Meiosis and genetic variation

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Meiosis and genetic variation

• (still Ch. 8 notes - 8.12-8.23)
• MEIOSIS ONLY OCCURS IN PRODUCTION OF GAMETES!

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Asexual Reproduction

• Many single-celled organisms reproduce by
  splitting, budding, binary fission, etc.
• GENETICALLY IDENTICAL OFFSPRING!
• Some multicellular organisms can reproduce
  asexually, produce clones.

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Sexual reproduction

• Fusion of 2 gametes (fertilization) to produce a
  single zygote.
• Introduces greater genetic variation, allows
  genetic recombination.
• With exception of self-fertilizing organisms
  (e.g. some plants), zygote is a combination of
  gametes from 2 different parents.

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Human Chromosomes

• ALL human body (somatic) cells have 23 pairs of
  homologous chromosomes
• 22 pairs of autosomes
• 1 pair of sex chromosomes
• EXCEPTION GAMETES! (reproductive cells)

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Fig. 11.3, p.171
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Review of Chromosome Numbers

• Gametes (sperm/ova cells)
• Each gamete is haploid (half the full set of
  chromosomes)
• contains 22 INDIVIDUAL autosomes and 1 sex
  chromosome Haploid number in humans n 23
• Fertilization results in zygote with 2 haploid
  sets of chromosomes - now diploid.
• Diploid cell 2n 46. (n23 in humans)
• Most cells in the body produced by mitosis.
• Only gametes are produced by meiosis!

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diploid germ cells in female
diploid germ cells in male
meiosis, gamete formation in both female and male
eggs
sperm
X
Y

X
X

fertilization
X
X
XX
XX
X
Y
XY
XY
sex chromosome combinations possible in the new
individual
Fig. 11.2, p.170
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Chromosome numbers
All are even numbers diploid (2n) sets of
homologous chromosomes!
Ploidy number of copies of each chromosome.
Diploidy
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Chromosomes Combining

• Once fertilization occurs, cells begin dividing
  and reproducing through MITOSIS.
• But how are the HAPLIOD gametes formed?
• THROUGH MEIOSIS!

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Meiosis key differences from mitosis

• Meiosis reduces the number of chromosomes by
  half.
• ALL daughter cells produced are genetically
  different from parent and each other.
• Meiosis involves two divisions, Meiosis I and
  Meiosis II.
• Meiosis I involves
• Synapsis homologous chromosomes pair up.
  Chiasmata form (crossing over of non-sister
  chromatids).
• In Metaphase I, homologous pairs line up at
  metaphase plate.
• In Anaphase I, sister chromatids do NOT separate.
• Overall, separation of homologous pairs of
  chromosomes, rather than sister chromatids of
  individual chromosome.

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Homologues

• Chromosomes exist in homologous pairs in diploid
  cells.

Exception Sex chromosomes (X, Y). Other
chromosomes are known as autosomes, they have
homologues.
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KEY DIFFERENCE! Homologous pairs separate
first
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CROSSING OVER!
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Sister Chromatids separate at centromeres
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Meiosis 1

• First division of meiosis
• Prophase 1 Each chromosome duplicated and
  remains closely associated. These are called
  sister chromatids. Crossing-over can occur during
  the latter part of this stage.
• Metaphase 1 Homologous chromosomes pair up and
  align in the middle.
• Anaphase 1 Homologous pairs separate with sister
  chromatids remaining together.
• Telophase 1 Two daughter cells are formed with
  each daughter containing only one chromosome of
  the homologous pair.

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Meiosis II

• Second division of meiosis (Gamete formation)
• Prophase 2 DNA does not replicate.
• Metaphase 2 Chromosomes align at the middle of
  cell.
• Anaphase 2 Centromeres divide and sister
  chromatids migrate separately to each pole.
• Telophase 2 Cell division is complete. Four
  haploid daughter cells, all genetically
  different, are formed.

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Mitosis vs. Meiosis
Just meiosis!
Comparing Mitosis and meiosis
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(No Transcript)
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Meiosis creates genetic variation

• During normal cell growth, mitosis produces
  daughter cells identical to parent cell (2n to
  2n)
• Meiosis results in genetic variation by shuffling
  of maternal and paternal chromosomes and crossing
  over.
• During sexual reproduction, fusion of the unique
  haploid gametes produces truly unique offspring.

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Independent assortment

• Number of combinations 2n

e.g. 2 chromosomes in haploid 2n 4 n 2 2n
22 4 possible combinations
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Independent assortment
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In humans
e.g. 23 chromosomes in haploid 2n 46 n 23 2n
223 8 million possible combinations!
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Random fertilization

• At least 8 million combinations from Mom, and
  another 8 million from Dad
• gt64 trillion combinations for a diploid zygote!!!

Mitosis after fertilization
Fertilization envelope
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Crossing over
Chiasmata sites of crossing over synapsis-
Exchange of genetic material between non-sister
chromatids. Crossing over produces recombinant
chromosomes.
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Alterations in chromosome number and individual
chromosomes

• Many mutations can occur during mitosis or
  meiosis that will affect the chromosome numbers
  or alter the information on individual
  chromosomes.
• Mutations can be harmful or beneficial to the
  organism
• Types of mutations include
• Nondisjunction
• Deletion
• Duplication
• Inversion
• Translocation

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Trisomy 21- Cause of Down Syndrome
Nondisjunction animation
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(No Transcript)
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• Various animations showing chromosomes alterations

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Review Questions

1.  What happens as homologous chromosomes pair up
   during prophase I of meiosis?
2. How does metaphase of mitosis differ from
   metaphase I of meiosis?
3. What is the sole purpose of meiosis?
4. What specific activities, involving DNA, occur
   during interphase prior to both mitosis and
   meiosis?

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• Compare mitosis and meiosis on the following
  points
• a. number of daughter cells produced.
• b. the amount of DNA in the daughter cells
  in contrast to the original cell.
• c. mechanism for introducing genetic
  variation.
• 6. What is a zygote and how is it formed?

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SHOW ME MEIOSIS!

• Using your phone and your magnetic chromosomes,
  show me ALL of meiosis, beginning with Interphase
  and ending with cytokinesis after meiosis 2.
• Take a picture of the chromosomes at every stage.
  
• Use chalk to draw nuclear envelopes, spindles and
  centrioles. Use labels in photos for phase names.
• When finished, raise your hand for me to check
  your photos for completion.
• IF ITS INCORRECT you need to DELETE photos and
  start AGAIN!

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Onion Root Tip

• Visuals of Cells undergoing mitosis
• Data Table
• of Cell Cycle of cells/total of cells

Phase of Cells on Card of Cell Cycle
Interphase
Prophase
Metaphase
Anaphase
Telophase
TOTALS