BIO 2, Lecture 8

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BIO 2, Lecture 8

• LIFES INFORMATION
• MOLECULE III TRANSLATION AND PROTEIN LOCALIZATION

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• In addition to mRNAs, genes can also code for
  other types of RNAs
• These other RNAs do not code for proteins
• Examples transfer RNA (tRNA), ribosomal RNA
  (rRNA)
• These RNAs fold into secondary and tertiary
  structures and perform important functions in the
  cell (much like proteins)

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tRNAs

• tRNAs are short RNA molecules (80 bases) that
  fold back upon themselves to form a
  cloverleaf-shaped structure
• Molecules of tRNA are not identical
• Each carries a specific amino acid (and will only
  carry that amino acid)
• Each has an anticodon on the other end that
  recognizes and base-pairs with a complementary
  codon on mRNA

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• Accurate translation requires two steps
• 1. An enzyme called aminoacyl-tRNA synthetase
  adds an amino acid to all the tRNAs that carry
  the anticodon that is complementary to the codon
  in the mRNA that codes for that amino acid
• Second The tRNA anticodon recognizes and
  base-pairs to its mRNA codon
• Flexible pairing at the third base of a codon is
  called wobble and allows some tRNAs to bind to
  more than one codon

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Aminoacyl-tRNA synthetase (enzyme)
Amino acid
P
P
P
Adenosine
ATP
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Aminoacyl-tRNA synthetase (enzyme)
Amino acid
P
P
P
Adenosine
ATP
Adenosine
P
P
P
i
P
i
P
i
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Aminoacyl-tRNA synthetase (enzyme)
Amino acid
P
P
P
Adenosine
ATP
Adenosine
P
tRNA
P
P
i
Aminoacyl-tRNA synthetase
P
i
P
i
tRNA
Adenosine
P
AMP
Computer model
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Aminoacyl-tRNA synthetase (enzyme)
Amino acid
P
P
P
Adenosine
ATP
Adenosine
P
tRNA
P
P
i
Aminoacyl-tRNA synthetase
P
i
P
i
tRNA
Adenosine
P
AMP
Computer model
Aminoacyl-tRNA (charged tRNA)
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• Ribosomes facilitate specific coupling of tRNA
  anticodons with mRNA codons in protein synthesis
• The two ribosomal subunits (large and small) are
  made up of a combination of proteins and
  ribosomal RNA (rRNA)

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30 S (small) subunit of the bacterial
ribosome Proteins blue, RNA pink
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• A ribosome has three binding sites for tRNA
• The P site holds the tRNA that carries the
  growing polypeptide chain
• The A site holds the tRNA that carries the next
  amino acid to be added to the chain
• The E site is the exit site, where tRNAs (which
  have now lost their amino acids) leave the
  ribosome

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• The three stages of translation
• Initiation
• Elongation
• Termination
• Requires energy (in the form of GTP) and
  additional proteins (called factors)
• In prokaryotes, initiation takes place when an
  rRNA in the small ribosomal subunit base-pairs to
  a region in the 5UTR of the mRNA

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• Then the small subunit moves along the mRNA
  (through the 5 UTR) until it reaches the start
  codon (always 5-AUG-3)
• Proteins called initiation factors bring in the
  large subunit that completes the translation
  initiation complex

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• During the elongation stage, amino acids are
  added one by one to the preceding amino acid
• Each addition involves proteins called elongation
  factors and occurs in three steps codon
  recognition, peptide bond formation, and
  translocation

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Amino end of polypeptide
E
3?
mRNA
P site
A site
5?
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Amino end of polypeptide
E
3?
mRNA
P site
A site
5?
GTP
GDP
E
A
P
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Amino end of polypeptide
E
3?
mRNA
P site
A site
5?
GTP
GDP
E
A
P
E
P
A
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Amino end of polypeptide
E
3?
mRNA
P site
A site
Ribosome ready for next aminoacyl tRNA
5?
GTP
GDP
E
E
P
A
A
P
GDP
GTP
E
P
A
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• Termination occurs when a stop codon in the mRNA
  reaches the A site of the ribosome
• The A site accepts a protein called a release
  factor
• The release factor causes the addition of a water
  molecule instead of an amino acid
• This reaction releases the polypeptide, and the
  translation assembly then comes apart

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• In both prokaryotes and eukaryotes, a number of
  ribosomes can translate a single mRNA
  simultaneously, forming a polyribosome (or
  polysome)
• Polyribosomes enable a cell to make many copies
  of a polypeptide very quickly from a single mRNA

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• Remember, though, that translation is not
  sufficient to make a protein (which is a
  functional molecule)
• Polypeptide chains are modified after translation
  and then targeted to the correct location in the
  cell before they can fold and, if the protein has
  quarternary structure, get together with other
  polypeptides, to function properly

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• Two populations of ribosomes are evident in
  eukaryotic cells free ribsomes (in the cytosol)
  and bound ribosomes (attached to the ER)
• Free ribosomes mostly synthesize proteins that
  function in the cytosol
• Bound ribosomes make proteins of the endomembrane
  system and proteins that are secreted from the
  cell
• Ribosomes are identical and can switch from free
  to bound

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• Polypeptide synthesis always begins in the
  cytosol
• Synthesis finishes in the cytosol unless the
  polypeptide signals the ribosome to attach to the
  ER
• Polypeptides destined for the ER or for secretion
  are marked by a signal peptide
• A signal-recognition particle (SRP) binds to the
  signal peptide
• The SRP brings the signal peptide and its
  ribosome to the ER

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• Mutations are changes in the sequence, number of
  copies, or location of a gene
• The smallest type of mutation is a point mutation
• Involves a very small change within a single gene
• Can involve the substitution of one base-pair for
  another OR the deletion or insertion of a small
  number of nucleotide pairs
• Even though they are small, can cause major
  changes to the function of a protein

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• Point mutations come in two main types including
• Single Base-pair substitutions
• Silent
• Missense
• Nonsense
• Insertions or deletions
• Frameshift leading to immediate nonsense
• Frameshift leading to extensive missense
• Missing amino acid(s)

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• A base-pair substitution replaces one nucleotide
  and its partner with another pair of nucleotides
• Silent mutations have no effect on the amino acid
  produced by a codon because of redundancy in the
  genetic code
• Missense mutations still code for an amino acid,
  but not necessarily the right amino acid
• Nonsense mutations change an amino acid codon
  into a stop codon, nearly always leading to a
  nonfunctional protein

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• Insertions and deletions are additions or losses
  of nucleotide pairs in a gene
• These mutations have a disastrous effect on the
  resulting protein even more often than
  substitutions do
• Insertion or deletion of nucleotides may alter
  the reading frame, producing a frameshift
  mutation most of which eliminate the proteins
  functional altogether

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