September 13, 2018
Making sense of Meiosis
REFERENCES:
Singh.R.J.
1993.
Plant Cytogenetics.
CRC Press. Chapter 3.Cell Division pp.31-37.
Molecular
Cell
Biology, Lodish et al., Chapter 8
Introduction
to
Genetic Analysis, Griffiths et al., Chapter 3
Learning Checklist:
- Meiosis is the formation of four
haploid gametes from a diploid cell.
- Meiosis creates genetic diversity in
a population through independent segregation of
chromosomes and through genetic recombination.
- The formation of haploid gametes
each bearing a full complement of chromosomes
requires chromosome pairing in meiotic prophase I.
Random assortment of chromosomes would result in
gametes that were missing some chromosomes and had
duplicates of others.
- Chromosome pairing is also required
for genetic recombination.
- Meiosis consists of two cycles: a
disjunctional division that separates the homologous
chromosomes, and a mitotic division, that separates
the two sister chromatids for each chromosome. Know
the stages of meiosis and be able to identify
cells in each stage. Also know the key events that
occur during the five stages of meiotic prophase I.
- Understand how C-value changes during
meiosis.
I. Why
Meiosis?
Meiosis is the physical basis of
Mendel's laws of independent segregation and independent
assortment. As we will see, it is also an elaborate,
metabolically costly, and sometimes faulty mode of reproduction.
Considering the costs involved, what are the benefits?
A. Sexual
reproduction creates genetic diversity
- Independent segregation of chromosomes
Because each gamete receives
chromosomes at random from one parent or the other, there
is a 50/50 chance of getting a given copy of any chromosome. Put
another way, there are 2 possible outcomes for getting either
the maternal or paternal chromosome. For a genome of n
chromosomes, there are 2 n possible gametes from a
single parent. For example, each human parent, with 23
chromosome pairs, can produce 2 23 = 8.4 x 10
6 possible gametes. Since each chromosome carries a
unique combination of alleles for thousands of loci, progeny are
always unique.
- Genetic recombination
The unique combination of alleles
contained on a given chromosome would be fixed if recombination
didn't occur. Given a large enough population, recombination
will occur between homologous chromosomes at all loci, so
that an almost limitless number of combinations of alleles, at
different loci, can be tested by evolution.
B. In organisms with a haploid stage,
meiosis may provide a "genetic cleansing" mechanism to
eliminate deleterious alleles.
Flowering plants and fungi
go through haploid stages as part of their life cycles. In
plants, this is the gametophyte generation. Metabolically-active
haploid cells must carry out all fundamental cellular functions.
Since only one copy of each gene is present, there is strong
selective pressure against deleterious alleles in the haploid
state.
C. The problem: production of haploid
gametes requires some mechanism to ensure that each
gamete gets a complete set of chromosomes.
In mitosis, the two homologues for each
chromosome can replicate and segregate independently and you'll
always come out with a balanced set of chromosomes in each
daughter cell. The strategy is simple: just attach a spindle fiber
to each centromere from pole to pole, and as long as one chromatid
migrates to each pole, each daughter cell will be complete.
But in meiosis, that wouldn't work.
In the figure, a diploid with two sets of chromosomes, I and II,
has just undergone a round of DNA replication, such that there
are now two copies of each homologue. A single reduction
division, as in mitosis, would still result in two balanced
diploid cells. The problem arises in the second division. The
figure at right shows one possible outcome for a meiosis in
which spindle fibers randomly attached to the kinetochore for
one copy of each chromosome, during the second meiotic division.
There is no orderly way to undergo a second cell division and be
sure that each cell gets one and only one copy each of
chromosomes I and II. This is why chromosome pairing (synapsis)
is neccessary in meiotic prophase I . By keeping all homologues
for a chromosome together, a balanced segregation can
occur. At the same time, synapsis provides an opportunity
for genetic recombination to occur.
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