Meiosis

Gene

Segment of DNA that codes for a protein/trait

Allele

Version of a gene

Meiosis

Cellular division that splits chromosomes so that the result is 4 daughter haploid cells

Meiosis I

separates homologous chromosomes while replicated sister chromatids stay together in the two daughter cells

Meiosis II

The two daughter cells divide again to separate the sister chromatids into two additional daughter cells each

What is the result of meiosis?

4 daughter haploid that are genetically distinct from the parents

The cell cycle of meiosis

G1 → S → G2 → Meiosis I → C → Meiosis II → C → G1

Where does genetic variation come from?

Crossing over and independent assortment

Crossing over: overview and genetic outcome

Crossing over (during prophase I) results in chromosomes with a combination of maternal and paternal segments and thus a mixture of maternal and paternal alleles

• Crossover is more likely to occur when genes are located far apart from each other (on the same chromosome).

Crossing Over: Molecular Mechanism

Synaptonemal complex: a lattice of proteins, including cohesion, that tightly pairs the homologous chromosome. Genes are aligned precisely with each other so segments can be exchanged exactly

Crossing Over; Molecular mechanism

Segments are exchanged exactly via large protein assemblies called recombination nodules

• Mark where CO will occur at chiasmata

• Cut and rejoin DNA

Independent Assortment: Overview and genetic outcome

Random alignment of chromosomes at Metaphase I plate contributes to the genetic diversity of gametes

• Homologous chromosomes assort and separate independently of each other during meiosis I so that the resulting cells have a mixture of chromosomes from mom and dad

• Resulting in two haploid cells from Meiosis I

Molecular Mechanism of Independent Assortment

Spindle fibers (composed of microtubules) attach to chromosomes via the kinetochore, a protein at the centromere.

• This attachment happens randomly = chromosome movement to the plate is also random

What is the result?: Homologous chromosomes physically connect at a chiasma at the beginning of meiosis I

Results in new combinations of alleles within a chromosome

• Crossing over/recombination occurs at chiasmata during prophase I

What is the result?: Homologous chromosomes are separated into two new daughter cells at the end of meiosis I

Results in either maternal or paternal copies of chromosomes in haploid cells

What is the result?: Sister chromatids are pulled apart at the end of meiosis II

Results in only one copy of each chromosome in haploid cells

Nondisjunction

Most often caused by:

• Inflexible cohesion protein = homologs fail to separate after crossing over (can also happen in anaphase II)

What usually happens to fertilized eggs with the wrong number of chromosomes?

Fail to develop viable embryos

Outcrossing

when gametes from different individuals combine to form offspring

Self-fertilization

when two gametes produced by the same individual fuse to form offspring

• offspring are still genetically distinct from parent, but there is far less genetic variation among offspring