meiosis and sexual life cycles. question? u does like really beget like? u the offspring will...
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Meiosis and Sexual Life Cycles
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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.
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Heredity
The transmission of traits from parents to offspring.
Comment - Humans have been aware of heredity for thousands of years.
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Genetics
The scientific study of heredity.
Comment - Genetics is only about 150 years old.
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Genes
The DNA for a trait. Locus - the physical location
of a gene in a chromosome.
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Reproduction
A method of copying genes to pass them on to offspring.
Two main types: Asexual reproduction Sexual reproduction
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Asexual Reproduction
Parent passes all of its genes to its offspring.
Uses mitosis. Also known as cloning. Comment - many organisms
reproduce this way.
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Asexual Bud
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Advantages
Only need 1 parent. Offspring are identical to the
parent. Good genetic traits are
conserved and reproduced.
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Disadvantages
No new DNA combinations for evolution to work on.
Clones may become extinct if attacked by a disease or pest.
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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.
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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.
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Disadvantages
Need two parents. Good gene combinations can
be lost. Offspring may not be an
improvement over the parents.
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Question ?
Do parents give their whole DNA copy to each offspring?
What would happen to chromosome number if they did?
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Chromosome Number
Is usually constant for a species. Examples:
Humans - 46 Corn - 20 Onions - 16 Dogs - 72
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Life Cycle - if Mitosis
Female 46 Male 46
egg 46 sperm 46
Zygote 92 mitosis mitosis
Mitosis
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Result
Chromosome number would double each generation.
Need a method to reduce the chromosome number.
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Life Cycle - if Meiosis
Female 46 Male 46
egg 23 sperm 23
Zygote 46 mitosis mitosis
Meiosis
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Result
Chromosome number will remain the same with each sexual reproduction event.
Meiosis is used to produce the gametes or sex cells.
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Meiosis - Purpose
To reduce the number of chromosomes by half.
Prevents doubling of chromosome numbers during sexual reproduction.
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Sexual Life Cycle
Has alternation of meiosis and fertilization to keep the chromosome numbers constant for a species.
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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
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Diploid
2 sets of chromosomes. Most common number in
body or somatic cells. Humans 2N = 46 Corn 2N = 20 Fruit Flies 2N = 8
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Haploid
1 set of chromosomes. Number in the gametes or
sex cells. Humans N = 23 Corn N = 10 Fruit Flies N = 4
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Polyploids Multiple sets of chromosomes. Examples
3N = triploid 4N = tetraploid
Common in plants, but often fatal in animals.
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Meiosis/Mitosis Preview of differences
Two cell divisions, not one. Four cells produced, not two. Synapsis and Chiasmata will
be observed in Meiosis
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Meiosis/Mitosis Preview of differences
1st division separates PAIRS of chromosomes, not duplicate chromosomes.
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Meiosis
Has two cell divisions. Steps follow the names for mitosis, but a “I” or “II” will be added to label the phase.
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Prophase I Basic steps same as in
prophase of Mitosis. Synapsis occurs as the
chromosomes condense. Synapsis - homologous
chromosomes form bivalents or tetrads.
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Prophase I
Longest phase of division.
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Metaphase I
Tetrads align on the metaphase plate.
Centromeres of homologous pairs point toward opposite poles.
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Anaphase I
Homologous PAIRS separate. Duplicate chromosomes are
still attached at the centromeres.
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Anaphase I possibilities
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Anaphase I
Maternal and Paternal chromosomes are now separated randomly.
All carry one allele for each gene.
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Telophase I
Similar to Mitosis. Chromosomes may or may
not unwind to chromatin. Cytokinesis separates
cytoplasm and 2 cells are formed.
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Meiosis II
Steps are the same as in Mitosis. Prophase II Metaphase II Anaphase II Telophase II
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Meiosis - Results
4 cells produced. Chromosome number halved. Gametes or sex cells made. Genetic variation increased.
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End of Part 1!!
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Sexual Sources of Genetic Variation
1. Independent Assortment of Chromosomes during Meiosis.
2. Random Fertilization.
3. Crossing Over.
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Independent Assortment
Each chromosome is sorted independently of the others.
It is random which one ends up at each pole.
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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.
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Gamete Possibilities
With 23 pairs of chromosomes, the number of combinations of chromosome types (paternal and maternal) are:
223 or 8,388,608
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Random Fertilization The choice of which sperm
fuses with which egg is random.
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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.
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Result
Is it any wonder that two offspring from the same human parents only resemble each other and are not identical twins?
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Crossing-Over
The exchange of sister chromatid material during synapsis.
Occurs ONLY in Prophase I.
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Chiasmata
The point of contact where two chromosomes are crossing-over.
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Importance
Breaks old linkage groups. Creates new linkage groups
increases genetic variation.
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Importance
Very common during meiosis.
Frequency can be used to map the position of genes on chromosomes.
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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
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Comments
Genes near the centromere do not cross-over very often.
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Nondisjunction
Members of the chromosomal pair fail to separate
End up with the wrong number of chromosomes
Can happen in I or II
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Alterations of chromosomes
Deletion Duplication Inversion Translocation
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Summary
Know how the chromosomes separate during Meiosis.
Know how Meiosis differs from Mitosis.
Know how sexual reproduction increases genetic variation.
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Sordaria
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Sordaria
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Sordaria
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Sordaria
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Sordaria
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Sordaria
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Sordaria