1. Meiosis and Sexual Life Cycles
Chapter 13

2. asexual vs sexual reproduction
Two parents give rise to offspring.
Each parent passes on half its genes to offspring.
Offspring have a unique combination of genes inherited from both parents.
Results in greater genetic variation; offspring vary genetically from their siblings and parents.
Asexual Reproduction
Single individual is the sole parent.
Single parent passes on all its genes to offspring.
Offspring are genetically identical to the parent (clones).

3. Meiosis and Fertilization
Somatic cell – Body cell; not sperm or egg cells.
Homologous chromosomes (homologues) -- A pair of chromosomes that have the same size, centromere position, and same gene locations.
Autosome -- A chromosome that is not a sex chromosome.
Sex chromosome -- chromosomes that determine an individual's sex.
• Females have a homologous pair of X chromosomes.
• Males have one X and one Y chromosome.
Humans have 22 pairs of autosomes and 1 pair of sex chromosomes.
The 46 somatic cell chromosomes are actually two sets of 23 chromosomes; one a maternal set and the other a paternal set.

4. Meiosis and Fertilization cont.
Diploid -- cells contain two sets of chromosomes; 2n; somatic cells.
Haploid -- cells contain one set of chromosomes; n; gametes (reproductive cells).
Diploid number is restored when two haploid gametes unite in the process of fertilization.
Zygote -- A diploid cell that results from the union of two haploid gametes.
Gametes are produced in the ovaries or testes by the process of meiosis.

5. Variety of Sexual Life Cycles

6. Stages of Meiosis
Replication: (Interphase) Chromosomes replicate as in mitosis.
Meiosis I: Separates the two chromosomes of each homologous pair, reducing the chromosome number by half.
Alignment: (Prophase I, Metaphase I)
Chromosomes condense, and synapsis occurs.
Each homologous pair in synapsis appears as a complex of four chromatids or a tetrad.
Homologous chromatids are linked by chiasmata where crossing-over occurs.

7. Crossing Over
Crossing over -- exchange of genetic material between homologues; increases genetic variation.
Produces chromosomes that contain genes from both parents.
In humans, there is an average of 2-3 crossovers per chromosome pair.

8. Stages of Meiosis (cont)
Separation: (Anaphase I, Telophase I, Cytokinesis)
Homologous chromosomes separate and move towards opposite poles of the cell.
Each pole now has a haploid set of chromosomes.
In some species, nuclear membranes and nucleoli reappear.
In other species, the daughter cells immediately prepare for meiosis II.
No DNA replication occurs before meiosis II.

10. Stages of Meiosis (cont)
Meiosis II: This second meiotic division separates sister chromatids of each chromosome (much like Mitosis).
Alignment: (Prophase II, Metaphase II)
Separation: (Anaphase II, Telophase II, Cytokinesis)
Four haploid daughter cells produced; genetically different from each other and from the original cell.

11. Meiosis animation

12. Meiosis in Humans
In males, each of the 4 haploid cells is called a spermatid, which will differentiate into 4 sperm (happens in testes).
In a female, meiosis freezes in Prophase I right before she is born (in ovaries); process continues at ovulation.
One cell will divide into only one egg cell and smaller polar bodies; Meiosis II is triggered by fertilization.