Download Introduction and Mendelian Analysis

Mitosis and Meiosis
Traits are controlled by genes
Each individual contains two copies of a gene.
Gene segregation
Each individual has thousands of genes
Independent assortment
What is the biological basis for Mendel’s laws?
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Genes reside on Chromosomes
Genes reside on chromosomes, understanding the behavior and
inheritance patterns of individual genes requires an
understanding of the behavior of inheritance patterns of
chromosomes.
The processes of mitosis and meiosis describe the two basic
patterns of chromosome behavior in higher eukaryotes
Mitosis: a form of cell division that produces two daughter cells
of identical genotypes
Meiosis: a form of cell division in a diploid cell that produces
four haploid cells
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Meiosis only occurs in a small specialized set of cells known as
the germ cells.
Development
Meiosis
2N ---->4N
---->N+N+N+N
Mitosis
2N ---->4N
----> 2N+2N
2N
|
4N
|
|
N
|
|
|
2N
|
|
4N
|
|
2N
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The Mitotic cell cycle
The mitotic cycle alternates between the replication of each
chromosome (S phase) and the segregation of the replicated
chromosomes to two daughter nuclei (M phase).
The intervals between these phases are known as gap phases and this
divides the cell cycle into four phases M, G1, S and G2. Interphase
consists of G1, S, and G2.
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Chromosome number
n=2
Species
Homologous
Chromosomes
(99.99% similar)
Human
Monkey
Mouse
Frog
Fruit fly
C. Elegans
Corn
S. Cerevisiae
S. Pombe
Indian muntjack
Haploid number
(n)
23
21
20
13
4
6
10
16
3
3
Haploids are 1N
Diploids are 2N
Tetraploids are 4N
Smallest number: The female of a subspecies of the ant,
Myrmecia pilosula, has one pair of chromosomes per cell. Its
male has only one chromosome in each cell.
Largest number: In the fern family of plants, the species
Ophioglossum reticulatum has about 630 pairs of chromosomes,
or 1260 chromosomes per cell.
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Mitosis
Mitosis is the period in which the chromosomes condense align
along the metaphase plate and migrate to opposite poles.
In part because this is the most visibly dramatic stage in the
cell cycle much research has focused on these mitotic events.
Net result: The creation of two daughter cells with identical
chromosome complements.
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Homologous
Chromosomes
99.99% identical
Mitosis
n=2
2N
Replication of DNA
n=2
4N
Each DNA mol is a chromatid
The two chromatids attached
to the centromere are called
sister chromatids
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Mitosis
n=2
4N
Sister chromatids line up at
the metaphase plate.
Sister chromatids separate
to opposite poles
n=2
2N
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Mitosis in haploid and diploid
a
A
n=1
2N
A
n=1
1N
Replication of DNA
Replication of DNA
A
n=1
4N
a
A
A
a
A
a
A
n=1
2N
A
A
A
a
n=1
2N
n=1
1N
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Cell cycle and cancer
Currently the cell cycle/mitosis is an intensively investigated
area of research. This is primarily due to the fact that:
1.
The structural and regulatory components governing the cell
cycle are conserved throughout the phyla. That is, the
same proteins are used in yeast, flies and humans.
2. A number of the mutations that produce cancer in humans
disrupt the genes involved in regulating cells during the cell
cycle.
Example:
Cancers result from uncontrolled and inappropriate division of
cells
Cells actually contain a set of genes whose job it is to prevent
cells from dividing inappropriately ( these genes are known
as tumor suppressor genes, anti-oncogenes, and more
poetically as the "guardians of the cell").
One gene known as p53 ensures that the chromosomes have
replicated properly before allowing the cells to proceed into
mitosis. Recently it has been found that lesions in this gene
are one of the most common in all human cancers.
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Chromosomes
Basic terms and key features of the chromosome:
Telomere:
Centromere:
Sister chromatids:
Homologue.
Metaphase plate:
Haploid (N)Diploid (2N):
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Karyotype
Description of length, number, morphology.
Karyotype analysis is extremely important in medicine.
Alternations in karyotypes are linked to birth defects and many
human cancers.
MetacentricAcrocentricTelocentric-
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Meiosis
Meiosis:
While the mitotic cycle is designed to produce two cells with the
identical genotype, the meiotic cycle is designed to produce four
cells each with half of the chromosome complement.
(Increase in ploidy leads to increase in size of cells.)
Meiosis allows the cell to maintain constant ploidy (following mating)
and at the same time to shuffle the genetic deck (in the progeny)
In meiosis:
Diploid cells undergo one round of chromosome replication followed
by two divisions thereby reducing ploidy and producing four haploid
cells. The two divisions are referred to as Meiosis I and Meiosis II.
2N
N
-----> 4N----->N
N
N
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Meiosis
Meiosis I:
Interphase I: chromosomes replicate
Prophase I: chromosomes condense members
of a chromosome pair (homologues) physically
associate with one another and lie side by side
on the metaphase plate. This process is known
as synapsis. The paired chromosome physically
overlap forming structures known as chiasma.
Metaphase I: the paired homologous
chromosomes, known as bivalents,move to the
center of the cell and line up along the
metaphase plate.
Anaphase I: in a process known as disjunction,
the members of a homologous pair migrate to
opposite poles. This effectively reduces the
total number of chromosomes by half and is
therefore called a reductional division.
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Meiosis
Telophase I: if this stage were equivalent to telophase of mitosis,
the nuclear envelope would reform and DNA synthesis would take
place. This does not occur and the anaphase meiotic products proceed
directly into Prophase II of meiosis
Net result: Four haploid meiotic products
Meiosis II is analogous to mitosis; chromosomes, rather than
homologous pairs align along the metaphase plate and the
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chromatids separate
MeiosisI
Chromosomes replicate
OR
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Homologous Chromosomes pair locate on metaphase plate at random
This is Mendels random assortment
Random assortment
OR
anaphaseI.
Centromeres do not separate
The two sister chromatids go to the same pole
OR
Reductional division
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MetaphaseIIa
The reduced number of chromosomes in each of the two cells align
on the metaphase plate (no pairing of homologous occurs), divide to
produce four haploid cells.
Cell division without intervening replication!!
Similar to mitotic metaphase
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25%
25%
MetaphaseIIb
The reduced number of chromosomes in each of the two cells align
on the metaphase plate (no pairing of homologous occurs), divide to
produce four haploid cells.
Cell division without intervening replication!!
Similar to mitotic metaphase
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25%
25%
Meiosis
a
A
B
b
A
Aa
a
B
Bb
b
A
Aa
a
B
Bb
b
A
A
a
a
B
B
b
b
A
A
a
a
20
B
B
b
b
1st mechanism for genetic diversity:
independent assortment of chromosomes
With 23 human
chromosomes, there is a
possible 223 = 8.4 x 106
distinct gametes.
Little Alberts 1st edition 9-36
© Garland Publishing
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Gene Shuffling
Unlike mitosis, the meiotic products are not genetically identical.
There are two reasons for this
1.
The arrangement of paired homologous on the plate at
Metaphase I is random. This random arrangement is the
mechanism behind Mendel's principle of independent assortment
2.
The paired homologues physically recombine (or crossover with
one another).
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Crossing over
There are two ways of generating variation:
Random assortment of chromosomes (shuffling of maternal and
paternal chromosomes)
Recombination between homologous chromosomes (crossing-over)
in metaphase I
Homologous chromosomes pair in metaphaseI
At least one crossover occurs per homologous pair
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Crossing over is the result of a physical exchange
between homologous chromosomes
Cytological studies in maize by Creighton and McClintock
(1931) were the first to demonstrate that recombination is
the result of a physical exchange between homologous
chromosomes
On chromosome 9 in corn there were two markers:
Endosperm composition:
Seed color:
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Crossing over is the result of a physical exchange
between homologous chromosomes
Cytological studies in maize by Creighton and McClintock
(1931) were the first to demonstrate that recombination is
the result of a physical exchange between homologous
chromosomes
On chromosome 9 in corn there were two markers:
Endosperm composition:
Seed color:
Wx waxy
C colored
wx starchy
c colorless
In addition, the chromosomes were morphologically distinct.
Some had a cytologically visible structure known as a knob
at the telomere and others had an interchange such that it
is longer
W
W
F1
C
C
w
w
X
W
w
C
c
c
c
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The genetic recombinants were also cytological recombinants.
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This strongly supported the model that recombination involves a
physical exchange between homologous chromosomes
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Mitosis and meiosis compared:
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