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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 = female X Y = male Scanning electron micrograph of an X and Y chromosome Image courtesy of Indigo Instruments, Canada X Chromosome Y Chromosome

Autsosomal DNA

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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= N= Fertilization- happens when a haploid sperm cell from the father fuses with a haploid egg (ovum) inside the mother. A human fertilized egg Source image: / Sperm fertilizing and egg Source 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 stage Source image A four day old human zygote Source:

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 mitosis 14. 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

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1. Meiosis I: Homologous Chromosomes Separate Copyright Pearson Prentice Hall 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.

3. Prophase I Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. There are 4 chromatids in a tetrad. Copyright Pearson Prentice Hall

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. Copyright Pearson Prentice Hall

4. Metaphase I- Spindle fibers attach to the chromosomes. Copyright Pearson Prentice Hall

5. Anaphase I- The fibers pull the homologous chromosomes toward opposite ends of the cell.

6. Telophase I Nuclear 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 II Meiosis 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 II The chromosomes line up in the center of cell. Metaphase II in pollen

11. Anaphase II The sister chromatids separate and move toward opposite ends of the cell.

12. Telophase II A 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

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Mitosis Meiosis Cells 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.

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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