Chapter 13 Meiosis and Sexual Life Cycles
Chapter 13 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.
Life Cycle Variations
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 (sister chromatids).
• 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.
Prophase I
• Chiasmata observed.
• Longest phase of division.
Metaphase I
• Tetrads or bivalents 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.
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 - 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:
2N = 223 = 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
70 trillion 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 non-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
Comment
• Genes near the centromere do not cross-over very often.
Summary
• Know how the chromosomes separate during Meiosis.
• Know how Meiosis differs from Mitosis.
• Know how sexual reproduction increases genetic variation.