Meiosis and Crossing Over8.12-8.18

I. Homologous Chromosomes

8.12

1. Somatic Cell- a typical body cell

a) Ex: muscle cells, skin cells

2. Each cell has a set number of chromosomes

3. The number chromosomes in each somatic cell depends on the species.

4. Humans have 46 chromosomes in each somatic cell

5. Homologous Chromosomes (homologs)- four chromosomes (2 sets one set comes from the mom the other set comes from the dad); that have same genes.

6. Locus- the specific location of a gene on a chromosome

7. Autosomes- a chromosome that is not a sex chromosome

a) Humans have 22 pairs or 44 autosomes

8. Sex Chromosomes- determines an individual’s gender.

X X = femaleX Y = male

Scanning electron micrograph of an X and Y chromosome Image courtesy of Indigo Instruments, Canada

X Chromosome

Y Chromosome

Autsosomal DNA

II. Gametes8.13

II. Gametes have a single set of chromosomes

8.13

Sperm & ova (egg)

1. Sexual Reproduction results in individuals having two sets of chromosomes; one set inherited from each parent.

2. Haploid n (half)- one set of chromosomes; 22 autosomes and an X or Y chromosome in the nucleus of the cell

3. Autosomes are in the diploid state

4. Diploid 2n (double)- there are 2 sets of homologous chromosomes in the nucleus of the cell

5. n= number of chromosomes

Discuss w/ Partner: Which set of

chromosomes is in the haploid state, set A or set B?How do you know?

Answer: Set A because there

is only one set of chromosomes. Set B is in the diploid state because it has two sets of chromosomes.

6. Gametes a.k.a sex cells: egg cells & sperm cells have a single set of chromosomes.

An Egg coming out it’s ovary were it is stored

Human sperm cells

Egg

Ovary

Discuss with Partner:What is 2n in

humans?Hint read page 139

Answer:In humans 2n= 46

7. In humans 2n= 468. N= 239. Fertilization-

happens when a haploid sperm cell from the father fuses with a haploid egg (ovum) inside the mother.

A human fertilized eggSource image:

www.wellcome.ac.uk/

Sperm fertilizing and eggSource image: sciencephoto.com

10.Zygote- A fertilized egg which is diploid (2n), has a set of c’somes from mom and one set from dad

A human zygote in the 8 cell stageSource image www.wellcome.ac.uk/

A four day old human zygote Source: www.wellcome.ac.uk/

11.All sexual life cycles involved alternation of generations; alterations of diploid & haploid stages

12.Haploid gametes prevents the c’some number from doubling in each generation.

13.Somatic cells are made by mitosis14.Gametes are made by meiosis which

only occurs in the ovaries & testes in animals

The Human Life Cycle

15.Meiosis- sperm and egg cells that are haploid are produced inside of an organism that is diploid.has two main divisions:

a) Meiosis I- homologous c’somes separate

b) Meiosis II- sister chromatids separate

III. Meiosis

8.14

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1. Meiosis I: Homologous Chromosomes Separate

Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis

Interphase I

Meiosis I

2. During meiosis the chromosome number is halved.

2. Interphase 1 Cells undergo a

round of DNA replication, forming duplicate chromosomes.

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3. Prophase IEach chromosome

pairs with its corresponding homologous chromosome to form a tetrad.

There are 4 chromatids in a tetrad.

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When homologous chromosomes form tetrads in prophase I, they exchange portions of their chromatids in a process called crossing-over.

Crossing-over produces new combinations of alleles.

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4. Metaphase I-Spindle fibers attach

to the chromosomes.

5. Anaphase I- The fibers pull the

homologous chromosomes toward opposite ends of the cell.

6. Telophase INuclear membranes form.

7. Cytokinesis The cell separates into

two cells.The two cells produced by

meiosis I have chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I.

8. Meiosis IIMeiosis I results in two

haploid (N) daughter cells, each with half the number of chromosomes as the original cell.

The two cells produced by meiosis I now enter a second meiotic division.

Unlike meiosis I, neither cell goes through chromosome replication.

Each of the cell’s chromosomes has 2 chromatids.

9. Prophase II- The centrioles duplicate. The two pairs of

centrioles separate into two centrosomes.The nuclear envelope breaks down, and the

spindle apparatus forms.

10.Metaphase IIThe chromosomes

line up in the center of cell.

Metaphase II in pollen

11.Anaphase IIThe sister chromatids

separate and move toward opposite ends of the cell.

12.Telophase IIA nuclear envelope forms

around each set of chromosomes

13.Cytokinesis II produces four daughter

cells (gametes, in animals), each with a haploid set of chromosomes.

14.Meiosis II results in four haploid (N) daughter cells.

15.Because of crossing-over, all four daughter cells or gametes are genetically different from each other and their parent cell

Telophase II

Cytokinesis

IV. Comparing Mitosis and Meiosis

8.15

Mitosis MeiosisCells produced by

mitosis have the same number of chromosomes and alleles as the original cell.

Mitosis allows an organism to grow and replace cells.

Some organisms reproduce asexually by mitosis.

Cells produced by meiosis have half the number of chromosomes as the parent cell.

These cells are genetically different from the diploid cell and from each other.

Meiosis is how sexually reproducing organisms produce gametes.

V. Crossing Over8.18

1. Crossing over- The exchange of parts of the chromatids of homologous chromosomes.

2. Crossing over takes place on the chiasma which is an x-shaped area of the c’some

3. Chiasma- the place were two homologous non-sister chromatids are attached to each other.

4. Crossing over leads to genetic recombination and variability in gametes

5. Genetic Recombination- the production of gene combinations different from those original chromosomes