1. Cells For Sexual Reproduction
Meiosis
Cells For Sexual Reproduction

2. Ch. 11.4 Meiosis for Sexual Reproduction
Sexual Reproduction - two parents
a. Offspring have NEW combination of genes
b. genetic mix of both parents
Advantage – genetic variation
a. Some may have traits that favor survival
b. Variation in individuals allows a species to evolve 

3. a. CONJUGATION b. Recipient cell gets new genes
Sexual Reproduction in bacteria and protists
a. CONJUGATION b. Recipient cell gets new genes

4. Complex organisms – make special cells
a. gametes – sperm and egg
b. Gametes combine in fertilization
- make a zygote  new organism

5. Chromosome Number: Diploid and Haploid
Homologous chromosomes
a. matched chromosome pairs
b. one member of pair from each parent
c. carry genes for the same traits
d. 22 pairs autosomes; one pair sex chromosomes X, Y
Gene for one trait

6. Locus – location of gene on a chromosome
Cells with paired chromosomes are diploid a. Somatic (body) cells are diploid b. 2n (n = number) Humans: 2n = 46
one chromosome pair
2 sets of chromosomes
- 2 of every gene
Locus – location of gene on a chromosome
Fruit fly 2n = 8

7. Chromosomes photographed during mitosis
Humans: 2n = 46
23 pairs of chromosomes
One set of 23 came from each parent
Chromosomes photographed during mitosis

8. Haploid cells have ONE set of chromosomes
(n) = ONE MEMBER from EACH homologous PAIR
human: n = gametes - sperm or egg

9. Meiosis is “Reduction Division”
Reduces chromosome number
from diploid to haploid
Cell DIVIDES TWICE

10. Meiosis - two cell divisions
2n parent cell
DNA replicates in interphase
First division – pairs separate
Second division – sister chromatids separate
 4 haploid daughter cells

11. Homologous pairs separate in MEIOSIS
TWO cell divisions
- Daughter cells have ½ parent chromosome number
Diploid cell - Has pairs
(2n=2)
Meiosis I - Pairs separate
(n = 1)
Meiosis II - copies separate
(n = 1)
Haploid cells - (n = 1)

12. Crossing over – only in meiosis
a. during Prophase or Metaphase I
b. Homologous chromatids trade pieces
c. Increases genetic variation

13. Two equally probable arrangements of chromosomes at metaphase I
INDEPENDENT ASSORTMENT – Each chromosome pair lines up randomly in meiosis, can separate differently
Combination 1
Combination 2
Combination 3
Combination 4
Gametes
Metaphase II
Two equally probable arrangements of chromosomes at metaphase I
Possibility 1
Possibility 2
Figure 8.16

14. Meiosis I Prophase I Metaphase I Interphase Anaphase I Telophase I
Cytokinesis

15. Meiosis II Haploid (n) cells Doubled chromosomes Prophase II
Metaphase II
Telophase II
Anaphase II
Haploid cells
Single chromosomes

16. Compare mitosis and meiosis

17. Making sperm and egg Sperm: 2n parent cell  4 haploid sperm Ovum:
 1 haploid egg
+ haploid polar bodies

18. Ovum needs all the cytoplasm
Sperm needs only nucleus(DNA)
- flagellum
- mitochondria for energy
Ovum and polar body

19. Causes of genetic variation
1. Homologous pairs have different genes
same traits, but may be different forms
2. Crossing over – homologs trade pieces before separating  new gene combinations
3. Pairs position in Metaphase I - independent
n pairs  2n possible combinations
4. Random fertilization of eggs by sperm
Any egg or sperm is equally likely to be used
5. Gene or chromosome mutation
- Error in replication or cell division

20. When meiosis goes wrong
Nondisjunction
- do not separate correctly
In mitosis  defective nucleus, cell usually dies
In meiosis  defective gamete
 wrong number of chromosomes in zygote

21. Accidents during meiosis  wrong chromosome number
Nondisjunction in meiosis I
Normal meiosis II
Gametes
n + 1
n 1
Number of chromosomes
Nondisjunction in meiosis II
Normal meiosis I
n -1 n
Number of chromosomes
Nondisjunction in meiosis I Nondisjunction in meiosis II
All gametes abnormal Some gametes normal

22. Wrong chromosome number in zygote  wrong number in every cell in organism
If cell is fertilized after nondisjunction  trisomy in zygote
Sperm cell
Egg cell
n (normal)
n + 1
Zygote
2n + 1
Trisomy = 3

23. KARYOTYPE picture of a person’s chromosomes
Photographed during mitosis
- sorted into homologous pairs
- largest-to-smallest
- sex chromosomes last

24. Normal female karyotype
Normal male karyotype
Normal female karyotype

25. Abnormal KARYOTYPE Look for: - missing or extra pieces
- pieces broken or moved
- pieces added or lost
autosomes
sex chrom.
Trisomy 21

26. Down Syndrome Trisomy chromosome # 21

27. Turner Syndrome XO Characteristic facial features Web of skin
Figure 8.22B
Characteristic facial
features
Web of
skin
Constriction
of aorta
Poor breast
development
Under developed
ovaries

28. Klinefelter Syndrome XXY

29. Other chromosomes mutations