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 sperm 46

Zygote 92

mitosis mitosis

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 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 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:

2N = 223 = 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.