Chapter 13 Meiosis and Sexual Life Cycles

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Chapter 13 Meiosis and Sexual Life Cycles
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Question?

• Reproduction is a characteristic of Life
• Does Like really beget Like?
• 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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Variation

• Is demonstrated by the differences in appearance
  that offspring show from parents and siblings.
• Offspring only resemble their parents and
  siblings.

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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 hasnt 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, sex cells
  or spores.

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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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Human Chromosomes

• Human somatic cells (any cell other than a
  gamete) have 23 pairs of chromosomes.
• A karyotype is an ordered display of the pairs of
  chromosomes from a cell.

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Human Chromosomes

• The two chromosomes in each pair are called
  homologous chromosomes, or homologs.
• Chromosomes in a homologous pair are the same
  length and carry genes controlling the same
  inherited characters.

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• Each pair of homologous chromosomes includes one
  chromosome from each parent.
• The 46 chromosomes in a human somatic cell are
  two sets of 23 one from the mother and one from
  the father.

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• A diploid cell (2n) has two sets of chromosomes.
• For humans, the diploid number is 46 (2n 46).

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Haploid

• Single set of chromosomes.
• Number in the gametes or sex cells.
• Humans N 23
• Corn N 10
• Fruit Flies N 4

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• A gamete (sperm or egg) contains a single set of
  chromosomes, and is haploid (N).
• For humans, the haploid number is 23 (N 23).
• Each set of 23 consists of 22 autosomes and a
  single sex chromosome.

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• In an unfertilized egg (ovum), the sex chromosome
  is X.
• In a sperm cell, the sex chromosome may be either
  X or Y.

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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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Life Cycle Variations
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Life cycle variation

• Plants and some algae exhibit an alternation of
  generations.
• This life cycle includes both a diploid and
  haploid multicellular stage.
• The diploid organism, called the sporophyte,
  makes haploid spores by meiosis.

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Plants

• Each spore grows by mitosis into a haploid
  organism called a gametophyte.
• A gametophyte makes haploid gametes by mitosis.
• Fertilization of gametes results in a diploid
  sporophyte.

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Another variation

• In most fungi and some protists, the only diploid
  stage is the single-celled zygote there is no
  multicellular diploid stage.
• The zygote produces haploid cells by meiosis.

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Fungi

• Each haploid cell grows by mitosis into a haploid
  multicellular organism.
• The haploid adult produces gametes by mitosis.

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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 - Uniqueness

• Three events are unique to meiosis, and all three
  occur in meiosis l
• Synapsis and crossing over in prophase I
  Homologous chromosomes physically connect and
  exchange genetic information.

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Continued

• 2. At the metaphase plate, there are paired
  homologous chromosomes (tetrads), instead of
  individual replicated chromosomes.
• 3. At anaphase I, it is homologous chromosomes,
  instead of sister chromatids, that separate.

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Meiosis/Mitosis Preview of differences

• 1st division separates PAIRS of chromosomes, not
  duplicate chromosomes.
• Interkinesis is present.

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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

• Chiasmata observed.
• Longest phase of division.

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Metaphase I

• Tetrads or bivalents 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.

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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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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.

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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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Sexual Sources of Genetic Variation

• 1. Independent Assortment of
  Chromosomes during Meiosis.
• 2. Random Fertilization.
• 3. Crossing Over.

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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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Summary

• Know how the chromosomes separate during Meiosis.
• Know how Meiosis differs from Mitosis.
• Know how sexual reproduction increases genetic
  variation.