meiosis

1. LESSON #6
Why are we so different from our sibilings?

2. MEIOSIS
Gametogenesis

3. GAMETOGENESIS
◦ Formation of gametes.
◦ Happens in the gonads.
◦ Basis of sexual reproduction
Source: Bitesize BBC

4. Gametes
◦ Reproductive cells
◦ In humans:
◦ Male gamete: sperm
◦ Female gamete: ovum
◦ Characteristics
◦ One set of chromosomes only haploid cells  In humans 23
chromosomes
◦ Formed in the gonads.
◦ A special diploid cell divides through a special process (meiosis) to
produce 4 haploid cells which will differentiate into 4 gametes .
Source: Scrape TV

5. Human karyotype
Human karyotype: (somatic cell)
◦ Shows 23 pairs of
chromosomes in total
◦ 22 pairs of autosomes and 1
pair of sex chromosomes.
◦ Sex chromosome pair can
be:
◦ XX : in females
◦ XY: in males

6. Human Gamete karyotype
Human gamete karyotype:
Only has one set of
chromosomes, one of each
type. 23 chromosomes in total.
◦ 22 autosomes and 1 sex
chromosome.
◦ Sperm may have sex
chromosomes X or Y
◦ Ova can only carry the X
chromosome.

7. DNA REPLICATION
◦ In order for gametes to form, the cell that is going to divide must
have replicated DNA
◦ Replication takes place during Interphase (S phase)

8. MEIOSIS: 2 steps
I. Reduction
division2n
n
n
n
n
n
n
II. Second
division
(mitotic division)
1 Diploid cell with
Replicated DNA
(chromosomes with sister
chromatids)
2 Haploid cells with
replicated DNA
(chromosomes with sister
chromatids)
4 Haploid cells
(chromosomes with just
one chromatid)
Special cell in the gonad

9. MEIOSIS I: Reduction division
◦ Prophase I : homologous chromosomes pair up. Crossing over occurs.
◦ Metaphase I: homologous chromosomes line up in pairs in the equatorial plate.
◦ Anaphase I : homologous chromosomes separate and migrate to opposite poles of
the cell.
◦ Telophase I: one chromosome of each type (with sister chromatids) reach opposite
poles, nuclear envelopes form and therefore 2 haploid nuclei finish forming.
Cytokinesis: the two haploid cells separate.

10. MEIOSIS I: Reduction division
Prophase I: crossing over
◦ Homologous chromosomes pair up
and swap different sections
randomly  crossing over
◦ Sister chromatids are no longer
genetically identical.
◦ Increases variability of the species.
◦ Allows for the 4 gametes to end up
carrying completely different
combination of genes.
◦ That’s why 2 siblings (non-twins) are
always different.
◦ The greater the variability, the
greater opportunities of survival of
the species.

11. MEIOSIS I : Reduction division
Metaphase I
◦ Homologous pairs align in the
equatorial plate.
◦ Spindle attaches to the
kinetochore of each
chromosome

12. MEIOSIS I : Reduction division
Anaphase I
◦ Spindle contracts and
shortens pulling homologous
chromosomes apart.
◦ If homologous chromosomes
travel to the same pole:
monosomies or trisomies may
occur.

13. MEIOSIS I: Reduction division
Telophase I
◦ Chromosomes reach opposite
poles.
◦ Nuclear envelopes form
◦ Two haploid nuclei are formed.
◦ Chromosomes still have 2
chromatids.
◦ There is only one chromosome
of each type in the nuclei.
◦ Cleavage furrow appears for
cytokinesis to take place.

14. MEIOSIS: 2 steps
I. Reduction
division2n
n
n
n
n
n
n
II. Second
division
(mitotic division)
1 Diploid cell with
Replicated DNA
(chromosomes with sister
chromatids)
2 Haploid cells with
replicated DNA
(chromosomes with sister
chromatids)
4 Haploid cells
(chromosomes with just
one chromatid)
Special cell in the gonad

15. MEIOSIS II: Second Division
◦ Happens in both haploid cells resulting from the reduction phase.
◦ Each of them go through 4 new phases of division.
◦ Prophase II: chromosomes coil. Mitotic spindle begins to appear.
◦ Metaphase II: Chromosomes line up in the equatorial plate. Spindle
attaches to kinetochore on both chromatids.
◦ Anaphase II: Chromatids migrate to opposite poles
◦ Telophase II: Single chromatid chromosomes reach opposite poles.
Nuclear envelope forms. Chromosomes uncoil. Nucleolus reappears.
Two new haploid nuclei are formed
◦ This division is more like the mitotic division.
◦ Cytokinesis separates the cytoplasms. Two new haploid cells form. Each
chromosome is made up of only 1 DNA molecule.
◦ All four cells, at the end of meiosis, have different combination of alleles.

16. MEIOSIS II: Second Division
Prophase II Metaphase II
◦ Chromosomes coil.
Mitotic spindle begins
to appear.
◦ Chromosomes line up in the
equatorial plate. Spindle
attaches to kinetochore on both
chromatids.

17. MEIOSIS II: Second Division
Anaphase II Telophase II
◦ Chromatids migrate to
opposite poles
◦ Single chromatid chromosomes
reach opposite poles. Nuclear
envelope forms. Chromosomes
uncoil. Nucleolus reappears. Two
new haploid nuclei are formed

20. Mitosis vs. Meiosis

22. Chromosomal Number
Abnormalities