Meiosis and Sexual Life Cycles

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

Heredity u The transmission of traits from parents to offspring. u Comment - Humans have been aware of heredity for thousands of years.

Genetics u The scientific study of heredity. u Comment - Genetics is only about 150 years old.

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

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

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

Asexual Bud

Advantages u Only need 1 parent. u Offspring are identical to the parent. u Good genetic traits are conserved and reproduced.

Disadvantages u No new DNA combinations for evolution to work on. u Clones may become extinct if attacked by a disease or pest.

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

Advantages u Offspring has a unique combination of DNA which may be an improvement over both parents. u New combination of DNA for evolution to work with.

Disadvantages u Need two parents. u Good gene combinations can be lost. u Offspring may not be an improvement over the parents.

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

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

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

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

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

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

Meiosis - Purpose u To reduce the number of chromosomes by half. u Prevents doubling of chromosome numbers during sexual reproduction.

Sexual Life Cycle u Has alternation of meiosis and fertilization to keep the chromosome numbers constant for a species.

Ploidy u Number of chromosomes in a "set" for an organism. u Or, how many different kinds of chromosomes the species has. u Usually shown as N = …… u Humans N = 23

Diploid u 2 sets of chromosomes. u Most common number in body or somatic cells. u Humans 2N = 46 u Corn 2N = 20 u Fruit Flies 2N = 8

Haploid u 1 set of chromosomes. u Number in the gametes or sex cells. u Humans N = 23 u Corn N = 10 u Fruit Flies N = 4

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

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

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

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

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

Prophase I u Longest phase of division.

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

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

Anaphase I possibilities

Anaphase I u Maternal and Paternal chromosomes are now separated randomly. u All carry one allele for each gene.

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

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

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

u End of Part 1!!

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

Independent Assortment u Each chromosome is sorted independently of the others. u It is random which one ends up at each pole.

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

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

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

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

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

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

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

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

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

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

Nondisjunction u Members of the chromosomal pair fail to separate u End up with the wrong number of chromosomes u Can happen in I or II

Alterations of chromosomes u Deletion u Duplication u Inversion u Translocation

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

Sordaria