IB HL Bio Notes.
What is Meiosis?
Meiosis is a reduction division of a diploid nucleus to form haploid nuclei. Most cells are diploid because their nucleus contrains 2 of every chromosome. Meiosis produces haploid cells that contain 1 of every chromosome. This type of cell division is necessary to produce sex cells which fuse during fertilization and double the number of chromosomes.
Meiosis consists of prophase I, metaphase I, anaphase I, telophase I, and repeation of each phase as a phase II.
During prophase I chromosomes supercoil, homologous chromosomes (bivalents) pair up (synapsis), crossing over occurs resulting in chiasma, centrioles move to opposite poles in animal cells and the nuclear membrane breaks down
During metaphase I spindle microtubules attach to centromeres, bivalents line up at the equator
During anaphase I the two chromosomes of each bivalent move to opposite poles, thus halving the chromosome number, each chromosome consists of two identical sister chromatids, if crossing over occured they are not identical
During telophase I the nuclear membranes form, the cell divides to form two haploid cells
During prophase II it is the same process as prophase I, except there are no homologous chromosomes to pair up, thus no crossing over occurs
During metaphase II it is similar to metahase I, except at the end of the phase the centromeres divide
During anaphase II the two chromatids of each chromosome move to opposite poles
During telophase II it is similar to telophase I except, two cells divide to form four haploid cells which will develop into gametes, thus chromatids become known as chromosomes