1. BIOLOGY Unit 4 Notes: Meiosis

2. (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).

5. (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.

6. (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.

8. (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.

10. (5) Meiosis Basics Meiosis-1: Meiosis-2:
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.

12. (6) Meiosis-1 Details Chromosomes form + Nucleus dissolves.
Homologous pairs of chromosomes line.
Crossing Over occurs.
Homologous pairs of chromosomes separate.
Cells divide and split, resulting in two cells.

14. (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.

17. (8) Meiosis-2: Pairs of sister chromatids line up.
Pairs of sister chromatids separate using the “Law of Segregation” and the “Law of Independent Assortment”.
Cells divide, resulting in 4 cells.

19. (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.

21. (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.