Nucleic Acids and Protein Synthesis

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Nucleic Acids and Protein Synthesis

• Modern BiologyChapter 10
• Biology Exploring Life
• Sections 11.1-11.5

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DNA (deoxyribonucleic acid)

• Genetic material
• Structure (double helix) discovered by Watson and
  Crick in 1953
• Two chains of nucleotide monomers, each of which
  has three parts

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DNA Nucleotides

• Deoxyribose a five-carbon sugar
• Phosphate group (phosphoric acid)
• Nitrogenous base--four possibilities

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Nitrogenous Bases

• Purines larger--two organic rings (adenine,
  guanine)
• Pyrimidines smaller--one organic ring (cytosine,
  thymine)

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Double Helix

• Two strands held together with hydrogen bonds
• Sugar-phosphate backbone
• Complementary nitrogenous base pairs in the
  middle like the rungs of a rope ladder
• Spiral formed from twisting of entire molecule

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Complementary Base Pairing

• Chargaffs Rule A T and G C
• A purine pairs with a pyrimidine because of size
  difference and number of bonding sites.
• A pairs with T G pairs with C.

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Replication

• How a DNA molecule makes an exact copy of itself
• Semi-conservative process
• Two identical copies from one original double
  helix
• One old strand and one new strand in each copy
• DNA synthesis in 5?3 direction only

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Semi-Conservative Replication

• 1. Enzymes unwind and unzip the double helix,
  forming a replication fork.
• 2. Each of the two half-ladders serves as a
  template (pattern) for putting together a new
  chain of complementary nucleotides.

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Semi-Conservative Replication

• 3. The new strand forms hydrogen bonds with the
  old strand.
• 4. The new DNA molecules twist to form two
  identical double helices.

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RNA (ribonucleic acid)

• Single-stranded molecules that play various roles
  in gene expression and protein synthesis
• Three types
• Messenger RNA (mRNA) carries genetic information
  from the nucleus to a ribosome during
  transcription
• Transfer RNA (tRNA) carries amino acids from the
  cytoplasm to a ribosome during translation
• Ribosomal RNA (rRNA) structural component of
  ribosomes

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RNA Nucleotides

• Sugar ribose (C5H10O5)
• Phosphate
• Baseone of four
• Adenine
• Guanine
• Cytosine
• Uracil, a pyrimidine which is complementary to
  adenine

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Transcription

• Genes (DNA segments) rewritten as mRNA
• Process
• 1. Enzymes unwind and unzip the DNA double
  helix.
• 2. RNA nucleotides form a chain whose base
  sequence is complementary to the DNA coding
  strand.
• 3. The mRNA leaves the nucleus for a ribosome,
  which will translate it into a protein.

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Transcription Steps

• Part of DNA double helix opens unwinds.

A
G
A
A
T
T
C
G
A
T
T
C
G
T
A
A
C
T
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Transcription Steps

• Only one strand is read or used in the open
  sectionsense strand (coding strand)

A
G
A
A
T
T
C
G
A
nonsense strand (noncoding strand) not used
T
T
C
G
T
A
A
C
T
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Transcription Steps

• RNA nucleotides enter and pair up by
  complementary base pairing

A
G
A
A
T
T
C
G
A
No Thymine in RNA Uracil (U) takes its place
T
T
C
G
T
A
A
C
T
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Transcription Steps

• RNA nucleotides enter and pair up by
  complementary base pairing

A
G
A
A
T
T
C
G
A
T
T
C
G
T
A
A
C
T
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Transcription Steps

• Enzyme called RNA polymerase connects all the new
  nucleotides in a strand of RNA

A
G
A
A
T
T
C
G
A
T
T
Single Strand of RNA
C
G
T
A
A
C
T
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Translation

• Process by which a protein is made when charged
  tRNA molecules bring amino acids to a ribosome,
  in the sequence encoded in mRNA
• The Genetic Code
• Codon group of 3 mRNA nucleotides that indicates
  a specific amino acid
• Anticodon group of 3 tRNA nucleotides that is
  complementary to a codon
• Tableuse and redundancy

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Protein Synthesis

• mRNA passes through ribosome, one codon at a time
  (start codon AUG)
• tRNA anticodon forms hydrogen bonds with mRNA
  codon
• tRNA releases its amino acid, which covalently
  bonds to other amino acids to elongate the
  polypeptide
• Empty tRNA leaves ribosome
• Process continues until one of three stop
  codons enters ribosome

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Translation (Protein Synthesis)

• mRNA comes together with ribosome.
• Door 1 or active site is over codon 1.
• tRNA grabs an amino acid.

Ribosome
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Translation (Protein Synthesis)

• tRNA w/ an amino acid and w/ correct anticodon
  moves into door or active site.

Ribosome
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Translation (Protein Synthesis)

• rRNA moves so Door 1 or active site is over the
  next codon.

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Translation (Protein Synthesis)

• Another tRNA grabs another amino acid. It has an
  anticodon to match the next codon on the mRNA.

Ribosome
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Translation (Protein Synthesis)

• Peptide bond forms between the 2 amino acids,
  connecting them together.

Ribosome
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Translation (Protein Synthesis)

• rRNA (ribosome) moves down again while another
  tRNA grabs another amino acid.

Ribosome
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Translation (Protein Synthesis)

• tRNA brings Amino Acid 3 to match up w/ the
  codon while tRNA 1 breaks away to be used again.

Ribosome
AA 1
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Translation (Protein Synthesis)

• Peptide bond forms connecting Amino Acid 3 to
  the chain.

Ribosome
AA 1
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Translation (Protein Synthesis)

• The process continues until signaled to end by a
  stop codon.

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Summary of Gene Expression

• Central Dogma of Molecular Biology
• DNA ? (transcription) ? RNA ? (translation) ?
  protein
• One Gene One Polypeptide concept