1. 8.12 Chromosomes are matched in homologous pairs
MEIOSIS AND CROSSING OVER
8.12 Chromosomes are matched in homologous pairs
Somatic( all cells except egg and sperm) cells of each species contain a specific number of chromosomes
Human cells have 46, making up 23 pairs of homologous chromosomes
Chromosomes
Centromere
Sister chromatids
Figure 8.12

2. 8.13 Gametes have a single set of chromosomes
Cells with two sets of chromosomes are said to be diploid
Gametes are haploid, with only one set of chromosomes

3. At fertilization, a sperm fuses with an egg, forming a diploid zygote
Repeated mitotic divisions lead to the development of a mature adult
The adult makes haploid gametes by meiosis
All of these processes make up the sexual life cycle of organisms

4. Multicellular diploid adults (2n = 46) Mitosis and development
The human life cycle
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

5. 8.14 Meiosis reduces the chromosome number from diploid to haploid
Meiosis, like mitosis, is preceded by chromosome duplication (DNA replication)
However, in meiosis the cell divides twice to form four daughter cells

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

7. 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

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

9. 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

10. 8.15 Review: A comparison of mitosis and meiosis
For both processes, chromosomes replicate only once, during interphase

11. 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

12. Each chromosome of a homologous pair comes from a different parent
8.16 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring
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

13. The large number of possible arrangements of chromosome pairs at metaphase I of meiosis leads to many different combinations of chromosomes in gametes
Random fertilization also increases variation in offspring

14. 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

15. 8.17 Homologous chromosomes carry different versions of genes
The differences between homologous chromosomes are based on the fact that they can carry different versions of a gene at corresponding loci

16. 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

17. 8.18 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
This increases variation further

18. Tetrad
Chaisma
Centromere
Figure 8.18A

19. 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