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MEIOSIS. Meiosis. The form of cell division by which gametes, with half the number of chromosomes, are produced. Diploid (2n)  haploid (n) Meiosis is sexual reproduction. Meiosis. SEX CELLS divide to produce gametes (sperm or egg) .

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meiosis1
Meiosis
  • The form of cell divisionby which gametes, with half the number of chromosomes,are produced.
  • Diploid (2n)  haploid (n)
  • Meiosis is sexual reproduction.
meiosis2
Meiosis
  • SEX CELLS divide to produce gametes(sperm or egg).
  • GAMETEShave half the # of chromosomes.
  • Occurs only in gonads (testes or ovaries).

Male: spermatogenesis

Female: oogenesis

Two divisions (meiosis I and meiosis II).

  • (Meiosis is similar to mitosis with some chromosomal differences.)
overview
Overview
  • Start with a diploid cell, with 2 copies of each chromosome, one form each parent. The two copies are called homologues. Chromosomes each with 2 chromatids attached at the centromere.
  • Use 2 cell divisions:
  • Meiosis 1. First separate the homologues
  • Meiosis 2. Then separate the 2 chromatids.
  • The stages of meiosis have the same names as in mitosis: prophase, metaphase, anaphase, telophase. Each of the 2 cell divisions has all of these stages.
  • Meiosis 1 is unusual and needs a bit of study, but meiosis 2 is just like mitosis
meiosis i four phases
Meiosis I (four phases)
  • Cell division that reduces the chromosome number by one-half.
  • four phases:

a. prophase I

b. metaphase I

c. anaphase I

d. telophase I

prophase i
Prophase I
  • Longest and most complex phase (90%).
  • Chromosomes condense.
  • SYNAPSIS occurs:homologous chromosomes come together to form a tetrad.
  • TETRAD is two chromosomes or four chromatids(sister and nonsisterchromatids).
prophase i synapsis

Homologous chromosomes

sister chromatids

sister chromatids

Tetrad

Prophase I - Synapsis
metaphase i
Metaphase I
  • Shortest phase
  • Tetrads align on the metaphase plate.
  • INDEPENDENT ASSORTMENT OCCURS:

1. Orientation of homologous pair to poles is random.

2. Variation

3. Formula: 2n

Example: 2n = 4

then n = 2

thus 22 = 4 combinations

How many combinations are there for humans????

metaphase i1

OR

metaphase plate

metaphase plate

Metaphase I
answer
Answer
  • Formula: 2n
  • Human chromosomes: 2n = 46

n = 23

  • 223 = ~8 million combinations
anaphase i
Anaphase I
  • Homologous chromosomes separate and move towards the poles.
  • Sister chromatids remain attached at their centromeres.
telophase i
Telophase I
  • Each pole now has haploid set of chromosomes.
  • Cytokinesis occurs and two haploid daughter cells are formed.
meiosis ii
Meiosis II
  • No interphase II

(or very short - no more DNA replication)

  • Remember: Meiosis II is similar to mitosis
prophase ii
Prophase II
  • same as prophase in mitosis
metaphase ii

metaphase plate

metaphase plate

Metaphase II
  • same as metaphase in mitosis
anaphase ii
Anaphase II
  • same as anaphase in mitosis
  • sister chromatids separate
telophase ii
Telophase II
  • Same as telophase in mitosis.
  • Nuclei form.
  • Cytokinesis occurs.
  • Remember: four haploid daughter cells produced.

gametes = sperm or egg

spermatogenesis
SPERMATOGENESIS
  • Differences between male and female gametes.
  • In males, all 4 products of meiosis develop into sperm cells. They lose most of their cytoplasm, remodel their cell shape, and grow a long flagellum (tail).
oogenesis
OOGENESIS
  • In females, the cell divisions of meiosis are asymmetric: most of the cytoplasm goes into 1 of the 4 meiotic products, which becomes the egg. Eggs also develop large amounts of yolk proteins, which are used to feed the developing embryo. The other 3 meiotic cells are small “polar bodies”, which degenerate.
fertilization

sperm

n=23

n=23

egg

2n=46

zygote

Fertilization
  • The fusion of a sperm and egg to form a ZYGOTE.
  • A zygote is a fertilized egg