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: 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.

(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.

(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: 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.

(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.