Lab objectives Mitosis

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Lab objectives (Mitosis Meiosis)

• Define terms
• Recognize different phases of Mitosis and Meiosis
• Compare the two different processes
• Prepare slides that demonstrate the different
  phases

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Mitosis and Meiosis

• Q) What the heck is a chromosome?
• A chromosome is a long strand of genes
• Humans have 23 pairs (each pair is called a
  homologous pair)
• We ALL get one SET of chromosomes from dad and
  one set from mom

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Chromosomes

• Whats a homologous pair?
• Chromos that code for the same traits
• They code for the same traits but each member of
  the pair has different information (different
  versions of the same genes)

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Chromosomes (n)

• n refers to the number of pairs (or the number of
  different types of chromosomes)
• 2n refers to the total number of chromosomes
• In humans n 23
• In chimpanzees n 24
• In the king crab n 104

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More on Chromosomes

• Humans reproduce sexually
• Sexual reproduction means simply that the
  offspring receive genetic material from BOTH the
  mother and father
• Not all organisms reproduce this way
• For many organisms the offspring receive genetic
  material from only 1 individual

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Meiosis

• Meiosis is the process where the genetic material
  is reduced to half (i.e. only ONE set of
  chromosomes per daughter cell)
• Because Meiosis creates cells with only half the
  normal number of chromosomes, (gametes), when
  these cells join (as in fertilization) the new
  individual has the same number of chromosomes as
  each parent.

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Meiosis

• So in fact not ALL human cells have the 23 pairs
  of chromosomes
• Humans gametes have 1 copy of each chromosome,
  not 23 pairs.
• Gametes are human reproductive cells
• Male gametes are sperm
• Female gametes are eggs

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Meiosis versus Mitosis

• Mitosis is essential for cell growth
• It is the process that distributes the genetic
  evenly material between two daughter cells, so
  that each daughter cell is genetically identical
  to the parent cell.
• Its simply a way to make more of the same cell

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Phases of Mitosis
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Mitosis
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Mitosis

• Interphase
• Prophase
• Metaphase
• Anaphase
• Telophase
• REPEAT
• (This process produces two identical cells. The
  process then repeats for each of the new cells.)

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Mitosis

• Interphase
• DNA replication
• Centrosomes form
• Prophase
• Chromosomes condense become visible as sister
  chromatids (Condensation)
• Centrosomes move away from each other
• Mitotic spindle forms

• Metaphase
• Centrosomes at opposite poles
• Chromosomes line up at metaphase plate
  (centromeres line up)
• Each chromatid is attached to one of the poles
  via microtubules
• Anaphase
• Each of the sister chromatids moves toward the
  opposite pole

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Mitosis

• Telophase
• Formation of cleavage furrow
• Cell divides ? 2 identical copies
• New nuclear envelope forms
• New Interphase begins
• And the process starts all over

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Homologues versus chromatids

• Homologous chromosomes
• One from Mom and One from Dad
• Have different versions of the same genes
• Sister chromatids
• Bound at center by centromere
• Chromatids are identical
• Product of DNA replication

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And nowPhases of Meiosis
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Meiosis I

• Interphase
• Cell spends 90 of its life in this phase (like
  mitosis)
• Chromosome replication takes place ? sister
  chromatids

• Prophase I
• Homologous chromosomes come together (synapsis)
• Form tetrads
• This is when Crossing Over occurs (at chiasmata,
  between NON-SISTER chromatids)
• Centrosomes move away from each other

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Meiosis I

• Metaphase I
• Chromosomes aligned on Metaphase plate (grouped
  in homologous pairs)
• Each chromosome is attached to opposite pole
• Anaphase I
• Sister chromatids remain attached at centromeres
• Chromosomes move to opp. poles

• Telophase I
• Chromosomes reach poles
• Cell starts to cleave
• Produces 2 haploid cells Each cell is NOT
  identical (crossing over)
• Cytokinesis

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Meiosis II

• Prophase II
• Spindle forms
• Chromosomes move toward metaphase plate
• Metaphase II
• Chromosomes line up along metaphase plate

• Anaphase II
• Sister chromatids move toward opposite poles
• Telophase II
• Nuclei form at opposite poles
• Cytokinesis (division of cell)
• Produces 4 haploid cells

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Meiosis I
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Meiosis II
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Haploidy and Diploidy

• A cell that has 1 copy of each chromosome is
  haploid
• Human gametes are haploid (n)
• A cell that has 2 copies of each chromosome are
  diploid
• Human somatic cells (body cells) are diploid (2n)

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The 4 Main differences between Meiosis and Mitosis

• In Meiosis (prophase I) homologous chromosomes
  pair up and crossing over occurs (Neither of
  these happens in Mitosis)
• In Meiosis, homologous pairs align on opposite
  sides of the metaphase plate

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The 4 Main differences between Meiosis and Mitosis

• 2) In Meiosis (Metaphase I) homologous pairs
  align on opposite sides of the metaphase plate

Meiosis (MetaphaseI) Mitosis (Metaphase)
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The 4 Main differences between Meiosis and Mitosis

• 3) In Meiosis (Anaphase I) sister chromatids do
  not separate

Meiosis (AnaphseI) Mitosis (Anaphase)
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The 4 Main differences between Meiosis and Mitosis

• Meiosis I separates homologous pairs of
  chromosomes NOT sister chromatids

Meiosis (AnaphaseI) Mitosis (Anaphase)
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Reproduction

• Asexual reproduction (1 parent gives rise to 1 or
  more clones)
• What are some examples of organisms that
  reproduce asexually?

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Reproduction

• Asexual reproduction (1 parent gives rise to 1 or
  more clones)
• Pros and Cons
• Dont require another individual
• Offspring genetically identical to parent
• No genetic variation

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Reproduction

• Sexual reproduction (2 parents contribute to
  create genetically unique offspring)
• Pros and Cons
• Require another individual
• Creates genetic variation

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Genetic variation

• Why is variation good?
• A genetically homogeneous population can be more
  easily invaded by pathogens and parasites
• Differential reproductive success due to variance
  (population survival wont be all or nothing)
• Cope with new conditions, adapt to new
  environments

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Genetic Variation

• Sources of Variation
• Crossing over (when homologues exchange genetic
  material during prophase)
• Independent assortment (all the possible
  chromosome combinations from one parent)
• The inclusion of one chromosome into the gamete
  doesnt affect the probability of another
• Humans have 23 pairs of chromosomes so a human
  has 223 possible combinations, or about 8 million
  possible gametes (w/o crossing over)

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Independent Assortment

• If humans had only 5 chromosomes, then how many
  possible gametes would there be for one person
  (again w/o considering crossing over)
• 25 or 32 possible combinations
• But this is only chromosomal combinations of
  GAMETES

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Genetic Variation Random Fertilization

• A given person has 8 million possible gamete
  combinations
• But it takes two gametes (one from mom and one
  from dad) to make a new person.
• SoThe number of possible chromosomal
  combinations that are possible to make a new
  person is 8million times 8 million 64 trillion