BIOLOGY

Unit 4 Notes:Meiosis

(1) Reproduction• Types of Reproduction:– Asexual– Sexual

• Sexual Reproduction: Two parent organisms exchange genetic information to produce genetically different offspring.

• Asexual Reproduction: A single celled organism which makes an identical copy of itself (becoming the offspring).

(2) Reproduction & Cell Division

• In multicellular organisms which complete sexual reproduction, they must make more “sex cells”.

• Meiosis = Cell Division for “sex cells”.

• Sex Cells:– Also called Gametes.– Sperm in male organisms.– Eggs in female organisms.

(3) 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) Genetic Variation• The Purpose of Meiosis is to:– Cut the amount of DNA in ½, so that when a sperm and egg

combine, the resulting baby will have a full set of DNA.– Increase genetic variation and diversity amongst all sperm and

egg cells.

• You have at least 2 variations for every gene in your DNA:– Gene = Specific sequence of DNA.– Allele = Variation of a gene.

• Each sperm/egg cell should have a unique combination of gene alleles.

(5) Meiosis Basics• Meiosis-1:– Start with 1 cell and end with 2– Start with copies of all gene variations (actually “4n”).– Crossing Over occurs– Involves Homologous Pairs of Chromosomes

• Meiosis-2:– Start with 2 cells and end with 4– Cells are now “2n”.– Involves Pairs of Sister Chromatids– The “Law of Independent Assortment” and the “Law of

Segregation” are applied.

(6) Meiosis-1 Details1. Chromosomes form + Nucleus dissolves.

2. Homologous pairs of chromosomes line.

3. Crossing Over occurs.

4. Homologous pairs of chromosomes separate.

5. Cells divide and split, resulting in two cells.

(7) 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) Meiosis-2:1. Pairs of sister chromatids line up.

2. Pairs of sister chromatids separate using the “Law of Segregation” and the “Law of Independent Assortment”.

3. Cells divide, resulting in 4 cells.

(9) 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) 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.