Question

1. How is the genetic code used in gene expression?

Answer 1

Decoding messages is a key step in gene expression, in which information from a gene is read out to build a protein.

Genes that provide instructions for proteins are expressed in a two-step process.

• In transcription, the DNA sequence of a gene is "rewritten" in RNA. In eukaryotes, the RNA must go through additional processing steps to become a messenger RNA, or mRNA.

• In translation, the sequence of nucleotides in the mRNA is "translated" into a sequence of amino acids in a polypeptide (protein chain).

During translation, the nucleotide sequence of an mRNA is translated into the amino acid sequence of a polypeptide. Specifically, the nucleotides of the mRNA are read in triplets (groups of three) called codons. There are 616161 codons that specify amino acids. One codon is a "start" codon that indicates where to start translation. The start codon specifies the amino acid methionine, so most polypeptides begin with this amino acid. Three other “stop” codons signal the end of a polypeptide. These relationships between codons and amino acids are called the genetic code.

The genetic code is written in linear form, using the ribonucleotides that compose mRNA molecules as letters. The ribonucleotide sequence is derived from the complementary nucleotide bases in the DNA template strand. Therefore, the nucleotide sequence is exactly the same as the DNA coding strand. Each genetic code consists of three ribonucleotide letters, thus referred to as a triplet code. As such, a genetic code is a triplet code in which a sequence of three bases is needed to specify one amino acid. The genetic code translates the RNA sequences into the amino acid sequence.Each group of three ribonucleotides, called a codon, specifies one amino acid. These codes are unambiguous, as each triplet specifies only a single amino acid. Thus, one would imagine that a codon would be at least three bases long. With three bases, there are 43 = 64 codons, which is more than enough to encode the 20 amino acids. Therefore, the genetic code is degenerate, which means more than one triplet can encode the same amino acid. Each amino acid can have more than one codon, but no codon can encode more than one amino acid.