MeiosisMeiosis

Forming haploid cells Forming haploid cells for sexual reproductionfor sexual reproduction

InterphaseInterphase

Crossing Over

ProphaseProphase

Prophase 1Prophase 1

Chromosomes ____________.Chromosomes ____________. Nuclear membrane Nuclear membrane

___________.___________. Centrioles move and spindles Centrioles move and spindles

formform

condensecondense

disappearsdisappears

Prophase 1 cont.Prophase 1 cont. _________ occurs: pairing of _________ occurs: pairing of

homologous chromosomes homologous chromosomes (pair (pair of the same type of chromosome of the same type of chromosome but one from each parent)but one from each parent)

Forms a ______: 4 sister chromatidsForms a ______: 4 sister chromatids _____________ occurs: portions of _____________ occurs: portions of

chromatids may break off and chromatids may break off and reattach on adjacent chromatidsreattach on adjacent chromatids

SynapsisSynapsis

tetradtetrad

Crossing-overCrossing-over

Crossing overCrossing over

Metaphase and AnaphaseMetaphase and Anaphase

Metaphase 1 and Anaphase Metaphase 1 and Anaphase 11

Homologous pairs line up at Homologous pairs line up at the equatorthe equator

Spindle fibers attach to the Spindle fibers attach to the _____________ of each _____________ of each homologous pairhomologous pair

Homologous pairs split and Homologous pairs split and move to _______ _____move to _______ _____

centromerecentromere

opposite polesopposite poles

Telophase and CytokinesisTelophase and Cytokinesis

Telophase 1Telophase 1

_____________reach opposite _____________reach opposite poles of the cell poles of the cell

___________ occurs: the cell ___________ occurs: the cell divides.divides.

The resulting cells are The resulting cells are ______, (2n).______, (2n).

ChromosomesChromosomes

CytokinesisCytokinesis

diploiddiploid

Prophase 2Prophase 2

Centromeres of _____ Centromeres of _____ ________ attach to spindle ________ attach to spindle fibers.fibers.

Nuclear Nuclear membrane__________.membrane__________.

sister chromatidssister chromatids

disappearsdisappears

Prophase 2Prophase 2

Metaphase 2Metaphase 2

_____ __________ line up on _____ __________ line up on the equatorthe equator

Sister chromatidsSister chromatids

Metaphase 2Metaphase 2

Anaphase 2Anaphase 2

Chromatids separate and Chromatids separate and move to _______ ______ of move to _______ ______ of the cell.the cell.

opposite polesopposite poles

Anaphase 2Anaphase 2

Telophase 2Telophase 2 ______ __________ forms around ______ __________ forms around

the chromosomes.the chromosomes. The cells divide by _________.The cells divide by _________. The resulting 4 cells are The resulting 4 cells are

________, (n).________, (n). These reproductive cells are These reproductive cells are

called _______.called _______.

Nuclear membraneNuclear membrane

cytokinesiscytokinesis

haploidhaploid

gametesgametes

Telophase Telophase and and

CytokinesisCytokinesis

MeiosisMeiosis The process of nuclear The process of nuclear

division that produces division that produces haploid cells with half the haploid cells with half the number of chromosomes as number of chromosomes as the original diploid cell.the original diploid cell.

Why have meiosis?Why have meiosis?

Results in genetic variation Results in genetic variation in the new offspring.in the new offspring.

Diagram of MeiosisDiagram of Meiosis

Diploid cell (2n)

Haploid cell (n)

More sources of genetic More sources of genetic variation:variation:

1.1. Crossing-over:Crossing-over: In prophase 1, when In prophase 1, when

homologous chromosomes homologous chromosomes exchange genetic informationexchange genetic information

2.2. Random fertilization:Random fertilization: A sperm and egg unite, forming A sperm and egg unite, forming

a a zygotezygote..

Only germ cells undergo Only germ cells undergo meiosis to produce haploid meiosis to produce haploid reproductive cells.reproductive cells.

What are germ cellsWhat are germ cells

What are somatic cells?What are somatic cells?

All other cells in the All other cells in the body are somatic cells. body are somatic cells. Normally diploid.Normally diploid.

Gamete developmentGamete development

Gametes are only formed Gametes are only formed in specialized in specialized reproductive organs in an reproductive organs in an organism.organism.

Spermatogenesis: Spermatogenesis: formation of sperm cells. formation of sperm cells.

Oogenesis: formation of Oogenesis: formation of egg cells. A parent cell egg cells. A parent cell produces only 1 mature produces only 1 mature egg, which receives most of egg, which receives most of the cytoplasm. The other 3 the cytoplasm. The other 3 cells are called polar bodies cells are called polar bodies and they degenerate.and they degenerate.

Let’s ReviewLet’s Review1.1. What are homologous What are homologous

chromosomes?chromosomes?

2.2. What happens to homologous pairs What happens to homologous pairs during metaphase 1?during metaphase 1?

3.3. How many chromosomes are in a How many chromosomes are in a cell at the end of telophase 1?cell at the end of telophase 1?

4.4. How many chromosomes are in a How many chromosomes are in a cell at the end of telophase 2?cell at the end of telophase 2?