1. Meiosis and genetic variation
Honors Biology
Unit 5

2. Genome Genome > Chromosome > DNA > Genes
Genome: Complete complement of an organism’s DNA.
Includes genes (control traits) and non-coding DNA organized in chromosomes.
Genome > Chromosome > DNA > Genes

3. Genes Eukaryotic DNA is organized in chromosomes.
Genes have specific places on chromosomes.

4. Heredity
Heredity – way of transferring genetic information to offspring
Chromosome theory of heredity: chromosomes carry genes.
Gene – “unit of heredity”.

5. Reproduction
Asexual: self-reproducing, produce clones (offspring genetically identical to parent)
Many single-celled organisms reproduce by splitting, budding, parthenogenesis.
Some multicellular organisms can reproduce asexually.

6. Sexual reproduction Fusion of two gametes to produce a single zygote.
With exception of self-fertilizing organisms (e.g. some plants), zygote has gametes from two different parents.

8. In humans … Total = 46 chromosomes (23 pairs)
44 autosomes + 2 sex chromosomes

9. Two kinds of Chromosomes
Autosomes:
Code for most genes in your body (not sex determining chromosomes)
In humans chromosome #1-22

10. Two kinds of Chromosomes
Sex Chromosomes:
chromosomes that determine what gender you will be.
#23 in humans
Females are: XX
Males are XY

11. Important Vocabulary
Gametes: sperm/egg
haploid (n)
Haploid= Contains a single set of chromosomes (23)
Zygote: fertilized egg
now diploid (2n)
Diploid= Contains a two sets of chromosomes (23x2=46)
Somatic cell: any cell other than gametes, most of the cells in the body. – All are diploid (2n)
Meiosis KM

12. Sex Chromosomes
Mammals use a chromosomal method of determining sex: XX is female and XY is male.
Birds use a ZW system: ZZ is male and ZW is female.
the evolutionary origin of mammalian and bird sex chromosomes is different
Some reptiles use developmental temperature to determine sex: depends on the species, but hot is male and cold is female in some.

13. Homologues Exception: Sex chromosomes (X, Y).
Chromosomes exist in homologous pairs in diploid cells.
homologous pairs: chromosomes that code for the same genes but possibly have different alleles (get one from each parent)
Exception: Sex chromosomes (X, Y).
All autosomes are in homologous pairs.

14. Homologous chromosomes after replication
Now each homologous chromosome has a sister chromatid
Still considered diploid (2n) cell because still just two copies of each chromosome (even though they now have a twin)
Meiosis KM

15. Meiosis KM

16. Meiosis KM

17. Chromosome numbers
All are even numbers – diploid (2n) sets of homologous chromosomes!
All even, as all are diploid, contain pairs of chromosomes.
Ploidy = number of copies of each chromosome. Diploidy

18. Meiosis: What is it?
Meiosis: cell division process by which the number of chromosomes per cell is cut in half
Why does it occur: Meiosis is used to produce the haploid(n) gametes (sperm and eggs)

19. Meiosis: Key differences from mitosis
Meiosis reduces the number of chromosomes by half.
Daughter cells differ from parent, and each other.
Meiosis involves two divisions, Mitosis only one.

20. Meiosis KM

21. Meiosis KM

22. Meiosis 1 First division of meiosis
Prophase 1: Duplicated homologous chromosomes match up forming tetrads. Crossing-over occurs at the chiasmata.
Metaphase 1: Tetrads align at the equator of the cell.
Anaphase 1: Homologous pairs separate with sister chromatids remaining together.
Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.

24. Crossing over
produces recombinant chromosomes, mixing the genes of the mother and father, recombining them.
Occurs during Prophase 1

25. Meiosis KM

26. Harlequin chromosomes

27. Meiosis II – Similar to mitosis
Second division of meiosis: Gamete formation
Prophase 2: DNA does not replicate.
Metaphase 2: Chromosomes line up at the equatorial.
Anaphase 2: Sister chromatids move separately to opposite ends of the cell.
Telophase 2: Cell division is complete. Four haploid(n) daughter cells are produced.

29. Mitosis vs. meiosis
Meiosis KM

30. Meiosis KM

31. Meiosis creates genetic variation
During normal cell growth, mitosis produces daughter cells identical to parent cell (2n to 2n)
Meiosis results in genetic variation by:
Shuffling of maternal and paternal chromosomes and crossing over.
No daughter cells formed during meiosis are genetically identical to either mother or father
During sexual reproduction, fusion of the unique haploid gametes produces truly unique offspring.

32. Independent assortment
Meiosis KM

33. Independent assortment
Number of combinations: 2n
e.g. if there are 2 chromosomes in haploid
2n = 4; n = 2
2n = 22 = 4 possible combinations

34. In humans e.g. 23 chromosomes in haploid 2n = 46; n = 23
2n = 223 = ~ 8 million possible combinations!

35. Random fertilization
At least 8 million combinations from Mom, and another 8 million from Dad … >64 trillion combinations for a diploid zygote!!!
Meiosis KM

36. Gamete Formation in Animals
Male gamete formation - Spermatogenesis
In males, all 4 products of meiosis develop into sperm cells. They lose most of their cytoplasm, remodel their cell shape, and grow a long flagellum (tail).

37. Gamete Formation in Animals
Female gamete formation - Oogenesis
In females, most of the cytoplasm goes into 1 of the 4 meiotic products, which becomes the egg.
The other 3 meiotic cells are small “polar bodies”, which degenerate.

38. Why Sexual Reproduction …
More genetic diversity: more potential for survival of species when environmental conditions change.
Shuffling of genes in meiosis
Crossing-over in meiosis
Fertilization: combines genes from 2 separate individuals
DNA back-up and repair.
Asexual organisms don't have back-up copies of genes, sexual organisms have 2 sets of chromosomes and one can act as a back-up if the other is damaged. (some organisms have more)
Sexual mechanisms, especially recombination, are used to repair damaged DNA - the undamaged chromosome acts as a template and eventually both chromosomes end up with the correct gene.

39. Review Questions
1. How does metaphase of mitosis differ from metaphase I of meiosis? 2. What happens as homologous chromosomes pair up during prophase I of meiosis? 3. What is the sole purpose of meiosis? 4. What specific activities, involving DNA, occur during interphase prior to both mitosis and meiosis? 5. Compare mitosis and meiosis on the following points: a. number of daughter cells produced. b. the amount of DNA in the daughter v. parent cell c. mechanism for introducing genetic variation. 6. What is a zygote and how is it formed?

40. Warm-up
Summarize the process of meiosis in 5 sentences or less (include the following in your summary)
What is the sole purpose of Meiosis?
What does it start with and what does it end with (include males and females)
Use and underline vocab words gamete, diploid, haploid, crossing over, and homologous pairs, and sister chromatids