How Cells Reproduce Chapter 7

DNA DNA serves as a storehouse of information for living things. DNA contains a code to make proteins

DNA Terminology Gene = physical unit of heredity; housed on a chromosome Made of DNA Human genome = 25,000 genes Chromosome = intricately folded unbroken strand of 40% DNA and 60% protein Humans = 23 pairs (each has 1,000-2,000 different genes! (23 is referred to as haploid number) Penicillin fungus = 1 pair Ferns = some > 500 pairs!

Human Chromosomes

The Cell Cycle This cycle keeps a record of the process of a cell over time It is made up of a repeating pattern of growth, genetic duplication and division A complete cycle is from the origin of a new cell until that cell divides. Note: A cell does not need to complete the cycle.

Cell Cycle Mitosis: cell division that occurs in non-reproductive cells (somatic cells) Meiosis: a second process that divides DNA in cells that participate in sexual reproduction (germ cells)

Mitosis Preparation phase of eukaryotic cells for division 5 Phases: G1 S G2 M C Interphase; preparation for cell division Prophase, Metaphase, Anaphase, Telophase

Interphase In general, the longest part of the cell cycle Cell increases in mass DNA is duplicated

Control of the cell cycle Figure 7.10

G1 Checkpoint Key checkpoint where the cell makes the decision of whether to continue with division or not Cell can delay division or enter a resting phase

The G1 Checkpoint Figure 7.11

G2 Checkpoint M Checkpoint Occurs right before mitosis Occurs right before cytokinesis after mitosis has occurred

Meiosis Cell division where gametes are produced Humans reproduce by sexual reproduction which allows for combinations of desirable traits and more variability Sexual reproduction presents a problem – of combining traits were accomplished by just combining two cells life would get incredibly complicated –more DNA is added to each generation

Figure 7.19

Genetic Background of Sexual Reproduction Somatic cells contain two of each homologous chromosome (2N, diploid number) Gametes have one of each of the homologous chromosomes (N, haploid number) Female gametes are the ova and are produced in the ovaries Male gametes are sperm and are produced in the testes The process by which a single diploid cell divides to produce haploid reproductive cells is meiosis

Meiosis Steps Meiosis reduces CH number by duplication of the CH and then two divisions Meiosis like mitosis begins with a diploid cell The DNA (CH) is duplicated

Meiosis Steps: Meiosis I Meiosis I : separation of homologous CH Prophase I CH become visible Homologous CH pair together = tetrad Crossing over occurs Homologous CH exchange recriprocal lengths of DNA This process takes what was called a “maternal” CH not has a portion of the “paternal” CH Allows for increased genetic diversity

Crossing Over Fig. 7.20

Meiosis Steps: Meiosis I Metaphase I Tetrads line up on the equatorial plate Alignment of one pair bears no relationship to the alignment of any other tetrad This random alignment is Independent Assortment Except for identical twins this process ensures that each offspring will be unique Anaphase I Homologous CH separate and move to poles Telophase I Cytokinesis creates two new haploid cells

Meiosis Steps: Meiosis II Stage where sister chromatids separate; events mimic mitosis Prophase II CH become visible Metaphase II CH align at the equatorial plate Anaphase II Sister chromatids separate and move to poles Telophase II Cytokinesis occus A total of four haploid cells each containing chromatin are formed

Prophase I Each duplicated chromosome pairs with homologue Homologues swap segments Each chromosome becomes attached to spindle

Metaphase I Chromosomes are moved to middle of cell Spindle is fully formed

Anaphase I Homologous chromosomes separate Sister chromatids remain attached

Telophase I Chromosomes arrive at opposite poles Usually followed by cytoplasmic division

Prophase II Microtubules attach to duplicated chromosomes

Metaphase II Duplicated chromosomes line up midway between spindle poles

Anaphase II Sister chromatids separate to become independent chromosomes

Telophase II Chromosomes arrive at opposite ends of cell Nuclear envelopes form around chromosome sets Four haploid cells

What is the Significance of Meiosis? The process generates genetic diversity by ensuring that the gametes formed are different from one another Crossing Over Independent Assortment

Genetic Diversity The above indicates that even though the two homologous chromosomes contain the same genetic information, the assortment of the chromosomes (the order they lie in) can determine what genetic information is present in each of the 4 gametes produced. With 23 chromosomes in a human gamete, their are 223 combinations (8388608 combinations)

What happens when Meiosis runs amuck? Genetic abnormalities Aneuploidy A condition in which an organism have more or fewer CH that normally exist in its species full set Responsible for large amount of human miscarriages In humans, live births will only result with one extra or fewer CH (2N +/- 1) Monosomy: 2N – 1 Trisomy: 2N + 1

What happens when Meiosis runs amuck? Aneuploidy The cause of aneuploidy is nondisjunction Homologous CH or sister chromatids fail to separate correctly in meiosis Eggs or sperm will then have an extra CH(s) or will be missing some Human aneuploid conditions: Down’s Syndrome: Trisomy 21 Turners: XO Metafemale: Trisomy X Klinefelter’s: XXY XYY males