Y do u need 61 codons to code 20 amino acids?

Well, as you probably know, the number of codons you can produce is dependent on the number of base pairs and the number of base pairs in each codon.

If you do the math to figure out how many possible codons you can make with codons of a given length, you find that the answer is 4^n, with "n" being the length of your codon, in nucleotides. This is because there are four possible base pairs, so for each base pair you add to a codon, you could add an A, T, C, or G. This multiplies the total number of possible codons by 4, each time you add a base pair.

So, if codons were only one base pair long, you would only be able to code for 4^1, or four different things you could code for. If you need at least one stop and start codon, that leaves you with only two amino acids you can code for.

If codons were increased to two base pairs long, that would give you 4^2, or up to 16 total possible codons. If you subtract two for one stop and one start codon, you can now code for 14 amino acids.

The maximum number of codons you can make having three base pairs each is 4^3, or 64. As you can see, this is actually the minimum codon length that can code for all 20 amino acids plus stop and start codons. And since you can make up to 64 codons with three amino acids, hey, you might as well use all of them. Having multiple codons for certain amino acids actually acts to reduce harmful mutations--if a base pair gets screwed up in the DNA, there's a chance the codon might still code for the same amino acids, in which case the mutation would not make any difference at all.

This is why 61 codons are used--amino acids of three codons in length are the minimum possible length of codons that can code for all 20 amino acids. And since you can produce 64 different codons of this length, you may as well use all of the possible codons to help guard against harmful mutations.

Hope this helps!

Remember, codons are 3 base sequences, of adenine, cytosine, guanine, and thymine. If there were only 2 bases per codon, you would have a possible total of 4 x 4 = 16 combinations, which isn't enough for the 20 naturally occurring amino acids. Having a 3rd base per codon lets you have 4 x 4 x 4 = 64 combinations. 61 of these happen to be coding for amino acids, creating degeneracy (multiple codons code for the same amino acid), while the other 3 are 'stop' sequences.

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RE:

Y do u need 61 codons to code 20 amino acids?

You speak about the genetic code, but do not reveal the nature of what is coded in thee nucleotide sequences. Simple mathematics, like 4^3, won't help here. Everything is determined in nature...