Meiosis

Meiosis is the type of division which: 1. Reduces the chromosome # from diploid to haploid 2. Is used to produce gametes: eggs and sperm

Diploid number: the total # of chromosomes in a normal body cell 1. chromosomes are in pairs 2. chromosomes are in 2 sets: one from dad, one from mom 3. symbol for diploid #: 2n Humans have 46 chromosomes in cells: diploid # = 2n = 46

Haploid number: # of chromosomes in an egg or sperm cell 1. chromosomes are not in pairs 2. chromosomes are in 1 set 3. symbol for haploid #: n Humans have 23 chromosomes in gamete cells: haploid # = n = 23

Egg (n) + Sperm (n) = Zygote (2n) 4. When an egg (n) is fertilized by a sperm (n), the result is a zygote which is 2n In humans: Egg (n) + Sperm (n) = Zygote (2n) 46 23 23

The purpose of meiosis is to divide cell chromosomes so that the diploid # is reduced to the haploid # There are two complete divisions: meiosis I and II

Interphase 2n cell

Stages of Interphase G1 = growth and development S = DNA replication - chromosomes become double G2 = preparation for division

Interphase of meiosis is exactly like interphase of mitosis, except that it occurs only in certain cells of the ovaries or testes

Meiosis I – Reduction division Prophase I: a. Nucleolus and nuclear envelope disappear b. Spindle forms c. Chromosomes shorten, thicken, and become visible d. Homologous pairs of chromosomes are together e. Crossing over occurs

Prophase I 2n cell

Crossing over: the exchange of portions of chromosomes between homologous partners (increases variation)

Metaphase I: a. Chromosomes line up at the equator b. Spindle fibers attach to centromeres c. Homologous chromosomes are together

Metaphase I 2n cell

Anaphase I: a. Homologous pairs separate b. Centromeres DO NOT divide c. Chromosomes remain double stranded

Anaphase I 2n cell 2 = n = 2 n number of chromosomes go to each pole

Telophase I: a. Nucleolus and nuclear membrane reappear b. Spindle disappears c. Nuclei are now haploid (n) but chromosomes are still double stranded (2 chromatids are joined at the centromere)

Telophase I Cytokinesis may or may not occur

Interkinesis: the period between the divisions of meiosis I and meiosis II May be short or long Chromosomes are already double stranded so they DO NOT replicate again

Meiosis II – 2nd division Purpose is to separate sister chromatids Phases are very similar to mitosis

Prophase II: each daughter cell is now haploid (n) with double stranded chromosomes

Metaphase II: Chromosomes line up at the equator

Anaphase II: chromosomes are now single stranded and moving to opposite poles

Telophase II and cytokinesis: 4 haploid daughter cells are produced

1 2 4 2n n Growth, asexual Sexual Yes No Mitosis Meiosis # of DNA replications 1 # of divisions 2 # of daughter cells 4 n # of daughter cells 2n n Purpose Growth, asexual Sexual Daughter cells like parent? Yes No Daughter cells like each other?