BIOL 3301 EXAM QUESTIONS AND ANSWERS
LATEST UPDATE (GRADED A+)
Features of the genetic code
Four mRNA nucleotides combine to form 64 different 3-letter combinations (codons)
- Of the 64 codons, 61 sense codons specify amino acids
- AUG is always the first codon read in an mRNA translation
- UAA, UAG, and UGA are stop codons that do not code for amino acids and stop polypeptide synthesis
The genetic code is universal?
Essentially the same in all living organisms and viruses
The genetic code is commaless?
Meaning with no indicators to mark the end of one codon and the beginning of the next
degeneracy
An amino acid is represented by more than one codon
Methionine and Tryptophan
Only two amino acids specified by a single codon
What are the 21st and 22nd amino acids?
Selenocysteine and Pyrrolysine
- Found in specialty enzymes
- Encoded by UGA and UAG codons, respectively
- Attached by tRNAs that carry them to the ribosome
- Codon and downstream sequences in mRNA are needed to incorporate these amino acids
How was the genetic code deciphered?
By two research groups, headed by Nirenberg and Khorana in the 1960s.
- The goal: Experiment aims to help decipher the relationship between base composition and particular
amino acids.
In the 1960s, what did Khorana and his collaborators develop?
Developed a novel method to synthesize RNA; they first created short RNAs (2-4 nucleotides long) that
had a defined sequence. These were then linked together enzymatically to create long copolymers.
They used these copolymers in a cell-free translation system.
- Example: synthetic RNA "UC"
~ forms polymer with codon possibilities UCU, CUC
~ incorporated amino acids serine and leucine
In 1964 what did Nirenberg and Leder discover?
, Discovered that a 3 nucleotide RNA could cause a ribosome to bind a tRNA
1. They mixed one triplet in 20 tubes each with a different radiolabeled amino acid
2. Samples were filtered
3. Only amino acids stuck to the ribosomes were retained
4. The tube with a large amount of retained radioactivity was the corresponding amino acid
How does recognition between mRNA-tRNA work?
During mRNA-tRNA recognition, the anticodon in tRNA binds to a complementary codon in mRNA
- tRNAs are named according to the amino acid they bear
NOTE: The anticodon is antiparallel to the codon
aminoacyl-tRNA synthetase enzymes
Specific to each amino acid, load amino acids onto tRNA
- There are 20 types
- It catalyzes a two-step reaction involving three different molecules:
Amino acid, tRNA, and ATP
Modified bases may affect
translation rates, recognition by aminoacyl-tRNA synthetases, codon-anticodon recognition
The genetic code is degenerate, w/ the exception of serine, arginine, and leucine.
Serine, Arginine, and Leucine, this degeneracy always occurs at the codon's third position.
Wobble hypothesis
Proposed by Francis Crick in 1966, a mechanism that explains how one anticodon can pair with more
than one codon.
In the codon-anticodon recognition process, the first two positions pair strictly according to the
A-U/G-C rule, however the third position:
Third position can actually "wobble" or move a bit
- Thus tolerating certain types of mismatches
Isoacceptor tRNAs
two or more tRNAs that differ at the wobble position but can recognize the same codon
Where does translation occur?
On the surface of a large macromolecular complex termed the ribosome
What kind of ribosomes does bacteria and eukaryotes have?
Bacterial cells have one type of ribosome
- found in their cytoplasm
Eukaryotic cells have two types of ribosomes
- One type is found in the cytoplasm
LATEST UPDATE (GRADED A+)
Features of the genetic code
Four mRNA nucleotides combine to form 64 different 3-letter combinations (codons)
- Of the 64 codons, 61 sense codons specify amino acids
- AUG is always the first codon read in an mRNA translation
- UAA, UAG, and UGA are stop codons that do not code for amino acids and stop polypeptide synthesis
The genetic code is universal?
Essentially the same in all living organisms and viruses
The genetic code is commaless?
Meaning with no indicators to mark the end of one codon and the beginning of the next
degeneracy
An amino acid is represented by more than one codon
Methionine and Tryptophan
Only two amino acids specified by a single codon
What are the 21st and 22nd amino acids?
Selenocysteine and Pyrrolysine
- Found in specialty enzymes
- Encoded by UGA and UAG codons, respectively
- Attached by tRNAs that carry them to the ribosome
- Codon and downstream sequences in mRNA are needed to incorporate these amino acids
How was the genetic code deciphered?
By two research groups, headed by Nirenberg and Khorana in the 1960s.
- The goal: Experiment aims to help decipher the relationship between base composition and particular
amino acids.
In the 1960s, what did Khorana and his collaborators develop?
Developed a novel method to synthesize RNA; they first created short RNAs (2-4 nucleotides long) that
had a defined sequence. These were then linked together enzymatically to create long copolymers.
They used these copolymers in a cell-free translation system.
- Example: synthetic RNA "UC"
~ forms polymer with codon possibilities UCU, CUC
~ incorporated amino acids serine and leucine
In 1964 what did Nirenberg and Leder discover?
, Discovered that a 3 nucleotide RNA could cause a ribosome to bind a tRNA
1. They mixed one triplet in 20 tubes each with a different radiolabeled amino acid
2. Samples were filtered
3. Only amino acids stuck to the ribosomes were retained
4. The tube with a large amount of retained radioactivity was the corresponding amino acid
How does recognition between mRNA-tRNA work?
During mRNA-tRNA recognition, the anticodon in tRNA binds to a complementary codon in mRNA
- tRNAs are named according to the amino acid they bear
NOTE: The anticodon is antiparallel to the codon
aminoacyl-tRNA synthetase enzymes
Specific to each amino acid, load amino acids onto tRNA
- There are 20 types
- It catalyzes a two-step reaction involving three different molecules:
Amino acid, tRNA, and ATP
Modified bases may affect
translation rates, recognition by aminoacyl-tRNA synthetases, codon-anticodon recognition
The genetic code is degenerate, w/ the exception of serine, arginine, and leucine.
Serine, Arginine, and Leucine, this degeneracy always occurs at the codon's third position.
Wobble hypothesis
Proposed by Francis Crick in 1966, a mechanism that explains how one anticodon can pair with more
than one codon.
In the codon-anticodon recognition process, the first two positions pair strictly according to the
A-U/G-C rule, however the third position:
Third position can actually "wobble" or move a bit
- Thus tolerating certain types of mismatches
Isoacceptor tRNAs
two or more tRNAs that differ at the wobble position but can recognize the same codon
Where does translation occur?
On the surface of a large macromolecular complex termed the ribosome
What kind of ribosomes does bacteria and eukaryotes have?
Bacterial cells have one type of ribosome
- found in their cytoplasm
Eukaryotic cells have two types of ribosomes
- One type is found in the cytoplasm
