1. Meiosis (gamete formation)
Topic 3
Meiosis (gamete formation)

2. Human chromosomes
Human somatic cells (any cell other than a gamete) have:
22 pairs of homologous chromosomes
1 pair of sex chromosomes
46 individual chromosomes
Individuals inherit one set from each parent
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

3. Human chromosomes Are these chromosomes from a male or female?
The sex chromosomes are called X and Y
Human females have a homologous pair of X chromosomes (XX)
Human males have one X and one Y chromosome, which is much smaller than an X chromosome
The 22 pairs of chromosomes that do not determine sex are called …?
Are these chromosomes from a male or female?
How do you know?
Sex chromosomes
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

4. How we describe cellular chromosome number
Unit 5 - Reproduction (Topics 3-4)
How we describe cellular chromosome number
Humans
Human gametes (sex cells)
Diploid (2n)
Haploid (n) l l l l l l l l l
Chromosome # = 6
Chromosome # = 3
Cells with 2 complete sets of chromosomes (1 set from mom, 1 set from dad – pairs!…this means that you have 2 versions of every gene.)
Cells with 1 complete set of chromosomes (only one of each gene) 4

5. n=23 egg 2n=46 zygote sperm n=23
Unit 5 - Reproduction (Topics 3-4)
Human gametes (n) are combined for reproduction.
n=23
egg
The fertilized egg is now 2n and is called a zygote.
sperm
n=23
2n=46
zygote
Fertilization 5 5

6. Meiosis & Sexual Reproduction
Start here with D

7. Double division of meiosis
DNA replication
Meiosis 1
1st division of meiosis separates homologous pairs
Meiosis 2
2nd division of meiosis separates sister chromatids

8. Figure 13.7 The stages of meiotic cell division: Meiosis I
Unit 5 - Reproduction (Topics 3-4)
Figure The stages of meiotic cell division: Meiosis I
OBJ 15
C-block Start here 8 8

9. Figure 13.7 The stages of meiotic cell division: Meiosis II
Unit 5 - Reproduction (Topics 3-4)
Figure The stages of meiotic cell division: Meiosis II
OBJ 15 9 9

10. Trading pieces of DNA creates variation
Crossing over
during Prophase 1, sister chromatids of homologous chromosomes intertwine
homologous pairs swap pieces of chromosome
DNA breaks & re-attaches
prophase 1
1-3 crossing over events per homologous pair
Providing variation for natural selection
tetrad

11. Variation from: Independent assortment of chromosomes
gametes of 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
Providing variation for natural selection
offspring
from Mom
from Dad
new gametes
made by offspring

12. Mitosis vs. Meiosis

13. Gametogenesis in humans
Oogenesis: formation of eggs
Spermatogenesis: formation of sperm
Occurs in the reproductive organs

14. Oogenesis During fetal development, oocyte pauses after Prophase I
Meiosis 1 completed during maturation
Meiosis 2 pauses in Metaphase II
Meiosis 2 completed after release from ovary

15. Sperm production
Sperm production begins after puberty and is continuous throughout a male’s lifetime. Spermatogenesis takes about 10 weeks.

16. Variation from random fertilization
Sperm + Egg = ?
any 2 parents will produce a zygote with over 70 trillion (223 x 223) possible diploid combinations
Providing variation for natural selection

17. Sexual reproduction creates variability
Sexual reproduction allows us to maintain both genetic similarity & differences.

18. When meiosis goes wrong: chromosomal disorders

19. Human chromosomes review
The sex chromosomes are called X and Y
Human females have a homologous pair of X chromosomes (XX)
Human males have one X and one Y chromosome, which is much smaller than an X chromosome
The 22 pairs of chromosomes that do not determine sex are called autosomes
Chromosomes in a homologous pair are the same length and carry genes controlling the same inherited characters
The banding pattern results from the coiling of the DNA and the histone proteins
Banding patterns are unique to individual chromosomes. Homologous chromosomes with the same genes have the same banding pattern.
Sex chromosomes
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

20. ACTIVITY INTERLUDE

21. Copyright © 2008 Pearson Education, Inc
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

22. Karyotype review
A karyotype is an ordered display of the pairs of chromosomes from a cell
Karyotypes can tell us the sex of a fetus
Karyotypes can also tell us if there are chromosomal abnormalities
Autosomal disorders result from extra autosomal chromosomes
Sex chromosome disorders result from extra sex chromosomes
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

23. Chromosomal disorders
Result from errors during meiosis
Abnormal number of chromosomes (nondisjunction)
Chromosomes fail to separate (either during meiosis 1 or meiosis 2)
Usually fatal if an autosome
Usually nonfatal if a sex chromosome (if there is at least one X)
Missing bits of chromosome (deletion)
Crossing over with nonhomologous chromosomes (translocation)

24. ACTIVITY INTERLUDE

25. Patient A

26. Patient B

27. Patient C

28. What’s the diagnosis? Patient A: Klinefelter’s syndrome
Extra sex chromosome (XXY)
Patient B: Patau syndrome (trisomy 13)
Extra autosomal chromosome 13
Patient C: Down’s syndrome (trisomy 21)
Extra autosomal chromosome 21

30. Variation from:
Crossing over: creates completely new combinations of traits on each chromosome
Can create an infinite variety in gametes

31. Sperm production Epididymis Testis germ cell (diploid) Coiled
seminiferous
tubules
primary
spermatocyte
(diploid)
MEIOSIS I
secondary
spermatocytes
(haploid)
MEIOSIS II
Vas deferens
spermatids
(haploid)
Sperm production begins after puberty and is continuous throughout a male’s lifetime. Spermatogenesis takes about 10 weeks.
spermatozoa
Cross-section of
seminiferous tubule

32. Oogenesis MEIOSIS I MEIOSIS II first polar body second polar body ovum
(haploid)
secondary
oocyte
primary
(diploid)
germinal cell
primary follicles
mature follicle with
secondary oocyte
ruptured follicle (ovulation)
corpus luteum
developing
follicle
fertilization
fallopian tube
About 200,000 follicles per ovary

33. The value of sexual reproduction
Sexual reproduction introduces genetic variation
genetic recombination
independent assortment of chromosomes
crossing over
random fertilization
Providing variation for natural selection
metaphase1