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Genetic code notes | Definition, chart and properties

What is Genetic code and its significance?

The genetic code or codon definition

A sequence of three nucleotides on mRNA specifying an amino acid or other information is called a codon or genetic code. For example Triplet code AAA in DNA codes for phenylalanine amino acid in the protein. Triplet code AAA in DNA during transcription is changed into UUU code of RNA. Thus, AAA and UUU  are DNA and RNA codon for amino acid phenylalanine.

Gene is actually a segment of the chromosome, composed of DNA thread of particular length controlling particular protein formation.

Genes and chromosomes

Gene is a DNA segment composed of a set of nucleotide. Nucleotides sequence within gene controls amino acid sequence within a protein. Four different types of DNA nucleotides within a DNA molecule direct the position of twenty different amino acids in the growing polypeptide chain of protein during the translation process. Actually, nucleotides within DNA chain make a code which directs the position of the specific amino acid within the protein chain. This code is called the Genetic code.

3 nucleotides code for an amino acid

Is it possible for one nucleotide to control the position of one amino acid? Absolutely not, because four types of nucleotides could control only four different amino acids out of 20. If we consider a code in which two bases  (e.g. AG ) make a set to control one amino acid, only 16 possible codons could be formed which are not enough to control 20 amino acid. But if there is a set of three bases (e.g. AGC) to control one amino acid, then 64 possible codons combinations of genetic code could be formed which are more than enough to control the position of 20 different amino acids.

       “SO, we conclude that the genetic code is triplet code and each codon consists of 3 nucleotides bases”

Genetic code examples

  •  AGA codes for arginine amino acids.
  • AUA codes for isoleucine amino acids.
  • AAA  codes for lysine amino acids. etc

Triplet genetic code confirmation

Triplet nature of genetic code was confirmed by different groups of scientists. In 1961 J. Heinrich Matthaei and Marshall Nierenberg were able to construct a synthetic RNA composed of Uracil base only. They used this RNA to form a protein. The protein was composed of phenylalanine amino acid only. It was concluded that phenylalanine amino acid is controlled by triplet codon UUU.

Later Nierenberg and Philip Leder developed a cell-free system, a test tube in which cytoplasmic contents of a cell were present, and assigned amino acid to each of the RNA codons.

These are genetic codes by which thousands of different types of genes within an organism are controlling the formation of thousands of different types of proteins, which control almost every aspect of organism i.e. growth, development, Physiology, etc.

                                   RNA Codon table showing 64 codons

Out of 64 RNA codons 61 code for some amino acid and rest three codons are stop codons, which terminate the translation process and stop protein synthesis.

genetic codon examples in chart
The genetic code chart with start codon and stop codons

Properties of Genetic code

  1. The genetic code is degenerate

Most amino acids have more than one codon, for example in the case of arginine, leucine, and serine amino acids each one of them has 6 different codons. In the case of leucine amino acid, these codons are CUA, CUC, CUG, CUU, UUA, and UUG.  This helps in codes against the harmful effect of the mutation.

2. The genetic code is unambiguous

Each code has only one meaning i.e it codes only one amino acid. For example, AUG  codes only one amino acid which is methionine. It can never code any other amino acid.

3. The genetic code has “start” and “stop” signals

There is only one start codon (AUG, initiation codon) which starts the translation process, but to stop this process three stop codons are present i.e.UAA, UGA and UAG.

4. A codon aways has an anticodon

A specific tRNA molecule contains a set of three consecutive nucleotides that can base pair with the codon of mRNA. This set of nucleotide that can base pair with codon is called Anticodon. For example for codon of UGC on mRNA, tRNA will have anticodon ACG.

  • Type of codon which starts protein synthesis is called Initiation codon i.e. AUG.
  • Type of codon which terminates protein synthesis is called stop codon i.e. UAA, UGA, and UAG.

5. Genetic codes are universal except in rare cases

These will code same amino acids in all organisms, even it may be plant-animal of fungi, etc.

Frequently asked questions about Genetic code

Why genetic code is Commaless?

Francis Crick and his coworkers in 1961 predicted the triplet nature of the genetic and concluded that the code is not overlapping and degenerate in nature. They said that triplet codes follow each other in a linear order without interrupting commas or noncoding regions in bacterial DNA i.e. Introns. So, Genetic code was called commaless, without Introns. But after the discovery of Introns in 1977 within DNA, showed that all genes are not commaless or degenerate and overlap with each other and this is very common in eukaryotes.

What are the letters of the genetic code?

The genetic code is a set of three nucleotides. Each nucleotide is named on the basis of one of the four nitrogenous bases found in each of them. Nucleotide having nitrogenous bases Adenine is assigned the “A” letter, and nucleotide containing  Guanine base is assigned letter  “G”. Similarly  Cytosine and Thymine containing nucleotides are assigned letter “C” and  “T” respectively. So. A, G, C, and T are letters of Genetic code.

What is meant by the universality of the codon?

University in genetic code means that with few exceptions in viruses rest all organisms use the same set of codes for directing their amino acid sequence. A single code found in all organisms will always code for the single specific amino acid.

 Because there are three nitrogenous bases in a single codon, so total possible triplet sets of four nucleotides will be 4 x 4 x 4 = 64, there are 64 possible combinations for a codon. As there are only 20 possible amino acids, this means that several different codons can encode for the single amino acid.

How a codon is formed?

The genetic code is composed by codons, which are the three-letter sequence of nucleotides. Each codon makes a link with specific anticodons of tRNA and in this way, it codes for one specific amino acid.

What is the genetic code used for?

The genetic code is set of three nucleotides which specifies one amino acid of the protein chain. Consecutive triplet code exactly define location and type of amino acid to form a specific amino acid chain. The organism uses its specific codon sequence in forming peculiar protein and hence these codes regulate and control organism at the cellular level.

 

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