The beginning of protein
synthesis
OVERVIEW



Uses a strand of nuclear DNA to produce a
single-stranded RNA molecule
Small section of DNA molecule (gene) unwinds
and free-floating nucleotides bind to the strand
The strands are complimentary
Eg. DNA - A T A T G C
RNA - U A U A C G
with U replacing T in RNA
1.
The enzyme RNA POLYMERASE binds to the
PROMOTER site on the DNA
2.
The DNA being transcribed is separated by
the RNA polymerase in the region where the
gene is to be transcribed
3.
RNA nucleotides pair with their
complimentary bases on one strand of the
DNA.
REMEMBER: Uracil pairs with adenine on the RNA
4.
RNA polymerase forms covalent bonds
between the nucleotides
5. RNA separates from the DNA and the double
helix reforms




The section of DNA being read is called the ANTISENSE or TEMPLATE strand
The opposite side of the DNA is called the SENSE
or COMPLIMENTARY strand
In eukaryotes, the immediate product of
transcription is known as PRE-mRNA
 it must go through several stages
of modifications before becoming
MATURE mRNA
One of these is called RNA SPLICING
 throughout the mRNA are sequences that will
not contribute to the formation of the polypeptide
 these pieces of mRNA are called INTRONS
 the introns must be removed and the
remaining coding positions of mRNA are called
EXONS
 the exons are spliced (joined) together to
form mature mRNA
 the ends are capped (METHYLATED CAP) and
tailed (POLY- A TAIL)
VIDEO:
TRANSCRIPTION
TRANSLATION
THE FORMATION OF A PROTEIN
Translation Overview
Translation is the process of protein production using mRNA as a guide
THE GENETIC CODE








Is the code used that enables the cells machinery
to convert the base sequence into an amino acid
The sequence of 3 bases on the mRNA is called a
CODON
Each codon (triplet) codes for a specific amino
acid which will be added to the polypeptide
Each amino acid is carried by a specific transfer
RNA (tRNA) which has a 3-base code
complimentary to the mRNA called an
ANTI-CODON
A tRNA with the correct anti-codon attaches to
the codon on the mRNA
There are 64 possible codons
Different codons may code for the same amino
acid
‘STOP’ codons code for translation to stop
RIBOSOMES





Translation takes place outside the nucleus at the
RIBOSOMES
They consist of protein combined with
RIBOSOMAL RNA (rRNA) which does not
contain a genetic message
Ribosomes contain 2 sub-units: a small 40s subunit and a larger 60s sub-unit
Generally proteins secreted from cells are
synthesised on ribosomes bound to the ER and
those that remain in the cytosol are made on free
ribosomes
May form POLYRIBOSOMES or POLYSOMES
that all bind to a single mRNA moving along it
like a production line. This allows more than one
ribosome to translate the mRNA at the same time
Translation Diagrams
RIBOSOME
POLYSOME
Translation, the process
1.
2.
3.
4.
5.
6.
7.
8.
mRNA binds to the small subunit (40s) of the
ribosome
tRNA molecules carry a specific amino acid
corresponding to its anti-codon
The tRNA binds to the ribosome at the site where its
anticodon matches the codon on the mRNA
The ribosome moves along the mRNA, adding amino
acids to the polypeptide chain, until a stop codon is
reached
The completed polypeptide is then released
Each tRNA molecule is recognised by a tRNAactivating enzyme that binds the specific amino
acid to the tRNA, using ATP for energy
The amino acid attaches at the 3’ end of the tRNA
which terminates with the sequence CCA
Each ribosome has 3 tRNA binding sites:
E – Exit site
P – Peptidyl site
A – Aminoacyl site
(see diagram previous)
A tRNA molecule
Translation diagram
VIDEO:
TRANSLATION
VIDEO:
CRASHCOURSE
PROTEIN SYNTHESIS
Every time a cell divides, the genetic information contained
in the chromosomes needs to be COPIED and DISTRIBUTED
to the DAUGHTER CELLS
STEP 1:
The double
stranded DNA
molecule is acted
upon by the enzyme
DNA HELICASE.
The enzyme breaks
the hydrogen bonds
between the base
pairs. The strands
move apart creating
replication forks.
STEP 2:
• Each of the strands acts as
a template for a new DNA
strand. A short sequence,
made of RNA called a
PRIMER attach to the
single DNA strands and
initiate replication
•Individual nucleotides are
added at the 3’ end of the
DNA strand according to
the base-pairing rules
•The process is catalysed by
the enzyme DNA
POLYMERASE by linking
the phosphate of the
newest nucleotide to the
sugar before it
STEP 3:
• One DNA strand (THE
LEADING STRAND) forms
continuously in the direction
of replication
•On the other DNA strand
(travelling in the opposite
direction) forms
discontinuously. This strand
is called the LAGGING
STRAND. The fragments on
the lagging strand are joined
by the enzyme DNA LIGASE
• The lagging strand must
wait for the DNA to unwind
before the DNA polymerase
can act on the template. This
results in the new DNA
strand forming in sections
called OKAZAKI
FRAGMENTS
STEP 4:
• Each of the double helix
molecules is an EXACT
copy of the original DNA
•The process is
SEMI-CONSERVATIVE
 each of the new double
stranded molecules
contains one of the
ORIGINAL DNA strands
and one NEW DNA strand
DNA
REPLICATION
ENZYMES