Mitosis

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Mitosis - a form of cell division that
produces two daughter cells, each having the
same genetic complement as the parent cell.
  - 2n --- briefly 4n (anaphase) ------2n   -
identical chromatids (sister chromatids)are
separated and then segregated to different
daughter cells.   - occurs in the somatic cells
of our bodies
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Meiosis form of cell division where there
are two successive rounds of cell division
following DNA replication - produces haploid
cells (n) - start with 46 double stranded
chromosomes (2n) After 1 division - 23 double
stranded chromosomes (n) After 2nd division - 23
single stranded chromosomes (n)   - occurs in our
germ cells cells that produce our gametes
- egg and sperm
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Why do we need meiosis?
- it is the fundamental basis of sex
What is the purpose of sex?

• - to bring two haploid gametes together to form a
  diploid zygote.
• n (mom) n (dad) 2n (offspring)

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Replication of chromosomes

• Replication is the process of duplicating a
  chromosome
• Occurs prior to division
• Replicated copies are called sister chromatids
• Held together at centromere

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Meiosisa cell division forming gametes

• Goal reduce genetic material by half
• Why?

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Meiosis cell division in two parts
Sister chromatids separate
Homologs separate
Result one copy of each chromosome in a gamete.
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A replicated chromosome
Gene X
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Meiosis I the reduction division
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Prophase I
Early prophase Homologs pair. Crossing over
occurs.
Late prophase Chromosomes condense. Spindle
forms. Nuclear envelope fragments.
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Metaphase I
Homolog pairs align along the equator of the cell.
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Anaphase I
Homologs separate and move to opposite
poles. Sister chromatids remain Attached at
their centromeres.
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Telophase I
Nuclear envelopes reassemble. Spindle
disappears. Cytokinesis divides cell into two.
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Meiosis II

• Only one homolog
• of each chromosome
• is present in the cell.

Gene X
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Meiosis II the equational division
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Prophase II
Nuclear envelope fragments. Spindle forms.
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Metaphase II
Chromosomes align along equator of cell.
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Anaphase II
Sister chromatids separate and move to opposite
poles.
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Telophase II
Nuclear envelope assembles. Chromosomes
decondense. Spindle disappears. Cytokinesis
divides cell into two.
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Results of meiosis
Gametes Four haploid cells One copy of each
chromosome One allele of each gene Different
combinations of alleles for different genes
along the chromosome
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Fig 2.19
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• Why do we have meiosis?
• to generate haploid gametes

• to make new combinations of genes
• How?
• random (independent) assortment
• recombination

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Independent Assortment
The homologue of one chromosome can be
inherited with either homologue of a second
chromosome.
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Fig 2.20
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Random assortment   Since the combination of
maternal and parental chromosomes received by a
gamete is random. And we have 23 pairs of
chromosomes The possible combinations in an egg
or a sperm are -   223
8,388,608 combinations in an offspring   223
X 223 70,368,744,177,664 Result
Generates new combinations of genes (alleles)
when the genes are located on different
chromosomes.
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Recombination   Crossing-over - the
physical exchange of chromosomal material between
chromatids of homologous chromosomes. - Result
Generation of new combinations of genes
(alleles) if the genes are located on the same
chromosome.
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Recombination (crossing over)

• Occurs in prophase of meiosis I
• Generates diversity

• Creates chromosomes with new combinations of
  alleles for genes A to F.

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Recombination (crossing over)

• Occurs in prophase of meiosis I
• Generates diversity

A
a
a
A
B
b
b
B
c
C
C
c
D
D
d
d
E
E
e
e
Letters denote genes Case denotes alleles
F
F
f
f

• Creates chromosomes with new combinations of
  alleles for genes A to F.

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Recombination (crossing over)

• Occurs in prophase of meiosis I
• Generates diversity

a
A
a
A
B
b
B
b
c
c
C
C
D
D
d
d
E
E
e
e
Letters denote genes Case denotes alleles
F
F
f
f

• Creates chromosomes with new combinations of
  alleles for genes A to F.

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Chiasmata

• Crossing over
• or recombination
• events create
• chiasmata.

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Fig 2.21
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Fig 2.22
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Gametogenesis - spermatogenesis -
oogenesis
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Spermatogenesis sperm formation
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Spermatogeneis

• Stem cells in testes divide mitotically to create
  a pool of spermatocytes.

Meiosis produces four spermatids.
Spermiogenesis is the process of forming the
mature sperm.
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Oogenesis
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Oogenesis ovum formation

• One of four meiotic products becomes an ovum.
• The three remaining meiotic products are polar
  bodies.

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Oogenesis versus Spermatogenisis
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Fertilization

• Fertilization is the joining of sperm and ovum.

• Meiosis II in the ovum is completed at the
  time of
• fertilization forming one ovum and one polar
  body.

• Following fertilization, chemical reactions
  occur
• preventing additional sperm from entering the
  ovum.