Mitosis (Revisited) & Meiosis
Chapter for today: Chap. 10
Major points for the day:
1. Meiosis
allows random segregation of genes to progeny
2. Maternal and paternal chromosomes segregate in Meiosis I reducing
the chromosomes to a haploid number
3. Meiosis II is a mitotic-like division separating sister chromatids
Reprise
Last Monday Dr. Kloetzel talked about mitosis, the process eukaryotic cells use to produce identical copies of cells.
• The
essential fact about mitosis is that at metaphase each duplicated
chromosome lines up on the metaphase plate and the two sister chromatids
separate and move to opposite poles of the cell.
• Since the two sister chromatids are the same this ensures
that each of the unique DNA molecules in the cell are transmitted to
the next generation.
• Nondisjunction (the name of this event) results in cells with
too few or too many chromosomes. Most of the resulting cells would be
dead.
— Down’s syndrome is an example of a relatively benign aneuploidy
So clearly, the process of mitosis is essential in the short term to the ability of organisms to survive.
Another process of cell division is essential to the long—term survival of a species-meiosis
Down’s Syndrome and other aneuploidy diseases really result from nondisjunction during the development of germ cells because of a failure of meiosis
The concept of homologous chromosomes
We have already discussed the idea that each of your cells contains two copies of each unique chromosome
This is easiest to envision when talking about the sex chromosomes:
• In
humans, the sex of each individual is determined by the identity of
the two sex chromosomes
• There are two types of sex chromosomes, the X and the Y
• A male receives an X chromosome from his mother and a Y from
his father
— This is the clearest example of the idea of there being a "mother’s" chromosome and a "father’s" chromosome
• A female receives an X chromosome from each parent
However, the existence of a maternal chromosome and a paternal chromosome is not limited to the sex chromosomes
Each of the somatic chromosomes (chromosomes not involved in sex determination, not "chromosomes involved in body formation") also comes as a maternal and a paternal copy
In fact, the two chromosomes in any pair are not identical since the parents are individuals with very different genetic make-ups
• This difference is at the level of genes, as we shall see. Mutations in particular genes cause differences that we can see-we call those phenotypes
The two chromosomes that make up a pair are called homologous, which literally means that they have a common origin (evolutionarily)
• It
does not mean that they are identical
• It means that almost all of their sequences are exactly alike,
with only small differences which generate phenotypic differences
Overview of Meiosis
The object of meiosis is to generate a random assortment of chromosome combinations in cells that have half the normal number of chromosomes
• In
a diploid organism that means the formation of haploid cells that carry
a random collection of chromosomes
• The process is very carefully regulated so that no cell receives
less than the full complement of chromosomes
— All
cells have one copy of each chromosome
— And no cell has more than one of any of those chromosomes
The only way to produce cells with half their normal number of chromosomes is to divide twice without allowing DNA replication
There are certainly many several ways the cell could accomplish this end.
• One
necessary part of such a scheme is that chromosomes would have to "pair
up" before cell division could occur
• In mitosis each pair of chromosomes is physically connected
at the centromere, so when the package that we call a chromosome moves
to the metaphase plate, the two identical chromosomes are held together.
If chromosomes didn’t pair it wouldn’t be possible to faithfully segregate one copy to each of the daughter cells
• The
chromosomes would have no way of knowing which pole of the cell their
homologue was going to.
• About half the time they would go to the same pole:
In meiosis pairing occurs between duplicated chromosomes (those involving sister chromatids.
• Pairing
of the duplicated chromosomes occurs on a metaphase plate
• The spindle then pulls the pairs apart
• As a result, each cell gets a pair of sister chromatids, not
one each of the paternal and maternal chromosomes
• This is the difference between meiosis and mitosis
— The first division creates a pair of cells which are different
— Each contains either the paternal or the maternal chromosomes, but not both
However, the direction that each of these chromosomes goes (that is to which cell they segregate) is random
• As
a result, there is a shuffling of the genes because a random selection
of chromosomes is made at each first meiotic metaphase
• In an organism with two chromosomes there are only four possible
outcomes for a given cell after this division:
|
Chromosome I |
Chromosome II |
|
P |
P |
|
P |
M |
|
M |
P |
|
M |
M |
• In an organism with three chromosomes there are only eight possible outcomes for a given cell after this division:
|
Chromosome I |
Chromosome II |
Chromosome III |
|
P |
P |
P |
|
M |
P |
P |
|
P |
M |
P |
|
P |
P |
M |
|
M |
M |
P |
|
M |
P |
M |
|
P |
M |
M |
|
M |
M |
M |
• Since humans have 23 chromosomes, the number of possibilities is 223 or 8,388,608 possibilities!
Remember that this is the number of alternative outcomes for a single individual-meiosis could occur in over 8 million ways. If you consider the genetic variability in a population, it is clear that it is extremely unlikely that any two gametes would be the same
The mechanics of meiosis
Meiosis consists of two successive divisions without an intervening DNA replication
Each of the divisions looks similar to a mitotic division, though the first involves a separation of chromosomes which is unlike mitosis (the second is simply a mitotic division)
The two divisions are called Meiosis I and Meiosis II. In each of these divisions the intervals within are designated with a I or II (e.g, Prophase I and Prophase II)
Meiosis I
• Meiosis I begins as Mitosis, with the disassembly of the nuclear membrane and condensation of the chromosomes (Prophase I)
— We will not be concerned right now with the process of recombination which occurs during Prophase I, but return to it in a minute.
• The duplicated chromosomes (each with two sister chromatids) attach to the spindle and move to the metaphase plate (Metaphase I)
— Note
that each pair of duplicated chromosomes is still paired
— They therefore align with each other at the metaphase plate
— The centromeric regions of each duplicated chromosome is
fused into one structure which is attached to one of the spindle poles
by microtubules
• The spindle pulls the duplicated chromosomes to the poles (Anaphase I)
— At
this stage the maternal and paternal chromosomes are separated from
each other
— This is the separation which is random, giving rise to the
large number of possible segregation patterns
• The chromosomes dissociate from the spindle (Telophase I)
Meiosis II (which occurs after cell division-two cells are now involved)
• The two resulting cells then proceed directly to the next division with the assembly of new spindles in each cell, and attachment of the still condensed chromosomes to it (Prophase II)
— Note that each sister chromatid must attach to a different spindle in preparation for division of the sister chromatids from each other
• The chromosomes line up on the metaphase plate (Metaphase II)
— Since
each of the duplicated chromosomes in these cells are unique they
can not pair
— Each chromosome lines up by itself on the plate
— This is like a mitotic division, in the sense that duplicated
chromosomes consisting of linked sister chromatids separate at metaphase
• Each pair of sister chromatids then separates and moves to opposite poles of the cell (Anaphase II)
—
In a normal mitosis there are a diploid number of duplicated chromosomes,
resulting in two diploid cells each receiving one of these chromatids,
now called chromosome (one maternal and one paternal)
—In Meiosis II, the division is of a haploid number of duplicated
chromosomes (some originally maternal and some paternal) that result
in cells with a haploid number of unduplicated chromosomes (again,
some maternal and some paternal)
• The chromosomes decondense and the nuclear membrane reforms (Telophase II)
The result is the production of four haploid cells each containing a single copy of each chromosome.
The second meiotic division resembles a mitotic division
During the second meiotic division duplicated chromosomes align on the metaphase plate and then separate to the opposite poles.
This results in each of the cells having one copy of each of the chromosomes.
The division resembles a mitotic division but the number of unique chromosomes is different
• Mitosis
occurs immediately after a round of DNA duplication, so the cells have
four copies of each DNA molecule, 2 in each of the pairs of duplicated
chromosomes.
• The cell is technically diploid because there are only two
types of chromosomes present, though each chromosome is duplicated
• After division the cell is still diploid, but there is only
one copy of each chromosome in the cell.
• In Meiosis II the cells begin as haploids because each cell
has only one copy of each unique chromosome, though it is duplicated
(two sister chromatids)
• At metaphase the two sister chromatids separate to the two
poles, as in mitosis, resulting in two haploid cells, each with one
copy of each chromosome.
DNA recombination and meiosis
There is another level at which the collection of genes in the genome are randomized: DNA recombination
• During
prophase I the chromosomes pair with each other so that the maternal
and paternal chromosomes are in close contact.
• This pairing is how the cell determines which chromosomes
are homologues
• Enzymes in the cell recognize regions of identical sequence
in the chromosomes leading to formation physical linkage between the
chromosomes
• Other enzymes catalyze the breakage and rejoining of DNA strands
to exchange sequences between non—sister chromatids
• This changes the linkage of genes on the chromosome
Remember that the two homologous chromosomes are not identical in every way.
• They
carry sequence differences between the genome of the mother and the
father
• These differences are called alleles
• The father’s chromosome will have a set of alleles on
each chromosome many of which will be different from those on the mother’s
chromosome
• Recombination puts some of the mother’s alleles and some
of the father’s alleles on the same DNA molecule
The take-home-lesson for meiosis is that random segregation of chromosomes at Anaphase I and the shuffling of sequences by recombination during Prophase I provides an immense variability in the products of meiosis.
• The
segregation provides over 8 million possible ways chromosomes can segregate
into the gametes
• Recombination greatly increases this number since each chromosome
probably undergoes at least one recombination per chromosome arm per
meiotic prophase
• In combination with the great diversity of alleles present
in a population, this random assortment of alleles provides for an immense
number of possible gametes
• Fusion of two such gametes creates a new individual who is
almost certainly unique genetically
