Meiosis Flashcards
Meiosis
Process that produces sex cells - occurs in the gonads (ovaries and testes), two divisions, result is four haploid cells (half the chromosomes needed to ensure correct number after fertilization), daughter cells genetically different
Differences from Mitosis
- Two divisions (eight stages)
- Occurs in gonads
- Produces sex cells
- Produces four daughter cells with half the chromosomes of the parent cells (haploid)
- Daughter cells are genetically different
The Human Life Cycle
In each generation, doubling of chromosome number from fertilization offset by halving number of chromosome during meiosis
Stages of Meiosis
Interphase I, Prophase I, Metaphase I, Anaphase I, Telophase & Cytokinesis I, Prophase II, Metaphase II, Anaphase II, Telophase & Cytokinesis II
Interphase I
Chromosomes replicates - each DNA strand (chromatin) unzips into two strands (loosely packed and not visible); two pairs of centrioles lie right outside nucleus
Prophase I
Chromosomes condense; homologous chromosomes pair up, forming tetrads (synapsis), and attach to nuclear membrane; enzymes cut out sequences of DNA (genetic material) from the chromosomes and exchange them between the two (crossing over); centrioles separate and spindle fibres begin to grow from them; nuclear membrane disappears
Metaphase I
Pairs of homologous chromosomes line up on metaphase plate, one on each side of plate; spindle fibres attach
Anaphase I
Pairs of homologous chromosome are pulled apart from each other to opposite ends by spindle fibres
Telophase & Cytokinesis I
Chromosomes arrive at opposite ends and new nuclear membranes start to form; rest of cell divides including cytoplasm; cleavage furrow forms; two haploid cells formed
Prophase II
Chromosomes condense, centrioles separate and spindle fibres begin to grow from them, nuclear membrane breaks down
Metaphase II
Chromosomes align along metaphase plate; spindle fibres attach to centromeres
Anaphase II
Sister chromatids are pulled apart to opposite ends by spindle fibres
Telophase & Cytokinesis II
Chromosomes arrive at opposite ends and new nuclear membranes start to form; rest of cell divides including cytoplasm; cleavage furrow forms; four haploid cells formed
Difference in Females (to produce eggs)
During telophase & cytokinesis I, most of cytoplasm is distributed into one of two emerging cells, resulting in one large cell and one smaller cell - larger cell divides again, smaller cell (polar body) degenerates; during telophase & cytokinesis II, same thing happens and large cell becomes egg, smaller cell (polar body) degenerates
