MeiosisMeiosisMeiosis – Type of cell division that happens only in Meiosis – Type of cell division that happens only in

sex cells (gametes)sex cells (gametes)

The resulting cells receive only half the number of The resulting cells receive only half the number of chromosomes present in somatic (body) cells.chromosomes present in somatic (body) cells.

Meiosis VideoMeiosis Video

How many chromosomes do How many chromosomes do human somatic cells have?human somatic cells have?

Karyotypes show that human somatic cells have Karyotypes show that human somatic cells have 46 46 chromosomes and these chromosomes come inchromosomes and these chromosomes come in pairs pairs..

Human somatic cells are said to be Human somatic cells are said to be diploid. diploid. DiploidDiploid – cells having pairs of chromosomes – cells having pairs of chromosomes

These pairs are said to be These pairs are said to be homologoushomologous..

HomologousHomologous – similar in structure; the two – similar in structure; the two members of each pair of chromosomes carry members of each pair of chromosomes carry DNA with information for corresponding traits.DNA with information for corresponding traits.

Why Pairs?Why Pairs?

Mitosis Mitosis

Fertilization

Mitosis

Mitosis

46

46

92

92

92

92

92

92

92

This means that the chromosome number has to This means that the chromosome number has to be 23 to produce gametes (sperm and egg).be 23 to produce gametes (sperm and egg).

How is the chromosome number reduced?How is the chromosome number reduced?

Meiosis!Meiosis!

Fertilization

Mitosis vs. MeiosisMitosis vs. Meiosis

ProphaseProphase

• Homologous chromosomes pair to form tetrad

Mitosis vs. MeiosisMitosis vs. Meiosis

MetaphaseMetaphase

• Homologous pairs line up on equatorial plane

Tetrads can be in any order as long as Tetrads can be in any order as long as homologous chromosomes are pairedhomologous chromosomes are paired

Random AssortmentRandom Assortment

Mitosis vs. MeiosisMitosis vs. Meiosis

AnaphaseAnaphase

• Homologous chromosomes of each tetrad separate and move to opposite poles

Mitosis vs. MeiosisMitosis vs. Meiosis

TelophaseTelophase

• Cytoplasm divides to form two cells.

Mitosis vs. MeiosisMitosis vs. Meiosis

Second Stage ofSecond Stage of Meiosis Meiosis ProphaseProphase

Chromosomes skip Chromosomes skip replicationreplication

Begin to move toward Begin to move toward middlemiddle

Mitosis vs. MeiosisMitosis vs. Meiosis

MetaphaseMetaphase Chromosomes line Chromosomes line

up on equatorial up on equatorial plateplate

Spindle fibers Spindle fibers attach at attach at centromerescentromeres

Mitosis vs. MeiosisMitosis vs. Meiosis

AnaphaseAnaphase

Sister chromatids Sister chromatids separate at separate at centromerecentromere

Mitosis vs. MeiosisMitosis vs. Meiosis TelophaseTelophase

Nuclear membrane Nuclear membrane reforms.reforms.

Chromosome Chromosome numbernumber is cut in is cut in half. Now half. Now haploidhaploid..

HaploidHaploid – cells having – cells having only one chromosome only one chromosome from each homologous from each homologous pair.pair.

Differences between Differences between Mitosis and MeiosisMitosis and Meiosis

1.1. Meiosis makes sex cells (gametes)Meiosis makes sex cells (gametes)

2.2. Meiosis cuts chromosome number in half. Meiosis cuts chromosome number in half. No more pairs. No more pairs.

3.3. Meiosis has two rounds of cell division Meiosis has two rounds of cell division while mitosis only has one.while mitosis only has one.

4.4. Chromosome pairs (tetrads) meet in the Chromosome pairs (tetrads) meet in the middle during first metaphasemiddle during first metaphase..

How many sperm are produced from one How many sperm are produced from one cell in meiosis?cell in meiosis?

FourFour

How many eggs are produced from one How many eggs are produced from one cell in meiosis?cell in meiosis?

OneOne

Timing of MeiosisTiming of Meiosis

FemalesFemales Begins before birth. Begins before birth.

Release one egg a month from puberty until Release one egg a month from puberty until menopausemenopause..

Timing of MeiosisTiming of Meiosis

MalesMales Begins at pubertyBegins at puberty

Continues until death. Slows down – sperm Continues until death. Slows down – sperm count lowers.count lowers.

Terms to be familiar Terms to be familiar with:with:

Diploid Diploid – cells having two sets of – cells having two sets of chromosomes; in humans this number is 46chromosomes; in humans this number is 46

HaploidHaploid – cells having only one – cells having only one chromosome fromm each homologous pair; chromosome fromm each homologous pair; in humans this number is 23.in humans this number is 23.

HomologousHomologous – similar in structure; – similar in structure; homologues in diploid cells are the two homologues in diploid cells are the two members of each pair of chromosomes that members of each pair of chromosomes that carry DNA with information for carry DNA with information for corresponding traits.corresponding traits.