1. MEIOSIS
TEJASWI VARALWAR

2. OVERVIEW
 Meiosis is a kind of cell division that leads to the production of gametes.
 gametes: egg cells and sperm cells
 contain half the number of chromosomes of an adult body cell
 Adult body cells (somatic cells) are diploid, containing 2 sets of chromosomes.
 Gametes are haploid, containing only 1 set of chromosomes.

3. OVERVIEW
Sexual reproduction includes the fusion of gametes
(fertilization) to produce a diploid zygote.
Life cycles of sexually reproducing organisms involve the
alternation of haploid and diploid stages.
Some life cycles include longer diploid phases, some
include longer haploid phases.

4. Spermatogenesis
2n=46
human
sex cell
diploid (2n)
n=23
n=23
n=23
n=23
n=23
n=23
sperm
haploid (n)

5. Meiosis – mouse testes

6. Features of Meiosis
 Meiosis includes two rounds of division – meiosis I and meiosis II.
 During meiosis I, homologous chromosomes (homologues) become closely associated with
each other. This is synapsis.
 Proteins between the homologues hold them in a synaptonemal complex.
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8. Features of Meiosis
 Crossing over: genetic recombination between non-sister chromatids
 physical exchange of regions of the chromatids
 chiasmata: sites of crossing over
 The homologues are separated from each other in anaphase I.
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9. Features of Meiosis
 Meiosis involves two successive cell divisions with no replication of genetic material
between them.
 This results in a reduction of the chromosome number from diploid to haploid.
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11. 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

12. Prophase I
 Longest and most complex phase (90%).
 DNA condenses to form chromosomes.
 Duplicated chromatids are joint together at centromere
 Synapsis occurs: homologous chromosomes come together to form a tetrad and exchange
chromosomal matter called recombination or crossing over
 Nuclear membrane begins to breakdown.
 Centrosomes migrate to opposite ends of the cell
 Microtubules appear and attach to chromosomes

13. Prophase I - Synapsis
Homologous chromosomes
sister chromatids sister chromatids
Tetrad

14. Homologous Chromosomes
 Pair of chromosomes (maternal and paternal) that are similar in shape and size.
 Homologous pairs (tetrads) carry genes controlling the same inherited traits.
 Each locus (position of a gene) is in the same position on homologues.
 Humans have 23 pairs of homologous chromosomes.
a. 22 pairs of autosomes
b. 01 pair of sex chromosomes

15. Karyotype
 A method of organizing the chromosomes of a cell in relation to number, size,
and type.

16. Homologous Chromosomes
Paternal Maternal
eye color
locus
eye color
locus
hair color
locus
hair color
locus

17. Humans have 23 Sets of Homologous Chromosomes
Each Homologous set is made up of 2 Homologues.
Homologue
Homologue

18. Prophase I
centrioles
spindle fiber
aster
fibers

19. 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

20. Metaphase I
terminal chiasmata hold homologues together following
crossing over
microtubules from opposite poles attach to each
homologue, not each sister chromatid
homologues are aligned at the metaphase plate side-by-
side
the orientation of each pair of homologues on the spindle
is random

21. Metaphase I
metaphase plate
OR
metaphase plate

22. Anaphase I
 Homologous chromosomes separate and move towards the poles.
 Sister chromatids remain attached at their centromeres.
-microtubules of the spindle shorten

23. Anaphase I

24. Telophase I
 Each pole now has haploid set of chromosomes.
 nuclear envelopes form around each set of chromosomes
 sister chromatids are no longer identical because of crossing over
 Cytokinesis occurs and two haploid daughter cells are formed.

25. Telophase I

26. Meiosis II
 No interphase II
(or very short - no more DNA replication)
 Meiosis II is similar to mitosis

27. Prophase II
 Again chromosomes condense
 Nuclear envelope dissolves and spindle apparatus form
 Difference between prophase 1 and 2 is that daughter cells have only one copy of each
homologous chromosome.
 No synapses

28. Metaphase II
 Chromosomes align on the metaphase plate or the equator of the cell.
 Since chromatids are no longer identical, there will be many possible ways to align.
metaphase platemetaphase plate

29. Anaphase II
 sister chromatids separate from each other as microtubules shorten

30. Telophase II
 Nuclear membrane reforms and cytoplasm is divided into two haploid daughter cells
 Nuclei form.
 Cytokinesis occurs.
 Four haploid daughter cells are produced
 gametes = sperm or egg

31. Telophase II

32. End Of Meiosis
 Meiosis II began with two cells and those further split into two cells each
 We now have 4 haploid cells at the end of meiosis
 These cells are gametes

33. Meiosis
2n=4
sex cell
diploid (2n)
n=2
n=2
meiosis I
n=2
n=2
n=2
n=2
sperm
haploid (n)
meiosis II

34. END OF
INTERPHASE
PROPHASE I METAPHASE I ANAPHASE I
MEIOSIS I
Genetic recombination results from crossing
over during prophase I of meiosis

35. TELOPHASE II
ANAPHASE II
METAPHASE IIPROPHASE IITELOPHASE I
MEIOSIS

36. METAPHASE I METAPHASE I
TELOPHASE II
METAPHASE II
INDEPENDENT ASSORTMENT

37. egg
polar
body
spermatogonium
primary
spermatocyte
secondary
spermatocyte
oogonium
primary
oocyte
secondary
oocyte
polar bodies
(will be degraded)
spermatids
meiosis ll
meiosis l
SPERMATOGENESIS OOGENESISa b

38. Meiosis vs Mitosis
 Meiosis produces haploid cells that are not identical to each other.
 Genetic differences in these cells arise from:
-crossing over
-random alignment of homologues in metaphase I (independent assortment)
 Mitosis produces 2 cells identical to each other.
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40. THANK YOU