1. AP BIOLOGY GENETIC VARIATION
CHAPTER 13: MEIOSIS

2. Meiosis 1

3. Meiosis 2

4. Mitosis vs. Meiosis

5. Mitosis vs. Meiosis Mitosis 1 cell division Meiosis 2 cell divisions
daughter cells genetically identical to parent cell and each other
produces 2 cells
2n  2n
diploid diploid
produces cells for growth & repair ; Also produces new organisms in asexual reproduction
no crossing over
Meiosis
2 cell divisions
daughter cells genetically different from parent and each other
produces 4 cells
2n  1n
diploidhaploid
produces gametes
crossing over occurs

6. Replicated chromosome
Fig. 13-9a
MITOSIS
MEIOSIS
MEIOSIS I
Parent cell
Chiasma
Chromosome
replication
Chromosome
replication
Prophase
Prophase I
Homologous
chromosome
pair
2n = 6
Replicated chromosome
Metaphase
Metaphase I
Anaphase
Telophase
Anaphase I
Figure 13.9 A comparison of mitosis and meiosis in diploid cells
Telophase I
Haploid
n = 3
Daughter
cells of
meiosis I 2n 2n
MEIOSIS II
Daughter cells
of mitosis n n n n
Daughter cells of meiosis II

7. Putting it all together…
meiosis  fertilization  mitosis + development
gametes 46 23 46 23 46 46 46 46 46 23
meiosis 46 46
egg 46 46 23
zygote
fertilization
mitosis & development
mitosis
sperm

8. GENETIC VARIATION
Sexual reproduction produces genetic variation in three ways:
A. independent assortment of chromosomes
B. crossing over
C. random fertilization
metaphase1

9. Independent assortment of chromosomes
- Occurs during metaphase I in meiosis.
- Each homologous pair of chromosomes
lines up along the metaphase plate
independently of other pairs.
- When pairs separate during anaphase I,
various combinations of chromosomes go to each pole.

10. Possibility 1 Possibility 2 Two equally probable arrangements of
Fig
Possibility 1
Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Figure The independent assortment of homologous chromosomes in meiosis
Daughter
cells
Combination 1
Combination 2
Combination 3
Combination 4

11. To calculate the number of possible
- Gametes from offspring do not have same combination of genes as gametes from parents.
Random assortment in humans produces 223 (8,388,608) different combinations in gametes
To calculate the number of possible
independent assortment combinations of gametes from a parent,
calculate 2n where n = haploid number.
offspring
from Mom
from Dad
new gametes
made by offspring

12. B. Crossing over
Crossing over creates completely new combinations of genes on each chromosome.
Creates an infinite variety of gametes.

13. Recombinant chromosomes
Fig
Prophase I
of meiosis
Nonsister
chromatids
held together
during synapsis
Pair of
homologs
Chiasma
Centromere
TEM
Anaphase I
Figure The results of crossing over during meiosis
Anaphase II
Daughter
cells
Recombinant chromosomes

14. C. Random fertilization
Sperm + Egg = ?
Any 2 parents will produce a zygote with over 70 trillion (223 x 223) possible diploid combinations.

15. Sexual reproduction creates variability
Sexual reproduction allows us to maintain both genetic similarity & differences.
Jonas Brothers
Baldwin brothers
Martin & Charlie Sheen, Emilio Estevez

16. E. Sperm production Spermatogenesis continuous & prolific process
Epididymis
Testis
germ cell
(diploid)
Coiled
seminiferous
tubules
primary
spermatocyte
(diploid)
MEIOSIS I
secondary
spermatocytes
(haploid)
MEIOSIS II
Vas deferens
spermatids
(haploid)
spermatozoa
Spermatogenesis
continuous & prolific process
each ejaculation = million sperm
Cross-section of
seminiferous tubule

17. F. Egg production Oogenesis eggs in ovaries halted before Anaphase 1
Meiosis 1 completed during maturation
Meiosis 2 completed after fertilization
1 egg + 2 polar bodies
unequal division
Meiosis 1 completed
during egg maturation
ovulation
Meiosis 2 completed
triggered by fertilization