PROTEIN SYNTHESIS
• The formation of new proteins
using the code carried on DNA
Gene is Selected
• A gene is a section of DNA coding for
a particular polypeptide or protein.
• When a protein is required by the cell
or the organism as a whole, the gene
for that protein is activated.
Gene
Introns and Exons
Intron
Exon
Intron
Exon
Intron
Exon
Intron
Exon
• DNA is made up of coding sequences
called exons and non coding
sequences called introns.
Transcription
• An enzyme RNA Polymerase
unwinds the section of DNA carrying
the desired gene.
• Only one strand of the DNA (the
template) is recognised.
• RNA Polymerase then binds to a
promoter site and begins to form an
RNA copy of the template strand
(called messenger RNA (mRNA)).
Naming the Strands
DNA
• The DNA strand is unwound exposing
the bases that carry the genetic code.
• An mRNA copy of the top coding
strand is made by base pairing with
the bottom template strand.
Coding strand
Primary RNA
Template strand
Messenger RNA
• Free nucleotides match up with the
coded sequence of DNA.
• When bases pair, Uracil is present in
RNA instead of Thymine.
• A single stranded primary transcript
that is complementary to the template
strand is formed.
• DNA template
TAC GCC AAA TCC
• mRNA codons AUG CGG UUU AGG
Removal of Introns
• The primary transcript contains a
copy of both introns and exons.
• Introns are removed and exons are
spliced (joined) to form mature mRNA.
Movement of mRNA
• The completed single strand of mRNA
migrates out of the nucleus through
pores in the nuclear membrane.
• mRNA binds to ribosomes.
• The mRNA is divided into triplets of
bases called codons.
• Different codons carry the code for a
particular amino acid (AA)
• Other codons carry start and stop
codes.
Important Codon Facts
• AUG is the start codon on all viable
mRNA molecules. It also codes for
the amino acid Methionine.
• UAA, UAG, UGA are stop codes.
• Many codons are degenerate – more
than one codon codes for an AA.
• The code is universal. The same
codes work in all organisms.
• 43 different combinations give 64
different codons.
Amino Acid Codons
Transfer RNA
• Transfer RNA (tRNA) is a carrier
molecule which picks up amino acids
from the cytoplasm and carries them
to the mRNA / ribosome complex.
AA
Methionine
U A C
anticodon
Translation
• A triplet of bases on the tRNA called
an anticodon matches with the
mRNA codon.
• tRNA carrying its AA binds to the
codon
• The ribosome moves along the
mRNA activating new codons.
• AA’s are joined by peptide bonds to
form long polypeptide chains.
Ribosome Action
• The ribosome acts as catalyst holding
all the components together while
bonds are formed.
AA1
AA2
U A C G C C
A U G C G G U U U A G G C C C U A G
Peptide Bond Formation
• As the ribosome moves relative to the
mRNA, amino acids are added until a
stop signal is reached.
AA1
AA2
AA3
AA4
AA5
stop
A U C
A U G C G G U U U A G G C C C U A G
Translation speed
• Rapid protein formation can be
achieved when the mRNA is
attached to more than one ribosome.
Electron Microscope View
• mRNA (purple) binds to a ribosome.
• Beads of AA’s (yellow) are joined to
form a polypeptide.
Questions
• What name is given to a section of DNA coding for a
polypeptide?
• The coding sequences of DNA are known as what?
• Name the enzyme responsible for unwinding the DNA
helix.
• How does the enzyme recognize the start of a
message?
• Which DNA strand is used in the base pairing process?
• What name is given to the RNA strand first formed,
that contains introns and exons?
• In RNA, which base binds with Adenine?
• What is the codon for the start of an mRNA?
• What are the three stop codons?
• What does the term “codons are degenerate” mean?
• Why is the codon a three base sequence?
• What does the term, “the genetic code is universal”
mean?
• What name is given to the process of forming mRNA?
• What is the full name for tRNA?
• What is the role of tRNA?
• mRNA has a codon, tRNA has an?
• What holds amino acids together in a polypeptide?
• What is the role of the ribosomes?
• How do the ribosomes operate that allows long
amino acid chains to form?
• What name is given to the process of converting an
mRNA code into an amino acid sequence?
• Describe one way in which the rate of translation can
be increased?