Lecture 6: Gene to protein Tranlsation Flashcards
Genetic information flows from _____ through the process of translation
mRNA to protein
A cell translates an mRNA message into a protein with the help of
transfer RNA (tRNA)
What do tRNAs do?
Transfer amino acids to the growing polypeptide in a ribosome
The tRNA molecule:
- A cell has all 20 amino acids available in its cytoplasm.
- Molecules of tRNA are not identical.
- Each carries a specific amino acid on one end.
- Each has an anticodon on the other end that base-pairs with a complementary codon on mRNA
What does the anticodon bind to? and the other end?
base pairs with with a complementary codon on mRNA
- an amino acid binds to the other end
The structure of tRNA:
a tRNA molecules consists of about ___ ____ folded back on itself
80 nucleotides
The structure of tRNA: includes a loop containing …
the anticodon and an attachment site at the 3’ end for an amino acid
The structure of tRNA: hydrogen bonds cause the molecule to
twist and fold into a three-dimensional molecule
Shape of tRNA
is roughly L shaped
Like other types of RNA, tRNA molecules are transcribed from
DNA templates
In both eukaryotic and bacterial cells are tRNA used repeatedly?
YES:
• It picks up its designated amino acid in the cytosol, and deposits it at the ribosome.
• The tRNA then returns to the cytosol to pick up another copy of that same amino acid.
Accurate translation requires two recognition steps:
- A correct match between a tRNA and an amino acid
2. A correct match between the tRNA anticodon and an mRNA codon
recognition step 1: 1. A correct match between a tRNA and an amino acid
- Each amino acid is joined to the correct tRNA by aminoacyl- tRNA synthetase
Aminoacyl-tRNA synthesise work:
- There are 20 different synthetases that match the 20 different amino acids.
- Each has an active site specific for a particular tRNA–amino acid combination.
- The synthetase catalyzes a covalent bond between them in a process driven by ATP hydrolysis.
- The result is an aminoacyl- tRNA or charged amino acid
Recognition step 2:
2. A correct match between the tRNA anticodon and an mRNA codon
- If each anticodon had to be a perfect match to each codon, we would expect to find 61 types of tRNA
- The actual number is about 45, because the anticodons of some tRNAs recognize more than one codon.
- Rules for base pairing between the third base of the codon and the anticodon are relaxed.
- This flexible base pairing is called wobble.
Site for translation:
RIBOSOME
Ribosomes facilitate
specific coupling of tRNA anticodons with mRNA codons in protein synthesis
A ribosome consists of
a large and a small subunit, each made up of proteins and ribosomal RNA (rRNA).
Why is rRNA the most abundant type of cellular RNA
because most cells contain thousands of ribosomes
A ribosome has 3 binding sites for tRNA::
– The P site holds the tRNA that carries the growing polypeptide
chain
– The A site holds the tRNA that carries the next amino acid to be added to the chain
– The E site is the exit site, where discharged tRNAs leave the ribosome
P site =
Peptidyl-tRNA binding site
A site -
Aminoacyl-tRNA binding site
E site -
Exit site
Ribosomes work in translation:
- The ribosome holds the tRNA and mRNA in close proximity and positions the new amino acid for addition to the carboxyl end of the growing polypeptide.
- It then catalyzes the formation of the peptide bond.
- As the polypeptide becomes longer, it passes through an exit tunnel and is eventually released.
3 stages of translation:
-initiation
-elongation
-termination
• All three stages require protein “factors” that aid in the translation process
Ribosome association and initiation of translation
-A small ribosomal subunit binds with mRNA and a special initiator tRNA which carries methionine binds to the start codon.
- Attachment of a large ribosomal subunit, forming the translation initiation complex then occurs.
• Energy in the form of a GTP molecule is invested in the formation of the initiation complex. GTP –> GDP + Pi
• Initiator tRNA is in the P site.
Ribosome association and initiation of translation: In BACTERIA
In bacteria, the binding occurs at a specific RNA sequence, just upstream of the start codon, AUG.
Ribosome association and initiation of translation: In EUKARYOTE
In eukaryotes, the small subunit, with the initiator tRNA already bound to the mRNA, binds to the 5ʹ cap of the mRNA and then moves downstream along the mRNA until it reaches the start codon AUG.
Elongation stage:
During the elongation stage, amino acids are added one by one to the preceding amino acid at the C-terminus of the growing chain
• Each addition involves proteins called elongation factors and occurs in three steps:
– codon recognition
– peptide bond formation
– translocation
• Translation proceeds along the mRNA in a 5′to3′ direction
Elongation:
1. Codon recognition: Incoming tRNA base pairs with 3ʹ complementary mRNA anti-codon in A site
- Peptide bond formation:
Polypeptide from tRNA in the P site is attached to the amino acid in the A site via a peptide bond. - Translocation:
The mRNA with its bound tRNAs moves along bring the next codon to be translated to the A site.
The tRNA from the A site moves to the P. The empty tRNA in the P site moves to the E site and is released.
Termination of trnalstion: Termination occurs when
when one of the three stop codons reaches the A site of the ribosome.
- A release factor binds to the stop codon and causes hydrolysis of the bond between the polypeptide and its tRNA in the P site.
- This frees the polypeptide, which is released through the exit tunnel of the ribosome’s large subunit.
- The translation complex disassembles.
- Break down of the translation assembly requires the hydrolysis of two more GTP molecules
Polyribosomes: A single mRNA may be used to make
many copies of a polypeptide simultaneously as multiple ribosomes, polyribosomes or polysomes, trail along the same mRNA.
Often translation is not sufficient enough to make ___
SO..
- Often translation is not sufficient to make a functional protein.
- Polypeptide chains are modified after translation or targeted to specific sites in the cell
During and after synthesis, a polypeptide chain spontaneously
could and folds into its 3D shape
Proteins may all require post-translational modification before doing their job
– Some polypeptides are activated by enzymes that cleave them
– Other polypeptides come together to form the subunits of a protein
Protein folding:
- During and after synthesis, a polypeptide chain spontaneously coils and folds into its three-dimensional shape
- Amino acids interact with each other to produce a well-defined three-dimensional structure
- Some neurogenerative diseases result from accumulation of amyloid fibrils formed when proteins fold incorrectly.
- Some allergies may result from misfolded proteins.
two populations of ribosomes are evident in cells:
- free ribosomes (in the cytosol)
- bound ribosomes (attached to the ER)
Free ribosomes mostly
synthesise proteins that function in the cytosol
Bound ribosomes make
proteins of the endomembrane system and proteins that are secreted from the cell
ribosomes are identical and can
switch from free to bound
polypeptides destined for the ER or for secretion are marked by
a signal peptide
Targeting polypeptides to specific locations:
1) Polypeptide synthesis always begins in the cytosol
2) A signal recognition particle (SRP) binds to the signal peptide of a polypeptide momentarily stopping translation
3) The SRP binds to a receptor protein in the ER membrane
4) The SRP leaves & polypeptide synthesis resumes with simultaneous translocation of the polypeptide across the membrane
5) a Signal leaving enzyme cuts off the signal peptide
6) The completed polypeptide is released
