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PROTEIN SYNTHESIS
CENTRAL DOGMA OF MOLECULAR
BIOLOGY:
• DNA is used as the blueprint to direct the
production of certain proteins
What is a gene?
• Gene = a segment of DNA coding for a RNA
segment.
– These RNA segments will be used to produce a
polypeptide (structural or enzymatic protein)
• Each strand of DNA can contain thousands of
genes
• Each gene has a precise beginning and an end
Gene Expression
• The DNA nucleotide sequence codes for the
order in which amino acids are put together to
form proteins
• Every three nucleotides on the mRNA (codon) codes
for a specific amino acid
Genetic Code
• 20 different amino acids
but 64 possible codons
– 43 = 64
– Some redundancy more then one codon
codes for the same
amino acid
• Code is universal in almost
all organisms
Transcription
• Information is
transferred from DNA
to RNA
• Occurs in the nucleus
Types of RNA
• All three types of RNA are transcribed from
DNA
– Messenger RNA – carries the coded
message from the DNA to the ribosome in
the cytoplasm
– Ribosomal RNA – reads the mRNA
– Transfer RNA – transfers the correct amino
acid to the ribosome
Overview of Transcription
• The segment of DNA that
contains the gene for a
specific protein or RNA
that the cell wants to
produce will unwind and
the complementary RNA
strand will be made by
incorporation the RNA
nucleotides
Stages in Transcription
• Each gene has a precise beginning known as
the promotor region and an end known as the
termination sequence
Stages in transcription of RNA
• Initiation
– Transcription factors bind
to the promoter region
(TATA box) of the DNA
– RNA polymerase then
initiates transcription by
binding to the transcription
factor
• Unwinds the DNA
• Elongates the RNA segment
Stages in transcription of RNA
• Elongation
– Nucleotides are added in the 5’ to 3’ direction by RNA
polymerase
– They form temporary hydrogen bonds with the DNA
template
– As the DNA helix reforms the RNA peels away
Stages in transcription of RNA
• Termination
– At the end of the gene the termination sequence
causes transcription to end
– Pre-RNA segment dissociates from the DNA
Terminator
Post-transcriptional Modifications
mRNA
• 5’ cap – a guanine triphosphate is added
– signal for ribosomal attachment in the cytoplasm
• 3’ poly A tail – polyA polymerase adds ~250
“A” nucleotides to the end
– protects RNA from being degraded by nucleases
Post-transcriptional Modifications
mRNA
• Splicing
– Exons – coding region
– Introns – noncoding region
• Cleaved out by snRNPs, and exons are spliced together
Post-transcriptional Modifications
Pre - mRNA
Mature mRNA
Post-transcriptional modifications
rRNA
• rRNA associates with proteins to form two
subunits (40s and 60s)
• Leaves the nucleus and enters the cytoplasm
Post-transcriptional Modifications
tRNA
– Folds into a three dimensional structure (clover
shaped)
Translation
• Going from the mRNA
nucleotide code to
amino acid code
• mRNA is read by a
ribosome (rRNA) to
determine the sequence
of amino acids
• Occurs in the cytoplasm
Translation
• Players in Translation
– mRNA strand
– Ribosomes (rRNA)
– tRNAs carrying
amino acids
– enzymes
rRNA
• rRNA has a mRNA binding site and three
tRNA binding sites
– A site (amino-acyl binding site)
– P site (peptidyl binding site)
– E site
tRNA
• Has an anticodon three base sequence that is
complementary to a codon on the mRNA
• 3’ end of the tRNA contains a binding site for a
specific amino acid
Stages of Translation
• Initiation
– mRNA binds to the 40s ribosome subunit
– The initiator tRNA binds to the mRNA start codon
(AUG) at the P site on the ribosome
– The arrival of the 60s subunit completes the initiator
complex
Stages of Translation
• Elongation
– The next tRNA enters at
the A site
– The enzyme peptidyl
transferase forms a
peptide bond between the
amino acid on the P site
and the new amino acid
on the A site.
Stages of Translation:
• Elongation
– The ribosome then
moves down the
mRNA (translocation)
– The tRNA that was at
the A site is now at
the P site and the
empty tRNA that was
at the P site now exits
at the E site
Charging of tRNA
• Amino acids are floating
freely in the cytoplasm
• The enzyme amino-acyl tRNA
synthetase attaches the amino
acids to the 3’end of the tRNA
• Requires ATP
Stages of Translation
• Termination
– Elongation continues until a stop codon on the
mRNA is reached (UAA, UAG, UGA)
– The polypeptide is then released from the
ribosome by a release factor
Polysomes
• Several ribosomes can simultaneously translate
the same mRNA strand to make multiple
copies of the same polypeptide
Post-translational modifications to
the Polypeptide
• The start methionine is removed by the enzyme
aminopeptidase
• Protein will under go folding or modifications
– Cleavage into smaller fragments or joined with other
polypeptides
– Chemical modifications: addition of carbohydrates
or lipids
– Transport to its destination
Bigger Picture:
What happens to
these proteins once
they are made?
Summary
Mutations and their consequences
• Mutation = a change in the sequence of bases
within a gene
– Caused by a mistake during DNA replication (rare)
– Or due to environmental factors called mutagens
• Mutations can be somatic or germinal
What is the main difference in the outcome of
germinal mutation compared to the outcome of
somatic mutation?
Types of Mutations
• Point mutations (substitutions) – change in a
single nucleotide
– Due to redundancy of the genetic code it may change
the amino acid, it may not “wobble”
– Silent mutations do not change the protein
Normal hemoglobin DNA
Sickle-cell hemoglobin
mRNA
Protein
Glu
Val
Types of Mutations
• Frame-shift mutation – caused by insertion
or deletion of a nucleotide
– Changes the reading frame of the codons,
usually results in a non-functional protein
Phe
Leu
Gly
Ala
Ala
His
Are all mutations bad?
• Although mutations are
sometimes harmful…..
– They are also the
source of the rich
diversity of genes in
the world
– They contribute to the
process of evolution
by natural selection