10.2 – Meiosis Sexual Reproduction Function: creates gametes (sperm, egg, pollen) for sexual reproduction.
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10.2 – MeiosisSexual Reproduction
Function: creates gametes (sperm, egg,
pollen) for sexual reproduction.
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. Diploid
1. 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
Diploid
1. 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
Haploid
1. Sex cells are called GAMETES (sperm and egg)a) Gametes join to form a ZYGOTE
i. sperm + egg = zygote
2. Gametes are haploid (n)a)Sperm = nb)Egg = n
3. n + n = 2n
Meiosis
1. 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
Meiosis
1. 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
Prophase I
1. 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 Meiosis
Genetic 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!
Metaphase I
1. During Metaphase I:a) Homologous chromosomes are
pulled to the middle of the cell by the spindle
Independent Assortment
1. 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
Anaphase I
1. During Anaphase I:a) Homologous chromosomes are pulled
towards opposite ends of the cell by the spindle
Telophase I
1. During Telophase I:a) Nuclear envelope reformsb) Spindle dissolvesc) Cell stretches and cytoplasm divides
(cytokinesis)
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-forms
2. Meiosis II is the division of the two diploid cells resulting from Meiosis Ia) 4 haploid cells
will be created
Anaphase II
1.During Anaphase II:a)Sister chromatids are
separated and pulled to opposite ends of the cell by the spindle
Telophase II
1.During Telophase IIa)Nuclear membrane re-
formsb)Spindle dissolvesc) Cells stretch and prepare for
cytokinesis
Cytokinesis
1. 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 Diversity
1. 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
Nondisjunction
1. 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
Down Syndrome
1. 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
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 Mitosis
2 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
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
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