Deciphering of the genetic code

description of the deciphering of the genetic code and genetic code table and explanation of characteristics of the genetic code and different scientists involved in cracking of the genetic code

1. Deciphering the genetic code D.INDRAJA

2. WHAT IT IS?  A codon is the Nucleotides sequence in mRNA which codes for a prticular amino acid. Genetic code is the sequence of nitrogenous bases in mRNA molecules which e ncloses information for the synthesis of protein molecule overview of protein synthesis A gene consists of a specific sequence of nucleotides Information about its amino acid sequence is present in a gene Every protein has got a unique amino acid sequence  The base sequence in the gene encodes a specific sequence of amino acids When a protein is to be synthesized, the corresponding gene is transcribed An mRNA molecule is formed This process is called translation

3. Deciphering the genetic code The genetic code is cracked by two different ways: T heoretical approach  triplet hypothesis practical approach

4. Theoretical approach -Triplet hypothesis It was proposed by G eorge G amow in the mid-1950’s The letters A, G, T and C correspond to the nucleotides found in DNA. They are organized into codons . As we know 20 amino acids are present the possibilities of how many nucleotides that may code for codon is Most suited for 20 aminoacids

5. Practical approach Gamow’s triplet hypothesis seemed logical and was widely accepted. However, it had not been experimentally proven, and researchers still did not know which triplets of nucleotides corresponded to which amino acids . The cracking of the genetic code began in 1961, with work from the American biochemist Marshall Nirenberg and his post-doctoral fellow,  J. Heinrich Matthaei  at the  National Institutes of Health  (NIH ) Their success relied on two experimental innovations : A way to make artificial mRNA molecules with specific, known sequences. A system to translate mRNAs into polypeptides outside of a cell (a "cell-free" system). Nirenberg's system consisted of cytoplasm from burst  E. coli  cells, which contains all of the materials needed for translation. artificial mRNA with poly U seq

6. First, Nirenberg synthesized an mRNA molecule consisting only of the nucleotide uracil (called poly-U). When he added poly-U mRNA to the cell-free system, he found that the polypeptides made consisted exclusively of the amino acid phenylalanine. To identify how many nucleotides code for a codon the investigators used RNA molecule with a radioactive containing 12 uracil containing nucleotides and found reactivity, they also found reactivity with six, five, four, three nucleotides however two nucleotides almost produced no reactivity Based on this we can say that the minimum no of nucleotides that produces the reactivity is also the number of nucleotides present in a codon

7. Nirenberg concluded that UUU might code for phenylalanine. Using the same approach, he was able to show that poly-C mRNA was translated into polypeptides made exclusively of the amino acid proline , suggesting that the triplet CCC might code for proline . Other researchers, such as the biochemist Har Gobind Khorana at University of Wisconsin, extended Nirenberg 's experiment by synthesizing artificial mRNAs with more complex sequences. For instance, in one experiment, Khorana generated a poly-UC (UCUCUCUCUC…) mRNA and added it to a cell-free system similar to Nirenberg 's. The poly-UC mRNA that it was translated into polypeptides with an alternating pattern of serine and leucine

8. Using the same technique, Severo Ochoa found that Poly-A resulted in the synthesis of poly-lysine This proved that: AAA is the codon for lysine By 1965, using the cell-free system and other techniques, Nirenberg, Khorana, and their colleagues had deciphered the entire genetic code.   It was found that all the 64 triplets codons The remaining three codons do not code for any amino acid They are called nonsense codons /stop codons The nonsense codons also have a function They act as stop signals (chain termination signals ) Nirenberg and Khorana (along with another genetic code researcher, Robert Holley) received the Nobel Prize in 1968. AAAAAAAAAAAA lys lys lys lys Poly A mRNA Poly lys polypeptide

9. Genetic code table

10. Genetic properties 1)The code is a triplet code. 2)The code is non overlapping code. 3)The code is commaless . 4)The code is non ambiguous . 5)The code is polarity . 6)The code is degenerate . 7)Some codes act as start code. 8)Some codes act as stop code. 9)The code is universal code.

11. The code is a triplet code The Nucleotides of mRNA are arranged as a linear sequence of codons each codon consisting of three successive nitrogenous bases .the code is a triplet codon The code is Non overlapping The genetic code is nonoverlapping , i.e.,the adjacent codons do not overlap. A nonoverlapping code means that the same letter is not used for two different codons . In other words, no single base can take part in the formation of more than one codon .

12. The code is commaless There is no signal to indicate the end of one codon and the beginning of the next. The genetic code is commaless (or comma-free ) The code is non ambiguous A particular codon will always code for the same amino acid . Thus one codon can not specify   more than one amino acid AUG  METHIONINE GUG VALINE, METHIONINE The code is polarity The genetic code has polarity, that is, the code is always read in a fixed direction, i.e., in the 5′ → 3′ direction. Exceptions

13. The code is degenerate The code is degenerate which means that the same amino acid is coded by more than one base triplet.  Example- the three amino acids arginine , alanine and leucine each have six synonymous codons . Some codes as act start code . Translation always begins at a  start codon , which has the sequence AUG and encodes the amino acid methionine (Met) in most organisms. Thus, every polypeptide typically starts with methionine , although the initial methionine may be snipped off in later processing steps Alternative start codons depending on the organism include "GUG" or "UUG"; these codons normally represent  valine  and  leucine , respectively, but as start codons they are translated as methionine or formylmethionine Some codes act as stop code The three  stop codons  have names: UAG is  amber , UGA is  opal  (sometimes also called  umber ), and UAA is  ochre . Stop codons are also called "termination" or "nonsense" codons . while the 61 codons are sense codons

14. The code is universal code. Although the code is based on work conducted on the bacterium Escherichia coli but it is valid for other organisms. This important characteristic of the genetic code is called its universality. It means that the same sequences of 3 bases encode the same amino acids in all life forms from simple microorganisms to complex, multicelled organisms such as human beings . Exceptions The genetic code, once thought to be identical in all forms of life , has been found to diverge slightly in certain organisms and in the   mitochondria  of some  eukaryotes