1. 1. Meiosis and chromosome number
Steps in meiosis
Source of genetic variation
Independent alignment of homologues
b. Recombination

2. Somatic cells are diploid.
Gametes are haploid, with only one set of chromosomes

3. Meiosis reduces the number of genomes from diploid to haploid
Haploid gametes (n = 23)
human life cycle
Meiosis creates gametes
Mitosis of the zygote produces adult bodies
Egg cell
Sperm cell
MEIOSIS
FERTILIZATION
Diploid zygote (2n = 46)
Multicellular diploid adults (2n = 46)
Mitosis and development
Figure 8.13

4. MEIOSIS I: Homologous chromosomes separate
Steps in 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

5. 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

6. 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

7. Meiosis II is essentially the same as mitosis
sister chromatids of each chromosome separate
result is four haploid daughter cells

8. MITOSIS MEIOSIS Diploid Diploid 1 somatic cell 2n 2n 2 2n 2n 3 2n 2n 4
gamete
precursor
somatic
cell 2n 2n
duplication 2 2n 2n 3 2n 2n 4 2n 2n
Figure: 10-01
Title:
Meiosis compared to mitosis.
Caption:
1. Both mitosis and meiosis begin with diploid cells, meaning cells that contain paired sets of chromosomes. The two members of each pair are homologous, meaning the same in shape and function. Two sets of homologous chromosomes are shown in both the mitosis and meiosis figures.The larger chromosome pairs in each cell represents one homologous pair, while the smaller chromosome pairs represent the other homologous pair. One member of each homologous pair (in red) comes from the mother of the person whose cell is undergoing meiosis, while the other member of the pair (in blue) comes from the father of this person. 2. In both mitosis and meiosis, the chromosomes duplicate. Each chromosome is now composed of two sister chromatids.
3. In mitosis, the chromosomes line up on the metaphase plate, one sister chromatid on each side of the plate. In meiosis, meanwhile, homologous chromosomes -- not sister chromatids -- line up on opposite sides of the metaphase plate. 4. In mitosis, the sister chromatids separate. In meiosis, the homologous pairs of chromosomes separate. 5 In mitosis, cell division takes place, and each of the sister chromatids from step 4 is now a full-fledged chromosome. Mitosis is finished. In meiosis, in the first of two cell divisions, one member of each homologous pair has gone to one cell, the other member to the other cell. Because each of these cells now has only a single set of chromosomes, each is in the haploid state. Next, these single chromosomes line up on the metaphase plate, with their sister chromatids on oppositesides of the plate. 6. The sister chromatids of each chromosome then separate. 7. The cells divide again, yielding four haploid cells.
division
diploid
haploid 5 2n 2n 1n 1n 6
division 7 1n 1n 1n 1n

9. Each chromosome of a homologous pair comes from a different parent
Homologous chromosomes carry different versions of genes (alleles) 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

10. Two equally probable arrangements of chromosomes at metaphase I
Independent alignment of homologous chromosomes
POSSIBILITY 1
POSSIBILITY 2
Two equally probable arrangements of chromosomes at metaphase I
Metaphase II
Gametes
Combination 1
Combination 2
Combination 3
Combination 4
Figure 8.16

11. Crossing over further increases genetic variability
the exchange of corresponding segments between two homologous chromosomes

12. Tetrad
Chaisma
Centromere
Figure 8.18A

13. MEIOSIS I
END OF
INTERPHASE
PROPHASE I
METAPHASE I
ANAPHASE I
Figure: 10-02a
Title:
Meiosis I.
Caption:
Genetic recombination results from crossing over during prophase I of meiosis

14. 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

15. INDEPENDENT ASSORTMENT
TELOPHASE II
METAPHASE II
METAPHASE I
METAPHASE I
Figure: 10-02d
Title:
Independent assortment.
Caption:

16. 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

17. Figure: 10-07
Title:
Big difference in size.
Caption:
A human egg surrounded by much smaller human sperm.

18. 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

19. 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

20. 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

21. Connection: An extra copy of chromosome 21 causes Down syndrome
This karyotype shows three number 21 chromosomes
An extra copy of chromosome 21 causes Down syndrome
Figure 8.20A, B

22. The chance of having a Down syndrome child goes up with maternal age
Figure 8.20C

23. Connection: Abnormal numbers of sex chromosomes do not usually affect survival
Nondisjunction can also produce gametes with extra or missing sex chromosomes
Unusual numbers of sex chromosomes upset the genetic balance less than an unusual number of autosomes

24. Table 8.22