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Meiosis
Joanna
George-Johnson, Contributor
Last
time, we spoke of a type of cell division that maintained the chromosome number
at the end of the process. This week, we will look at a next type of cell division.
However, this cell division will decrease the number of chromosomes by half, that
is, Haploid cells are made. 
Meiosis
is the process by which gametes (sperm/egg/pollen) are made. Therefore, this process
only occurs in organisms that carry out sexual reproduction. In
human females, the production of eggs occurs before they are born. However, at
birth they are immature. At puberty, the eggs mature, one each month. In males,
the production of sperms starts at the onset of puberty. (More of this will be
discussed when reproduction is covered.) - Meiosis
is a process of cell division which results in the production of four daughter
cells from a single-parent cell. The daughter cells are not identical to one another
or to the original parent cell. Meiosis is a process to convert a diploid cell
to a haploid gamete, and cause a change in the genetic information to increase
diversity in the offspring.
The
rationale for meiosis - Formation
of new individual happens by a combination of two haploid sex cells (gametes).
- Fertilisation:
This is a combination of genetic information from two separate cells that have
one half the original genetic information.
- Gametes
for fertilisation usually come from separate parents.
- If
both gametes are haploid (n), then the individual that is formed will have the
full complement of chromosomes (2n).
Meiosis
is a very complex process so for simplicity the process is normally studied in
two stages with four phases each. This is an area where students tend to get confused
especially with keeping the number of chromosomes straight. I will attempt to
give clarity. So in an effort to diffuse the confusion I will use a table to describe
the process. | STAGE | PHASE | DESCRIPTION | #
Chromosomes (humans) | #
Chromatids' (humans) |  | Prophase
I | Already
duplicated Homologous² chromosomes pair and stands cross over. Forming chiasmata.
Chromosomes condense thus can be viewed with microscope. Nucleus starts to disappear | 46 | 92 | | Metaphase
I | Homologus
chromosomes align at the equator of cell. The alignment is random, with either
parental homologue on a side. | 46 | 92 | | Anaphase
I | The
crossed over sections separate. Chromosomes, each with two chtomatids, move to
opposite side of cell | 46
(23 on either side) | 92
(46 on either side) | | Telophase
I | Nuclear
envelopes from around each set of chromosomes. Forming two cells. In preparation
for Meiosis II | 23
in each if the two cells | 46
in each of the two cells |  | Prophase
II | The
chromosomes in each cell shorten and become visible under the microscope. The
nuclear envelope disappears | 23
in each of the two cells | 46
in each of the two cells | | Metaphase
II | The
chromosomes (still connected by a centromere) line up on the middle of the cell.
Spindle fibres emanating from the centrioles connect to the centromeres from either
side. | 23
in each of the two cells | 46
in each of the two cells |
1.
Cromatids: Strands of newly-duplicated chromosome still joined together.
Therefore, 1 duplicated chromosome = 2 cromatids. 2.
Homologous chromosome: Two chromosomes of the same type. (one from your
mom and the other from your dad). For example, a pair of chromosome 'number 6'
in the same cell is called homologous. | | Anaphase
II | The
centromeres that join the sister chromatids split. The sister chromatids separate.
They move to opposite sides of the cell until they have separated into two groups
near the poles of the spindle. | 23
in each of the two cells | 46
in each of the two cells Not
called chromatids by late anaphase called chromosomes instead. | | | Telophase
II | In
telophase, the chromosomes, which were distinct and condensed, begin to disperse
and become less visible under the microscope. A nuclear envelope re-forms around
each cluster of chromosomes. The spindle begins to break apart, and a nucleolus
becomes visible in each daughter nucleus. | 23
in each of the four cells | not
called chromatids anymore | | | | | | |
Questions
1.
A human cell has 46 total or 23 pairs of chromosomes. Following mitosis, the daughter
cells would each have a total of ______ chromosomes. After meiosis I, the two
daughter cells would have _____chromosomes, and after meiosis II ______ chromosomes.
2. The
process of meiosis produces four cells with non-identical chromosomes. At what
stage does this diversification takes place? 3.
Which stage of meiosis do cells become haploid? Joanna
George-Johnson teaches at Ardenne High School.
Email: Masterbio@gmail.com.
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