Genetic code Flashcards
what is an mRNA?
A messenger mRNA that carries genetic code from DNA to ribosomes
what is a tRNA?
A transfer RNA that plays a crucial role in protein synthesis
Each tRNA molecule is specifically designed to carry a particular amino acid. This specificity is determined by the tRNA’s anticodon, a sequence of three nucleotides that recognizes and binds to a complementary codon on the mRNA
How do mRNA and tRNA work together to make a protein?
mRNA have specific codons
tRNA have complementary anticodons
when the mRNA is in the ribosome, the tRNA binds to the mRNA codons. This brings the amino acids together in the correct order to make a protein. The amino acids are then joined together by peptide bonds to form a protein chain
what is a cognate tRNA?
the correct codon or amino acid is referred as cognate tRNA
What did scientists Crick and Brenner discover about genetic code?
- The code works in sets of three letters (triplets or “codons”)
- Every letter in the genetic sequence matters and is read as part of these triplets
- Multiple different triplets can code for the same amino acid
What are the three key properties of the genetic code discovered through genetic studies?
- It is based on triplet codon sequences
- It is nonpunctuated (all nucleotides have genetic meaning)
- It is degenerate (multiple codons have the same meaning)
Define Aminoacylation?
The process of attaching an amino acid to a tRNA molecule. It can be referred to as “charging” the tRNA.
How is the protein sequence determined?
its inferred from the DNA sequence using the genetic code
how did scientists crack the code?
By using ribosome binding assays and vitro translation
Describe the process of the Ribosomes binding assay?
They would put together ribosomes, a specific mRNA with exposed codons, and a bunch of different tRNA molecules with different amino acids associated with them.
Let the mix set for a bit
Then, they would observe which aminoacyle-tRNA molecule binds to which RNA codon. This allows them to find the codons that code for each amino acid.
They also found that aminoacyl-tRNA would knly form complexes with ribosomes if it was a cognate (matching) tRNA to the mRNA codon.
What is Inosine (I)?
- Inosine (I) is a modified version of the base adenine.
- It can be found in the first position of the tRNA anticodon.
- It’s involved in wobble base pairing because it can pair with multiple other bases.
- It can form base pairs with A / U / C
Describe the structure of tRNA?
tRNAs fold up into a common “cloverleaf” secondary structure
The amino acid is attached to the 3’ hydroxyl group of the 3’ terminal A nucleotide.
All tRNAs have the 3’ terminal nucleotides CCA
How many types of codons are there in the genetic code and what do they encode?
Total of 64 codons:
61 sense codons (encode specific amino acids)
3 stop/termination codons
AUG serves as the start/initiation codon (almost always)
What does synonymous codons mean?
Codons that code for the same amino acid
THEY TEND TO DIFFER AT THE 3RD NUCLEOTIDE
The genetic code is degenerate. what does that mean?
Degenerate - Multiple codons code for the same amino acid (64 triplet codons / 20 standard amino acids)
there are more codons than amino acids so multiple codons code for the same amino acid
Describe the process of in vitro translation
They radioactively labelled amino acids and placed it in a tube with a known sequence of RNA. For example, UUU repeated
Then, they worked to see which amino acids were incorporated in the protein formed by looking at the radioactivity of the isotopes in the product. For example, amino acid, phenylalanine, was incorporated.
This allowed them to match the codons to the amino acid it codes for. i.e. the RNA codon UUU codes for Phenylalanine.
Who established the general nature of the genetic code?
Francis Crick and Sydney Brenner
What enzyme is used to form synthetic RNA?
Polynucleotide Phosphorylase
what are the 3 termination codons?
UAA
UGA
UAG
What is Inosine (I)?
- Inosine (I) is a modified version of the base adenine.
- It can be found in the first position of the tRNA anticodon.
- It’s involved in wobble base pairing because it can pair with multiple other bases.
- It can form base pairs with A / U / C
What do AUG and UGG encode for?
AUG = Methionine
UGG = Tryptophan
what is wobble base pairing?
Wobble base pairing is a non-standard base pairing between two nucleotides in RNA molecules that doesn’t strictly adhere to the conventional Watson-Crick base pairing rules (A-U, G-C).
This phenomenon primarily occurs at the third position of the codon in mRNA during protein synthesis.
what is the acceptor stem loop on the secondary tRNA structure?
Remember it is on the 3’ end and the sequence is CCA
The acceptor stem loop is where the amino acid joins to the tRNA
Why is the poly(U) experiment historically significant?
The poly(U) experiment showed that UUU specifically encoded phenylalanine, providing one of the first direct pieces of evidence for codon-amino acid correspondence and helping to crack the genetic code.
What is coaxial stacking and why is it important for tRNA structure?
Coaxial stacking is when short helices stack on top of each other. This increases the thermodynamic stability of the RNA molecule, helping maintain the L-shape structure.
How is the same amino acid incorporated into protein when there are multiple codons with the same genetic meaning?
Different tRNAs - Same Amino Acid:
Some synonymous codons are recognized by different “isoacceptor” tRNAs
These different tRNAs are all charged with the same amino acid
The aminoacyl-tRNA synthetase for that amino acid recognizes and charges all these different tRNAs
Same tRNA - Multiple Codons (Wobble Base-Pairing):
One tRNA can actually recognize multiple different codons that differ at their 3rd base
This happens through “wobble base-pairing”
Many tRNAs have a modified nucleotide called inosine (I) at the 1st position of their anticodon
Inosine can base-pair with three different bases:
U (Uracil)
A (Adenine)
C (Cytosine)
what enzyme are tRNA charged with? Does this reaction require ATP?
aminoacyl- tRNA synthetase
Yes this reaction requires ATP
Where and how is an amino acid attached to tRNA?
tRNA is charged by aminoacyl tRNA synthetase enzyme - all types of tRNA use the same enzyme - and the reaction requires ATP
In the reaction, ATP is converted to AMP
and the amino acid is linked to the 3’ hydroxyl group of the terminal adenosine nucleotide via its carboxyl group by an ester linkage.
