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BIO 184
Fall 2006
LECTURE 6
Lecture 6:
Reproduction and Chromosome Transmission
Drawing of the life cycle of Homo sapiens. The germ line cells of the adult male produce
haploid sperm, while the adult female’s produce haploid eggs. During fertilization, a diploid
zygote is formed, undergoes mitosis, and develops into a juvenile and then an adult.
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookmeiosis.html#Life%20Cycles
I. Chromosomes
Chromosomes are structures within living cells that contain the genetic material –
i.e. they contain genes.
 Mendel knew nothing about chromosomes or genes; however, his laws
predicted their behavior as they were passed through the generations
Biochemically, chromosomes are comprised of DNA (the genetic material) and
proteins (which provide a scaffolding, or organized structure, for the DNA to fold
into a chromosome)
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The chromosomes are different in prokaryotes and eukaryotes.

Prokaryotes usually have a single, circular chromosome located in the
nucleoid of the cell. The nucleoid is not surrounded by a membrane.

In eukaryotes, there are usually many linear chromosomes housed inside a
membrane-bound nucleus.
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II. Cytogenetics
Cytogenetics is the field of genetics that studies the structure and behavior of
chromosomes. The first cytogeneticists worked in the 1880s, when microscopes
finally became powerful enough to see the chromosomes inside living cells and
observe their behavior.
Multicellular organisms generally have two types of cells:
 Somatic cells (e.g. meristem, flower petals, liver, skin)
o These cells have no potential immortality and are diploid in most
multicellular eukaryotes
o Diploid cells have two sets of chromosomes (two copies of the
organism’s genome)
 Germ line cells (eggs and sperm)
o These cells are potentially immortal and are haploid
o Haploid cells have only one set of chromosomes (one copy of the
organism’s genome)
In a cytogenetics laboratory, the microscopes are equipped with cameras.
Microscopic images of the chromosomes from a donor sample are scanned into a
computer and organized in pairs (if the donor cells are diploid) from largest to
smallest. This produces a karyotype – a photographic image of the chromosomes
inside a cell.
A karyotype prepared from the white blood cells of a normal male human. Note that since
blood cells are somatic, they are also diploid and contain two complete sets of chromosomes
(two of each kind). Humans have 23 chromosomes per set, so a normal diploid cell contains 46.
http://aspin.asu.edu/geneinfo/karyo.htm
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Organisms differ in the number of chromosomes per genome:
 Humans: 23 per set
 Dogs: 39 per set
 Fruit flies: 4 per set
Thus, a human egg (haploid) has 23 chromosomes but a human liver cell (diploid) has
46 (two sets of 23).
III. Homologous Chromosomes
Eukaryotic chromosomes are usually inherited in sets, one from the mother (via the
egg) and one from the father (via the sperm).
Members of a pair of chromosomes (e.g. the copy of chromosome 7 you received
from your father and the copy you received from your mother) are called
homologoues.
The two chromosomes in a homologous pair:
 Are identical (or nearly so) in size
 Have the same banding pattern and centromere location
 Carry the same genes in the same order
o But not necessarily the same alleles
 Are nearly identical in DNA sequence (differ at about 1 in every 500 bp)
The physical location of a gene on a
chromosome is called its locus.
Note that the centromeres of homologous chromosome are NOT connected. This
fact will become important later in understanding the difference between sister
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LECTURE 6
chromatids (which are connected at the centromere and are exactly alike) and
homologous chromosomes (which are never connected at the centromere and
differ slightly from one another).
IV. Cell Division
Cells divide for many reasons:
a. In single-celled organisms, asexual reproduction
 Bacteria, amoeba, yeast
b. In multicellular organisms:
 Growth (esp. during embryogenesis in animals but throughout the life cycle
of plants)
 Repair and replacement of old/unhealthy cells
These processes does not involve the contributions from two parent cells but are
basically a cloning of a cell to create two identical daughter cells.
1. BINARY FISSION
Prokaryotes divide quickly (once every 20 minutes in E. coli) by binary fission.
 Single circular chromosome is replicated
 Cell divides and each daughter cell contains one copy of the circular genome
 Does not involve contributions from two parent cells, only one
See Figure 3.4, Brooker
2. MITOSIS
Eukaryotic cells divide more slowly (some rarely divide at all) and do so in a much
more complex and orderly way.
See Figure 3.5, Brooker
In actively dividing cells, G1, S, and G2 are collectively called interphase. Cells that
are not actively dividing (e.g. human neurons) are said to be “resting” in G0. Cells do
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not generally undergo division unless they receive a signal from outside the cell to
do so.
During G1, a cell receives an external signal to divide and commits itself to
replicating its chromosomes and dividing its cytoplasm.
In S phase, the cell replicates its chromosomes.
 Replicates are held together at the
centromere and each replicate is called a
chromatid. The two chromatids are called
sister chromatids.
 A better term for sister chromatids might
be “identical twin chromatids” because
sister chromatids are products of the most
recent round of DNA replication and are
thus likely to be exactly identical.
See Figure 3.6(b), Brooker
Note that at the end of S phase, a cell has twice
as many chromatids as there are chromosomes in
G1. For example, a human cell has 46
chromosomes in G1 and 46 pairs of sister
chromatids (also called chromosomes) at the end
of S phase.
Therefore, the term chromosome can refer
either to:
 A single unreplicated chromatid in G1
 A replicated pair of sister chromatids (end of S phase, G2, and entering
mitosis).
In G2, the cell grows and prepares to divide. It also checks its DNA to make sure
that no major errors occurred during S phase. If the DNA is not healthy, the cell
will usually “commit suicide” along a specialized path called apoptosis. Just before
the cell enters mitosis, the centrosome (which will later give rise to the spindle
apparatus that seprates the sister chromatids) also replicates.
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Mitosis is divided into five phases:


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Prophase
o Chromosomes begin to condense and become visible by light
microscopy
o Nuclear envelope dissociates into small vesicles
o Centrosomes separate to opposite poles of the cell
o The spindle apparatus is formed
Prometaphase
o The centrosomes have reached opposite poles of the cell
o The chromosomes are not yet maximally condensed and are perfect at
this point for staining (banding) and karyotyping
Metaphase
o Spindle fibers attach to centromeres (at a specialized structure
called the kinetochore) and move the sister chromatid pairs to the
center of the cell
o The sister chromatid pairs line up on the “metaphase plate”, an
imaginary plane that slices across the equatorial midline of the cell
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o Homologous pairs ignore one another during this process and line up on
the metaphase plate independently
Anaphase
o The connection between sister chromatids is broken at the
centromere and the chromatids are now independent chromosomes
o The former sister chromatids move to opposite sides of cell, pulled by
spindle fibers
Telophase
o The chromosomes reach their opposite poles and begin to decondense
o The nuclear membrane reforms around each new daughter nucleus
o A cleavage furrow (in animals) or a cell plate (in plants) begins to form
across the metaphase plate
Telophase is then quickly followed by cytokinesis, or the division of the cytoplasm
to form two independent daughter cells.
Mitosis and cytokinesis ultimately form two daughter cells that have the same
number and kind of chromosomes as the mother cell. Thus, barring rare mutations,
the daughter cells are exact genetic clones of one another.
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Thus, mitosis ensures genetic continuity from one generation of cells to the next
and does not give rise to genetic variation (except through rare mutations that
occur during DNA replication).
3. MEIOSIS
Organisms that reproduce sexually also undergo a specialized kind of cell division
called meiosis. Meiosis does give rise to genetic variation and is quite different
from mitosis, though the two processes have many superficial similarities.
There are two main events that occur during meiosis:
a. The number of sets of chromosomes is halved
b. Allelic combinations are scrambled to produce genetic variation
In order to accomplish this, meiosis differs from mitosis in key ways:



Meisois has two cell divisions (meiosis I and meiosis II), while mitosis has
only one
Homologues exchange parts during meiosis, while they ignore one another
during mitosis
In meiosis, homologues move to the metaphase plate together during meiosis
I and segregate from one another during anaphase I; sister chromatids
segregate in meiosis II. In mitosis, homologues never segregate from one
another.
Meiosis I
Like mitosis, meiosis I and meiosis II can be broken down into 5 phases. Note,
however, that very different events are occurring in meiosis I than occurred in the
same phases of mitosis.

Prophase I
o Chromosomes condense and become visible by light microscopy
o Homologous chromosomes find one another, pair up, and physically
exchange parts in a process called crossing over.
o Your text goes into a great deal of detail about this process, but all
you need to know for this course is the overall picture.
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LECTURE 6

Prometaphase I
o At the start of prometaphase I, the homologues are still connected
together and thus are moved to the metaphase plate by the spindle
fibers as a unit
o Meanwhile, the nuclear envelope has completed its fragmentation

Metaphase I
o Homologues are lined up across the
metaphase plate from one another
and prepare to segregate. Note
that this is quite different from
mitosis, where only one pair of
sister chromatids is aligned on the
plate at any given spot.
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LECTURE 6

Anaphase I
o Homologoues segregate to opposite poles. This is the point during
meiosis that the chromosome number is halved. Note that each
daughter cell will receive only one chromosome of each type.

Telophase I
o Nuclear envelopes reform, cleavage furrow (or cell plate) is visible,
and cell prepares for cytokinesis
Meiosis II
Meiosis II involves the mitotic divisions of the two haploid cells produced by
meiosis I. In meiosis II, the sister chromatids of each chromosome segregate and
enter new cells.
At the end of the entire process of meiosis, then, there are four haploid cells, all
of which are different from one another and from the original parent cell.
The four haploid products of meiosis are called gametes. In higher organisms
(plants and animals), two very different types of gametes are formed: eggs and
sperm.
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LECTURE 6
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