The Genetic Code

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Questions and Answers

In 1955, who proposed the need for an adaptor molecule and what role did this adaptor play in the Central Dogma?

  • Watson proposed the need for tRNA to carry amino acids to the ribosome.
  • Pauling proposed the need for an adaptor to catalyze the formation of peptide bonds.
  • Franklin proposed the need for an adaptor to stabilize mRNA structure.
  • Crick proposed the need for an adaptor between nucleic acid and protein 'languages'. (correct)

What is the general size range, in number of nucleotides, of a typical tRNA molecule?

  • 73-93 ntds (correct)
  • 100-120 ntds
  • 50-60 ntds
  • 20-30 ntds

In the context of codon-anticodon interactions, which base of the codon pairs with the first base of the anticodon?

  • The second base of the codon pairs with the second base of the anticodon.
  • The first base of the codon pairs with the third base of the anticodon.
  • The pairing sequence does not matter.
  • The third base of the codon pairs with the first base of the anticodon. (correct)

Which of the following is NOT a stop codon?

<p>AUG (A)</p> Signup and view all the answers

What is meant by the term '3rd position degeneracy' in the context of the genetic code?

<p>The third position in the codon is more flexible in its base-pairing. (B)</p> Signup and view all the answers

How many different codons specify the amino acid methionine (Met) in most organisms?

<p>1 (A)</p> Signup and view all the answers

If a cellular mRNA transcript has the sequence 5'-AUGCGUAGGUCGAUUGG-3', what is the maximum number of amino acids that could be encoded by this sequence?

<p>4 (A)</p> Signup and view all the answers

What accounts for the ability of a single tRNA molecule to recognize multiple codons coding for the same amino acid?

<p>Wobble base-pairing (D)</p> Signup and view all the answers

Which of the following is true regarding the role of inosine in tRNA function?

<p>Inosine allows one tRNA to bind to three codons. (B)</p> Signup and view all the answers

What is the direct role of aminoacyl-tRNA synthases in protein synthesis?

<p>They add amino acids to their corresponding tRNAs. (D)</p> Signup and view all the answers

During tRNA charging, in what form is the amino acid activated before it is transferred to the tRNA?

<p>Amino acid-AMP (A)</p> Signup and view all the answers

What aspect of tRNA enables aminoacyl tRNA synthetases to select the correct tRNA molecule?

<p>The overall shape of the tRNA (B)</p> Signup and view all the answers

Why is accurate tRNA charging essential for protein synthesis?

<p>Ribosomes do not check whether the tRNAs have the proper amino acid. (B)</p> Signup and view all the answers

What is the function of a suppressor tRNA?

<p>To insert an amino acid at a stop codon (B)</p> Signup and view all the answers

What is a potential consequence of a suppressor tRNA inserting an amino acid at a normal stop codon?

<p>The protein will be elongated, potentially affecting its function. (B)</p> Signup and view all the answers

What is the effect of shifting the reading frame during translation?

<p>It introduces stop codons and alters the amino acid sequence. (C)</p> Signup and view all the answers

How might a cell recover from a mutation that introduces a premature stop codon in an essential gene?

<p>By using a suppressor tRNA (A)</p> Signup and view all the answers

What characteristic defines triplet expansion diseases?

<p>Expansion of trinucleotide repeats in DNA (A)</p> Signup and view all the answers

What is a notable exception to the universality of the genetic code?

<p>Mitochondrial genomes (B)</p> Signup and view all the answers

In some fungi, such as Candida albicans, what unusual translation occurs with the codon CUG?

<p>CUG codes for serine instead of leucine (A)</p> Signup and view all the answers

Which of the following best describes the function of the SECIS element?

<p>It facilitates the incorporation of selenocysteine at a UGA codon. (C)</p> Signup and view all the answers

Why are special strains sometimes needed to express proteins with codons that are rare in E. coli?

<p>To provide extra copies of tRNAs that recognize rare codons (A)</p> Signup and view all the answers

What is the role of plasmids in special E. coli strains used to express proteins with rare codons:

<p>They encode extra copies of the rare tRNAs. (B)</p> Signup and view all the answers

If a tRNA normally recognizes the codon 5'-CAG-3', but a mutation arises that creates a suppressor tRNA that recognizes 5'-UAG-3', what is the most likely change in the anticodon sequence of the suppressor tRNA?

<p>The anticodon sequence changes to 3'-AUC-5'. (B)</p> Signup and view all the answers

What determines the reading frame of an mRNA during translation?

<p>The start codon (AUG) (C)</p> Signup and view all the answers

If a mutation causes a frameshift early in the coding sequence of a gene, what is the most likely outcome?

<p>A protein with a completely different amino acid sequence after the frameshift and likely non-functional (A)</p> Signup and view all the answers

Consider a tRNA with an anticodon that includes inosine. The presence of inosine may allow the tRNA to recognize:

<p>Multiple codons with different third bases. (D)</p> Signup and view all the answers

Why is it necessary for aminoacyl-tRNA synthetases to have proofreading capabilities?

<p>To prevent the ribosome from adding the wrong amino acid to the polypeptide chain (B)</p> Signup and view all the answers

Flashcards

Adaptor Molecule

An adaptor molecule needed because nucleic acids and proteins speak different 'languages'.

Anticodon

A specific sequence of three nucleotides on tRNA that base-pairs with a codon on mRNA during translation.

Inosine

A modified nucleoside found in tRNA that can base pair with U, C, or A.

Wobble Base Pairing

The flexibility in base pairing between a codon and anticodon, especially at the 3rd position.

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Aminoacyl-tRNA Synthetases

Enzymes that catalyze the attachment of a specific amino acid to its corresponding tRNA.

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tRNA Charging

The process of attaching the correct amino acid to its tRNA by aminoacyl-tRNA synthetases.

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Proofreading

A mechanism in aminoacyl-tRNA synthetases to ensure the correct amino acid is attached to the tRNA.

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Suppressor tRNAs

tRNAs that can suppress the effect of a stop codon by inserting an amino acid, allowing translation to continue.

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Genetic Code

The set of rules by which information encoded in genetic material (DNA or RNA) is translated into proteins.

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Codon

A sequence of three nucleotides in mRNA that specifies a particular amino acid or a stop signal during translation.

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UAG

A stop codon that signals the termination of translation. Also known at the amber codon.

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Wobble Hypothesis

Allows one tRNA to recognize multiple codons due to non-standard base pairing at the third codon position.

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Inosine

A modified nucleobase that is found in tRNA and can base pair with U, C, and A.

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Aminoacyl-tRNA Synthetases

Enzymes that attach the correct amino acid to its corresponding tRNA, essential for translation.

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tRNA Charging

The process where aminoacyl-tRNA synthetase adds the correct amino acid to the matching tRNA.

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Proofreading

A safeguard mechanism in aminoacyl-tRNA synthetases that makes sure the proper amino acid is attached to tRNA.

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Suppressor tRNAs

tRNAs that can override a stop codon by inserting an amino acid, allowing translation continuation.

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Suppressor Mutation

Mutation where a stop codon is replaced with a codon for an amino acid, continuing translation.

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Suppression

Using a tRNA with a mutated anticodon to add a specific amino acid at a stop codon site.

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Selenocysteine tRNA

Special tRNA used to incorporate selenocysteine at UGA codons, requiring specific RNA sequences.

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Universal Code

The near universality means the same codons specify the same amino acids across most species.

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Degeneracy

The phenomenon where different codons specify the same amino acid, making the code redundant.

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Cloverleaf tRNA

Each tRNA molecule is shaped like a cloverleaf because of its secondary structure.

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Third Base Degeneracy

The point at which the third base of a codon can vary and still code for the same amino acid.

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Aminoacyl tRNA Synthetases

Enzymes that catalyze the esterification of a specific tRNA with its cognate amino acid.

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Study Notes

  • Exam 2 scores, as a percentage of possible points, for students who took the exam on schedule had a mean of 61, with 73 students

The Genetic Code

  • Crick proposed in 1955 that there was a need for an adaptor between nucleic acid and protein "languages" when he proposed the Central Dogma
  • Adaptor = tRNA
  • Crick also proposed that mRNA is read three nucleotides at a time
  • tRNA cloverleaf structure consists of 73-93 nucleotides with many modified bases
  • tRNA's include a D arm, TΨC arm, Acceptor arm, Variable loop, and Anticodon loop
  • tRNA’s have a modified L shape
  • Base 3 of codon binds base 1 of anticodon
  • Codons and anticodons are antiparallel
  • There are 3 stop codons, and 3rd position degeneracy
  • mRNA stop codons: UAG (amber), UAA (ocher), UGA (opal)
  • UAG stop codon is naturally a bit leaky
  • There are multiple codons for abundant amino acids
  • Methionine has only one codon
  • Not all organisms use all codons with the same frequency
  • The genetic code could be read with only 31 tRNAs because of 3rd position degeneracy
  • Typically, ~45 tRNAs are present, one or more for each amino acid
  • There are no stop tRNAs!
  • Multiple codons can be recognized by the same tRNA because of wobble base-pairing
  • tRNA structure allows for more flexibility in base pairing at the 3rd position
  • One tRNA can have Inosine which can read three codons
  • The genetic code is directly read by amino acyl-tRNA synthases
  • There is one tRNA for each amino acid, responsible for charging all the corresponding tRNAs
  • Chemistry of tRNA "charging" Step 1: adenylation resulting in amino acid-AMP
  • Chemistry of tRNA "charging" Step 2: amino acid-AMP + tRNA results in charged tRNA
  • If 2'OH attacks, it migrates to 3'OH by transesterification, thus all have amino acid bound to 3’OH when charging is complete
  • Aminoacyl tRNA synthetases recognize proper tRNAs based on overall shape, not anticodon sequence
  • Accurate tRNA charging is important because ribosomes do not check whether the tRNAs have the proper amino acid
  • Ribosomes can incorporate "unnatural” engineered amino acids into proteins
  • Normally, there is only one open reading frame
  • Shifting by 1 or 2 nucleotides from this open reading fram results in stop codons
  • Suppressor tRNAs can insert an amino acid at a stop codon
  • "amber" suppressors suppress UAG (amber) stop codon
  • Sometimes 2 (or 3) wrongs can make it right
  • The long (CAG)n regions can form heterochromatin, but does preserve open reading frame
  • Recall triplet expansion diseases from chapter 12
  • The genetic code is almost universal
  • Mitochondria typically have a "simplified" code that requires only 22 tRNAs
  • Mammalian mitochondria encode only 13 proteins
  • In some fungi, including human pathogen candida albicans CUG will encode Ser rather than Leu
  • CUG is a rare codon in related species, thus it might have been altered without a great deal of damage
  • Selenocysteine is incorporated into few enzymes using a special tRNA that binds UGA(normally a stop codon) with the help of a special RNA sequence and RNA binding protein

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Molecular Biology: The Genetic Code - Chapter 17 PDF

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