Messenger RNA carries genetic information from DNA in the nucleus to the ribosomes in the cytoplasm for protein production. Transfer RNA molecules ferry individual amino acids to the ribosome according to the three-nucleotide codon sequences on mRNA. The ribosome joins the amino acids together into a protein chain according to these codons. The genetic code is nearly universal and uses 64 possible triplet codon combinations to specify 20 different amino acids.
1. Translation: From Messenger RNA to Protein
•The information encoded in DNA is transferred to
messenger RNA and then decoded by the ribosome to
produce proteins.
•The major molecules involved in translation are:
-Messenger RNA (mRNA)
-Transfer RNA (tRNA)
-Ribosomes
-Enzymes, other factors, energy sources
3. •Carries information copied
from DNA in nucleus to
ribosomes in cytoplasm
•Will become associated
with ribosomes
•Will direct protein
synthesis, putting amino
acids in the proper order on
the growing chain
Messenger RNA
4. Transfer RNA
• Carrier molecule which transfers amino acids to the protein
chain
• tRNA is made in the nucleus from a DNA template
• Each tRNA is specific to carry one of the 20 amino acids
used to make proteins
• tRNA’s sit attached to their specific amino acids in a pool
in the cytoplasm
• tRNA are about 80 nucleotides long, are folded uniquely,
due to hydrogen bonding
• Are recyclable, can be used again and again
7. Ribosomes
• Very small particles (25nm) that are key
components in protein synthesis
• Made up of 2 subunits, a large and a small.
• Are found as subunits in the cytoplasm and
attached to ER. They assemble on the
mRNA when translation begins
10. Getting Started: The Genetic Code
– The bases of DNA code for the placement of amino
acids on a growing chain
– 20 different amino acids must be coded for:
• How long is the code?
– a.) If code is 1 base long – only code for 4 amino acids
– b.) If code is 2 bases long – only 16 amino acids can be coded
for
– c.) *If code is 3 bases long – 64 amino acids can be coded for*
– This means the code is a triplet. The 3 mRNA bases are
known as the codon, the 3 tRNA bases that match up
are the anti-codon
11. Getting Started: The Genetic Code
– Facts about the code:
• It is continuous (no comma’s)
• It is non-overlapping
• It is almost universal
• It shows degeneracy – with 2 exceptions, most
amino acids have more than one codon
– this degeneracy is a built in guard against point mutations
– The code has start and stop signals
13. How Translation Works
– tRNA becomes “charged” by picking up an amino acid.
– This is done by the enzyme “amino-acyl tRNA
synthetase”
-One synthetase for each amino acid
-A single synthetase may recognize multiple tRNAs
for the same amino acid
– 5’ end of mRNA comes together with 2 ribosome
subunits:
14.
15. Steps in Translation
• Once the tRNA is charged, translation can
begin
• It will occur in 3 steps:
1.) initiation
2.) elongation
3.) termination
16. 1.) Initation
•needs energy (GTP) + proteins called initiation factors
•“AUG” is always first codon added. tRNA binds to “p-site”
(has anticodon “UAC”)
Three tRNA binding
sites:
A site = amino-acyl
tRNA binding site
P site = peptidyl-tRNA
binding site
E site = exit site
17. In addition to the APE sites there is an mRNA binding groove
that holds onto the message being translated
18. •Next step in translation
is elongation
•Amino acid binds to
A-site
•2 amino acids
undergo peptide-bond
synthesis; catalyzed
by the enzyme
peptidyl transferase
•Dipeptide is attached
to A-site, empty t-
RNA shifts to e-site,
called translocation;
requires the enzyme
translocase.
19.
20. Termination of translation
is triggered by stop codons
When a stop codon is
reached, the following
happens:
1.) protein chain is
released
2.) mRNA +
ribosomes separate
(ribosome splits
into subunits)
requires “release
factors” + GTP
21. Release of the protein causes
the disassociation of the
ribosome into its constituent
subunits.
