Meiosis Notes

CP BiologyMs. Morrison

Chromosome Number

Every organism gets half of chromosomes from one parent and half from the other parent

Two sets of chromosomes = homologous chromosomes

Chromosome Number, pg 2

Cells with homologous chromosomes = diploid (2N)

Gametes (egg, sperm) – contain only one set of chromosomes = haploid (N)

Humans – 23 pairs (46 chromsomes)

Meiosis Form of cell division to reduce the

number of chromosomes in cell to half (2N N)

Two stages to process: Meiosis I and Meiosis II

At end of Meiosis II – one diploid cell has become four haploid cells

Used to produce gametes (does not occur in regular body cells)

Meiosis I

Picture overview:

Meiosis I, pg 2

Interphase I – all chromosomes are replicated

Prophase I – each chromosome pairs with its homologous chromosome to form a tetrad (4 chromatids)

Meiosis I, pg 3

Prophase I – during this phase, chromosomes in tetrads may exchange portions of their chromatids in process = crossing over (gives new gene combinations

Meiosis I, pg 4

Metaphase I – spindle fibers attach to chromosomes along center of cell

Anaphase I – spindle fibers pull the homologous chromosomes toward opposite ends of the cell

Meiosis I, pg 5

Telophase I/Cytokinesis – nuclear membranes form and the cell separates into two cells

At end of Meiosis I – two daughter cells that are haploid because they contain one set of chromosomes (duplicated, though)

Meiosis II Two haploid daugther cells go through another

division similar to mitosis but does NOT have replication prior to prophase II

Meiosis II, pg 2

Prophase II – nuclear membranes disappear and spindle forms

Metaphase II – chromosomes line up along center of cell like those in mitosis

Meiosis II, pg 3

Anaphase II – sister chromatids separate and move towards opposite sides of the cell

Telophase II/Cytokinesis – nuclear membranes reform and the two cells separate into four cells – all are haploid

Gamete Formation

Males – haploid gametes = sperm, always produce four sperm through meiosis

Females – haploid gamete = egg, uneven divisions through Meiosis I & II so egg receives most of cytoplasm, remaining three cells = polar bodies (not used in reproduction)