Chapter 10

Meiosis and Sexual Reproduction Chapter 10

Introducing Alleles • Asexual reproduction- all offspring inherit the same number and kinds of genes from single parent. • Genes- are sequences of chromosomal DNA. • Sexual reproduction- process involving meiosis, formation of gametes, and fertilization (a union of two gametes). • The first cell of a new individual holds a pair of genes from each parent. • Allele- a unique molecular form of the same gene. • Sexual Reproduction- offspring inherit new combinations of alleles, which lead to

Meiosis • Meiosis- sorts out chromosomes into parcels not once but twice. • Chromosome number- the sum total of chromosomes in cells of a given type. • Diploid number- 2n it has a pair of each type of chromosome, often from two parents. • Homologous Chromosomes- are the same length, shape, and assortment of genes. • Meiosis reduces the number of chromosomes from diploid to haploid

Cont. • Meiosis is similar to mitosis. – In meiosis germ cell duplicate themselves in interpahse. – The 2 DNA strands stay attached at the centromere. (called sister chromatids) – The microtubules of the spindle apparatus move the chromosomes to different locations. – Chromosomes go through 2 consecutive divisions that end with the formation of 4 haploid cells. – Meiosis has two phases meiosis 1 which includes prophase 1, metaphase 1, anaphase 1, and telophase 1 and meiosis 2 which

Cont. • Once the two chromosomes have aligned they separate. • The cytoplasm starts to separate forming two new cells. • Meiosis two begins after this stage. • The two sister chromatids of each chromosome are separated from each other. • This makes 4 parcels of 23 chromosomes and each has 1 chromosome of each type. • The nuclear envelope starts to form making 4 new haploid cells.

Meiosis Introduces Variations in Traits • During prophase 1 the homologous chromosomes swap parts of themselves. • The chromatids of one become stitched point by point along their length to the chromatids of the other, with little space between them. • This tight junction favors crossing over a molecular interaction between a chromatid of one chromosome and a chromatid of the homologous partner. • DNA strands break and seal in ways so that genes are swapped. • When genes are in a slightly different form they are called alleles. • The number of alleles on 1 chromosome will not be identical to their partners. • Each crossover event gives the genes a slightly

Cont. • Crossing over leads to recombination's among genes of homologous chromosomes. • Metaphase 1 microtubules from both poles have now aligned all of the duplicated chromosomes at the spindle equator. • Because tethering is random either partner can end up at either pole. • There are many different combinations that can occur with crossing over with

Gametes to Offsprings • Sporophyte- a muilticelled spore-producing body that makes sexual spores by meiosis. • Spore- is a haploid reproductive cell that is not a gamete and that does not take part in fertilization. • Gametophyte- is spores undergoing mitotic cell divisions. • Diploid germ cells give rise to gametes. • Male germ cells give rise to primary spermatocyte, the cells then goes through meiosis and cytoplasmic division and four haploid cells develop into spermatids. • The cells then undergo changes to produce sperm, which are mature gametes.

Cont. • In females germ cells become primary oocytes. • Primary oocyte divides after meiosis 1, producing a secondary oocyte which gets nearly all of the cytoplasm. • The other cell becomes a polar body. • Both cells enter meiosis 2, then one of the daughter cells becomes the second polar body. • The other daughter cell gets most of the cytoplasm and develops into a gamete. • The mature female gamete is an ovum or egg.

Cont. • Fertilization is the time when a female and male gamete unite and their haploid nuclei fuse. • During prophase 1 every human chromosomes undergoes an average of two or three crossovers.