2. Mitosis vs. Meiosis

3. Meiosis
The form of cell division by which gametes, with half the number of chromosomes, are produced.
Diploid (2n) → haploid (n)
Meiosis is sexual reproduction.
Two divisions (meiosis I and meiosis II).

4. Fertilization n=23 egg 2n=46 zygote
The fusion of a sperm and egg to form a zygote.
A zygote is a fertilized egg
n=23
egg
sperm
2n=46
zygote

5. Meiosis Sex cells divide to produce gametes (sperm or egg).
Gametes have half the # of chromosomes.
Occurs only in gonads (testes or ovaries).
Male: spermatogenesis
Female: oogenesis
Meiosis is similar to mitosis with some chromosomal differences.

6. Spermatogenesis
sperm
meiosis II
human
sex cell
meiosis I

8. Meiosis I (four phases)
Cell division that reduces the chromosome number by one-half.
four phases:
a. prophase I
b. metaphase I
c. anaphase I
d. telophase I

9. Prophase I - Synapsis
Homologous chromosomes
sister chromatids
Tetrad

10. Homologous Chromosomes
Pair of chromosomes (maternal and paternal) that are similar in shape and size.
Homologous pairs (tetrads) carry genes controlling the same inherited traits.
Each locus (position of a gene) is in the same position on homologues.
Humans have 23 pairs of homologous chromosomes.
a. 22 pairs of autosomes (somatic cells)
b. 01 pair of sex chromosomes

11. Trait- feature or characteristic of a living thing
Gene- unit of heredity that is transferred from parent to offspring and is held to determine characteristics of the offspring
Trait- feature or characteristic of a living thing
Locus- specific location of a gene, or DNA sequence, on a chromosome

12. Homologous Chromosomes
Paternal
Maternal
eye color
locus
hair color

13. Karyotype
A method of organizing the chromosomes of a cell in relation to number, size, and type.

14. Humans have 23 Sets of Homologous Chromosomes Each Homologous set is made up of 2 Homologues.

15. Autosomes (The Autosomes code for most of the offspring’s traits)
In Humans the “Autosomes” are sets

16. In Humans the “Sex Chromosomes” are the 23rd set
Sex Chromosomes The Sex Chromosomes code for the sex of the offspring. ** If the offspring has two “X” chromosomes it will be a female. ** If the offspring has one “X” chromosome and one “Y” chromosome it will be a male.
In Humans the “Sex Chromosomes” are the 23rd set
XX chromosome - female
XY chromosome - male

17. Boy or Girl? The Y Chromosome “Decides”
X chromosome
Y chromosome

18. Crossing Over
Crossing over (variation) may occur between nonsister chromatids at the chiasmata.
Crossing over: segments of nonsister chromatids break and reattach to the other chromatid.
Chiasmata (chiasma) are the sites of crossing over.

19. Crossing Over - variation
nonsister chromatids
chiasmata: site of crossing over
variation
Tetrad

20. Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over
Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

21. Question:
In terms of Independent Assortment -how many different combinations of sperm could a human male produce?

22. Answer 223 = ~8 million combinations Formula: 2n
Human chromosomes: 2n = 46
n = 23
223 = ~8 million combinations

23. Anaphase I Homologous chromosomes separate and move towards the poles.
Sister chromatids remain attached at their centromeres.

24. Non Disjunction Occurs when chromosomes fail to separate properly
Some cells end up with one more or one less pair of chromosomes

25. 21 trisomy – Downs Syndrome
Can you see the extra 21st chromosome?
Is this person male or female?

26. Telophase I Each pole now has haploid set of chromosomes.
Cytokinesis occurs and two haploid daughter cells are formed.

27. Telophase I

28. Meiosis II No interphase II (or very short - no more DNA replication)
Remember: Meiosis II is similar to mitosis

29. Prophase II
same as prophase in mitosis

30. Metaphase II
same as metaphase in mitosis
metaphase plate

31. Anaphase II
same as anaphase in mitosis
sister chromatids separate

32. Telophase II Same as telophase in mitosis. Nuclei form.
Remember: four haploid daughter cells produced.
gametes = sperm or egg
Cytokinesis occurs.

33. Telophase II

34. Variation
Important to population as the raw material for natural selection.
Question:
What are the three sexual sources of
genetic variation?

35. Answer: Remember: variation is good! 1. crossing over (prophase I)
2. independent assortment (metaphase I)
3. random fertilization
Remember: variation is good!

36. Question:
A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

37. Answer:
10 chromosomes (haploid)

38. Question:
A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

39. Answer:
10 chromosomes