Introduction to Bioinformatics II Lecture 9 By Shumaila Azam

Genetic Code The genetic code is the set of rules by which information encoded within genetic material (DNA or mRNA sequences) is translated into proteins by living cells.

Sequence reading frame A codon is defined by the initial nucleotide from which translation starts. Example: the string GGGAAACCC GGG, AAA, and CCC GGA and AAC GAA and ACC Every sequence can, thus, be read in three reading frames

Start/Stop Codon Translation starts with a chain initiation codon or start codon Initiation factors are also required to start translation The most common start codon is AUG, which is read as methionine "GUG" or "UUG"; these codons normally represent valine and leucine The three stop codons :UAG, UGA, and UAA Stop codons are also called "termination" or "nonsense" codons. there is no cognate tRNA that has anticodons complementary to these stop signals

Effect of Mutations During the process of DNA replication, errors occasionally occur in the second strand called mutations. can have an impact on the phenotype of an organism if they occur within the protein coding sequence of a gene.

RNA Codon Table

DNA Codon Table

How to read a codon table Use in protein synthesis for translating the mRNA code into amino acid sequence

You need a sequence of mRNA From the transcription of DNA --> mRNA Example: DNA sequence: TAC GGA CAT AAC ACC TGC ATC mRNA sequence: AUG CCU GUA UUG UGG ACG UAG

Transcription mRNA sequence leaves the nucleus and travels to the cytoplasm to a free floating ribosome or to the rough ER. It will attach to the ribosome and begin the second step of protein synthesis, translation.

Translation mRNA is read as a series of codons (three letters) within the ribosome. tRNA molecules have an anticodon sequence of letters that are complements to the mRNA ex: mRNA CGA UCC (codon) tRNA GCU AGG (anticodon)

So now we get to the codon table! Locate the first letter of codon using the left side of the table. Ex. AUG look for the A

Now move to the second letter of codon which is ‘U’ Look at the top of the table where is the title ‘2nd letter’ Find the letter ‘U’ and follow it down until it intersects with the letter ‘A’ from the left side. four amino acids (isoleucine, isoleucine, isoleucine, and (start) methionine.

Down to the last letter of the codon! Look to the right hand side for the third letter. Find the letter ‘G’ which will intersect with the box that had four choices. Methionine (start)

Try the codon CAC

What do these codons have to do with proteins? Each codon represents an amino acid that will eventually form a protein that is used within a cell. Proteins are made up of hundreds of amino acids in a specific sequence. Long string of amino acids will form