Genetics

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Genetics

• AP Biology

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The Discovery of DNA Structure

• Rosalind Franklin x-ray diffraction photographs
  of DNA
• Watson Crick built model based on x-ray
  diffraction photos

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DNA Structure

• Deoxyribose sugar backbone, alternating with
  phosphate groups
• Nitrogenous bases held together by hydrogen
  bonds Adenine, Thymine, Cytosine and Guanine
• Arranged in a double helix
• Chargaffs base pairing rules A-T G-C

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Anti-parallel nature of DNA
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DNA Replication

• Makes an exact copy
• Double helix separates at Origins of Replication
• Each strand serves as the template for a new
  strand
• Semi-conservative replication

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DNA polymerase builds new strand 5 to 3 (adds
nucleotides onto 3 end (OH)
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DNA replication occurs in a 5 to 3 direction
Leading strand Other side is constructed in 5 to
3 fragments called Okasaki fragments Lagging
strand
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From Gene to Protein
DNA contains information of amino acid sequence
in proteins
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Transcription
Synthesis of RNA using the DNA as a template
contains genes protein building instructions
RNA Structure Contains ribose sugar, Base
thymine is replaced with uracil, single stranded
Types of RNA Ribosomal RNA (rRNA) manufactured
in nucleolus Transfer RNA (tRNA) Messenger RNA
(mRNA)
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Single stranded RNA molecules fold into secondary
structures
Characteristic secondary structure of tRNA
Uracil
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RNA Editing

• RNA molecule made by transcription premRNA
• Contains stretches of bases introns that do
  not code for proteins
• Introns are removed and coding sequences exons
  are spliced together

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Role of RNA Splicing

• One gene, One polypeptide hypothesis a gene of
  DNA codes for one protein.
• Several proteins can be manufactured from a
  single gene due to alternative splicing

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Translation
tRNAs bring amino acids to the ribosome to
assemble proteins
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Translation the specifics

• Amino acids joined to tRNAs by a specific
• aminoacyl-tRNA synthetase
• Ribosomal Structure
• P site holds tRNA carrying amino acid the
  growing polypetide chain
• A site holds the tRNA carrying the next amino
  acid to be added to the chain
• E site discharged tRNAs leave through this site

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Translation the specifics continued

• Initiation mRNA, initiator tRNA, ribosomal
  subunits assemble. Initiator tRNA sits in P site
  and A site is vacant
• Elongation
• Codon recognition H bonding between tRNA and
  codon in A site. Requires elongation factors
  GTP
• Peptide bond formation an rRNA molecule
  catalyzes formation of peptide bond growing chain
  from P site to the new amino acid in the A site
• Translocation the tRNA in the A site (now has
  polypetide chain) moves to P site. Blank tRNA in
  P site moved to E site.
• Termination stop codon release factor binds to
  A site causes the addition of water
  hydrolyzes tRNA from protein chain.

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DNA Mutations - Effects on Translation

• Point mutations
• Insertions and Deletions
• Frameshift mutations
• Base pair substitution
• Missense still codes for amino acid
• Nonsense prematurely codes for a stop codon

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Control of Transcription in Eukarytoic cells

• Transcription is most often the result of a
  chemical signal
• Some genes are constitutively active meaning
  they are always turned on in a cell
• Chemical signals often in the form of hormones
  either steroidal or peptide
• Peptide hormones cannot diffuse into the cell and
  bind hormone receptors on the cell membrane
• Steroid hormones are able to diffuse easily into
  the nucleus where they bind steroid hormone
  receptors that function as transcription factors
  (transcription factors turn on/off the
  transcription of a gene

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Peptide hormones
Binding of peptide hormone to cell membrane
receptor
Activation of cellular signaling molecules that
carry signal to the nucleus
Examples of peptide hormones insulin,
adrenaline, vasopressin, etc.
Change in transcription of a gene
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Steroid hormone signaling
Examples of steroid hormones estrogen,
testosterone, glucocorticoids
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Intronic RNA

• Once thought to be completely nonfunctional
• Within the last 5-7 years microRNAs
• Now found that microRNAs can bind to newly
  transcribed mRNA and target them for degradation
• Another way that the cell controls what proteins
  are made

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Can be synthesized in the laboratory and can be
used to shut down production of a protein
possible therapeutic uses