Meiosis and Sexual Reproduction

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Chapter 12

• Meiosis and Sexual Reproduction

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Haploid and Diploid

• Haploid number of chromosomes in gametes
  (single set)

- Diploid- number of chromosomes in somatic cells
(double set)
- In most organisms sex cells (gametes) have
only half the number of chromosomes
http//www.emc.maricopa.edu/faculty/farabee/BIOBK/
gametogen_2.gif
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• Haploid number n

• diploid number 2n

• In humans n 23 therefore 2n 46

• In every diploid cell each chromosome has a
  partner

• These pairs are known as homologous pairs Or
  Homologues

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http//www.emc.maricopa.edu/faculty/farabee/BIOBK/
BioBookmeiosis.htmlMeiosis

• Special kind of nuclear division -meiosis

• Diploid set of chromosomes (containing two
  homologues of each pair)
• Is reduced to a haploid set (containing only one
  homologue of each pair)

• Meiosis counterbalances the effects of
  fertilization (reduction division)

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Meiosis and the Life Cycle
Meiosis occurs at different times during the life
cycle of different organisms.
In many protists and fungi- meiosis occurs
immediately after fusion of the mating cells.
In plants- a haploid phase and diploid phase
alternate
Alternation of generations
Process in which a haploid phase is followed by
a diploid phase and again by a haploid phase
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Human Life Cycle

• Typical animal life cycle, in which the diploid
  individual produces haploid gametes by meioses
  immediately preceding fertilization.

NOTE

• Meiosis in plant or human has the same result
• At some point during life cycle of a sexually
  reducing
• organism, it reduces the diploid chromosome set
  to the
• haploid chromosome set.

Although meiosis in animals produces gametes,
meiosis in plants produce SPORES.
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Homo Sapien life cycle
Censor
Censor
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Meiosis vs Mitosis

• Differences of Mitosis and mitosis

Mitosis can occur in haploid or diploid
cells meiosis occurs only in cells with diploid
number of chromosomes
During meiosis, each diploid nucleus divides
twice, producing a total of four nuclei.
haploid nuclei produced by meiosis contain new
combinations of chromosomes. This is the
homologous chromosomes, originally derived from
the organism's parents, are assorted randomly
among the four new haploid nuclei.
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Phases of Meiosis
Consists of two successive nuclear divisions,
conventionally designated meiosis I and meiosis
II.
Meiosis I- homologous chromosomes pair and then
separate from one another
Meiosis II- chromatids of each homologue separate
Interphase precedes meiosis - chromosomes
replicated - by beginning of meiosis each
chromosome consists of two identical sister
chromatids held together at centromere
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PROPHASE I

• Chromatin condense chromosomes come into view
• Homologous chromosomes come together in pairs
• Synapsis- chromatids come out of the two
  homologues

Crossing over is an important mechanism for
recombining the genetic material from the two
parents.
Crossing over- involves the exchange of segments
of one chromosome with corresponding segments
from its homologous chromosome.
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• As homologues begin to pull away from each other-
  the cross over places do not move, these places
  are called chiasmata.

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Metaphase I

• Homologous pair line up along the equatorial
  plane of the cell.
• Centromere region of each homologue has doubled
  by the end of metaphase
• Spindle fibers have become associated with the
  kinetochores

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Anaphase I

• Homologues separate
• Two sister chromatids of each homologue do not
  separate as they did in mitosis.

TELOPHASE I
Homologues have moved to the poles Each
chromosome group now contains only ½ the number
of chromosomes as the original nucleus
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Meiosis II

• Resembles mitosis except that it does not precede
  replication of the chromosomal material
• Beginning of 2nd meiotic division, the
  chromosomes (if dispersed) condense.

• PROPHASE II
• Nuclear envelopes disintegrate and new spindle
  fibers begin to appear.

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Metaphase II

• Three chromatid pairs in each nucleus line up on
  the equatorial plane
• Spindle fibers are once again associated with the
  kinetochores

Anaphase II - sister chromatids separate
from one another -Each chromatid (now a
chromosome) moves toward one of the poles
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Telophase II

• Spindle microtubules disappear and a nuclear
  envelope forms around each set of chromosomes
• Cytoplasmic division proceeds
• Final end up
• Begin with 1 cell containing 6 chromosome
• End up with four cells each with three chromosomes

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Comparing Mitosis and Meiosis
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Meiosis in the Human Species

• In all vertebrates
• Meiosis takes place in reproductive organs
• Testes and ovaries
• Females
• Meiotic division also produce haploid nuclei but
  the cytoplasm is apportioned unequally during
  cytokinesis in both meiosis I and II

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Cytology and Genetics meet Suttons Hypothesis

• Sutton
• Chromosomes paired at the beginning of first
  meiotic division
• Principle of Segregation
• Chromosomes carried genes
• Alleles occurred on homologous chromosomes