GENETIC CODE BY Dr. PRASANNA - PowerPoint Presentation

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GENETIC CODE

BYDr. PRASANNAS P T SCIENCE COLLEGE(SHRI GOVIND GURU UNIVERSITY)

Genetic code is the term

used for the way the four bases of DNA--the A, C, G, and Ts--are strung/arranged together in a way that the cellular machinery, the ribosome, can read them and turn them into a protein.

In the genetic code, each three nucleotides in a row count as a triplet and code for a single amino acid. 

The final step in the expression of genes that encode proteins is translation. The mRNA nucleotide sequence is translated into the amino acid sequence of a polypeptide chain. Protein synthesis is called translation because it is a decoding process. The information encoded in the language of nucleic acids must be rewritten in the language of proteins

Establishment of the Genetic Code

DNA is the genetic materialIt stores genetic

information

The genetic code is the DNA base sequence that corresponds to the amino acid sequence of the polypeptide specified by the gene.The nucleotide and amino acid sequences are colinear. The genetic code is universal with few exceptionsMutations in DNA cause changes of amino acids in a polypeptide chain

Because only

20 amino acids normally are present in proteins, there must be at least 20 different code words in DNA. Therefore the code must be

contained in some sequence of the four nucleotides

commonly found in DNA.If the code words were two nucleotides in length, there would be only 16 possible combinations (42) of the four nucleotides This would not be enough to code for all 20 amino acids. Therefore a code word, or codon, had to consist of at least nucleotide triplets

This

would give 64 possible combinations (

4

3

) which are more than the minimum of 20 needed to specify the common amino acids.The code was deciphered/decoded in the early 1960s by Marshall Nirenberg, Heinrich Matthaei, Philip Leder, and Har Gobind Khorana. In 1968 Nirenberg and Khorana shared the Nobel Prize with Robert Holley, the first person to sequence a nucleic acid (phenylalanyl-tRNA).

Organization of the Code

The genetic code is universal. They are read in frames called reading framesThey are nonoverlapping

There is code degeneracy. There are up to six different codons for a given amino acid. Only 61 codons, the sense codons, direct amino acid incorporation into protein.The remaining three codons (UGA, UAG, and UAA) are involved in the termination of

translation

They are

called stop

or nonsense

codons. There are not 61 different tRNAs, one for each codon. The 5′ nucleotide in the anticodon can varyIf the nucleotides in the second and third anticodon positions complement the first two bases of the mRNA codon, an aminoacyl-tRNA with the

proper amino

acid will bind to the mRNA-ribosome complex.

This somewhat loose base pairing is known as wobble and relieves cells of the need to synthesize so many tRNAs. Wobble also decreases the effects of DNA mutations.

Code degeneracy

Crick suggested that the genetic code is degenerate. 

It

means that each of the 64 triplets must have some meaning.So some amino acids must be specified by two or more different triplets. If only 20 triplets are used (with the other 44 being nonsense, in that they do not code for any amino acid), then most mutations will produce nonsense words, which will stop the protein-building process. Crick reasoned that many or all amino acids must have several different names in the base-pair code; 

Overlaping and nonoverlaping

of codons

Gene code

tRNA structure

I is inosineSimilar to G

Codon-Anticodon Pairings Allowed by the Wobble Rules

Questions

General characters of genetic codeShort note on genetic codonDefine gene, gentic code, translation,

degenracy

, anticodon, wobble hyphothesis, History/ scientist contribution related to genetic code

Thank you

REFERENCES

Principles Of Microbiology , Atlas R.M. Microbiology Gerard J. Tortora

Foundations In Microbiology 

Kathleen Park TalaroGeneral Microbiology , Roger Y. Stanier Macmillan, 1987Michael J. Pelczar Jr. Chan Ecs And Krieg Nr (2004) Microbiology , 5th Edition. Tata Mcgraw Hill.

15 May 2020

Dr. Prasanna , SHRI GOVIND GURU UNIVERSITY, GODHRA, GUJARAT INDIA

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Reference

Prescott’s Microbiology, Eighth Edition Reviewed By Joanne J. Dobbins Joanne M. Willey ,  Linda M. Sherwood , And  Christopher J. Woolverton

 .  2011.  

Mcgraw-Hill Higher Education,  New York, NY.Medigan M T And Martinko JM (2014), Brock Biology Of Microorganisms, 14th Edition. Parker J. Prentice Hall International Inc15 May 2020Dr. Prasanna , SHRI GOVIND GURU UNIVERSITY, GODHRA, GUJARAT INDIA

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