1. 1. Meiosis and chromosome number
Life cycle and ploidy levels
Steps in meiosis
Source of genetic variation
Independent alignment of homologues
b. recombination

2. 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.

3. Figure: 10-06
Title:
Mobile cells, bearing chromosomes.
Caption:
Human sperm, colored to show detail.

4. Gametes have a single set of chromosomes
Gametes are haploid, with only one set of chromosomes
Somatic cells are diploid.

5. Multicellular diploid adults (2n = 46) Mitosis and development
The human life cycle
Meiosis creates gametes
Mitosis of the zygote produces adult bodies
Haploid gametes (n = 23)
Egg cell
Sperm cell
MEIOSIS
FERTILIZATION
Diploid zygote (2n = 46)
Multicellular diploid adults (2n = 46)
Mitosis and development
Figure 8.13

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

7. Meiosis reduces the chromosome number from diploid to haploid
Chromosomes are duplicated before meiosis, then the cell divides twice to form four daughter cells.

8. 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

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

10. 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

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

12. 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

13. 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

14. Each chromosome of a homologous pair comes from a different parent
Genetic variation among offspring is a result of 1) Independent orientation of chromosomes in meiosis 2) random fertilization
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

15. Two equally probable arrangements of chromosomes at metaphase I
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

16. Homologous chromosomes carry different versions of genes at corresponding loci

17. C E C E C E c e c e c e Coat-color genes Eye-color genes Brown Black
White
Pink
Tetrad in parent cell (homologous pair of duplicated chromosomes)
Chromosomes of the four gametes
Figure 8.17A, B

18. 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, which increases variation further

19. tetrad
Figure: 10-02c
Title:
Crossing over during prophase I.
Caption:

20. Tetrad
Chaisma
Centromere
Figure 8.18A

22. How crossing over leads to genetic recombination
Coat-color genes
Eye-color genes
How crossing over leads to genetic recombination
Tetrad (homologous pair of chromosomes in synapsis) 1
Breakage of homologous chromatids 2
Joining of homologous chromatids
Chiasma
Separation of homologous chromosomes at anaphase I 3
Separation of chromatids at anaphase II and completion of meiosis 4
Parental type of chromosome
Recombinant chromosome
Recombinant chromosome
Parental type of chromosome
Figure 8.18B
Gametes of four genetic types

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

24. 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

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

26. a SPERMATOGENESIS b OOGENESIS spermatogonium oogonium primary
spermatocyte
primary
oocyte
meiosis l
secondary
spermatocyte
secondary
oocyte
Figure: 10-05
Title:
Sperm and egg formation in humans.
Caption:
a. 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. b 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

27. Figure: 10-08
Title:
Reproduction in bacteria.
Caption:
There is no fusion of sperm and eggs in bacteria, just a division of one cell into two. Pictured is a Staphylococcus aureus bacterium undergoing such cell division, referred to as binary fission.

28. 8.21 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

29. 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

30. 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

31. 8.20 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

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

33. 8.22 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

34. Table 8.22

35. 8.23 Connection: Alterations of chromosome structure can cause birth defects and cancer
Chromosome breakage can lead to rearrangements that can produce genetic disorders or cancer
Four types of rearrangement are deletion, duplication, inversion, and translocation

36. Homologous chromosomes
Deletion
Duplication
Homologous chromosomes
Inversion
Reciprocal translocation
Nonhomologous chromosomes
Figure 8.23A, B

37. Translocation
Figure 8.23Bx

38. Chromosomal changes in a somatic cell can cause cancer
A chromosomal translocation in the bone marrow is associated with chronic myelogenous leukemia
Chromosome 9
Reciprocal translocation
Chromosome 22
“Philadelphia chromosome”
Activated cancer-causing gene
Figure 8.23C