Meiosis chapter 6

Mitosis Cell Division Used for growth and repair Occurs in somatic (body) cells Produces two new cells identical to the parent cell

Chromosomes

Chromosomes Chromosomes store genetic information Human cells have 46 chromosomes: 23 pairs

Chromosomes

Called Sister Chromatids Chromosomes Duplicated chromosomes are called chromatids chromatids held together by centromere Called Sister Chromatids

Body cells are diploid = 2n Gametes are haploid = n The pairs are called homologs

How can the cell divide and cut its chromosome number in ½? The Problem: How can the cell divide and cut its chromosome number in ½? How can we go from 2n to 1n?

Meiosis

Facts About Meiosis Two divisions- Meiosis I and II reduction- division Original cell is diploid (2n) Four daughter cells are haploid (1n)

How do you cut the chromosome number in ½? Here is the problem: How do you cut the chromosome number in ½? How do you divide and get the right chromosomes into each cell?

A major difference between the divisions of mitosis and meiosis: The HOMOLOGS pair up before dividing http://www.pbs.org/wgbh/nova/baby/divi_flash.html

Meiosis: Two Cell Divisions Sister chromatids separate Meiosis I Meiosis II Homologs separate Diploid Diploid Haploid

Prophase I Late prophase Early prophase Chromosomes condense. Spindle forms. Nuclear envelope fragments. Early prophase Homologs pair. Crossing over occurs.

Crossing-Over Homologous chromosomes in a tetrad cross over each other Pieces of chromosomes or genes are exchanged Produces Genetic recombination in the offspring

Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment

Homologous pairs of chromosomes align along the equator of the cell Metaphase I Homologous pairs of chromosomes align along the equator of the cell

Anaphase I Homologs separate and move to opposite poles. Sister chromatids remain attached at their centromeres.

Telophase I Nuclear envelopes reassemble. Spindle disappears. Cytokinesis divides cell into two.

Prophase II Nuclear envelope fragments. Spindle forms.

Metaphase II Chromosomes align along equator of cell.

Anaphase II Sister chromatids separate and move to opposite poles. Equator Pole Sister chromatids separate and move to opposite poles.

Telophase II Nuclear envelope assembles. Chromosomes decondense. Spindle disappears. Cytokinesis divides cell into two.

Results of Meiosis Gametes (egg & sperm) form Four haploid cells with one copy of each chromosome One allele of each gene Different combinations of alleles for different genes along the chromosome

Comparing Mitosis and Meiosis

Mitosis Meiosis WHAT it is used for How many daughter cells Are the daughters identical to the parent? If the parent is diploid with 16 chromosomes, what will the daughter be? Does the DNA need to replicate before division? Does crossing over occur?

Comparison of Divisions Mitosis Meiosis Number of divisions Number of daughter cells Genetically identical? Chromosome # Where When Role

Comparison of Divisions Mitosis Meiosis Number of divisions 1 2 Number of daughter cells 4 Genetically identical? Yes No Chromosome # Same as parent Half of parent Where Somatic cells Germ cells When Throughout life At sexual maturity Role Growth and repair Sexual reproduction