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.
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