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.

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.

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

Longest phase of division.

Metaphase I

Tetrads 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.

All carry one allele for each gene.

Telophase I

Similar to Mitosis. Chromosomes may or may

not unwind to chromatin. Cytokinesis separates

cytoplasm and 2 cells are formed.

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.

End of Part 1!!

Sexual Sources of Genetic Variation

1. Independent Assortment of Chromosomes during Meiosis.

2. Random Fertilization.

3. Crossing Over.

Independent Assortment

Each chromosome is sorted independently of the others.

It is random which one ends up at each pole.

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:

223 or 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 64 million 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 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

Comments

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

Nondisjunction

Members of the chromosomal pair fail to separate

End up with the wrong number of chromosomes

Can happen in I or II

Alterations of chromosomes

Deletion Duplication Inversion Translocation

Summary

Know how the chromosomes separate during Meiosis.

Know how Meiosis differs from Mitosis.

Know how sexual reproduction increases genetic variation.

Sordaria

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