1. Meiosis and Sexual Reproduction

2. Asexual Reproduction Sexual Reproduction
“Germ cells” undergo meiosis and cytokinesis
Daughter cells become gametes (sex cells)
Gametes meet (hopefully!) at fertilization
Produces genetic variation among offspring
This variation in traits is the basis for evolutionary change
Single “parent” produces offspring
Growth and repair
Mitosis produces daughter cells genetically identical to one another and to the parent = “clones”

3. Chromosome Structure
Explore the “chromosome viewer” at this site based on information from the Human Genome project (& the source of this figure):

4. Homologous Chromosomes may carry different Alleles
Each diploid cell has two of each chromosome (23 pairs in humans)
One chromosome in each pair from mother, other from father
Paternal and maternal chromosomes may carry different alleles
Mom
Dad
Perhaps different forms of the same gene?

5. Ploidy: Chromosome Number
Sum total of chromosomes in a cell
Germ cells are diploid (2n)
Gametes are haploid (n)
Meiosis halves chromosome number, leaving one complete set of 23 in each gamete
Are these chromosomes replicated?
1-5
6-12
13-18
19-22
23: XX

6. Meiosis: Two Divisions
Two successive nuclear divisions
Meiosis I
Meiosis II
Four haploid nuclei are formed
All (potentially) genetically different
Events are similar to those in mitosis
(prophase, metaphase, anaphase, telophase)
(Without DNA replication in between)

7. Meiosis I (one homologous pair of chromosomes)
What is the ploidy level of this nucleus?
Are the chromosomes replicated?
Each homologue in the
cell pairs with its partner,
then the partners
separate
What is the ploidy level of these nuclei after meiosis I?

8. Meiosis II
The two sister chromatids of each duplicated chromosome are separated from each other
two chromosomes
(unduplicated)
one chromosome
(duplicated)

9. Meiosis I - Stages Prophase I Metaphase I Anaphase I Telophase I

10. Meiosis II - Stages Prophase II Metaphase II Anaphase II Telophase II

11. 3 main reasons for genetic variability as a result of sexual reproduction:
Crossing over
Prophase I
Independent Assortment (of chromosomes)
Metaphase I
Random Fertilization
random re-combination of genes in fertilized gamete

12. Crossing Over All four chromatids are closely aligned
p. 144 E E e e
All four chromatids are closely aligned
Homologous chromosomes exchange segments
New combinations of alleles created
What would result if sister chromatids exchanged segments? l L L l
Germ cell  “synapsis”
big eyes, small eyes
no light light
Big Eyes (E)
small eyes (e)
Light (L)
no light (l)
Bioluminescenc and eye size in viper fish. What traits would be linked after crossing over?

13. Independent Assortment
random alignment of chromosomes during Metaphase
number of possible combinations of chromosomes in a gamete is: 2n
n = # chromosome types
p. 145