Meiosis and Sexual Life Cycles

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

Starter Questions? Reproduction is a characteristic of Life Does Like really beget Like? 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.

Variation Is demonstrated by the differences in appearance that offspring show from parents and siblings. Offspring only “resemble” their parents and siblings.

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. “Big Idea” – produces a population of organisms less able to deal with environmental change.

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 sperm 46 Zygote 92 mitosis mitosis Mitosis

Result Chromosome number would double each generation. Need a method to reduce the chromosome number. Learning Check: How many chromosomes does a human male & female have? How many does their offspring have? What is a zygote?

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

Result Chromosome number will remain the same with each sexual reproduction event. Meiosis is used to produce the gametes, (sex cells or spores). Check for understanding: What is the male gamete? Female gamete? (in Animals first then plants).

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

Human Chromosomes Human somatic cells (any cell other than a gamete) have 23 pairs of chromosomes. A karyotype is an ordered display of the pairs of chromosomes from a cell.

Human Chromosomes The two chromosomes in each pair are called homologous chromosomes, or homologs. Chromosomes in a homologous pair are the same length and carry genes controlling the same inherited characters.

Each pair of homologous chromosomes includes one chromosome from each parent. The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father.

A diploid cell (2n) has two sets of chromosomes. For humans, the diploid number is 46 (2n = 46).

Haploid Single set of chromosomes. Number in the gametes or sex cells. Humans N = 23 Corn N = 10 Fruit Flies N = 4

A gamete (sperm or egg) contains a single set of chromosomes, and is haploid (N). For humans, the haploid number is 23 (N = 23). Each set of 23 consists of 22 autosomes and a single sex chromosome.

In an unfertilized egg (ovum), the sex chromosome is X. In a sperm cell, the sex chromosome may be either X or Y.

Polyploids Multiple sets of chromosomes. Examples 3N = triploid 4N = tetraploid Common in plants, but often fatal in animals.

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

Meiosis - Uniqueness Three events are unique to meiosis, and all three occur in meiosis l (detailed descriptions are level 2 items): Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information.

Continued… 2. At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes. 3. At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate.

Meiosis/Mitosis Preview of differences 1st division separates PAIRS of chromosomes, not duplicate chromosomes. 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.

Homologous Pliody (haploid & Diploid) Gamete Somatic cells Karyotype

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.

Homework 1. Make a table of comparison in your copy comparing and contrasting Mitosis to Meiosis. (You should BOTH explain in words and through clear labelled illustrations). Explain The process of crossing over and discuss its importance to life.

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

Chiasmata (level 3 material) 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.

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

End of Class discussion What are the advantages for an organism to reproduce sexually?

Summary Know how the chromosomes separate during Meiosis. Know how Meiosis differs from Mitosis. Know how sexual reproduction increases genetic variation (more on this Thursday).