Chapter 13: Meiosis and Sexual Life Cycles

Resourses

PBS Advantage of sexual reproductionhttp://www.pbs.org/wgbh/evolution/sex/advantage/

meiosis tutorial (U of Arizona)http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/main.html

Meiosis tutorial 2http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookmeiosis.html

meiosis tuorial 3http://www.meiosistutorial.net

Can you pick out the kids for each couple?

Heredity and Genetics Heredity

The transmission of traits from parents to offspring. Gene: The DNA for a trait. Locus - the physical location of a gene in a chromosome

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.

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

Asexual Reproduction 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.

Sexual Reproduction 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?

Life Cycle - if Mitosis

Female 46 Male 46

egg 46 sperm 46

Zygote 92

Mitosis

mitosis mitosis

Life Cycle - if Meiosis

Female 46 Male 46

egg 23 sperm 23

Zygote 46

Meiosis

mitosis mitosis

Chromosome number will remain the same with each sexual reproduction event.

Chromosome Number Is usually constant for a species. Examples:

Humans - 46 Corn - 20 Onions - 16 Dogs - 72

Sexual Reproduction Meiosis - Purpose

To produce haploid gametes or sex cells. 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.

Chromosome Number 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 Karyotype

Chromosome Number 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 usually fatal in animals.

Interphase

Prophase I

Prophase I Basic steps same as in prophase of Mitosis. But also, synapsis occurs as the chromosomes

condense. Synapsis - homologous chromosomes form bivalents

or tetrads. Crossing over occur (the exchange of sister

chromatid material during synapsis) may occur only during this phase.

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

Longest phase of division.

Metaphase I

Metaphase I Tetrads or bivalents align on the metaphase

plate. Centromeres of homologous pairs point

toward opposite poles.

Anaphase I

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

the centromeres. Maternal and Paternal chromosomes are now

separated randomly. (Independent Assortment) The chance to inherit a single chromosome

(maternal or paternal) of each pair is 1/2.

Telophase I

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 II

Meiosis - Results 4 cells produced. Chromosome number halved. Gametes or sex cells made. Genetic variation increased. How?1. Independent Assortment of Chromosomes

during Meiosis.2. Random Fertilization.3. Crossing Over.

1. Independent Assortment Gamete Possibilities

With 23 pairs of chromosomes, the number of combinations of chromosome types (paternal and maternal) are:

223 or 8,388,608

2. Random Fertilization

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

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: two offspring from the same human

parents only resemble each other (except identical twins).

3. Crossing-over Very common during meiosis. In fact, even multiple cross-overs are common,

especially on large chromosomes. Breaks old linkage groups. Creates new linkage groups increases genetic

variation. Frequency can be used to map the position of genes

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

Crossing-over

Summary Know how the chromosomes separate during

Meiosis. Know how Meiosis differs from Mitosis. Know how sexual reproduction increases genetic

variation. Chapter 46 will examine the differences in Meiosis

between human males and females. AP Lab 3 http://www.phschool.com/science/biology_place/lab

bench/lab3/intro.html