The genetic code and transfer RNA

What is the genetic code?

The dictionary that tells us which codon represents which amino acid. 

How did Nuremberg crack the genetic code?

• Synthesised RNA containing only U bases

• Incubated RNA with extract of E.Coli cells

• Found protein was made entirely of phenylalanine.

What are the features of the genetic code?

Non-overlapping, degenerate, almost universal, presence of start and stop codons

Why is the genetic code non-overlapping?

One nucleotide belongs in one codon.  

Why is the genetic code degenerate?

64 different codons but only 20 amino acids. This means most amino acids are represented by more than one codon. Only methionine and tryptophan are represented by a single codon. 

What are start and stop codons?

AUG is the initiation codon, therefore all newly-made proteins begin with methionine. 3 termination codons: UGA, UAA and UAG.

Why is the genetic code almost universal?

The same genetic code is used by almost all prokaryotes and by genes in the nuclei of almost all eukaryotes. Minor variations seen – in come mycoplasma, in some protists, in mitochondria.

What is an open reading frame?

The region between an initiation codon and a termination codon

How many possible reading frames does a sequence have?

3

What is tRNA and what are its features?

• Adaptor molecules deliver amino acids to the appropriate codons, these adaptors are tRNA.

• Small – 73-90 nucleotides long

• Contain unusual bases – thymine, dihydrouracil, pseudouracil (all U like), methylguanine (G-like).

• All tRNAs have an anticodon

• All tRNAs have the unpaired sequence CAA at their 3' end. All amino acids can be linked to the 3'-OH the adenosine at the 3' end.

What is an anticodon?

3 exposed bases complementary to a codon. tRNAs recognise codons due to base pairing between codon and anticodon.

What does the TψC arm contain?

Thymine and pseudouracil

What does the D arm contain?

Dihydrouracil

What are tRNAs attached to their amino acid called?

Aminoacyl tRNAs or charged tRNAs. 

What is tRNAgly?

The tRNA for glycine

What is gly-tRNAgly?

The aminoacyl tRNA. 

How do some tRNAs recognise more than one codon?

Some tRNAs recognise more than one codon due to base wobble

What is base wobble?

The ability of some bases at the 5' end of anticodon to pair with more than one base at the 3' end of codon. 

What does the genetic code require to operate?

The RNA whose anticodon recognises a codon representing a particular amino acid must be linked only to that amino acid. 

What are enzymes that link tRNA to amino acids called?

Aminoacyl-tRNA sythetases

How many different types of aminoacyl tRNA synthetases are there?

20

What are the steps for the joining of an amino acid to a tRNA molecule?

• Amino acid + ATP → Aminoacyl-AMP + PPi

• AA-AMP + tRNA → Aminoacyl-tRNA + AMP+ PPi