Major Points
1. Meiosis allows random segregation of genes to progeny
2. Maternal & paternal chromosomes segregate in Meiosis I
3. Meiosis II is a mitotic division
Mitosis
Essential fact about mitosis:
* Duplicated chromosomes lineup on the metaphase plate; sister chromatids
separate
so Each daughter cell receives an identical set of chromosomes
What happens if they don't separate?
Homologous Chromosomes
* Each diploid cell contains two copies of each chromosome
- They are nearly identical
- Some differences exist which cause differences in "phenotype"
* Such pairs of chromosomes are called homologous (similar by descent),
and each is called a homologue
Overview of Meiosis
Object of meiosis: to generate a random assortment of chromosomes
* The products are gametes; they fuse to produce diploid cells
so They must be haploid
* Meiosis reduces chromosome number to haploid
- This requires two successive divisions of a tetraploid cell
(after DNA replication)
Meiosis I
* The first meiotic division requires duplicated chromosomes to pair at
metaphase
* Each daughter cell receives one of the two chromosomes from each pair
of homologues.
Why is pairing important?
Random Segregation
Pairing at Metaphase I
* Pairing ensures that each cell gets one chromosome of each type
* Which one goes to each cell is random
How many combinations are possible?
Meiosis with Two Chromosomes
Chromosome I Chromosome II
P P
P M
M P
M M
Meiosis with Two Chromosomes
Chrom. I Chrom. II Chrom. III
P P P
M P P
P M P
P P M
M M P
M P M
P M M
M M M
Meiosis Creates 2n Possibilities
* The number of possible arrangements increases geometrically * For 23 human
chromosomes there are 223 possibilities, or 8,388,608! - Remember that each
individual results from fusion of gametes from different genomes * Clearly,
the chances that two individuals would be identical is very small
Summary of Meiosis I
DNA replication: increase to tetraploid
Prophase I: chromosome pairing & recombination; nuclear breakdown
Metaphase I: paired homologues separate
Anaphase I: chromosomes to opposite poles
No intervening DNA replication
Products are diploid
Summary of Meiosis II
Prophase I: reattachment to spindle
Metaphase I: chromosomes align on metaphase plate
Anaphase I: sister chromatids separate to poles
Products are haploid
Effect of Recombination on Allele Segregation
* During Prophase I DNA is recombinead
- Paired chromosomes undergo breakage and rejoining at identical
sites
- Changes linkage of paternal and maternl genes
How does this affect segregation?