Meiosis is a type of cell division that produces gametes, or sex cells, like sperm and eggs. It involves two cell divisions starting with one diploid cell that has two sets of each chromosome and resulting in four haploid cells that each have one set. The two divisions are meiosis I and meiosis II. Meiosis I separates the homologous chromosome pairs and crossing over can occur, while meiosis II separates the sister chromatids. Errors in meiosis cell division can result in gametes with too many or too few chromosomes, leading to genetic disorders.

1. BIOLOGY
Unit 4 Notes:
Meiosis

2. (1) What is Meiosis?
• Cell division for sex-cells.
• Sex Cells:
– Are also called Gametes.
– Sperm in Males
– Eggs in Females

3. (2) What happens during Meiosis?
• There are 2 divisions.
• Start with 1 cell and end up with 4 genetically
unique cells.
• Start “2n” or “Diploid”  End “1n” or “Haploid”
• 1n = Haploid = ½ Set of Genetic Information
– In Humans that means that each sperm or egg cell
should have 23 chromosomes.

4. (3) Meiosis Phases
• Meiosis-1:
– Start with 1 cell and end with 2
– Prophase, Metaphase-1, Anaphase-1, Telophase-1,
Cytokinesis-1
– Crossing Over occurs
– Homologous Pairs of Chromosomes
• Meiosis-2:
– Start with 2 cells and end with 4
– Metaphase-2, Anaphase-2, Telophase-2, Cytokinesis-2
– Pairs of Sister Chromatids
– Law of Independent Assortment
– Law of Segregation

6. (4) Meiosis-1 Details
• Prophase:
Chromosomes form + Nucleus dissolves.
• Metaphase-1:
Homologous pairs of chromosomes line up at the metaphase
plate.
Crossing Over occurs.
• Anaphase-1:
Homologous pairs of chromosomes separate.
• Telophase-1:
Chromosomes are fully separated.
• Cytokinesis-1:
Cytoplasm splits and the cell membranes divide.

7. (5) Meiosis-1 Vocabulary:
• Homologous Pairs of Chromosomes 
– Pairs of sister chromatids with similar segments of DNA
(or genes.)
– Looks like 2 X’s.
• Crossing Over 
– When nearby chunks of DNA are exchanged between
homologous pairs of chromosomes.
– Used to increase genetic diversity.

8. (6) Meiosis-2:
• Metaphase-2:
Pairs of sister chromatids line up at the metaphase plate.
• Anaphase-2:
Pairs of sister chromatids separate.
Law of Segregation.
Law of Independent Assortment.
• Telophase-2:
Chromosomes are fully separated.
Nuclei reform.
• Cytokinesis-2:
Cytoplasm splits and the cell membranes divide up.

9. (7) Meiosis-2 Vocabulary:
• Sister Chromatids:
Identical copies of chromosomes.
• Law of Segregation:
Chromosomes split up so that each sex cell
receives a different variation of each gene.
• Law of Independent Assortment:
Chromosomes split up completely independent of
every other chromosome and gene variation.

10. (8) Meiosis Errors
• When chromosomes do not separate correctly
during meiosis this can cause:
– Sex Cells to have too many chromosomes (more than
23).
– Sex cells to have too few chromosomes (less than 23).
• Individuals born from these sex cells will acquire a
genetic disorder from having the wrong number of
sex chromosomes.