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Why is our genetic code degenerate?

To answer this question, it is first important to understand the meaning of the word degenerate.

So being an A level Biology student, you must know that genes code for amino acids. (these are the basic components of protiens)

Now, each amino acid is coded for by a triad sequence of bases , namely Adenine, Cystine, Thyamine and Guanine (Thyamine is replaced by Uracil in RNA, if we are referring to codons instead of anticodons). These bases are referred to by the capitalised form of their first letter i.e. A,C,T,G,U. Hence, an amino acid is coded for by an codon with the sequence: AUG, or GUA, or GGA (Any combination really of A,G,C,U)

There are 20 amino acids that need to be coded for in humans. Therefore there should be 20 sets of triplet sequences, however because of simple maths ( that I will show shortly) , there are actually 64 combinations/triad codes available which means that some amino acids have a repetitive code i.e. the code is degenerate. For example, the amino acid Cystiene is coded for by the codons UGU AND UGC. Some amino acids have up to six combinations for just themselves.

So here is the maths:

We have four bases available: A , G , C and U

And we have to code for 20 Amino acids.

If each Amino acid was coded for by 2 bases, we would only have 16 possible combinations, which is four short of how many we need: 4 x 4 = 16

_ _

(Four options in the first blank, four options in the second blank as the code can be repeated and be in any arrangement, there fore it is 4 x 4 )

Now the next option is to have three bases code for each amino acid :

_ _ _

So four options in the first, four options in the second and four options in the third. Therefore 4 x 4 x 4 or 4which is 64 ! Thats how many we have -which is more than the 20 we need and hence the code is degenerate!

Obviously four bases coding for an amino acid would mean 44 and so on... many many more than we actually need!

2 years ago

Answered by Amna , an A Level Biology tutor with MyTutor

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