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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