Meiosis and Sexual Lifecycles Chapter 13

Objectives Understand the meaning of “SEX” Understand the process of meiosis Understand the need to undergo meiosis as sexual organisms

Need For Sex Sex: results in the recombination of genetic information Organisms with sexual lifecycles have a greater range of genetic variation that may result in a greater ability to adapt to an environment Meiosis is key to enhancing this variation

Sex Terms Diploid: having 2 of each chromosome type Haploid: having a single representative of each chromosome type Homologous chromosomes (homologs): chromosomes from different parents that are of the same type (contain similar information) Sister Chromatids: 2 “identical” strands of DNA connected by a centromere that contains a kinetochore. Makes up each member of a homologous pair

Comparison Mitosis Meiosis produces 2 daughter cells daughters are clones of the parent daughter cells are diploid growth, replacement, repair Meiosis produces up to 4 daughter cells daughters are distinct from parents daughter cells are haploid generation of gametes (sex cells): egg & sperm

Meiosis (the process) Meiosis: nuclear division with a reduction in chromosome number by half Two parts to the process Meiosis I: separation of homologous pairs Meiosis II: separation of sister chromatids (just as in mitosis)

Meiosis I Prophase I: same as mitosis Metaphase I: both members of a homologous pair arrange themselves as a tetrad (synapsis) along the metaphase plate; crossover (chiasma) may occur (may begin in Prophase I) Anaphase I: Centromere remains intact and homologous pairs are separated as spindle fibers shorten

Meiosis II Telophase I: same as mitosis (includes cytokinesis) Meiosis II is the same process as mitosis except starting from Telophase I instead of Interphase * no doubling of DNA has occurred Result is up to four daughter cells

Genetic Variation Variation in the DNA of a zygote can be introduced by three different methods occurring during the sexual cycle Random fertilization: 8+ million possible combinations of sperm and egg = 70,368,744,000,000 possible combinations Independent Assortment Crossing over

Independent Assortment There are about 8 million possible combinations of chromosomes based on random assortment of the different types in the human genome (8,388,744). This occurs during Anaphase I of meiosis as homologous pairs are separated Genes that do not reside on the same chromosome may sort independent of one another

Crossing Over During tetrad formation (Prophase I) chromosomes may cross (chiasma). Sometimes genetic information may be exchanged between homologs further shuffling the genetic information of the eventual gamete (4,951,760,200,000,000,000,000,000,000 different zygotes with a single crossover)