Translation of mRNA - Structure and function of tRNA

14 important questions on Translation of mRNA - Structure and function of tRNA

What is Francis Crick's adaptor hypothesis?

The hypothesis that the position of an amino acid within a polypeptide is determined by the binding between the mRNA and an adaptor molecule carrying a specific amino acid.

What are the two functions of tRNA during translation?

1. It recognizes a three-base codon sequence in mRNA;
2. It carries an amino acid specific for that codon.

In what way does the recognition between tRNA and mRNA take place?

The anticodon in a tRNA binds to a codon in mRNA in an antiparallel manner.
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How does the tRNA determine which amino acid to bind?

The anticodon on the tRNA corresponds with the amino acid that it carries (in reverse, beause it's an anticodon).

How many different tRNA molecules does a cell have and how does the cell manage to make these molecules?

61, for every sense codon 1. Therefore, the DNA contains many tRNA genes that encode tRNA molecules with different sequences.

Which 3 structural features do all different tRNA molecules have?

- tRNA's have a cloverleaf pattern (because of the 3 stem-loops, in which one of them the anticodon is located);
- The amino acids binds to the acceptor stem of the molecule, at the 3' end tRNA's have a CCA sequence;
- tRNA's have variable sites which can differ in the number of nucleotides they contain.

Which enzymes in the cell catalyze the attachment of amino acids to tRNA molecules?

Aminoacyl-tRNA synthetases

How many different aminoacyl-tRNA synthetase enzymes does a cell have?

20, for each of the 20 amino acids 1,
for example: alanyl-tRNA synthetase recognizes tRNA^ALA and attaches alanine to it.

How does aminoacyl-tRNA synthetase bind the amino acid to tRNA (steps)?

- The correct amino acid and is bound to the aminoacyl-tRNA synthetase;
- AMP binds to the synthetase and pyrophosphate is released;
- The correct tRNA binds with its 3' end to the amino acid and AMP is released;
- The charged tRNA is released.

How does the aminoacyl-tRNA synthetase recognize the correct tRNA molecule?

It recognizes the anticodon of tRNA and other regions in tRNA.

On which principle is the wobble rule based?

On the degeneracy of the genetic code. Multiple codons code for one amino acid. The first two places in these different codons are almost always the same. However, for some amino acids, the third place can either be U, C, A or G.

What is the wobble rule?

In the codon-anticodon recognition process (mRNA-tRNA), the first two positions of the codon pair strictly according to the AU/GC rule, but the third position can tolerate mismatches.

How are two tRNA molecules that differ at the wobble position but are able to recognize the same codon, called?

Isoacceptor tRNA's.

According to the wobble rule, a single tRNA can recognize more than one codon. What kind of practical adventage does the cell have because of this?

It is unnecessary for a cell to make 61 different tRNA molecules.

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