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.
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