tRNA: the Adapter

1
tRNA the Adapter

• Adapter means a connector for joining parts.
• Crick first came up with the hypothesis of an
  adapter molecule.
• Later, it was discovered that transfer RNA
  (tRNA)s are that adapter.
• Each amino acid gets attached to a very specific
  tRNA which brings it to the ribosome where it
  gets connected to a growing chain of polypeptide.
  
• Figure shows the structure of yeast alanine tRNA.

2
tRNA what's under the bonnet??

• It has a 3' terminal site for amino acid
  attachment.
• It also contains a three base region called the
  anticodon. It complements to the three bases of
  the codon on the mRNA. Each tRNA contains a
  specific anticodon triplet sequence that can
  base-pair to one or more codons for an amino
  acid. For example, one codon for lysine is AAA
  the anticodon of a lysine tRNA might be UUU. Some
  anticodons can pair with more than one codon due
  to a phenomenon known as wobble base pairing.

3
Codon translation by tRNA

• Each amino acid gets attached to tRNA by specific
  aminoacyl-tRNA systhetase enzyme.
• Now, this amino acid specific enzyme links it
  (i.e. the amino acid) only to those tRNAs that
  recognize the codons for that particular amino
  acid.
• Figure shows two binding sites of an
  aminoacyl-tRNA specific for alanine. One site is
  for alanine and the other is for its related
  tRNA.
• Explanation.

4
Throw caution to the wind!

• Amino acids are illeterate
• Its the tRNA adapter's responsibility to
  recognize the mRNA codons and attach amino acids
  appropiately.

5
Degeneracy

• The term degeneracy comes from degenerate
  code.
• The reason for such genetic variation is not
  known. But....
• 1. Because of degeneracy, most amino acids
  can be brought to the ribosome by several
  alternative tRNA types, each with its
  different anti-codon binding to different mRNA
  codons.
• 2. Wobble base pair.

6
Wobble base pair......why oh why?

• The fact that there are 61 amino acid-coding
  codons and roughly 40 tRNA molecules
  presented a problem.
• In 1966 Francis Crick proposed the Wobble
  hypothesis to account for this.
• He postulated that the 5' base on the anti-codon
  was not as spatially confined as
  the other two bases, and could thus have
  non-standard base pairing. This would account
  for 60 codons for 40 tRNA.

7
Wobble base pair......behind the scenes

• The third nucleotide can form hydrogen bonds
  either with its normal complementary nucleotide
  in the third position of the codon or with a
  different nucleotide (through a loose kind of
  base pairing) in that position.
• In the figure, we see that the ability to
  wobble enables a single tRNA (related to
  serine) to recognize two codons UCU and UCC in
  the mRNA.

8
Codonanticodon pairings allowed by the Wobble
Rules

• The letter I stands for inosine, one of the rare
  bases found in tRNA, often in the anticodon.