CHAPTER 11 NOTES-MEIOSIS
Definition: a process of reduction division that produces gametes in which the number of chromosomes per cell is cut in half (haploid) through the separation of homologous chromosomes.
Homologous chromosomes: chromosomes that have a corresponding chromosome from the opposite-sex parent.
· A cell that contains both sets of homologous chromosomes is diploid, which means “two sets”. (2n)
· A cell that contains only one set of chromosomes is haploid. (n)
Ø Meiosis I and Meiosis II
INTERPHASE: just like interphase before mitosis
G1: cells grows, protein production
S: DNA replication
G2: productions of new organelles including centrioles
· Chromatin coils into chromatids and the homologous pairs of chromatids come together to form a tetrad.
· Crossing over occurs were portions of the homologous chromosomes are exchanged.
· The rest of prophase is very similar to mitosis.
Why is crossing over so important?
Crossing over allows for the random mixing of genes (genetic recombination) which adds genetic variety to a species so that no two individuals are exactly the same.
· Tetrads line up at the equator of the spindle.
Original chromosomes from mom (maternal) and dad (paternal) can line up randomly and independent of each other on either side of the equator. Provides genetic variation.
· Homologous chromosomes separate and are pulled to the poles (the centromeres do NOT split).
· Chromosomes uncoil, spindle breaks down. The nuclear membranes and nucleoli reform.
· Cytoplasm divides (cytokinesis) to form two cells.
NOTE: There is NO interphase between meiosis I and meiosis II.
Why is this important?
So that each new cell will only have half (haploid) the number of chromosomes as the original parent cell.
PROPHASE II: *just like mitosis
METAPHASE II: except that there
ANAPHASE II: is only half the
TELOPHASE II/ number of
Prophase II Metaphase II Anaphase II
NOTE: Random fertilization of the egg and sperm is the third source of genetic variation.
What is the end result of meiosis?
Four genetically different haploid cells that contain half the number of chromomsomes as the original parent cell.
What was the end result of mitosis?
Two genetically identical diploid cells that are identical to the original parent cell.