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Meiosis (11C)

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Mark Polko

on 7 March 2014

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Transcript of Meiosis (11C)

Meiosis and mitosis
Cell division
Meiosis
Process: meiosis (like mitosis) is preceded by the replication of chromosomes or DNA. However, this single replication is followed by
two consecutive cell divisions,
called meiosis I and meiosis II. These divisions result in four daughter cells (rather than the two daughter cells of mitosis), each with only half as many chromosomes as the parent.
So what is the difference?
Meiosis I: consists of prophase I, metaphase I, anaphase I and telophase I
During
metaphase I
, the homologous pairs are
randomly arranged
on the metaphase plate: sometimes the paternal chromosome is on the right and the maternal on the left and it could also be the other way around. This way
many different and diverse combinations can happen
(t
his is the second variability source of meiosis
: for example, a new daughter cell could have chromosomes 1, 2’, 3, 4, 5’ etc, while the other would have 1’, 2, 3’, 4’, 5 etc).
Metaphase I
Anaphase I
and telophase
In anaphase I and telophase I the homologous chromosomes
migrate toward the opposite poles of the cells
. Segregation of chromosomes (
each pole now has a haploid chromosome set
, but each chromosome still has two chromatids) and finally cytokinesis, usually occurring simultaneously with telophase I forms two daughter cells each with only one of the homologous chromosomes.
There is no further replication of the genetic material prior to the second division of meiosis II.
Meiosis II
During the 2nd meiotic division,
the two chromatids of each chromosome separate into the daughter cells in a very similar way as mitosis.
At the end of meiosis II there will be four daughter cells, each with the haploid number (n) of chromosomes and genetically different from one another and from the mother cell (genetic variation).

SUMMARY
: meiosis is a cellular division necessary for organisms with sexual reproduction. Meiosis reduces the number of chromosomes by half (which will be restored during fertilization) and allows for genetic variation (the daughter cells are genetically different from each other, and also from the mother cell).
Mitosis
Meiosis I: consists of prophase I, metaphase I, anaphase I and telophase I
During
prophase I
homologous chromosomes, each made up of two chromatids,
come together as pairs
(forming a tetrad, a complex of four chromatids). At numerous places along their length, nonsister chromatids (chromatids belonging to homologous chromosomes, in contrast to sister chromatids belonging to the same chromosome) are
criss-crossed and recombined (crossing over)
. As a result of these crossings,
mixed chromatids are formed
with fragments from the mother and the father chromosomes (
this is the first source of variability in meiosis
).
http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120074/bio17.swf::Comparison%20of%20Meiosis%20and%20Mitosis
http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120073/bio14.swf::Mitosis%20and%20Cytokinesis
Cellular division in eukaryotic cells consists of two phases: first the nucleus divides (mitosis) and then the cytoplasm divides (cytokinesis). The following scheme shows the stages of mitotic cell division in an animal cell.
Differences with plant cells:
a.There are
no centrioles
in plant cells.
b.Cytokinesis, in animal cells occurs by a process known as
cleavage
, where the formation of a cleavage furrow pinches the cell in two. However, cytokinesis in plant cells, which have walls, is very different. A structure called t
he cell plate made of cellulose will separate both daughter cells
. The cell plate will later give rise to the new cell wall.
SUMMARY: mitosis is a cell reproduction process by which
multicellular organisms regenerate lost or damaged cells, or simply make new cells.
In the case of unicellular organisms it can be considered as asexual reproduction.
It does not generate genetic variability
, as the new daughter cells are identical to each other and to the mother cell. This is how all somatic cells divide (epithelial cells, liver cells, etc.
All but sex cells
).
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