Chapter 13 Meiosis and Sexual Life Cycles

Chapter 13 Meiosis and Sexual Life Cycles

Question? Does Like really beget Like? The offspring will resemble the parents, but they may not be exactly like them. This chapter deals with reproduction of life.

Heredity The transmission of traits from parents to offspring. 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.

Genes The DNA for a trait. Locus - the physical location of a gene in a chromosome.

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

Advantages 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.

Advantages 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?

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

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

Result Chromosome number would double each generation. Need a method to reduce the chromosome number.

Life Cycle - if Meiosis Female 46 Male 46 egg 23 Meiosis sperm 23 mitosis Zygote 46 mitosis

Result Chromosome number will remain the same with each sexual reproduction event. Meiosis is used to produce the gametes or sex cells.

Meiosis - Purpose To reduce the number of chromosomes by half. 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.

Ploidy 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

Haploid 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 often fatal in animals.

Life Cycle Variations

Meiosis/Mitosis Preview of differences Two cell divisions, not one. Four cells produced, not two. Synapsis and Chiasmata will be observed in Meiosis

Meiosis/Mitosis Preview of differences 1st division separates PAIRS of chromosomes, not duplicate chromosomes (sister chromatids). Interkinesis is present.

Meiosis Has two cell divisions. Steps follow the names for mitosis, but a I or II will be added to label the phase.

Prophase I Basic steps same as in prophase of Mitosis. Synapsis occurs as the chromosomes condense. Synapsis - homologous chromosomes form bivalents or tetrads.

Prophase I Chiasmata observed. Longest phase of division.

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

Anaphase I Homologous PAIRS separate. Duplicate chromosomes are still attached at the centromeres.

Anaphase I possibilities

Anaphase I Maternal and Paternal chromosomes are now separated randomly.

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 - Results 4 cells produced. Chromosome number halved. Gametes or sex cells made. Genetic variation increased.

Sexual Sources of Genetic Variation 1. Independent Assortment of Chromosomes during Meiosis. 2. Random Fertilization. 3. Crossing Over.

Independent Assortment There are 23 pairs of chromosomes in humans. The chance to inherit a single chromosome (maternal or paternal) of each pair is 1/2.

Gamete Possibilities With 23 pairs of chromosomes, the number of combinations of chromosome types (paternal and maternal) are: 2 N = 2 23 = 8,388,608

Random Fertilization The choice of which sperm fuses with which egg is random.

Random Fertilization Therefore, with 8,388,608 kinds of sperms and 8,388,608 kinds of eggs, the number of possible combinations of offspring is over 70 trillion kinds.

Result Is it any wonder that two offspring from the same human parents only resemble each other and are not identical twins?

Crossing-Over The exchange of non-sister chromatid material during synapsis. Occurs ONLY in Prophase I.

Chiasmata The point of contact where two chromosomes are crossing-over.

Importance Breaks old linkage groups. Creates new linkage groups increases genetic variation.

Importance Very common during meiosis. Frequency can be used to map the position of genes on chromosomes.

Comments With crossing over, offspring can never be 100% like a parent if sexual reproduction is used. Multiple cross-overs are common, especially on large chromosomes

Comment Genes near the centromere do not cross-over very often.

Summary Know how the chromosomes separate during Meiosis. Know how Meiosis differs from Mitosis. Know how sexual reproduction increases genetic variation.