Translation- Part 1

Describe the genetic code.

The genetic code, in a nutshell, shows which codons code for which amino acids. (If you search it up on Google Images, you'll get plenty of tables.) It also shows which three codons are "stop" codons, which signal the end of translation. You'll never need to memorise the table- just knowing how to use it is enough.

Explain the history and experimental design associated with the discovery of the genetic code.

The genetic code was discovered through the work of several different scientists in the 1960s. In 1961, Nirenberg and Matthaei created artificial mRNA all made up of one kind of nucleotide- for example, they had one that only contained uracil, one that only contained adenine and so on. From this, they could create long strings of the same amino acid.

Later on, Khorana discovered a method that could make specific repeating mRNA strands. This helped scientists learn that the genetic code relied on series of three bases (codons). Nirenberg and Leder later used this knowledge to decipher all of the remaining codon sequences.

Explain the redundancy in the genetic code at the mRNA level.

There are 64 possible codons, as each codon is made up of 3 bases, and there are 4 different bases (4 x 4 x 4 = 64). However, there are only 20 different amino acids. A lot of amino acids can actually be coded for by multiple different codons: for example, UUU and UUC both code for phenylalanine. Hence there is redundancy in the genetic code at the mRNA level.

Explain the redundancy in the recognition of the genetic code by tRNAs using the wobble hypothesis.

As I mentioned, there are 64 codons. However, there are only around 40-50 tRNAs. How, then, can each codon find a corresponding tRNA to match up with?

The wobble hypothesis suggests that the third position of a codon is the least important, and inaccurate binding here may have less of an effect. For example, as I mentioned earlier, both UUU and UUC code for phenylalanine. Thus, it wouldn't be detrimental to the cell if phenylalyl-tRNA had an anticodon that could bind to both UUU and UUC.

One factor contributing to the wobble hypothesis is that tRNA often contain modified bases, such as inosine. Inosine can bind to U, A or C. Hence if phenylalyl-tRNA carried the anticodon 5'-IAA-3', it could bind to both UUU and UUC (remember, tRNA is antiparallel to mRNA, so the anticodon is going to look "backwards" compared to the codon).