1. Outline for today’s lecture (Ch. 13)
Sexual and asexual life cycles
Meiosis
Origins of Genetic Variation
Independent assortment
Crossing over (“recombination”)

2. Heredity Transmission of traits between generations
Molecular basis of heredity is DNA replication
Gene is a specific segment of DNA
Physical location on the chromosome is called a genetic LOCUS (plural = “loci”)
e.g., the “eye-color locus”, Adh locus

3. Asexual Life Cycles Single (diploid) individual is the parent
Parent passes copies of ALL its genes to its offspring (reproduces “clonally”)
Various mechanisms
Mitotic cell division in unicellular Eukaryotes
Vegetative reproduction, e.g., plant cuttings, hydra budding
Parthenogenesis

4. Sexual Life Cycles Two (diploid) parents give rise to offspring
Offspring differ genetically from their parents and their siblings
GAMETES are haploid reproductive cells that transmit genes across generations

5. Sexual Life Cycles Key Point: Sexual reproduction → Genetic variation
MOST eukaryotes reproduce sexually at least sometimes
Most prokaryotes (e.g., bacteria) exchange genes at least occasionally

6. Sexual Life Cycles – Human Example
46 Chromosomes
22 Homologous pairs, called “autosomes”
Same length
Same centromere position
Same sequence (+/-)
SAME GENES!!

7. Sexual Life Cycles – Human Example
One pair of “sex chromosomes”
i.e., “sex-determining gene(s)” reside on these chromosomes
Females are XX
Males are XY
Only small region of homology (= same genes) between X, Y X Y

8. Schematic drawing of a chromosome

9. Diploid cell, n=3 BEFORE DNA replication
3 Homologous Pairs
2 autosomes
1 sex chromosome (XX)
One homologous chromosome from each parent
DNA content = 2C
Ploidy = 2n X 2 1

10. Diploid cell, n=3, AFTER DNA replication
3 Homologous Pairs
One homologous chromosome from each parent = TWO SISTER CHROMATIDS
DNA content = 4C
Ploidy = 2n X 1 2

11. Sexual Life Cycles - animals
Free-living stage is diploid
Gametes formed by meiosis
Haploid gametes merge genomes to form diploid zygote (“syngamy”)

12. Sexual Life Cycles - Plants
Diploid sporophyte forms haploid spores by meiosis
Spores form gametophyte by mitosis
Gametophyte forms gametes by mitosis
Gametes merge to form diploid zygote

13. Sexual Life Cycles - Fungi
Free-living, multicellular organism is haploid
Gametes formed by mitosis
Gametes merge to form diploid zygote
Zygote undergoes meiosis to form haploid cells

14. Meiosis
RECALL: Function of MITOSIS is to faithfully replicate the parental genome in each daughter cell with no change in information content
Function of MEIOSIS is to produce haploid cells from diploid cells
Necessary for the formation of gametes
Necessary for sexual reproduction

15. Meiosis – an overview Interphase 1 –
Begin with two homologous chromosomes,
DNA content = 2C
Ploidy = 2n (diploid)

16. Meiosis – an overview Interphase 1 – Chromosomes replicate
DNA content = 4C
Ploidy = 2n

17. Meiosis – an overview “Meiosis I” Homologous chromosomes separate
Cell Division #1
Result is TWO haploid (ploidy = n) cells with TWO SISTER CHROMATIDS of one of the two homologs

18. Meiosis – an overview “Meiosis II” Sister chromatids separate
Cell Division # 2
Result is FOUR haploid daughter cells, each with an unreplicated chromosome (= 1C)
Half as many chromosomes as the parent cell

19. Meiosis I – early Prophase I
Tetrad
Chiasmata
Homologous chromosomes pair
Synaptonemal complex (proteins) attaches homologs
“synapsis”
Homologs form tetrad

20. Meiosis I – late Prophase I
Chiasmata
Spindle fiber
Chromosomes cross over, form “chiasmata”
Exchange of DNA between homologs occurs at chiasma
Spindles form and attach to kinetochores as in mitosis
Tetrad

21. Meiosis I – Metaphase I
Chromosomes lined up on metaphase plate in homologous pairs
Spindles from one pole attach to one chromosome of each pair
Spindles from the other pole attach to the other chromosome of the pair
Kinetochore

22. Meiosis I – Anaphase I
Homologous chromosomes separate and move along spindle fibers toward pole
Sister chromatids remain attached at centromeres
Note that recombination has occurred!

23. Meiosis I – Telophase and cytokinesis
Homologous chromosomes reach (opposite) poles
Each pole has complete haploid complement of chromosomes
Each chromosome consists of two sister chromatids

24. Meiosis II – Prophase II
Spindle forms
Chromosomes move toward metaphase plate

25. Meiosis II – Metaphase II
Chromosomes reach metaphase plate, as in mitosis
Kinetochores of sister chromatids attach to spindle fibers from opposite poles

26. Meiosis II – Anaphase II
Centromeres of sister chromatids separate
Sister chromatids move toward opposite poles

27. Meiosis II – Telophase and cytokinesis
Mechanism as before
Note that now FOUR HAPLOID DAUGHTER CELLS formed from each parent cell
Note that some chromosomes are recombinant, some are not

28. Meiosis I - Summary Chiasma (site of crossing-over) Tetrad formed by
synapsis of homologs

29. Meiosis I - Summary
Tetrads align at metaphase plate

30. Meiosis I - Summary Homologous chromosomes separate
Sister chromatids remain
paired

31. Meiosis II - Summary Sister chromatids separate
Haploid daughter cells result

32. Origins of Genetic Variation
Independent Assortment of Chromosomes
Recombination among chromosomes
Crossing over
Recombination within chromosomes
Random fertilization

33. Independent Assortment of Chromosomes

34. Independent Assortment of Chromosomes
Number of possible combinations of chromosomes within a gamete
Two homologs A, B: Mom = A1B1, Dad = A2B2
Four combinations: A1B1, A1B2, A2B1, A2B2
Three homologs: Mom = A1B1C1, Dad = A2B2C2
Eight combinations:
A1B1C1, A1B1C2, A1B2C1, A1B2C2, A2B1C1, A2B2C1, A2B1C2, A2B2C2
n homologs: 2n combinations

35. Crossing-over – Recombination within chromosomes
Averages ≥ 2 per chromosome per meiosis in humans, flies
If no crossing-over, genes on same chromosomes would always be inherited together

36. Crossing-over – Recombination within chromosomes
Human genome has ~20K genes. Suppose each gene
assorts independently. How many combinations?

37. Review: Mitosis vs. Meiosis
Event Mitosis Meiosis
DNA Replication Interphase Interphase I
# Cell Divisions
# Daughter cells
“Ploidy” of daughters 2n (diploid) n (haploid)
Synapsis of homologs? No Yes
Crossing-over No Yes
(recombination)
Biological Purpose Duplicate cells Generate
faithfully gametes

38. Meiosis, Genetic variation, and Evolution
Role of segregation
Role of crossing-over
What about LIMITS to evolution?
E.g., body size

39. For Thursday: Introduction to Mendelian Genetics
Read Chapter 14 through p. 260