1. 3.1 Meiosis

2. 3.1 Meiosis
Multicellular organisms that reproduce sexually have two types of cells: Somatic & Sex cells
Somatic Cells (AKA body cells)
reproduce by cell division
Make up the majority of the cells in the organism
Have 23 pairs of chromosomes (total of 46)
Sex cells
Called gametes
½ the chromosomes of the parent cell
Makeup tiny fraction of organisms cells
Male gametes = sperm, female gametes = eggs or ova

3. Individuals of the same species have the same number of chromosomes.
Ex. Goldfish have 94, fruit flies 8, humans 46
To maintain the same number of chromosomes from generation to generation a parent creates gametes through meiosis.
Only happens in reproductive cells

4. Diploid (2n) have a complete set of chromosomes
Haploid (n) are cells with half the number of chromosomes of the parent
Diploid (2n) have a complete set of chromosomes
Homologous Chromosomes are pairs of chromosomes
Each parent gives one copy during meiosis to form a gamete.
Ex. Humans have 23 pairs

5. Homologous Chromosomes

6. Meiosis There are two stages:
Meiosis I
Meiosis II
They produce 4 haploid (23 chromosomes) gametes
Each stage is divided into four parts:
Prophase I & II
Metaphase I & II
Anaphase I & II
Telophase I & II

7. Meiosis Diagram
Meiosis I separates the homologous chromosomes and meiosis II separates the pairs of chromatids.

8. Meiosis I
Homologous chromosomes separate & move to opposite poles along with their sister chromatid.
The sister chromatids do not separate.
Results in two daughter cells, with only one of the homologous chromosomes

9. Crossing over
Exchange of genetic material
Genetically different daughter cells

10. Meiosis II Similar to mitosis Sister chromatids line up at the equator
The chromatids are pulled to the opposite pole
Nuclear membranes reform & cytokinesis occurs
Results in 4 haploid daughter cells called gametes (sex cells)

11. Meiosis Animations How Meiosis Works Comparison of Mitosis & Meiosis
hill.com/sites/ /student_view0/chapter28 /animation__how_meiosis_works.html
Comparison of Mitosis & Meiosis
hill.com/sites/ /student_view0/chapter28 /animation__comparison_of_meiosis_and_mitosis__quiz _1_.html

12. Atypical Meiosis
Once in a while errors do occur , the most common being nondisjunction.
Nondisjunction occurs when the homologous chromosomes do not separate during Meiosis
The result is gametes with the wrong number of chromosomes
Can occur in meiosis I or meiosis II
E.g.: Down syndrome results in a zygote with 47 chromosomes (3 copies of Chromosome #21)