Dr' Natalie Halbert

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Dr. Natalie Halbert nhalbert_at_cvm.tamu.edu 862-4774
Weekly Tutorial by Ahmed Elhameky ahmed_at_cvm.tamu.
edu Tuesdays 3pm-5pm VMRB (Building 1197) Room
208
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From Genes to PhenotypesPart II Translation

• From Chapter 10
• Human Genetics Concepts and Applications, 6th
  edition
• by Ricki Lewis

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Lets review

• How many steps in transcription?
• What is the name of the main enzyme that
  catalyzes transcription?
• How many nuclear RNA polymerases are in
  eukaryotes? What are their functions?
• In which part of the cell does transcription
  occur?
• In what ways can mRNA be modified after synthesis
  of the initial transcript?

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Translation

• The process of reading the RNA sequence of an
  mRNA and creating the amino acid sequence of a
  protein is called translation.

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The Central Dogma at the cellular level in
eukaryotes.
RNA polymerase II
Intron removal from pre-mRNA
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Basic components of translation
Messenger RNA fully processed and transported
from nucleus to cytoplasm
Ribosomes protein subunits rRNA(s)
transfer RNAs - fully processed, transported from
nucleus to cytoplasm, and charged with amino
acids
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What is the correspondence between the mRNA
nucleotides and the amino acids of the protein?

• Proteins are formed from 20 amino acids in humans.

Codons of one nucleotide A G C U
Codons of two nucleotides AA GA CA UA AG GG
CG UG AC GC CC UC AU GU CU UU
Can only encode 4 amino acids
Can only encode 16 amino acids
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What is the correspondence between the mRNA
nucleotides and the amino acids of the protein?

• Proteins are formed from 20 amino acids in humans.

Codons of three nucleotides AAA AGA ACA AUA
AAG AGG ACG AUG AAC AGC ACC AUC AAU AGU
ACU AUU GAA GGA GCA GUA GAG GGG GCG
GUG GAC GGC GCC GUC GAU GGU GCU GUU CAA
CGA CCA CUA CAG CGG CCG CUG CAC CGC CCC
CUC CAU CGU CCU CUU UAA UGA UCA UUA UAG
UGG UCG UUG UAC UGC UCC UUC UAU UGU UCU
UUU
Allows for 64 potential codons gt sufficient!
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The Genetic Code.

• The code is written in triplets (CODONS).
• The code is without punctuation (continuous).
• The code is non-overlapping.
• The code is almost universal.
• All organisms share the same language, the major
  exception is the mitochondrial genome.
• The code is degenerative.
• All but 2 amino acids are coded for by more than
  1 codon.
• Exceptions include methionine (AUG) and
  trytptophan (UGG).
• For a given amino acid, the first and the second
  codon positions are less variable than the third
  position.
• The Wobble Hypothesis.

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The genetic code is non-overlapping
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The Genetic Code.

• The code has start and stop signals.
• AUG (methionine) is the common start codon
• Methionine can also be used WITHIN a
  polypeptide
• GUG may also be used as a start codon.
• There are three stop codons.
• UAG
• UAA
• UGA
• All three are chain termination codons.

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The genetic code
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Which RNA polymerase synthesizes tRNAs in
eukaryotes?
The correct amino acid is added to the growing
polypeptide only if 1 - The appropriate amino
acid is added to the tRNA by aminoacyl-tRNA
synthetases. 2 Proper binding occurs between
the codon of the mRNA and the anticodon of the
tRNA.
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Composition of ribosomes from pro- and
eukaryotes. In humans, the rRNA genes are at p12
positions on chromosomes 13, 14, 15, 21 and 22.
Sites of transcription and assembly of rRNA seen
in interphase nucleus are called NORs. The 28S,
5.8S and 18S rRNAs are derived from a precursor
45S transcript. Several hundred rRNA genes exist
in human genome.
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Translation (An Overview)

• Translation is defined as protein synthesis.
• Occurs on ribosomes, where the genetic
  information is translated from the mRNA to a
  protein.
• mRNA is translated in the 5 to 3 direction.
• Amino acids are brought to the ribosome bound to
  a specific tRNA molecule.
• The mRNA and tRNA are responsible for the correct
  recognition of each amino acid in the growing
  polypeptide

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Initiation of Translation

• Usually begins with a AUG codon (methionine amino
  acid), may also start with a GUG codon.
• The initiation complex contains all the
  components (ribosomal proteins) that are required
  for translation.
• In prokaryotes, ribosomes bind to the
  Shine-Dalgarno sequence (approx 7 nt upstream of
  start codon) to align and locate the correct
  reading frame of the mRNA.
• In eukaryotes, a eukaryotic initiator factor
  (several different proteins) binds to the cap at
  the 5 end of the mRNA.

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Elongation

• Binding of the aminoacyl-tRNA to the ribosome
• formation of a peptide bond
• The movement (translocation) of the ribosome
  along the mRNA, one codon at a time.

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Termination

• Termination is usually signaled by one of the
  three stop codons UAG, UAA or UGA.
• There are a number of helper proteins involved
  (e.g. termination factors and release factors).

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Translation initiation
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Translation initiation
Assembling to begin translation
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Translation Elongation
Ribosome
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Translation Elongation
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Translation Elongation
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Translation Elongation
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Translation Elongation
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Translation Elongation
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Translation Elongation
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Translation Elongation
Release factor
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Translation Termination
Ribosome reaches stop codon
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Translation Termination
Once stop codon is reached, elements disassemble.
Release factor
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10_14.jpg
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Translation multiple copies of a protein are
made simultaneously
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Protein structure
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Levels of protein structure
Primary structure sequence of amino
acids
Secondary structure shapes formed within

regions of the protein
(helices, coil, sheets)
Tertiary structure shape of entire
folded
protein due to interactions
between particular peptides
Quaternary structure structures formed by
interaction
of several proteins together
e.g. Functional
hemoglobin is
two alpha-hemoglobin proteins
and
two beta-hemoglobin proteins

(heterotetramer)
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Four levels of protein structure
Amino acids Hydrogen
bonding S-S linkages, side groups
Sub-units


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Secondary structures
Tertiary structures
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luckiest man on the face of the earth
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Misfolding of protein impairs function

• Misfolded prion protein disrupts functions of
  other
• normally folded prion proteins.
• Aberrant conformation can propagate like
• an infectious agent.

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10_18.jpg
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Kuru
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What about mutations?
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Mutations at the nucleotide level

• Synonymous mutation is change to another codon
  that specifies the same amino acid.
• Missense mutation is change from one sense codon
  to a codon that specifies a different amino acid.
• Nonsense mutation is change from sense codon to
  stop codon.
• Frameshift mutation is a small deletion of one or
  two bases (or multiples thereof) that changes the
  reading frame of the mRNA results in premature
  stop codon.

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Analogies to effects of mutations on gene sequence

• Sense The one big fly had one red eye.
• Missense Thq one big fly had one red eye.
• Nonsense The one big.
• Frameshift The one qbi gfl yha don ere dey.
• Insertion The one big wet fly had one red
  eye.
• Deletion The one big had one red eye.
• Duplication The one big big fly had one red
  eye.
• Expansion The one big fly fly fly fly fly had
  one
• red eye.

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A codon of three nucleotides determines choice of
amino acid