Genetics

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Genetics
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DNA

• Our genetic blueprint (instructions)
• Used by all organisms to store hereditary
  information
• DNA segments that code for the construction of
  cell components are called genes (hereditary
  units, whats passed down in generations)

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Structure of DNA

• Double stranded helix
• Circular in prokaryotes
• Arranged in chromosomes in eukaryotes
• Supercoiling of the DNA allows in to be packed
  tightly in the nucleus (conserves space)

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Nucleotide

• The basic unit of DNA
• Composed of
• A sugar (deoxyribose)
• A phosphate group
• A nitrogenous base

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Nucleotide (cont)

• 4 nitrogenous bases
• Adenine
• Guanine
• Cytosine
• Thymine

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Bonding in DNA

• Bonds between sugar and phosphate group are
  called phosphodiester bonds
• Complimentary base pairing between nitrogenous
  bases (hydrogen bonds)
• Purines bond with pyramidines
• Adenine and Thymine (2 H-bonds)
• Guanine and Cytosine (3 H-bonds)

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Homework

• Read p. 186-190
• P. 198 1, 2, 4
• P. 204 1
• P. 205 3, 9

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Chromosome
Gene Sequence of bases that code for a
particular trait. Each chromosome has 100s to
1000s of genes
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Cell Division

• In unicellular organisms, cell division is
  reproduction. Called binary fission.
• In multicellular organisms, cell division is for
  reproduction (meiosis) or for growth and repair
  (mitosis)

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Cell Division

• 4 Major Events
• Reproductive signal
• This signal, which may come from inside or
  outside the cell, initiates the cellular
  reproductive events
• DNA Replication
• Production of an identical copy of all chromatin
  in nucleus
• Mitosis
• Separation of chromatin copies into the 2 new
  cells
• Cytokinesis
• Division of cytoplasm, fluid and organelles
  between the 2 new cells

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DNA Replication

• Before cell division can occur, DNA must make an
  exact copy of itself
• This means each cell will have an identical copy
  of the DNA

1. Unzipping the double helix
2. Complimentary base pairing
3. Reforming of hydrogen bonds

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The Cell Cycle
Replication
Growth metabolic processes
Preparing for division
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Mitosis

• Cell division in which the end result is 2 new
  cells with nuclei identical to the original
  nucleus

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Interphase

• Cells only spend a small amount of time in
  mitosis
• 90 of the time is spend in interphase
• This is the time period between phases of mitotic
  divisions
• It appears that the nucleus is inactive, but it
  is performing regular metabolic activity

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Interphase

• During interphase, chromosomes exist as chromatin
  (long, thin strands) and are not visible under a
  compound microscope
• Near the end of interphase, after receiving a
  reproductive signal, the chromatin replicates and
  the nucleus is now ready for mitosis

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Prophase

• Series of events preparing the nucleus to
  distribute its replicated chromatin into 2
  separate and identical sets
• Early Prophase
• Nucleolus begins to disintegrate
• Thin strands of chromatin begin to shorten and
  thicken

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• Middle Prophase
• Chromatin becomes visible
• Nucleolus is completely disintegrated
• Nuclear membrane breaks down
• Microtubules called spindle fibres appear at
  poles of the cell
• Resulting array of fibres is called the spindle

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• Late Prophase
• Nuclear membrane is gone
• Thickened chromatin has formed double-stranded
  chromosomes
• Double-stranded chromosomes move towards the
  centre of the cell

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Metaphase

• Double-stranded chromosomes line up along the
  equator of the cell
• Each pair of chromatids is attached by its
  centromere to a spindle
• Late Metaphase
• Centromeres split, double-stranded chromosomes
  begin the pull apart in opposite directions
• Once this happens, chromatids are called
  single-stranded chromosomes
• Each has half of a centromere

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Anaphase

• Single-stranded chromosomes form a V-shape,
  pointing toward the poles of the cell, still
  attached to spindle by centromere
• Chromosomes are pulled along by the contraction
  of the protein molecules in the spindles fibres
• At the end of anaphase, there is a complete set
  of single-stranded chromosomes at each pole

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Telophase

• Nuclear membrane reforms around each set of
  chromosomes
• Nucleolus reappears in each nucleus
• Chromosomes begin to unravel into chromatin
• Final product 2 complete nuclei identical to
  each other and the original
• At the end of telophase cell division is not
  complete

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Cytokinesis
The division of the cytoplasm to form two new
cells

• Animals
• As chromosomes reach opposite poles, the nuclear
  membrane begins to reform
• The cell membrane begins to pinch together at the
  equator
• This groove is called the cleavage furrow
• The cleavage furrow becomes deeper and deeper
  until eventually the cytoplasm splits into two
  masses

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Cytokinesis

• Plants
• A structure known as the cell plate forms from
  the fusion of tiny vesicles produced by the golgi
  apparatus
• Appears as a faint, thin line along the equator
  and gradually becomes thicker as each new cell
  constructs its own cell wall from molecules of
  cellulose

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Plant vs. Animal Cells

• Plant cells do not have centrioles or astral rays
  (star arrangement around centriole)
• Metaphase and anaphase are the same in plants and
  animals
• During late telophase, centrioles in animal cells
  replicate so each new cell has a pair

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Quickie Quiz

• The 4 major events of cell division (in order)
  are
• Draw and label the following centomere,
  chromatin, chromosome, sister chromatids
• Cytokinesis is marked by the formation of a(n)
  _____________ in animal cells and a(n)
  _______________ in plant cells.
• For the following statements identify the mitotic
  stage
• Nuclear membrane disintegrates
• Chromosomes are pulled by spindles to the poles
• Chromosomes begin to unravel into chromatin
• Chromatin becomes visible
• Chromosomes line up along the equator.

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Meiosis

• In sexual reproduction, 2 gametes (egg sperm)
  unite to form the first cell (zygote) of the new
  individual
• If the gametes were produced by normal mitosis,
  the zygote produced would have double the number
  of chromosomes
• E.g. 46 46 92 Not Viable!
• Gametes are formed by meiosis, a form of cell
  division in which the number of chromosomes in
  the gametes are reduced by half

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Meiosis

• Haploid (n) Single set of chromosomes (23 in
  humans)
• Diploid (2n) Paired set of chromosomes in humans
  (46 in humans)
• Homologous Chromosomes
• Paired chromosomes are called homologous
  chromosomes
• Homologous chromosomes pair during meiosis
• One from mom, one from dad
• Have the same sequence of genes, but a given gene
  may exist in a slightly different form (allele).
  This allows for genetic recombination.

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Meiosis involves DNA replication followed by two
nuclear divisions Meiosis I II

• Meiosis I
• Reductional division - 1 Diploid (2n) cell to 2
  haploid (n) daughter cells
• During prophase I, synapsis and crossing over
  occur
• Crossing over allows for the exchange of genetic
  material

Tetrad
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• During metaphase I, homologous chromosome pairs
  (tetrads) line up at equatorial plate
• During anaphase I, each chromosome separates from
  its homologue

• Two non-identical nuclei result from telophase I
• Cells are considered haploid, the homologous
  pair has been split

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Meiosis II

• No replication!
• 2 haploid (n) cells, from Meiosis I, give rise to
  4 haploid (n) daughter cells)
• Similar to mitosis, sister chromatids separate
• Because of crossing over, each cell has a
  slightly different genetic make-up

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Quickie Quiz

1. Gametes are (haploid/diploid) and contain ___
   chromosomes in humans.
2. Somatic cells (body cells) are (haploid/diploid)
   and contain ____ chromosomes in humans
3. Synapsis, the pairing of ___________ to form a(n)
   _______ occurs during __________. Here, the
   chromatids often intertwine and sometimes break
   off and exchange genetic material in a process
   known as ______________.
4. What is the end result of meiosis? Why is this
   necessary?