DNA Deoxyribonucleic Acid

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DNADeoxyribonucleic Acid

• (a little background, more in cell biology
  Chapter 16)

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Background

• Genes are portions of DNA molecules that contain
  genetic information that controls protein
  synthesis. DNA is one of the nucleic acids and
  RNA is the other.
• Each chromosome is made up of DNA and associated
  proteins. The DNA of each chromosome contains
  many genes.

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Background continued

• The process whereby genes direct protein
  synthesis is called gene expression.
• DNA gt RNA gt protein
• DNA is transcribed into RNA
• RNA is translated into proteins
• DNA is also replicated (duplicated)

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Central Dogma
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DNA structure

• DNA is composed of subunits, nucleotides, that
  consist of a 5Carbon sugar (deoxyribose),
  phosphate group(s), and nitrogenous bases Adenine
  (A), Thymine (T), Cytosine (C), and Guanine (G).
• Nucleotides are linked by sugar-phosphate bonds
  that basically form a backbone (like the sides of
  a ladder).
• Base pairing between A and T and between C and G
  connects two strands (sides of ladder) together
  (like the steps of a ladder).
• The 3-D structure of DNA is a twisted spiral (or
  more commonly called a helix) consisting of two
  strand of DNA, thus it is called a double helix.

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DNA nucleotide
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The Nitrogenous bases
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Base pairing
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DNA Double Helix
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III. DNA Replication

• In addition to serving as the code for proteins,
  DNA also undergoes replication.
• Replication is the duplication of DNA within a
  cell prior to cell division (occurring before
  mitosis before meiosis I)
• The original strands of the double helix serve as
  a template for the new strands (replication is
  semi-conservative)
• 1. The double strand of DNA unwinds and unzips.
• 2. Then new DNA nucleotides base pair with
  those of the two original strands (C with G, and
  A with T)
• Enzymes are involved in replication (e.g.,
  helicase and DNA polymerase)

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DNA replication
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Original and New -

• T-A T-A T-A
• A-C A-C A-C
• C-G C-G C-G
• C-G C-G C-G
• T-A T-A T-A
• G-C G-C G-C
• G-C G-C G-C
• A-T A-T A-T
• T-A T-A T-A

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Replication Animation

• http//video.google.com/videoplay?docid-879370417
  9367037205qDNAreplicationtotal90start0num
  10so0typesearchplindex0

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Chromosome structure.

• DNA (about 40) associated proteins (about 60)
  chromosomes
• Histones are the chromosomal proteins of
  eukaryotes
• Nucleosomes are bead-like particles containing
  DNA wound around clusters of histone proteins
• DNA can either be wound tightly (condensed) or
  relaxed (when replication or transcription are
  occurring but in different ways)

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Chromosome Structure
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• Mitosis
• (Chapter 12)

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Background information

• Mitosis is a part of the cell cycle when the
  chromosomes of a cell (parent cell) divide
  followed by cytokinesis forming two identical
  daughter cells.
• Meiosis is the two stage (two divisions Meiosis
  I and Meiosis II) process whereby four (or in
  some cases only one) daughter cells receive ½ of
  the chromosomes of the parent cell. It is a
  special process that occurs in the formation of
  the sex cells or gametes in sexually reproducing
  organisms (animals and plants).

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• In eukaryotes, replication results in the
  presence of two sets of each chromosome in the
  cell. These replicated chromosomes are most
  closely attached by a structure called a
  centromere and are known as sister chromatids.

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Sister chromatids
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In most higher animals and plants the chromosomes
are paired

• You have 23 pairs of chromosomes, a total of 46
  23 came from your mom 23 of them came from your
  dad. The members of these pairs are homologues.
  The two chromosomes of each pair carry genes
  controlling the same inherited characteristics.
• After replication there will be 46 x 2 92 total
  chromosomes present in a given human cell.
  Remember that the replicates will be physically
  attached at a centromere, so that it will still
  look like you have only 46 chromosomes
• In humans and other animals, there are two types
  of chromosomes
• sex chromosomes which differ between the males
  and females of species
• autosomal chromosomes or autosomes are the
  other chromosomes

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Normal human (male) karyotype
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Normal human female karyotype
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Before the tech pairs the chromosomes
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• If an organism has two pairs of chromosomes, then
  this organism is said to be diploid (2n). If an
  organism or a cell (like our gametes) has only
  one set of unpaired chromosomes, then that
  organism or cell is said to be haploid (n). Some
  organisms are triploid, tetraploid (or just plain
  polyploid meaning many sets).

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• The gametes (in human sperm and ova) are haploid
  (in humans n 23 chromosomes) and thus when
  fertilization or syngamy occurs the diploid
  number is restored in the diploid zygote.
• Germ cells are those that undergo meiosis and
  somatic cells (non reproductive cells) are those
  that divide only by mitosis.

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Mitosis

• Mitosis can be divided into phases based on the
  appearance of the chromosomes through a compound
  light microscope. However, Interphase is the
  period of time before mitosis begins. During
  this phase the chromatin is spread out and is not
  visible using a compound light microscope. The
  nucleus and nucleolus are visible and DNA
  replication occurs. The actual phases of mitosis
  are as follows

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Prophase

• chromosomes shorten thicken become visible
  using a compound light microscope. The sister
  chromatids are attached at the centromeres.
• The nuclear membrane breaks down and the nucleoli
  disappear.
• In animals, the centrioles move to the opposite
  ends of the cell
• Spindle fibers begin to form and attach to the
  kinetochore .
• The nucleolus (or nucleoli) disappear

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Metaphase

• The chromosomes line up at the equator (or
  metaphase plate) of the cell (note the
  arrangement and compare it to Metaphase I of
  meiosis)
• The spindles are complete and the centrioles are
  at the opposite poles of the cell.
• The kinetochores attach the chromosomes to the
  mitotic spindle

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Anaphase

• The centromeres separate because the kinetochore
  microtubules shorten.
• The sister chromatids separate and are pulled
  toward the opposite poles
• The polar microtubules elongate preparing the
  cell for cytokinesis

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Telophase

• The sister chromatids have reached the opposite
  ends of the cell
• The nuclear membranes form around each set of
  chromosomes and the nucleolus (or nucleoli)
  reappear
• In most cases, the cytoplasm begins to divide
• The chromosomes become less tightly coiled.
• Kinetochores disappear

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Mitosis in a Plant cell
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Mitosis in an Animal cell
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Mitosis animation

• http//video.google.com/videosearch?qmitosishle
  nsitesearch

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• MEIOSIS

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• Like mitosis, meiosis can also be divided into
  phases, but it is more complex than mitosis.
  Meiosis involves two stages.
• Be able to compare and contrast mitosis and
  meiosis.
• The process involves

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Prophase I (lasts longer and is more complex
than prophase of mitosis in fact it is divided
into five stages, but we will not cover them)

• The chromosomes shorten thicken become
  visible using a compound light microscope
• The nuclear membrane breaks down
• In animals, the centrioles move to the opposite
  ends of the cell
• Spindle fibers appear
• The homologues pair up (remember these are not
  the same as sister chromatids which are already
  paired up). Crossing over or synapsis occurs.
  This process involves the swapping of DNA between
  the homologues. It is an important source of
  variation

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Metaphase I

• The chromosomes line up at the equator of the
  cell, but the homologues not the sister
  chromatids are arranged toward the opposite ends
  of the cell.
• The spindles are attached to the homologues

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Anaphase I

• The chromosomes begin to separate
• But, the homologues, not the sister chromatids
  are pulled apart

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Telophase I

• Now the homologues are positioned at the opposite
  ends of the cell
• In some organisms, nuclear membranes reform
• Cytokinesis

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Prophase II

• If the chromsomes lengthened, after cytokinesis
  they now shorten again

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Metaphase II

• Spindles form and attach
• the chromosomes line up. Note the arrangement

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Anaphase II

• Now the sister chromatids separate

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Telophase II

• Now the sister chromatids of half of the original
  number of chromosomes are present in each of the
  haploid nuclei
• Nuclear membranes form

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• Be able to compare and contrast mitosis and
  meiosis. See and know figure 13.9.

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Spermatogenesis
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Oogenesis
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• http//video.google.com/videoplay?docid1107883966
  519514525qmeiosistotal160start0num10so0
  typesearchplindex0