MeiosisBiology 3 Kawashima

Philippine Science High SchoolSY 2009-2010

Homologous chromosomes condense at prophase I

• Homologous chromosomes pair along their lengths, aligned gene by gene

• Synaptonemal complex forms between homologues, holding them together along their lengths

Homologous chromosomes condense at prophase I

• Synaptonemal complex disassembles in late prophase I, and each chromosome pair become visible as a tetrad

• A tetrad is a group of four chromotids

• Each tetrad has one or more chiasmata, cris-crossed regions where crossing-over has occured

Tetrads align on the metaphase plate at metaphase I

• Chiasmata are still intact• One chromosome of

each pair face the pole• Chromatids of

homologous chromosomes are attached to kinetochore microtubules from one pole

Homologous chromosomes separate at anaphase I

• Sister chromatids remain attached at the centromere

• Sister chromatids move towards the same pole

• Homologous chromosomes, each composed of two sister chromatids, move to opposite poles

Each half has a complete haploid set of chromosomes at telophase I

• Each chromosome is still composed of two sister chromatids

• Cytokenesis begins to form two daughter cells, each with a haploid set of chromosomes

• No chromosome doubling occurs between telophase I and prophase II, as they have already been replicated

Meiosis I

• Homologous chromosomes condense at prophase I

• Tetrads align on the metaphase plate at metaphase I

• Homologous chromosomes separate at anaphase I

• Each half has a complete haploid set of chromosomes at telophase I