Download L3 - DNA Translation (Protein Synthesis

The Central Dogma Continued:
Translation (Protein Synthesis)
Translation
• The translation of the information
contained in mRNA into protein requires a
supply of amino acids, tRNA molecules,
mRNA, ribosomes, and a number of
enzymes.
• Translation occurs in four steps:
1.
2.
3.
4.
Translation, cont.
Activation of tRNA
Initiation
Elongation
Termination
Translation, cont.
Activation
Initiation
• In translation, tRNA molecules carry amino
acids to mRNA bound to ribosomes. A tRNA
molecule carrying its specified amino acid is
termed an activated tRNA.
• The initiation of protein synthesis begins as a
tRNA molecule carrying the N-terminal amino
acid (usually a form of methionine) meets with
the mRNA molecule on a ribosome forming a
complex at a specific site called the peptidyl site,
or P site.
• Activation occurs by the reaction of tRNA
molecules with ATP. Once activated,
enzymes in the cell match the correct amino
acid with the correct tRNA molecule.
• Each cell contains a complete set of enzymes
for all of the AA/tRNA combinations.
Translation, cont.
Structure of tRNA:
• The amino acid
attachment site is at
the open end of the
cloverleaf (the 3’ end),
and the anticodon is
located in the loop
opposite the open end.
• The P site represents the initiation site on a
ribosome where the initiation codon on the
mRNA must match up with the anticodon on the
tRNA. The binding occurs as H-bonds are
formed between the bases of the mRNA codon
and the anticodon on the tRNA meet.
Translation, cont.
Initiation
• To form the initiated complex, mRNA and a
small ribosomal subunit join so the initiating
codon (AUG) is aligned with P site of subunit.
• tRNA brings in methionine (eukaryotes) or Nformylmethionine (prokaryotes).
• Large ribosomal subunit attaches to complete
ribosome.
1
Translation, cont.
Elongation
• The next incoming tRNA bonds at a second
binding site called the A site on the mRNA.
• Once the tRNA binds at the A site with the
correct amino acid, a peptide bond is formed
between the amino acids attached to the 3’ end
of the two tRNA molecules catalyzed by
peptidyl transferase.
• The ribosome then moves along the mRNA
molecule to the next binding codon, known as
translocation, and releases the “empty” tRNA
from the p site as the tRNA from the A site
move in to make the A site available to receive
the next tRNA.
Translation, cont.
Elongation
Translation, cont.
Translation, cont.
Termination
• Once the ribosome reaches a termination
codon, termination begins
• The now empty ribosome then dissociates
and can then bind to another strand of
mRNA to begin again.
• Several ribosomes can be attached to an
mRNA strand at any one time resulting in
the formation of many peptide chains at the
same time. Such complexes of many
ribosomes on one mRNA molecules are
called polysomes or polyribosomes.
Mutations
A mutation is any change resulting in an
incorrect base sequence on DNA.
Occur naturally during DNA replication.
Induced by environmental factors.
• Ionizing radiation (X-rays, UV, γ)
• Mutagens – chemical agents.
Not necessarily harmful.
Could lead to the production of beneficial,
harmful or inactive proteins.
Rough Endoplasmic Reticulum
with Riobosomes along outer
surface.
Ribosomes along two
long mRNA Molecules.
Upper: emerging
Nascent Polypeptides.
They emerge from
each ribosome during
translation
Genetic Engineering
Genetic engineering, recombinant DNA
technology, genetic modification/manipulation (GM)
and gene splicing are terms that are applied to the
direct manipulation of an organism's genes.
Genetic engineering uses the techniques of
molecular cloning and transformation to alter the
structure and characteristics of genes directly. Since
a protein sequence is specified by a segment of DNA
called a gene, novel versions of that protein can be
produced by changing the DNA sequence of the
gene.
2
i.e. Recombinant DNA
Genetic Engineering
There are a number of ways through which
genetic engineering is accomplished.
Essentially, the process has four main steps:
1.Isolation of the genes of interest.
2.Insertion of the genes into a transfer vector.
3.Replication of cellular genome for
production of modified gene.
4.Separation of the genetically modified
organism or protein of interest.
Bacteria
plasmids
are often
used as
vectors
Recombinant DNA Con’t.
Genetically modified bacterium can then be
separated from unmodified bacterium and
reproduced to harvest products of modified
DNA.
VIDEO
3