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Mitosis and Meiosis
Traits are controlled by genes
Each individual has thousands of genes and each gene has two
copies in an individual.
What are the physical entities that carry the genes during
segregation and independent assortment?
What is the biological basis for Mendel’s laws?
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Genes reside on Chromosomes
Genes reside on chromosomes, Therefore 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 (not identical)
Meiosis only occurs in a small specialized set of cells known as
the germ cells.
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Development
Mitosis
2N ---->4N
----> 2N+2N (somatic cells)
Meiosis
2N ---->4N
---->N+N+N+N (germ cells)
The segregation and assortment of chromosomes in germ cells
is important in the transmission of traits
2N
|
4N
|
|
N
meiosis
|
|
|
2N
|
|
4N
|
|
2N
mitosis
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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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Digression: Chromosome number
n=2
Species
Human
Monkey
Mouse
Frog
Fruit fly
C. Elegans
Corn
S. Cerevisiae
S. Pombe
Haploid number
(n)
23
21
20
13
4
6
10
16
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Haploids are
Diploids are
Tetraploids are
Smallest number: The female of the ant, Myrmecia pilosula,
has one pair of chromosomes per cell. Its male has only one
chromosome in each cell.
Largest number: The fern Ophioglossum reticulatum has about
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630 pairs of chromosomes, or 1260 chromosomes per cell.
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 MITOTIC
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: end of chromosomes
Centromere: It is the constricted region where the
microtubules attach and help pull the sister chromatids apart
during mitosis
Sister chromatids: replicated chromatids in G2. The two sister
chromatids are identical to one another. During prophase and
metaphase they look like:
A
A
Homologue- chromosome pair in a diploid. They are similar but
not identical.
A
A
a
a
Metaphase plate: the region midway between the two spindle
poles in which the chromosomes align during metaphase
Haploid (N)- the condition in which each chromosome is
present in one copy (found in gametes)
Diploid (2N): the condition in which each chromosome is
present twice as members of a homologous pair
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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.
Metacentric- centromere in the middle
Acrocentric- centromere towards the end
Telocentric- centromere at the end
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Downs
Staining and specific banding pattern allows you to line up and
identify various chromosomes
Down's syndrome results from an individual possessing three
copies of chromosome 21 rather than the normal two. It is the
most common of all human defects and occurs in 1/200
conceptions and 1/900 births.
Females over 35 years- segregation defect leading to three
copies of chromosome 21
Karyotype analyses of fetus indicates whether the child has Downs.
Trisomies occur with other chromosomes as well but usually lead to
the death of the fetus
XXY and XYY individuals are also found.
Normal Male
Downs Male
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cri-du-chat
The syndrome cri-du-chat is the result of a deletion in the
short arm of human chromosome 5
The affected infant is mentally retarded and has abnormal
development of the larynx. Consequently, the most
characteristic symptom of the disorder is that their cry
resembles that of a cat.
sound recording of:
Normal,
infant
Cat,
cri-du-chat
infant
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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.
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
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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
A
A
a
a
B
B
b
b
b
b
B
B
A
A
a
a
A
A
a
a
B
B
b
b
b
b
B
B
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1st mechanism for genetic diversity
independent assortment of
chromosomes
Unlike mitosis, the meiotic
products are not genetically
identical
The arrangement of paired
homologous chromosomes on
the plate in METAPHASE-I is
RANDOM. Random
arrangement leads to
independent assortment of
genes on different
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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Second mechanism of genetic diversity
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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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
Mitosis and meiosis compared:
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Chromosome theory of inheritance
As you all know genes reside on chromosomes. This basic fact
is called the chromosome theory of inheritance. However
earlier in this century, the issue of where the units of
heredity resided was fiercely debated.
The notion the genes were located on the chromosomes came
from the recognition that the behavior of Mendel's particles
during meiosis parallels the behavior of chromosomes at
meiosis.
1
2
3
Mendel’s Laws of independent assortment imply that genes on
the same chromosome are inherited together and genes on
different chromosomes are inherited independently.
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chromosome theory of inheritance
As you all know genes reside on chromosomes. This basic fact
is called the chromosome theory of inheritance. However
earlier in this century, the issue of where the units of
heredity resided was fiercely debated.
The notion that genes were located on chromosomes came from
the recognition that the behavior of Mendel's particles during
meiosis parallels the behavior of chromosomes during meiosis.
1. Genes are in pairs, so are chromosomes
2. Alleles of a gene segregate equally into gametes, so do the
members of a homologous chromosome pair
3. Different genes act independently, so do different
chromosomes
Mendel’s Laws of independent assortment imply that genes on
the same chromosome are inherited together and genes on
different chromosomes are inherited independently.
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