Meiosis

Meiosis is the type of division which:1. Reduces the chromosome # from diploid to haploid2. Is used to produce gametes: eggs and sperm

Diploid number: the total # of chromosomes in a normal body cell1. chromosomes are in pairs2. chromosomes are in 2 sets:

one from dad, one from mom3. symbol for diploid #: 2n

Humans have 46 chromosomes in cells: diploid # = 2n = 46

Haploid number: # of chromosomes in an egg or sperm cell1. chromosomes are not in pairs2. chromosomes are in 1 set3. symbol for haploid #: n

Humans have 23 chromosomes in gamete cells: haploid # = n = 23

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)

23 4623

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 InterphaseG1 = 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

1. Prophase I:a. Nucleolus and nuclear envelope disappearb. Spindle formsc. Chromosomes shorten, thicken, and become visibled. 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)

2. Metaphase I:a. Chromosomes line up at the equatorb. Spindle fibers attach to centromeresc. Homologous chromosomes are together

Metaphase I

2n cell

3. Anaphase I:a. Homologous pairs separateb. Centromeres DO NOT dividec. Chromosomes remain double

stranded

Anaphase I

2n cell

2 = n = 2n number of chromosomes go to each

pole

4. Telophase I:a. Nucleolus and nuclear membrane reappearb. Spindle disappearsc. 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

a. May be short or longb. 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

1. Prophase II:each daughter cell is now haploid (n) with double

stranded chromosomes

2. Metaphase II:Chromosomesline up at the equator

3. Anaphase II:chromosomesare now singlestranded and

moving to opposite poles

4. Telophase II and cytokinesis:4 haploiddaughter cellsare produced

Mitosis Meiosis

# of DNA replications

1 1

# of divisions 1 2# of daughter cells

2 4

n # of daughter cells

2n n

Purpose Growth, asexual

Sexual

Daughter cells like parent?

Yes No

Daughter cells like each other?

Yes No