chapter 13 meiosis and sexual life cycles. question? u does like really beget like? u the offspring...
TRANSCRIPT
Question?
Does Like really beget Like? The offspring will “resemble”
the parents, but they may not be “exactly” like them.
Heredity
The transmission of traits from parents to offspring.
Comment - Humans have been aware of heredity for thousands of years.
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.
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
Result
Chromosome number would double each generation.
Need a method to reduce the chromosome number.
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 usually 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.
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.
Metaphase I
Tetrads or bivalents align on the metaphase plate.
Centromeres of homologous pairs point toward opposite poles.
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 - 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:
223 or 8,388,608
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?
Importance
Very common during meiosis.
Frequency can be used to map the position of genes on chromosomes.
Comments
Offspring can never be 100% like a parent if sexual reproduction is used.
Multiple cross-overs are common, especially on large chromosomes
Summary
Know how the chromosomes separate during Meiosis.
Know how Meiosis differs from Mitosis.
Know how sexual reproduction increases genetic variation.