1. Meiosis and Sexual Lifecycles
Chapter 13

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

3. 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

4. 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

6. 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

8. 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)

9. 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

10. 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

11. 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

12. 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

14. 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)