Meiosis

Meiosis a specialized type of cell division that results in four haploid daughter cells, each with half the number of chromosomes as the parent cell and all genetically distinct from the parent cell and each other.

Meiosis In sexually reproducing organisms, gametes (sperm and egg) must be produced The gametes must have only HALF the normal number of chromosomes in order to be reproductive cells

Meiosis Advantage: Meiosis is crucial in ensuring genetic variation (biological diversity) among populations which helps a population survive changes in the environment. Disadvantages: Requires more energy than asexual reproduction because gametes must be produced then have the opportunity to meet Embryo must be protected and nurtured

Understanding Ploidy Diploid (2n): 2 sets of chromosomes in somatic cells (body cells) mitosis is one diploid cell to two diploid cells Haploid (n): 1 set of chromosomes in gametes (sperm and egg cells) meiosis is one diploid cell to four haploid cells

Stages of Meiosis For meiosis to occur, the DNA must replicate once then divide twice Homologous chromosomes: a pair of chromosomes with same size, shape, and carrying the same alleles for the same traits

Interphase The diploid cell (2N) is in a neutral state cell is growing and DNA replicates

Prophase I Replicated chromosomes condense becoming visible nuclear membrane breaks down spindles appear develop, centrioles move to poles of cell; 2 more things occur: Pairing of homologous chromosomes: the 2 pairs of homologous chromosomes move together and align end-to-end to form a tetrad Crossing over: segments of tetrads break and reattach to the other chromosome

Metaphase I Nucleus has completely disintegrated Tetrads align at the equatorial plane Spindle fibres attach the centriole to the centromere of each chromosomes in the homologous pair

Anaphase I The spindle fibres begin to reel in the homologous chromosomes to the poles The sister chromatids are not separated in the first division, each chromosome within the homologous pair goes to opposite poles of the cell Disjunction: the normal separation of the homologous chromosomes

Telophase I Chromosomes have reached the poles of the cell and the chromosomes uncoil back into chromatin Nuclear membrane begins to form around each group of divided chromosomes Cytokinesis occurs to split the cell into two daughter cells (cell plate in plates and cleavage furrow in animal cells)

Prophase II Duplicated chromosomes condense and become visible under the microscope Nuclear membrane breaks down Centrioles migrate to poles of the cell and begin to develop spindle fibres

Metaphase II Nucleus has completely disintegrated Duplicated chromosomes align at the equatorial plane Spindle fibres attach the centriole to the centromere of each chromosome in the duplicated pair

Anaphase II The spindle fibres begin to reel in the chromosomes to the poles The sister chromatids are separated in this second division, and each sister chromatid is reeled to the opposite pole of the cell

Telophase II Chromosomes have reached the poles of the cell and the chromosomes uncoil back into chromatin Nuclear membrane begins to form around each group of divided chromosomes Cytokinesis occurs to split each cell into two daughter cells (cell plate in plates and cleavage furrow in animal cells)