RNA has four kinds of bases, and proteins are built from 20 kinds of amino acids…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
RNA has four kinds of bases, and proteins are built from kinds of amino acids.
- If each codon were made of two bases, how many different codons could there be?
- With codons of three bases, how many different codons are there?
- How many of these codons code for amino acids?
- A synthetic RNA is made with only uracil and cytosine. How many different codons can it contain?
- The translational unit of an mRNA, from the first base of its start codon to the last base of its stop codon, is nucleotides long. How many amino acids does the polypeptide have?
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
(a) Two bases. codons, fewer than the amino acids. This is why George Gamow argued that the code must use more than two bases.
(b) Three bases. codons, many more than are needed.
(c) Codons for amino acids. codons (UAA, UAG and UGA) are stop codons and code for no amino acid: codons. With codons for amino acids, some amino acids have more than one codon: the code is degenerate.
(d) Only U and C. Each of the three positions has choices: codons. RNA molecules with defined combinations of bases, like this one, helped to decipher the code.
(e) Amino acids. codons. The last one is the stop codon, so amino acids.
Answer: (a) codons. (b) codons. (c) codons. (d) codons. (e) amino acids.
Check. In (d), list them: UUU, UUC, UCU, UCC, CUU, CUC, CCU and CCC, eight codons.
Watch out. The untranslated regions lie outside the translational unit, before the start codon and after the stop codon, so they add no amino acids.
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