Bellringer 12/02

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Bellringer 12/02

• What does AIDS and HIV stand for?
• What are some possible ways to get HIV?
• REMINDERS HW 14 due Friday, Notebook check this
  week, LAB FEE anytime, 15 week progress reports
  next Monday!
• Pass your old bellringer sheets forward!

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Copy this in your notebook

• Name of Exchange 1______________________
• Name of Exchange 2______________________
• Name of Exchange 3______________________
• Testing status Positive or Negative ____________
• Patient 0 the original, first carrier of the
  virus
• Patient 0 Name? _________________________

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Bellringer Wed, 12/03

• What is meiosis?
• True or False A muscle cell performs meiosis
• Fill in the blank
• The two sex cells in humans are
• In Males_________ and In Females__________
• REMINDERS HW 14 due Fri, Notebook check this
  week, 15 wk progress reports Mon, LAB FEE anytime!

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Meiosis
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Meiosis A Source of Original Distinction
Why do you share some but not all characters of
each parent?
What are the rules of this sharing game?
At all levels, the answers lie in
meiosis. Meiosis- for sexual reproduction,
special, two-step cell division only found in sex
cells (female- egg and male- sperm)
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Reminders, Review and Something New

• Chromosomes condensed DNA, they form when cell
  is ready to divide
• 46 in Humans, 23 pairs
• Made up of 2 halves held together in the center
  by the centromere
• Sister chromatids- exact copies of a chromosome
  held together by a centromere (this is what we
  see in the S phase when DNA is copied)
• Chromatin- loose DNA
• Genes- sections of a chromosome responsible for
  one trait
• Genome- A cells complete set of an organisms
  genetic material (chromosomes and DNA)

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Boy or Girl? The Y Chromosome Decides
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Both male and female have 46 chromosomes 23-mater
nal (mother) and 23- paternal (from father) BUT-
when it comes to the sex chromosomes (23) THE
MALE sperm decides Male XY Female XX
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Meiosis performs 2 functions-
1) Meiosis takes 1 cell with two copies of every
chromosome (diploid) and makes 4 cells with a
single copy of every chromosome (haploid).
This is a good idea if youre going to combine
two cells to make a new organism. This trick is
accomplished by halving chromosome number first
before conception.
So In meiosis, one diploid cells produces four
haploid cells.
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2) Meiosis scrambles the specific forms of each
gene (parts of a chromosome) that each sex cell
(egg or sperm) receives.
This makes for a lot of genetic diversity. This
trick is accomplished through independent
assortment and crossing-over. Well find out what
these mean in a second!
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Why do we need meiosis again?

• Meiosis is necessary to halve the number of
  chromosomes going into the sex cells!
• Why halve the chromosomes in gametes (sperm and
  egg)?
• At fertilization the male and female sex cells
  will provide ½ of the chromosomes each so the
  offspring has half the genes from both parents
  making one complete set of its own!

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Meiosis
Parent cell chromosome pair
Chromosomes copied
1st division - pairs split
2nd division produces 4 gamete cells with ½ the
original no. of chromosomes
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Meiosis PMAT I and II
Parent cell
1st division
2nd division
4 gametes
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Stages of Meiosis PMAT I and II

• The key is Reduction Division- as the cell
  divides, it reduces the number of chromosomes in
  half.
• Reminder Diploid parent cells (46 chromosomes,
  or 23 X2) to Haploid daughter cells (23
  chromosomes)

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Meiosis I Separates Homologous Chromosomes

• Interphase
• Each of the chromosomes replicate
• The result is two genetically identical sister
  chromatids which remain attached at their
  centromeres- like mitosis!

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Prophase I

• During this phase each pair of chromatids dont
  move to the equator alone, they match up with
  their homologous pair and fasten together
  (synapsis) in a group of four called a tetrad.
  Exception x and y!
• Extremely IMPORTANT!!! It is during this phase
  that crossing over can occur.
• Crossing Over the exchange of segments during
  synapsis.

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Crossing Over
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Metaphase I

• The chromosomes line up at the equator attached
  by their centromeres to spindle fibers from
  centrioles.
• Still in homologous pairs!
• REMINDER What are homologous pairs?

homologous chromosomes
sister chromatids
sister chromatids
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Anaphase I

• The spindle guides the movement of the
  chromosomes toward the poles
• Sister chromatids remain attached
• Move as a unit towards the same pole
• The homologous chromosome moves toward the
  opposite pole
• Contrasts mitosis chromosomes appear as
  individuals instead of pairs (meiosis)

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Telophase I

• This is the end of the first meiotic cell
  division.
• The cytoplasm divides, forming two new daughter
  cells.
• Each of the newly formed cells has half the
  number of the parent cells chromosomes, but each
  chromosome is already replicated ready for the
  second meiotic cell division

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Cytokinesis

• Occurs simultaneously with telophase I
• Forms 2 daughter cells
• Plant cells cell plate
• Animal cells cleavage furrows
• NO FURTHER REPLICATION OF GENETIC MATERIAL PRIOR
  TO THE SECOND DIVISION OF MEIOSIS. NO NEW
  INTERPHASE!!!

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Figure 13.7 The stages of meiotic cell division
Meiosis I
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Meiosis II Separates sister chromatids

• Proceeds similar to mitosis
• REMINDER THERE IS NO INTERPHASE II !

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Prophase II

• Each of the daughter cells forms a spindle, and
  the double stranded chromosomes move toward the
  equator

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Metaphase II

• The chromosomes are positioned on the metaphase
  plate in a mitosis-like fashion

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Anaphase II

• The centromeres of sister chromatids finally
  separate
• The sister chromatids of each pair move toward
  opposite poles
• Now individual chromosomes

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Telophase II and Cytokinesis

• Nuclei form at opposite poles of the cell and
  cytokinesis occurs
• After completion of cytokinesis there are four
  daughter cells
• All are haploid (n)

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The stages of meiotic cell division Meiosis II
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One Way Meiosis Makes Lots of Different Sex Cells
(Gametes) Independent Assortment
Independent assortment produces 2n distinct
gametes, where n the number of unique
chromosomes.
In humans, n 23 and 223 6,000,0000.
Thats a lot of diversity by this mechanism alone.
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Another Way Meiosis Makes Lots of Different Sex
Cells Crossing-Over
Crossing-over multiplies the already huge number
of different gamete types produced by independent
assortment.
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Mitosis vs. Meiosis
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The Key Difference Between Mitosis and Meiosis is
the Way Chromosomes Uniquely Pair and Align in
Meiosis
Mitosis
The first (and distinguishing) division of meiosis
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Mitosis vs. Meiosis
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Boy or Girl? The Y Chromosome Decides
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Meiosis division error
Chromosome pair
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Meiosis error - fertilization
Should the gamete with the chromosome pair be
fertilized then the offspring will not be
normal. In humans this often occurs with the
21st pair producing a child with Downs Syndrome
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21 trisomy Downs Syndrome
Can you see the extra 21st chromosome? Is this
person male or female?