BIOLOGY

Unit 4 Notes:Meiosis

(1) What is Meiosis?

• Cell division for sex-cells.

• Sex Cells:– Are also called Gametes.– Sperm in Males– Eggs in Females

(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.

(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

(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.

(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.

(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.

(Drawing Time)

(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.

(Drawing Time)

(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.