Meiosis:
produces the sex cells, which contain half the regular # of chromosomes:
in humans, instead of their being 46 chromosomes in sex cells, as there
are in regular cells, there are only 23. So when they come together, there
will be the appropriate #: 23 + 23 = 46 (2 pairs of 23 homologous chromosomes).
Write right at the top of your syllabus: -- sexual reproduction = meiosis
+ fertilization--. The purpose of meiosis is to reduce the chromosome #
by half.
- Mechanics of meiosis
 12.1
Meiosis
- Initiation of meiosis
An organism's reaching sexual maturity initiates
the production of sex cells (sperm and eggs). In humans, meiosis occurs
in the male testes and in the female ovaries. (Females are born with all
their eggs already produced: 300,000 or so in prophase I; the 2 meiotic
sequences are completed upon fertilization.
- DNA replication
Chromosomes replicate, forming sister chromatids,
which will eventually separate into chromosomes.
- Division of the DNA (meiosis) and
cytoplasm (cytokinesis)
List of 2 things: 1 - meiosis results in the reduction
of the chromosome # in half. 2 - meiosis introduces genetic variation through
crossing-over (coming up.)
- Basic meiotic sequence
Meiosis involves 2 sequences:
- 1st meiotic sequence
The chromosome sequence is reduced in half and crossing-over
occurs.
Prophase I:
(the first meiotic event: interphase is NOT a phase of meiosis.) Homologous
chromosomes pair up. Crossing-over may occur.
Metaphase I: Chromosomes line up in the middle of the cell. Spindle fibers
attach to centromeres.
Anaphase I: Homologous chromosomes are moved to opposite ends. Anaphase
I is where reduction division occurs: when a cell's genetic material changes
from diploid to haploid; (the pairs are divided between daughter cells;
each cell gets one sock from each of the 23 pairs of matching, homologous
socks). (In humans, diploid (di=two) = 46 chromosomes (23 homologous pairs);
haploid (half) = 23 chromosomes.)
Telophase I: Nuclear membranes form and cytokinesis takes place. (There
may or may not be an interphase between telophase I and prophase II.)
- 2nd meiotic sequence
Prophase II: Nuclear membranes break down again.
Spindle fibers begin to form.
Metaphase II: Chromosomes line up, spindle fibers attach.
Anaphase II: Chromatids separate into chromosomes and move to opposite
sides.
Telophase II: Nuclear membranes form and cytokineses leaves 4 (haploid)
daughter cells. These cells will undergo some further modification, becoming
gametes: sex cells: of the four daughter cells in females, one will become
an egg (the others: polar bodies) and in males, the 4 haploid cells will
become sperm.
Upon fertilization, a sperm, cell and egg cell combine, restoring the chromosome
from a haploid (23 non-paired, non-homologous chromosomes in humans) to
a diploid state (23 homologous pairs: 46 total).
- Significant events associated with
meiosis
Here are three:
- Reduction of chromosome number
by half
(If the number were not reduced by half, how many
chromosomes would result at fertilization: twice as many, each time, which
would not function.)
- Formation of sex cells (gametes)
No sex cells = no sexual reproduction = no offspring
= no future.
- Genetic recombination through crossing-over
12.2
Crossing-over
Crossing-over occurs during prophase I of meiosis. Crossing-over is
the exchange of DNA segments between non-sister chromatids of homologous
chromosomes. Crossing-over may introduce genetic variation: new gene sequences
on a chromosome.
- Comparison of mitosis and meiosis
Here it is:
- Mitosis
- 1 cell ----> 2 cells
- No change in chromosome number
(total DNA content remains constant)
Diploid cells produce diploid daughter cells.
- No consistent mechanism for introducing
genetic variation
(no CONSISTENT mechanism; there is a possibility
of mutation/error in the process)
- Meiosis
12.3
Meiosis: Summary
The production of sex cells (in males: occurs in the testes. in females:
occurs in the ovaries.)
- 1 cell ----> 4 cells
One diploid cell produces 4 haploid cells.
- Chromosome number (total DNA content)
reduced by half
(Keep this in mind during the upcoming genetics lectures.)
Sex cells are haploid.
- Significant genetic variation introduced
through crossing-over
An important point to be further elaborated upon
in the evolution lectures.
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