10.2 – MeiosisSexual Reproduction

Function: creates gametes (sperm, egg,

pollen) for sexual reproduction.

Meiosis: Cell division for sexual reproduction.

Males - testes Females - ovariesSex Cells

Homologous Chromosomes

1. A normal human body cell contains 46 chromosomes in 23 pairsa) 23 from mother; 23 from father

2. Each pair of chromosomes are called HOMOLOGOUS CHROMOSOMES

3. In a homologous pair, one chromosome is from the mother and one is from the father

Haploid vs. Diploid1. Haploid cell – a cell with only one copy of

each chromosome (23 total in humans) a) ½ the number of chromosomes as the parent

or normal body cells.b) n haploid; n = 23

2. Diploid cell – a cell with 2 copies of each chromosome (46 total; 23 pairs in humans) a) 2n diploid; 2n = 46

Diploid1. All body cells are diploid (2n)

a) Skin, liver, bone, lung, etc.i. Mother 23 chromosomesii. Father 23 chromosomes

2. Body cells are called SOMATIC cells

3. Diploid body cells divide via mitosis to form two identical diploid daughter cells

46 chromosomes total

Haploid1. Sex cells are called GAMETES (sperm and egg)

a) Gametes join to form a ZYGOTEi. sperm + egg = zygote

2. Gametes are haploid (n)a)Sperm = nb)Egg = n

3. n + n = 2n

Meiosis1. Meiosis is the process of making

haploid gametes from a diploid cell

a) Each haploid gamete will have HALF the genetic material as the diploid parent cell

b)2n n

Meiosis1. Meiosis happens in two stages:

a) Meiosis Ib) Meiosis II

2. There are two cell divisionsa) 4 gametes are created

Phases of Meiosis

1st Division1.Prophase I2.Metaphase I3.Anaphase I4.Telophase I

2nd Division1.Prophase II2.Metaphase II3.Anaphase II4.Telophase II

Cytokinesis

Interphase

Interphase1. During

Interphase:a) Cell grows in

sizeb) DNA is

duplicatedNuclear membrane

Chromatin

Prophase I1. During Prophase I:

a) Chromatin condenses into chromosomesb) Nuclear membrane dissolvesc) Spindle formsd) Homologous chromosomes pair up and

CROSSING OVER occurs2. Crossing Over: homologous chromosomes pair

up and randomly exchange genetic information during Prophase 1

Mistakes in MeiosisGenetic Recombination:

AKA Crossing Over

1. Occurs during meiosis, chromosomes actually cross over each other, genetic material gets switched!

2. This leads to genetic variability!

3. Can be an endless number of different chromosomes possible!

Spindle

HomologousChromosomes(Crossing Over)

Metaphase I1. During Metaphase I:

a) Homologous chromosomes are pulled to the middle of the cell by the spindle

Spindle

HomologousChromosomes

Independent Assortment1. Chromosomes line up randomly

during metaphase

2. There is a 50% chance of the maternal or paternal chromosomes going to either side of the cell

= MOTHER= FATHER

Anaphase I1. During Anaphase I:

a) Homologous chromosomes are pulled towards opposite ends of the cell by the spindle

Spindle Chromosomes

Telophase I1. During Telophase I:

a) Nuclear envelope reformsb) Spindle dissolvesc) Cell stretches and cytoplasm divides

(cytokinesis)

Chromosomes

NO Interphase!1. There is NO Interphase between

Meiosis I and Meiosis II

2. Each gamete must receive HALF of the parent’s genetic material so no duplication is necessary

Prophase II1. During Prophase II:

a) Nuclear membrane dissolves

b) Spindle re-forms2. Meiosis II is the

division of the two diploid cells resulting from Meiosis Ia) 4 haploid cells

will be created

ChromosomesSpindle

Metaphase II1.During Metaphase II:

a)Chromosomes are pulled to the middle of the cell by the spindle

Chromosomes

Spindle

Anaphase II1.During Anaphase II:

a)Sister chromatids are separated and pulled to opposite ends of the cell by the spindle

Spindle

Chromosomes

Telophase II1.During Telophase II

a)Nuclear membrane re-forms

b)Spindle dissolvesc) Cells stretch and prepare for

cytokinesis

Chromosomes

Cytokinesis1. The cytoplasm divides resulting in

a total of 4 unique haploid cells

2. All gametes are different because of crossing over during Prophase I and independent assortment during Metaphase I & II

Meiosis: IPMAT cytokinesis; 2nd phase of PMAT producing 4 (haploid-single) daughter cells

I II

Prophase Synapsis occurs and crossing over of chromosomes

same

Metaphase Pairs of chromosomes line in the plates

2 celled-pair of chromosomes line in the plates

Anaphase Homologous pairs separate

Sister chromatids separates

Telophase Division occurs Division occurs

Genetic Diversity1. Meiosis increases genetic diversity because

every sperm or egg is a little bit different. No two offspring are exactly the samea)Crossing Overb) Independent Assortment

2. Genetic diversity causes some organisms to be more fit to survive and reproduce (evolution)

Meiosis and Gametes

How does meiosis create genetic diversity?

1) crossing over

2) random assortment of chromosomes

and later …

3) random pairing of gametes

Nondisjunction1. Nondisjunction - an error in Meiosis when

homologous chromosomes fail to separate properly during Anaphase I or II

2. The resulting gametes will have an extra chromosome or be missing a chromosomea) 3 copies instead of 2b) 1 copy instead of 2

3. Most zygotes will not survive

Nondisjunction during Meiosis II

Nondisjunctionduring Meiosis I

Non-Disjunction

1. In humans, a person with an extra 21st chromosomes will have Down’s

Syndrome

Down Syndrome1. Trisomy – when an organism has 3 copies of

a chromosome where it should only have two

2. Trisomy is caused by nondisjunction

3. Trisomy of the 21st chromosome results in DOWN SYNDROMEa) 3 copies of the 21st chromosome

Trisomy 21causes Down Syndrome

Mitosis vs. Meiosis

1. Makes EXACT copiesa) No new genetic

diversity2. 2n 2n (diploid

diploid)3. Happens in body

(somatic) cells4. One cell division5. Asexual

1. Makes UNIQUE gametesa) Increases genetic

diversity2. 2n n (diploid

haploid)3. Happens in gamete

producing cells (GONADS: testes and ovaries)

4. Two cell divisions5. Sexual

= Sexual Reproduction

Diploid Cell – full set of chromosomes

Same as Mitosis2 daughter cells are identical to parent cell.Chromosomes do NOT duplicate here. Each cell divides- chromosomes separating randomly!

The Result: 4 daughter cells with ½ the number of chromosomes needed for an individual.

Haploid Cells –½ set of chromosomesAlso called GametesEx. Sperm, egg, pollen

Eggs are created

Sperm are created Resulting Cell when 2

gametes fuse = Fertilization

Mitosis vs. MeiosisMitosis Meiosis

Function

Type of Reproduction

# of Cells Produced

Type of Cell Produced

# of Cellular Divisions

# of Chromosomes in Daughter cells

Mitosis vs. MeiosisMitosis Meiosis

Function Create new cells for growth & replace

old cells

Create gametes for sexual

reproduction

Type of Reproduction Asexual Sexual

# of Cells Produced 2 4Type of Cell Produced Diploid Haploid

# of Cellular Divisions 1 2

# of Chromosomes in Daughter cells

Same as parent cell ½ as parent cell

somatic cell - undergoes mitosis, non-sex cell, diploid, identical with the parent

germ cell - undergoes meiosis, sex-cell, haploid, non-identical with the parent, brings variety of genetic information

A germ cell is a reproductive cell, like an ovum, pollen, sperm, or spore. It is made in special tissues and contains the haploid chromosome number, which, in humans, is 23.

"Somatic cells" includes all the other cells; somatic cells are "body cells" (SOMA=body). They are diploid; human somatic cells have 46 chromosomes