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)
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