – does not
involve genetic contributions from two
different gametes
Eukaryotic Cell Cycle
Te
lop
ha
Anaphase
se
•  The capacity of bacteria to divide is
really quite astounding
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op
ha
se
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•  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
•  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
A pair of sister chromatids (a dyad)
•  S phase
o
o
o
o
Centromere
(DNA that is
hidden beneath
the kinetochore
proteins)
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
Kinetochore
(proteins attached
to the centromere)
•  G2 phase
o  The cell accumulates the material for nuclear and
cell division
One
chromatid
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2.3 Mitosis and Cytokinesis
•  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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Structure and function of the mitotic spindle
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Phases of mitosis
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One
chromatid (a monad)
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Key differences in cytokinesis between plants and
animals
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•  The mitotic spindle has three types of microtubules
The Mitotic Spindle Apparatus
o  Aster microtubules
•  From the centrosome to the plasma membrane
•  Help position the spindle
•  Mitotic spindle apparatus
o  Organizes and sorts eukaryotic chromosomes
o  Forms from microtubule-organizing centers
(MTOCs)
o  Polar microtubules
•  Project from the centrosomes to the middle
•  Help to push the spindle poles away from each other
•  In animals, the two MTOCs are
called centrosomes
o  Kinetochore microtubules
•  Attach to the kinetochore on the centromere of the chromosomes
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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Mitosis
•  In these diagrams, the original mother cell is diploid
(2n)
•  Mitosis is the process of organizing and sorting the
chromosomes into two daughter cells during the cell
cycle
o  It contains a total of 6 chromosomes
o  Three per set (n = 3)
•  Before mitosis begins, the cell is in Interphase
•  Mitosis takes place in five phases
o
o
o
o
o
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
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•  Refer to Figure 2.8 for the phases of mitosis
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•  Mitosis and cytokinesis ultimately produce two
daughter cells with the same number of
chromosomes as the mother cell
•  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
o  In plants
•  Formation of a cell plate
G1
C
•  The two daughter cells are genetically identical
G2
s
ine
o  Except for rare mutations
is
k
yto
•  Thus, mitosis ensures genetic consistency from
one cell to the next
Cleavage
furrow
•  The development of multicellularity relies on the
repeated processes of mitosis and cytokinesis
150 mm
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Meiosis
•  Meiosis produces haploid cells from a cell that was
originally diploid
2.4 Meiosis
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Phases of meiosis
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Key differences between mitosis and meiosis
•  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
•
•
•
•
•
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Prophase
Prometaphase
Metaphase
Anaphase
Telophase
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Prophase of Meiosis I
Prophase of Meiosis I
•  Leptotene
•  Prophase I is further subdivided into periods known as
o
o
o
o
o
o  Replicated chromosomes begin to
condense
Leptotene
Zygotene
Pachytene
Diplotene
Diakinesis
•  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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Crossing Over
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Prophase of Meiosis I
•  During pachytene of Prophase I
•  Diplotene
o  The synaptonemal complex starts to dissociate, allowing the bivalent to
separate slightly
•  Crossing over involves a physical exchange of
chromosome pieces that result in exchange of
genetic information
•  Diakinesis
•  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
o  The synaptonemal complex has disappeared
•  During crossing over, a chiasma forms (plural:
chiasmata)
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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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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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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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Mitosis vs. Meiosis o  Mitosis produces two diploid daughter cells
o  Meiosis produce four haploid daughter cells
2.5 Sexual Reproduction
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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Definition of sexual reproduction
How animals make sperm and egg cells
How plants alternate between haploid and diploid
generations
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•  Some simple eukaryotic species are isogamous
Sexual Reproduction
o  They produce gametes that are morphologically similar
•  Example: Many species of fungi and algae
•  The most common way for eukaryotic organisms
to produce offspring
•  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
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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Spermatogenesis
•  The production of sperm
•  In male animals, it occurs in the testes
•  Gametes are typically haploid
o  They contain a single set of chromosomes
•  A diploid spermatogonial cell divides mitotically to
produce two cells
•  Gametes are 1n, while diploid cells are 2n
o  A diploid human cell contains 46 chromosomes
o  One remains a spermatogonial cell
o  The other becomes a primary spermatocyte
o  A human gamete only contains 23 chromosomes
•  During meiosis, haploid cells are produced from diploid
cells
•  The primary spermatocyte progresses through meiosis I
and II
MEIOSIS I
o  Chromosomes must be correctly distributed
•  Each gamete must receive one chromosome from each pair
MEIOSIS II
Secondary spermatocyte
Primary
spermatocyte
(diploid)
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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
Spermatids
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
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MEIOSIS I
•  In human males, spermatogenesis is a continuous
process
MEIOSIS II
Secondary spermatocyte
o  A mature human male produces several hundred million sperm per day
Primary
spermatocyte
(diploid)
Spermatids
Sperm cells
(haploid)
(a) Spermatogenesis
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•  The primary oocytes initiate meiosis I
Oogenesis
•  However, they enter into a dormant phase
o  They are arrested in prophase I until sexual maturity
•  The production of egg cells
•  In female animals, it occurs in the ovaries
•  At puberty, primary oocytes are periodically activated
to progress through meiosis I
•  Early in development, diploid oogonia produce
diploid primary oocytes
•  The division in meiosis I is asymmetric producing two
haploid cells of unequal size
o  In humans, one oocyte per month is activated
o  In humans, for example, about 1 million primary oocytes per ovary are
produced before birth
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o  A large secondary oocyte and a small polar body
Secondary oocyte
Primary
oocyte
(diploid)
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First polar body
•  The secondary oocyte enters meiosis II but is quickly
arrested in it
•  The haploid egg and sperm nuclei then fuse to
create the diploid nucleus of a new individual
•  It is released into the oviduct
o  An event called ovulation
•  Note that only one of the four cells produced in this
meiosis becomes an egg
•  If the secondary oocyte is fertilized
o  Meiosis II is completed
o  A haploid egg and a second polar body are produced
Secondary oocyte
Second polar body
Second polar body
Primary
oocyte
(diploid)
Egg cell
(haploid)
Egg cell
(haploid)
First polar body
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Plants Alternate Between Haploid and Diploid Generations
•  Meiosis produces haploid cells called spores
o  Spores divide by mitosis to produce the gametophyte
•  The life cycles of plant species alternate between two
generations
•  In simpler plants, like mosses:
o  Spores develop into gametophytes that have large numbers of cells
o  Haploid generation is called the gametophyte
•  In flowering plants:
o  Diploid generation is called the sporophyte
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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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Animals vs. Plants
•  Meiosis occurs within two different
structures of the sporophyte
•  Animals produce gametes by meiosis
o  Anthers
•  Produce the male gametophyte – the
pollen grain
•  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
o  Ovaries
•  Produce the female gametophyte – the
embryo sac
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