1. Reduction Division Production of Gametes
Meiosis
Reduction Division
Production of Gametes

2. Vocabulary Meiosis
Homologous Chromosomes - chromosomes in a biological cell that pair (synapse) during meiosis
Crossing over - when two chromosomes break and then reconnect but to the different end piece. The result is an exchange of genes, called genetic recombination
Chromatid - is one of two identical strands of DNA making up a chromosome that are joined at their centromeres
Centromeres – holds two chromatids together to make a chromosome
Tetrad - consists of two homologous chromosomes attached together each composed of two sister chromatids.
Haploid – (n) half the normal number of chromosomes
Diploid – (2n) the normal number of chromosomes
Loci - the position of a gene on a chromosome

3. Figure: 09-07
Title:
A karyotype displays a full set of chromosomes.
Caption:
One member of each chromosome pair comes from the individual’s father and the other member from the mother. Each paired set of chromosomes is said to be “homologous,” meaning the same in size and function. (The two chromosomes over the number 1 are a homologous pair, the two over number 2, and so forth.) Homologous chromosomes are not exactly alike, however; the genes on them may differ somewhat, meaning the effects they produce will differ. Because this karyotype set is from a human male, there are 22 pairs of homologous chromosomes and then one X and one Y chromosome (which are not homologous). All the chromosomes are in the duplicated state.

4. Gametes have a single set of chromosomes
Gametes (egg and sperm) are haploid, with only one set of chromosomes
Somatic (body) cells are diploid.
Meiosis reduces the chromosome number from diploid to haploid

5. Meiosis creates gametes Mitosis of the zygote produces adult bodies
The human life cycle
Meiosis creates gametes
Mitosis of the zygote produces adult bodies
Figure 8.13

6. MEIOSIS I: Homologous chromosomes separate
1. Chromosomes are duplicated in meiosis I (4n)
2. Crossing over occurs (meiosis I)
MEIOSIS I: Homologous chromosomes separate
INTERPHASE
PROPHASE I
METAPHASE I
ANAPHASE I
Centrosomes (with centriole pairs)
Microtubules attached to kinetochore
Sites of crossing over
Metaphase plate
Sister chromatids remain attached
Spindle
Nuclear envelope
Chromatin
Sister chromatids
Tetrad
Centromere (with kinetochore)
Homologous chromosomes separate
Figure 8.14, part 1

7. 3. Then cell divides once to form two cells that are diploid (2n) and then those two cells divide again to form four daughter cells which are haploid (n) gametes. (meiosis II)
MEIOSIS II: Sister chromatids separate
TELOPHASE I AND CYTOKINESIS
TELOPHASE II AND CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE II
Cleavage furrow
Sister chromatids separate
Haploid daughter cells forming
Figure 8.14, part 2

8. Meiosis number of chromosomes

9. Mitosis chromosome number

10. In meiosis I, homologous chromosomes are paired
While paired, they cross over and exchange genetic information (DNA)
homologous pairs are then separated, and two daughter cells are produced

11. Meiosis II is essentially the same as mitosis without the doubling of the chromosomes
The two daughter cells divide
sister chromatids of each chromosome separate
result is four haploid daughter cells each with ½ the genetic information

12. PARENT CELL (before chromosome replication) Site of crossing over
MITOSIS
MEIOSIS
PARENT CELL (before chromosome replication)
Site of crossing over
MEIOSIS I
PROPHASE
PROPHASE I
Tetrad formed by synapsis of
homologous chromosomes
Duplicated chromosome (two sister chromatids)
Chromosome replication
Chromosome replication
2n = 4
Chromosomes align at the metaphase plate
Tetrads align at the metaphase plate
METAPHASE
METAPHASE I
ANAPHASE I
TELOPHASE I
ANAPHASE TELOPHASE
Sister chromatids separate during anaphase
Homologous chromosomes separate during anaphase I; sister chromatids remain together
Haploid n = 2
Daughter cells of meiosis I 2n 2n
No further chromosomal replication; sister chromatids separate during anaphase II
MEIOSIS II
Daughter cells of mitosis n n n n
Daughter cells of meiosis II
Figure 8.15

13. Each chromosome of a homologous pair comes from a different parent
Homologous chromosomes carry different versions of genes at corresponding loci
Each chromosome of a homologous pair comes from a different parent
Each chromosome thus differs at many points from the other member of the pair
Crossing over mixes the genes from the parents increasing variation

14. Crossing over further increases genetic variability
Crossing over is the exchange of corresponding segments between two homologous chromosomes
Genetic recombination results from crossing over during prophase I of meiosis
This increases genetic variability

15. MEIOSIS I END OF INTERPHASE PROPHASE I METAPHASE I ANAPHASE I
Figure: 10-02a
Title:
Meiosis I.
Caption:

16. MEIOSIS TELOPHASE I PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II
Figure: 10-02b
Title:
Meiosis II.
Caption:
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II

17. Spermatogenesis vs Oogenesis
Spermatogenesis goes though meiosis and produces 4 sperm for every one follicle cell.
Oogenesis goes though meiosis and produces ONLY 1 mature egg cell and 3 polar bodies for every one follicle cell.
The polar bodies will be broken down and reabsorbed.

18. a SPERMATOGENESIS b OOGENESIS spermatogonium oogonium primary
spermatocyte
primary
oocyte
meiosis l
secondary
spermatocyte
secondary
oocyte
Sperm and egg formation in humans.
In sperm formation (spermatogenesis), diploid cells called spermatogonia produce primary spermatocytes. The primary spermatocytes are the diploid cells that go through meiosis, yielding haploid secondary spermatocytes. These spermatocytes then go through meiosis II, yielding four haploid spermatids that will develop into mature sperm cells.
In egg formation (oogenesis), cells called oogonia, produced before the birth of the female, develop into primary oocytes. These diploid cells will remain in meiosis I until they mature in the female ovary, beginning at puberty. (Only one oocyte per month, on average, will complete this maturation process.) Oocytes that mature will enter meiosis II, but their development will remain arrested there until they are fertilized by sperm. An unequal meiotic division of cellular material leads to the production of three polar bodies from the original oocyte and one well-endowed egg. The egg can go on to be fertilized, but the polar bodies will be degraded.
polar
body
meiosis ll
spermatids
polar bodies
(will be degraded)
egg

19. Accidents during meiosis can alter chromosome number
Abnormal chromosome count is a result of nondisjunction
Either homologous pairs fail to separate during meiosis I
Nondisjunction in meiosis I
Normal meiosis II
Gametes
n + 1
n + 1
n – 1
n – 1
Number of chromosomes
Figure 8.21A

20. Or sister chromatids fail to separate during meiosis II
Normal meiosis I
Nondisjunction in meiosis II
Gametes
n + 1
n – 1 n n
Number of chromosomes
Figure 8.21B

21. Fertilization after nondisjunction in the mother results in a zygote with an extra chromosome
Egg cell
n + 1
Zygote 2n + 1
Sperm cell
n (normal)
Figure 8.21C