Meiosis produces gametes through two rounds of cell division beginning with one diploid cell and ending with four haploid cells. In meiosis I, homologous chromosomes pair and separate, reducing the chromosome number by half. Meiosis II separates sister chromatids, further dividing the cell and its genetic material. The steps of crossing over and independent assortment in meiosis I contribute to genetic variation among gametes and offspring.

1. 11-4 Meiosis
P275

2. Homologous chromosomes
• Pair of chromosomes that
one comes from the male
parent and the corresponding Homologous
one from the female parent chromosomes
Homologous ≠ identical Alleles
same genes in same loci
same length
same shape
may not the same alleles
for one gene

3. Diploid & Haploid Cells
• two copies of each • one copy of each
chromosome chromosome
• homologous pairs • no homologous pairs
• body cells • gametes (sex cells)
Sperm
Egg

4. ?
meiosis Sperm
Egg
Egg
Zygote
fertilisation
Sperm

5. The purpose of meiosis
is to produce gametes.
A (haploid) gamete
(sex cell, sperm or egg
cell) has half the
number of
chromosomes compared
to a (diploid) somatic
cell (body cell). A male
and a female gamete
may then fuse to form a
zygote, which will have
the same number of
chromosomes as a
somatic cell.

6. Human chromosomes
– Diploid cells: 23
pairs of homologous
chromosomes
– 1st-22nd pairs:
autosomes
– 23rd pair: sex
chromosome

7. Human chromosomes

8. 1-22 pairs : autosomes, 23 pair : sex chormosome

9. Karyotype (the type of nucleus)
• The number and appearance of the chromosomes in an
organism

10. Karyotyping
• Arrange the chromosomes in pairs
according to their size and structure
• Collect cells
• Culture cells
• Stop cell division at metaphase
• Staining chromosome
• Pair (Length, Shape, Position of
centromeres, Bands)
• Diagnosis (number, gender,
abnormality)

11. How many chromosoems
8
How many pairs of
homologous
chromosomes ？
4
How many pairs of
autosomes ？
3
How many pairs of
Fruit fly’s somatic cells
sex
chromosomes ？
1 : x,y

12. A B
germ cell
C D
What is the correct gamete
produced by the germ cell

13. Meiosis Overview
• reduction division
• number of chromosomes per cell halved
• separation of homologous chromosomes
• 1 diploid  4 genetically different
haploid
• DNA copied once, cell divide twice

15. Sperms are produced by … meiosis
Diploid parent
cell The
chromosomes in
the nucleus are
copied
Four haploid
daughter cells

16. Diploid Eggs are also produced by
parent cell meiosis
Haploid

17. Phases of meiosis
MEIOSIS I MEIOSIS II
PROPHASE I PROPHASE II
interphase no interphase
(DNA replication METAPHASE I (no DNA replication METAPHASE II
before meiosis I) ANAPHASE I before meiosis II) ANAPHASE II
TELOPHASE I TELOPHASE II

18. Meiosis I
(reduction division)
Interphase I Prophase I Metaphase I Anaphase I Telophase I
Cells undergo a round Each chromosome pairs Spindle fibers attach to The fibers pull the
of DNA replication, with its corresponding the chromosomes. homologous chromosomes
forming duplicate homologous chromosome toward the opposite ends
Chromosomes. to form a tetrad. of the cell.

20. Prophase 1
• Supercoil of
chromosomes
• Spindle form
• Synapsis
(homologous pair to
form bivalents)
• Crossing over
• Nuclear envelope
breaks down

21. Synapsis & Crossing over
• The key to the process of
meiosis is the Synapsis in
Prophase I and the splitting of
the homologues in Anaphase I.
• One event that can take place
during meiosis is crossing
over.
• Crossing over is the exchange
of genetic material between two
homologous chromosomes
which have paired up during
Prophase I.
• The result of crossing over is
genetic recombination.

22. Metaphase 1
1. Spindle network
complete
2. Bivalents line up on
equator
3. Centromere
attached to
microtubules

23. Anaphase 1
1. Centromeres do not
split
2. Microtubules
contract to separate
homologous pairs

24. Telophase 1
1. Chromosomes uncoil
2. Spindle breaks down
3. Nuclear envelope
reform

25. Meiosis II
(similar to mitosis)
There is no
DNA
replication
Prophase II Metaphase II Anaphase II Telophase II
Meiosis I results in two The chromosomes line up The sister chromatids Meiosis II results in four
haploid (N) daughter cells, in a similar way to the separate and move toward haploid (N) daughter cells.
each with half the number metaphase stage of opposite ends of the cell.
of chromosomes as the mitosis.
original.

26. Prophase 2
1. Supercoil of
chromosomes
2. Spindle form
3. Nuclear envelope
breaks down

27. Metaphase 2
1. Chromosomes move
to equator
2. Spindle network
complete
3. Centromere attach
to spindle

28. Anaphase 2
1. Centromere split
2. Microtubules
contract and sister
chromatids separate

29. Telophase 2
1. Chromosomes
uncoil
2. Spindle breaks down
3. Nuclear envelope
reforms
Cytokinesis 2
1. 4 haploid cells
2. 1 or many undergo
differentiation into
gametes

30. Which steps in meiosis are
important in producing
genetic variety?

31. 1. Crossing over in prophase 1
• Homologous chromosomes exchange nonsister
chromatids
chiasmata

32. 2. Independent assortment in
metaphase 1
• Bivalents line up on equator in a random way

33. Possibilities of Crossing over

34. e.g. 46 chromosomes, 23 pairs
2n = 46; n = 23
2n = 223 = ~ 8 million possible combinations!

35. 3. Random fertilization

36. Gamete formation in animals
• Male gametes (sperms) are formed through
spermatogenesis
• Female gametes (egg) are formed through oogenesis

37. a) Spermatogenesis
spermato- primary secondary spermatids
gonium spermatocyte spermatocytes sperm (n)
(n)
(2n ) (2n) (n)
Growth Meiosis I Meiosis II

38. b) Oogenesis
polar body polar
(n) bodies (n)
oogonium primary oocyte
(2n) (2n) secondary
oocyte (n)
ovum (n)
Growth Meiosis I Meiosis II

39. Comparing mitosis and meiosis
• Mitosis: 2 genetically identical diploid cells
• Meiosis: 4 genetically different haploid cells