Ploidy and sexual life cycle Human example

1
Ploidy and sexual life cycle Human example

• Each parent produces a haploid gamete (n).
• - Male (?) ? sperm
• - Female (?) ? ova (sing. ovum)
• Gametes are produced during a special type of
  cell division called Meiosis. Since the adult
  individual is diploid (2n) and the gametes
  produced by meiosis are haploid (n), meiosis is
  said to be a reductional division. (in contrast,
  mitosis of somatic cells is not reductional since
  daughter cells will be identical to parent cell).
• In organisms such as humans, gametes must fuse
  with another gamete, or die shortly after. They
  are produced exclusively for reproduction.

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Production of gametes Meiosis (not Mitosis)

• Differences between meiosis and mitosis
• 1) cells divide twice, but the chromosomes are
  duplicated only in the first division. Thus,
  chromosome numbers start as 2n (original cells),
  and end as n (gametes)
• 2) Meiosis mixes maternal and paternal
  chromosomes into each gamete, resulting in new
  genetic combinations.
• 3) There is crossing-over (individual chromosomes
  are broken the pieces mixed randomly). Each new
  chromosome is a mixture of paternal and maternal
  genes.
• Two divisions occur during meiosis (meiosis I,
  and meiosis II).
• - 4 cells are made (not 2 as in mitosis)
• - Each division has the same 4 stages (pro-,
  meta-, ana- and telophase)
• - Process begins with duplication of chromosomes
  into sister chromatids (as in mitosis)

3
What is the Crossing-over of meiosis?
Prophase I

• Crossing over occurs during prophase I of
  meiosis.
• Each homologous chromosome has created a copy of
  itself (these are sister chromatids). Since there
  are 2 chromosomes and each has done the same
  thing, there are 4 chromatids altogether, thus
  called a tetrad.
• The 2 pairs of sister chromatids remain attached
  by the centromeres, and wont separate even in
  anaphase I.
• Crossing-over is the process by which homologous
  chromosomes pair side by side, and exchange
  portions by having their ends attach to one
  another, and then breaking them off and
  reattaching them on another of the 4 chromatids.
  This is equivalent to mixing genetic information.
• The places at which sister chromatids attach to
  another (and where crossover will take place) are
  called chiasma (chiasmata). After cross-over,
  these same places will be called cross-over
  points.

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Meiosis (continued)

• Differences between meiosis and mitosis
• In prophase I, homologous chromosomes pair and
  cross-over
• Sister chromatids do not separate during anaphase
  I. Instead, homologous chromosomes (each with its
  2 sister chromatids) will migrate to opposite
  poles. 2 cells are produced, each containing a
  single set of chromosomes (but mixed), and each
  still consists of a pair of sister chromatids.
• During Meiosis II, the sister chromatids
  separate, resulting in 4 daughter cells, each
  with a haploid (n) number of chromosomes.
• Meiosis accomplishes
• Reduces chromosomes from 2n to n
• Provides genetic variation
• Gives each cell the correct ploidy (n)

5
Better view of meiosis (animals)
5-Anaphase I centromeres dont Split. Sister
chromatids to poles.
6-Telophase I Haploid nucleus visible
4-Metaphase I All chromosomes at metaphase plate
3-Late Prophase I tetrads visible
with chiasmata. Spindle well developed.
1-Early prophase I replication has
occurred. chromosomes visible
2-Mid Prophase I hom.chromosomes lined-up
cross-over
7- Interkinesis no replication of genetic
material occurs
8-Prophase II Material visible again
9- Metaphase II As normal
11- Telophase II cytokinesis of 2 cells) ending
in 4.
12- Interphase
10- Anaphase II Sister chromatids do separate.

• Meiosis consists of 2 consecutive cell divisions
  (each with the same 4 stages of mitosis),
  resulting in the production of 4 haploid daughter
  cells (gametes).
• The first division (Meiosis I) makes enough DNA
  for 4 haploid cells, but it doesnt separate it.
  The second division (Meiosis II) separates the
  DNA into the 4 gametes.
• As in mitosis, meiosis is preceded by
  replication, which creates chromatid pairs. The
  result is that the gamete-producing cell ends
  metaphase I with 4 times as much DNA as each
  gamete will need, so there will be enough DNA for
  them. In addition, crossing over has occurred so
  genetic variation has been introduced.

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Prophase I Members of each homologous pairs move
together and Lie side by side. Cross-over takes
place.
Metaphase I Each chromosome consists of 2
sister chromatids since the homologous are
together, there are 4 chromatids. They wont
separate as in mitosis.
Anaphase I Separation occurs between the 2
chromosomes of each pair, not between the
sister chromatids of each chromosome

• Meiosis 2 proceeds from each of
• the 2 cells produced in Meiosis I.
• It follows the normal process
• as described for mitosis, but since
• there are 2 original cells involved,
• results in 4 gametes produced.
• No crossover occurs
• sister chromatids separate in
• Anaphase as in mitosis.

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Meiosis in male and female animals

• In most male (?) animals, meiosis results in 4
  cells (sperm) that are of the same size (they may
  actually have differences in genetic composition)
• In most female (?) animals, the 4 cells that are
  produced are not of equal size. This is because
  one of them (the ovumegg) is very large and
  expensive to make. Thus the other (polar bodies),
  are very small and are actually non-functional
  and will disintegrate later. In some animals 2 or
  3 are produced, all of which will disintegrate.
• In the zygote, the majority of the cytoplasm and
  nutrition for the embryo comes from the ovum. The
  sperm provides almost only genetic material
• In plants, one polar body is not disintegrated
  and will actually be fertilized as well and
  becomes the endosperm (tissue that nourishes the
  plant embryo). See plants in detail later.

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