Meiosis 15 October, 2004 Text Chapter 13

In asexual reproduction, individuals give rise to genetically identical offspring (clones). All cell division in this process is mitotic. Asexual Reproduction

genome: all of a cell’s DNA chromosome: individual DNA molecule chromatin: chromosomal DNA and its protein scaffold sister chromatids: the two identical copies of a duplicated chromosome centromere: the point of attachment for two sister chromatids somatic cells: (most cells) have two copies of each chromosome (diploid) gametes: (reproductive cells; sperm and eggs) have one copy of each chromosome (haploid) Important Terms (review) locus: the location of a gene on a chromosome (sometimes = gene)

homologous chromosomes: the two similar but nonidentical chromosomes that make up a pair. sex chromosomes: in humans, X and Y chromosomes. Not truly homologous autosomes: all other chromosomes fertilization: the uniting of two gametes to form a zygote (n + n goes to 2n) meiosis: the process of gametogenesis (2n goes to n + n + n + n) More Important Terms

Describing Chromosomes

Some organisms have free-living multicellular haploid phases. Timing of Meiosis and Fertilization

Meiosis reduces the chromosome number from diploid to haploid. Cells resulting from the first meiotic division are haploid duplicated. First, the DNA in a diploid cell is duplicated in S phase. The products of meiosis are haploid gametes. Chromosomes and Meiosis

Meiosis I with Crossing Over

The products of meiosis are genetically different from each other and from the parent cell. Meiosis II -

Meiosis differs from mitosis in the number of divisions, the process of synapsis, and the number and genetic composition of daughter cells Mitosis and Meiosis

Independent assortment of chromosomes during the first meiotic division contributes to genetic variation. Here, there are 2 2 = 4 possible gametes In humans, there are 2 23 = about 8 million possible gametes Random fertilization allows for (8 million) 2 = 64 trillion possible offspring from any two parents. Crossing over occurs two to three times per chromosome, and increases the number of possible offspring even further. Independent Assortment