2.2: Genetic Code

Deciphering RNA Code

• in 1961 at the same time the RNA Tie Club was running, 2 non-members (Marshall Nirenberg, and Johann Matthaei) used a call-free system (study protein synthesis outside of living cell) to decipher the first part of the genetic code

• They placed all components necessary for the process into tubes such as RNA template, ribosomes, nucleotides, etc...

• this allowed them to determine which specific RNA nucleotide sequences produce which AA

• they discovered that a simple nucleic acid string of uracils produces a repeating polypeptide chain of phenylalanine amino acid

follow up research concluded that 3 nucleotides code for one AA

Defining a Codon

• sequence of three nucleotides in the mRNA molecule that either signify the beginning/end of translation or specify which amino acids should be added to the growing polypeptide chain.

• George Gamow founded the RNA Tie Club, which consisted of 20 members, representing the 20 amino acids, and four honorary members, representing the four nucleotides found in nucleic acids.

• he realized that one base code is not enough to code for all 20 amino acids becuase 4 bases would then equal 4 amino acids and there's 20 of them.

• even though 4 x 4 x 4 gives us 64 and there are only 20 amino acids, multipe codons can code for the same amino acid in a process called genetic code redundancy.

Deletion or Addition

• Removal of 3 codon aka an entire AA from the sequence

• usually leaves the sequence unchanged, preserving most of the protein's identity

• Adding 3 codons also produces similar results but instead adds an AA

Frameshift Mutations:

• removal/additions of nucleotides

• sometimes if one nucleotide is removed, and later a compensatory one is as well, it may end up restoring the actual sequence and freeing it of mutations

Open-Reading Frame (ORF)

• coding reading of a gene

• Scanning DNA for open reading frames that begin with a start codon and end with a stop codon is one way to identify potential genes within a DNA sequence.

• Depending on which nucleotide we start with, the sequence can be divided into three different reading frames. These are called the first, second, and third reading frames.

• Each reading frame groups the nucleotides into different codons, resulting in each reading frame producing a different sequence of amino acids and, ultimately, a different protein.

• therefore, if nothing is known about a double-stranded DNA, either of the 2 can be treated as a template strand, so there are actually six reading frames then.

Template vs. Non-Template Strands

• The non-template strand = coding strand -> contains the same codons as the mRNA

• non-coding strand = template strand -> contains the complementary sequence; this is what is read by the RNA polymerase

• AUG codes for methionine, which is the start codon in every protein synthesis

• tryptophan (UGG) is also unique in this sense

• genetic code also has 3 codons that don't code for AA called stop codons to signa when translation must stop