Chapter 13: Meiosis and Sexual Life Cycles

Resourses PBS Advantage of sexual reproduction http://www.pbs.org/wgbh/evolution/sex/advantage/ meiosis tutorial (U of Arizona) http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/main.html Meiosis tutorial 2 http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookmeiosis.html meiosis tuorial 3 http://www.meiosistutorial.net

Can you pick out the kids for each couple?

Heredity and Genetics Heredity The transmission of traits from parents to offspring. Gene: The DNA for a trait. Locus - the physical location of a gene in a chromosome Comment - Humans have been aware of heredity for thousands of years. Genetics The scientific study of heredity. Comment - Genetics is only about 150 years old.

Reproduction A method of copying genes to pass them on to offspring. Two main types: Asexual reproduction Sexual reproduction

Asexual Reproduction Parent passes all of its genes to its offspring. Uses mitosis/Also known as cloning. Comment - many organisms reproduce this way. Asexual Bud

Asexual Reproduction Advantages Disadvantages Only need 1 parent. Offspring are identical to the parent. Good genetic traits are conserved and reproduced. Disadvantages No new DNA combinations for evolution to work on. Clones may become extinct if attacked by a disease or pest.

Sexual Reproduction Two parents contribute DNA to an offspring. Comment - most organisms reproduce this way, but it hasn’t been proven in some fungi and a few others.

Sexual Reproduction Advantages Disadvantages Offspring has a unique combination of DNA which may be an improvement over both parents. New combination of DNA for evolution to work with. Disadvantages Need two parents. Good gene combinations can be lost. Offspring may not be an improvement over the parents.

Question ? Do parents give their whole DNA copy to each offspring? What would happen to chromosome number if they did?

Life Cycle - if Mitosis Mitosis Female 46 Male 46 egg 46 sperm 46 Zygote 92 Mitosis mitosis mitosis

Life Cycle - if Meiosis Meiosis Female 46 Male 46 egg 23 sperm 23 Zygote 46 Meiosis Chromosome number will remain the same with each sexual reproduction event. mitosis mitosis

Chromosome Number Is usually constant for a species. Examples: Humans - 46 Corn - 20 Onions - 16 Dogs - 72

Sexual Reproduction Meiosis - Purpose Sexual Life Cycle To produce haploid gametes or sex cells. Prevents doubling of chromosome numbers during sexual reproduction. Sexual Life Cycle Has alternation of meiosis and fertilization to keep the chromosome numbers constant for a species.

Life Cycle Variations

Chromosome Number Ploidy Diploid Number of chromosomes in a "set" for an organism, or how many different kinds of chromosomes the species has. Usually shown as N = …… Humans N = 23 Diploid 2 sets of chromosomes. Most common number in body or somatic cells. Humans 2N = 46 Corn 2N = 20 Fruit Flies 2N = 8

Human Karyotype

Chromosome Number Haploid Polyploids 1 set of chromosomes. Number in the gametes or sex cells. Humans N = 23 Corn N = 10 Fruit Flies N = 4 Polyploids Multiple sets of chromosomes. Examples 3N = triploid 4N = tetraploid Common in plants, but usually fatal in animals.

Interphase

Prophase I

Prophase I Basic steps same as in prophase of Mitosis. But also, synapsis occurs as the chromosomes condense. Synapsis - homologous chromosomes form bivalents or tetrads. Crossing over occur (the exchange of sister chromatid material during synapsis) may occur only during this phase. The point of contact where two chromosomes are crossing-over is called Chiasmata. Longest phase of division.

Metaphase I

Metaphase I Tetrads or bivalents align on the metaphase plate. Centromeres of homologous pairs point toward opposite poles.

Anaphase I

Anaphase I Homologous PAIRS separate. Duplicate chromosomes are still attached at the centromeres. Maternal and Paternal chromosomes are now separated randomly. (Independent Assortment) The chance to inherit a single chromosome (maternal or paternal) of each pair is 1/2.

Telophase I

Telophase I Similar to Mitosis. Chromosomes may or may not unwind to chromatin. Cytokinesis separates cytoplasm and 2 cells are formed.

Interkinesis No DNA synthesis occurs. May last for years, or the cell may go immediately into Meiosis II. May appear similar to Interphase of Mitosis.

Meiosis II Steps are the same as in Mitosis. Prophase II Metaphase II Anaphase II Telophase II

Meiosis II

Meiosis - Results 4 cells produced. Chromosome number halved. Gametes or sex cells made. Genetic variation increased. How? 1. Independent Assortment of Chromosomes during Meiosis. 2. Random Fertilization. 3. Crossing Over.

1. Independent Assortment Gamete Possibilities With 23 pairs of chromosomes, the number of combinations of chromosome types (paternal and maternal) are: 223 or 8,388,608

2. Random Fertilization The choice of which sperm fuses with which egg is random. Therefore, with 8,388,608 kinds of sperms and 8,388,608 kinds of eggs, the number of possible combinations of offspring is over 64 million kinds. Result: two offspring from the same human parents only resemble each other (except identical twins).

3. Crossing-over Very common during meiosis. In fact, even multiple cross-overs are common, especially on large chromosomes. Breaks old linkage groups. Creates new linkage groups increases genetic variation. Frequency can be used to map the position of genes on chromosomes. Genes near the centromere do not cross-over very often.

Crossing-over

Summary Know how the chromosomes separate during Meiosis. Know how Meiosis differs from Mitosis. Know how sexual reproduction increases genetic variation. Chapter 46 will examine the differences in Meiosis between human males and females. AP Lab 3 http://www.phschool.com/science/biology_place/labbench/lab3/intro.html