Meiosis and Sexual Meiosis and Sexual ReproductionReproduction

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”

Asexual Reproduction

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

http://www.ornl.gov/sci/techresources/Human_Genome/posters/chromosome/index.shtml

Chromosome Structure

Homologous ChromosomesHomologous Chromosomes may carry different AllelesAlleles

• 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 allelesPerhaps different forms of the same gene?

Mom Dad

PloidyPloidy: 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

19-22

13-18

6-12

23: XX

p. 141

Meiosis: Two Divisions

• Two successive nuclear divisions

1. Meiosis I

2. 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)

Meiosis I(one homologous pair of chromosomes)

Each homologue in the cell pairs with its partner,

then the partners separate

What is the ploidy level of this nucleus?Are the chromosomes replicated?

What is the ploidy level of these nuclei after meiosis I?

p. 141

Meiosis II

• The two sister chromatids of each duplicated chromosome are separated from each other

one chromosome (duplicated)

two chromosomes (unduplicated)

Meiosis I - Stages

Prophase I Metaphase I Anaphase I Telophase I

p. 142

Meiosis II - Stages

Prophase II Metaphase II Anaphase II Telophase II

p. 143

3 main reasons for genetic variability as a result of sexual reproduction:

1. Crossing over• Prophase I

2. Independent Assortment (of chromosomes)

• Metaphase I

3. Random Fertilization• random re-combination of genes in fertilized

gamete

Crossing Over•All four chromatids are closely aligned

•Homologous chromosomes exchange

segments

•New combinations of alleles createdWhat would result if sister chromatids exchanged segments?

big eyes, small eyesno light light

Big Eyes (E)small eyes (e)

Light (L)Light (L)no light (l)no light (l)

E E

LL LLllll

e e

Bioluminescenc and eye size in viper fish. What traits would be linked after crossing over?

Germ cell “synapsis”

p. 144

Independent Assortment

• random alignment of chromosomes during Metaphase

• number of possible combinations of chromosomes in a gamete is:

2n

n = # chromosome types

p. 145