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, 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