Topic 4.2 & 10.1 Notes

Meiosis• A reduction in

chromosomes to form HAPLOID cells.

• 2N to 1N

Homologous pairs • They associate with each other during prophase 1

through synapsis.

Chromosomes replicate in which phase?

• S phase!

Prophase 1• The homologous chromosomes form BIVALENTS.

These are replicated homologous chromosomes.• The sister chromatids are next to each other,

joined at the centromere.

Crossing Over• Non sister chromatids may cross over during

prophase 1 leading to recombinant alleles.

More detail on Prophase 1

Synapsis-the chromosomes associateCrossing Over-The Chiasma are the points at which they cross over. A Holliday junction forms when they anneal.This increases genetic diversity!

Metaphase 1• Bivalents line up at

the equator double file.

• There are many positions that they can align themselves in.

• This adds to the genetic variation…more on this later!

Anaphase 1• This is where the reduction takes place. The pairs

separate, but the sister chromatids remain attached!

Non Disjunction

Telophase 1• The cells are no longer diploid here.• The chromosomes may not be exact matches of

each other due to crossing over.

Cytokinesis 1

Prophase II• The nuclei break down again, but the

chromosomes do not cross over again.

Metaphase II• Random orientation again contributes to variation

in gametes.

Anaphase II• Chromatids are pulled to the opposite poles.

Telophase II and Cytokinesis

Oogenesis

Spermatogenesis

Mendel and Independent Assortment

• The presence of an allele of one of the genes in a gamete has no influence over which allele of another gene is present.

• This is only true for unlinked genes.• Click here for a more detailed explanation.

HOMEWORK 4.2/10.1• Make a T Chart to compare/contrast

mitosis and meiosis.• Please include as many as possible,

but a minimum of 6.• You may need to do some research

ON YOUR OWN.• Due Friday