The Genetic Code

The genetic code.Alvarez Beltran Enrique

Aguilar Tirado Jordan Onel Lau Solorio Mariel

Aviña Peña Raul Alejandro

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

What is the genetic code?

The genetic code is the relation between the

sequence of bases in DNA (or its RNA transcripts) and the sequence of amino acids in proteins.

Not all genetic information is stored using the genetic code. All organisms' DNA contains regulatory sequences, intergenic segments, chromosomal structural areas, and other non-coding DNA that can contribute greatly to phenotype.

The genetic code

1. Three nucleotides encode an amino acid.

2. The code is nonoverlapping.

3. The code has no punctuation.

4. The genetic code is degenerate.

Genetic code characteristics

Because the code is highly degenerate, only

tryptophan and methionine are encoded by just one triplet each. Each of the other 18 amino acids is encoded by two or more. Indeed, leucine, arginine, and serine are specified by six codons each. The number of codons for a particular amino acid correlates with its frequency of occurrence in proteins.

Major features of the genetic code

What is a codon?

Sequences of three nucleotides that specify which amino acid will be added next during protein synthesis.

Codons that specify the same amino acid. Note that synonyms are not distributed

haphazardly throughout the genetic code.

Most synonyms differ only in the last base of the triplet.

Synonyms

If the code where not degenerate, 20 codons would designated amino acids and 44 would lead to chain termination.

The probability of mutating to chain termination would therefore be much higher with a non degenerate code.

What is the biological significance of the extensive degeneracy of the genetic code?

Genetic Code Table

mRNA is translated into proteins on ribosomes

Start signal is complex in bacteria Polypeptide chains in bacteria start with a

modified amino acid (formylmethionine, fMet)

fMet recognizes the codon AUG or GUG

mRNA contains start & stop signals for protein synthesis

Is the genetic code the same in all organism?

The base sequence of many wild-type and mutant genes are known, as are the amino acid sequences of their encoded proteins.For each mutant, the nucleotide change in the gene and the amino acid change in the protein are as predicted by the genetic code.

Serious efforts to understand how proteins are encoded began after the structure of DNA was discovered by James Watson and Francis Crick, who used the experimental evidence of Maurice Wilkins and Rosalind Franklin (among others)

Discovery

Discover

 George Gamow postulated that a three-letter code must be employed to encode the 20 standard amino acids used by living cells to encode proteins. With four different nucleotides, a code of 2 nucleotides could only code for a maximum of 16 amino acids. A code of 3 nucleotides could code for a maximum of 64 amino acids.

The Crick, Brenner et al. experiment was the first to demonstrate that codons consist of three DNA bases. 

Marshall Nirenberg and Heinrich J. Matthaei were the first to elucidate the nature of a codon in 1961 at the National Institutes of Health.

Discover

BiochemestrySixth editionJeremy M. BergJohn L. TymoczkoLubert Stryler

http://en.wikipedia.org/wiki/Genetic_code

Bibliography