Final Flashcards
What are the four main traits of the genetic code?
Nonoverlapping, Continuous, Degenerate, and Read as Triplets
What is degeneracy? How can it help shield against harmful mutations?
The third position in the codon is often redundant. For example, UCU, UCA, UCG, and UCC all code for the same thing: Serine.
If there is a mutation in the third position of this codon, it will not affect its function, shielding it from harmful mutations.
Explain Nirenberg & Matthei, 1961 (The test that showed what UUU codes for)
They synthesized a long string of just U’s (3’-U-U-U-U-U-U-5’). They then set up 20 reactions, each with a different radioactive amino acid. The only one that gave a reaction was Phenylaline, and they concluded that UUU coded for Phe.
What is at the 3’ end of every tRNA?
The sequence CCA-3’. The amino acid attaches to this end.
How does the Wobble position allow tRNA to decode more than one codon?
The third base in a codon is called the ‘wobble position’ (AKA the 5’ position on the anticodon) because it often doesn’t determine the amino acid; that is what the first and second bases do. A modified nucleotide, ‘I,’ is sometimes put in this third position, and can be read as A, U, or C.
Because it can be read as three different bases, it allows the tRNA to decode more than one codon.
What is an anticodon?
Each tRNA has only one anticodon. In the secondary structure, one end of the tRNA contains its anticodon, which matches to a codon that specifies a certain amino acid. On the other end of the tRNA, there is an attachment site for the corresponding amino acid.
This is how tRNA translates from a codon to an amino acid.
What is tRNA’s secondary structure? What does the tRNA do in this structure?
tRNA’s secondary structure is a cloverleaf. In this structure, the tRNA can pair with a codon and translate it to an amino acid.
What is a tRNA’s tertiary structure?
tRNA’s tertiary structure is an upside-down L shape.
What do aminoacyl-tRNA synthases do? (3 things)
1) They are an enzyme that adds the correct amino acid to the correct tRNA
2) They define the genetic code
3) They use proof-reading to get high specificity
What is the basic structure of a ribosome?
1) Small subunit: Reads the RNA
2) Large subunit: Joins amino acids to make a polypeptide chain.
Both contain multiple rRNA molecules and protein components.
What does the Shine-Dalgarno sequence do?
It is a ribosomal binding site on the mRNA of prokaryotes. It initiates protein synthesis.
It is about 8 bases upstream of the start codon AUG, and it is typically a “AGGAGG” sequence (a purine-rich sequence).
How does the ribosomal RNA bind to the mRNA?
The rRNA has a complementary sequence at it’s 3’ end (on the 16S rRNA) for the small subunit to bind to the mRNA.
What does the A site do in a ribosome?
The A site is the point of entry for the aminoacyl tRNA (except for the first aminoacyl tRNA, which enters at the P site).
What does the P site do in a ribosome?
The P site is where the peptidyl tRNA is formed in the ribosome
What does the E site do in a ribosome?
The E site is the exit site of the now uncharged tRNA after it gives its amino acid to the growing peptide chain
How does termination occur?
A termination codon moves into the A site, but because the stop codons aren’t recognized by tRNA, a protein (called a release factor) is required for termination.
This hydrolyzes the ester bond and releases the newly synthesized protein.
What does the 5’ cap do for eukaryotes?
The 5’ cap is part of the small subunit, and it helps bind to ribosome to the mRNA.
How does initiation occur?
The small subunit scans along the mRNA until it finds the AUG start codon. This also establishes the reading frame for the mRNA strand.
What is the polyA tail binding protein? What is its importance?
The polyA tail binding protein binds onto the 3’ polyA tail in eukaryotes. Eukaryotes require this because it ensures that ribosomes only bind to intact mRNAs.
How does the ribosome know what amino acids to use?
The ribosome uses the mRNA as a set of instructions to tell it which amino acids to grab. The ribosome grabs tRNAs that are attached to the proper amino acids, take the amino acid off, and then let the tRNA go back into the cell to attach to another amino acid.
How are the amino acids joined together to make the polypeptide chain?
Amino acids are linked together by the peptidyl transferase reaction. The a-amino group of the A-site tRNA attacks the ester bond linking the P-site tRNA to its amino acid. That amino acid from the P-site tRNA is transferred to the amino acid of the A-site tRNA through a peptide bond
What component of the ribosome is peptide bond formation catalyzed by?
Peptide bond formation is catalyzed by the RNA component of the ribosome. This is known because X-Ray Crystallography was done to completely visualize the ribosome, and no protein was found near the active site.
