Meiosis and Sexual Reproduction

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Chapter 10

• Meiosis and Sexual Reproduction

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• Chromosome Numbers
• 1. All sexually reproducing organisms have pairs
  of chromosomes.
• 2. Homologous Chromosomes are pairs (2) of
  chromosomes that are similar in size, shape, and
  genetic content
• Each homologous chromosome comes from one of
  the two parents.
• Example The 46 chromosomes in human
  somatic cells (body cells) are actually 2 sets
  of 23 chromosomes1 set from the mother - -1
  set from the father.

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• 3. Diploid (2n) When a cell, such a somatic
  cell, contains sets of chromosomes, it is said to
  be diploid.
• The diploid number in a somatic cell can be
  written as 2n46.
• Biologists use the symbol 2n to represent 2
  sets of chromosomes.
• How many homologous pairs of chromosomes do
  humans have?____
• 4. Haploid (n or 1n) When a cell, such as a sex
  cell (gamete), contains one set of chromosomes,
  it is said to be haploid.
• n 1 set of chromosomes
• The haploid number in a gamete23
• n 23 (half of 46)

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Meiosis

• Organism Haploid Chromosome
• Mosquito 3
• Corn 10
• Human 23
• Horse 32
• Calculate the diploid chromosome number for each
  organism .....

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Chromosomes

• Fertilization is the joining of gametes (sex
  cells) from opposite sexes. A zygote is a
  fertilized egg cell, the first cell of a new
  individual.

Fertilization
Zygote
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• Chromosomes Determine Your Sex
• Of the 23 pairs of chromosomes in human somatic
  cells, 22 pairs are called autosomes.
• Autosomes are chromosomes that are not directly
  involved in determining the sex (gender) of an
  individual.
• The sex chromosomes, one of the 23 pairs of
  chromosomes in humans, contain genes that
  determine the sex of an individual.
• In humans the two sex chromosomes are X and Y
  chromosomes.
• XY chromosomes male
• XX chromosomes female
• Because a female can donate
• only an X chromosome to her
• offspring, the sex of an offspring
• is determined by the male, who
• can donate either an X or a Y.

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Meiosis Forms Haploid Cells

• Meiosis form of cell division that halves the
  number of each chromosome when forming
  specialized reproductive cells (gametes or
  spores)
• Involves two divisions of the nucleus ? Meiosis I
  and Meiosis II

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Meiosis I

• Prophase I Homologous chromosomes pair all
  along their length and then cross-over.
  Crossing-over occurs when parts of a chromatid on
  one homologous chromosome are broken and
  exchanged with the corresponding parts on one of
  the chromatids of the other h.chromosome.

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• Metaphase I the pairs of h.chromosomes line up
  along the equator as in mitosis
• Anaphase I H.chromosomes separate like mitosis.
  Chromatids do not separate at the centromereeach
  chromosome is still composed of 2 chromatids!
• Telophase I Individual chromosomes gather at
  each pole forming two new cells.
• The chromosomes DO NOT replicate between Meiosis
  I and Meiosis II !

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Meiosis II

• Prophase II A new spindle forms around each
  single chromosome
• Metaphase II Once again, chromosomes line up
  along the equator
• Anaphase II The centromeres divide, and the
  chromatids move to the opposite sides of the
  cell.
• Telophase II A nuclear envelope forms around
  each set of chromosomes. Result is 4 haploid
  cells.

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Meiosis Contributes to Genetic Variation

• 3 mechanisms make key contributions to this
  genetic variation independent assortment,
  crossing-over, and random fertilization.
• Chromosome pairs separate independently
• Law of Independent Assortment states that the
  alleles of different genes separate independently
  of one another during meiosis. Meaning that
  different traits are inherited separately. (Ex.
  Human n 23. 223 8 million possibilities)

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Independent Assortment

Metaphase of Meiosis I
Metaphase of Meiosis II
Gametes with different possible combos
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Crossing-over

• The DNA exchange that occurs during crossing-over
  adds even more recombination (variety) to the
  independent assortment of chromosomes that occurs
  later in meiosis.
• There are many genes on a single chromosome each
  with its own locus, or place, on the chromosome.
  Two genes on the same chromosome may be close
  together of far apart.
• Genes located close together tend to be inherited
  together, which is called genetic linkage.
• This is essential to evolution

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Genetic Linkage

• A linkage map shows the relative locations of
  genes on a chromosome (not the actual location).
  The percentage of cross-overs has been converted
  into a unit of measurement called map units.

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Random Fertilization

• Random fertilization- which sperm wins the race
  to the egg?

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Male and Female Gamete Formation involves Meiosis

• Spermatogenesis process by which sperm are
  produced (original single cell called germ cell
  final product is four undifferentiated sperm
  cells.)
• Oogenesis process by which gametes are produced
  in females (original cell called a germ cell
  final product is a single egg cell or ovum. The
  other 3 polar bodies will be reabsorbed)

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Types of Reproduction

• Reproduction, the process of producing offspring,
  can be asexual or sexual.
• Asexual reproduction single parent passes
  copies of all its genes to each of its offspring
  no fusion of haploid cell. New individual is a
  clone (genetically identical)
• Sexual reproduction two parents each form
  haploid reproductive cells, which join to form
  offspring. Offspring has traits of both parents,
  but are not exactly like each parent.

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Types of Asexual Reproduction

• Fission separation of a parent into two or more
  individuals of about equal size (exs. Amoebas,
  bacteria- binary fission)
• Fragmentation reproduction in which the body
  breaks into several pieces.
• Budding new individuals split off from existing
  ones (ex. Hydra)

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Advantages and Disadvantages

• Asexual is the simplest and most primitive
  method.
• Advantage In a stable environment, it allows
  organisms to produce many offspring in a short
  period of time w/o using energy to produce
  gametes or find a mate.
• Disadvantage DNA does not vary which could cause
  adaption problems if the environment changes.

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Advantages and Disadvantages

• Sexual reproduction thought to have evolved as
  a way to originally repair damaged DNA
• Advantages powerful means of quickly making
  different combinations of genes among
  individuals.
• Disadvantages takes up energy in finding mates