Genetic code

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The genetic code has four main features[1]:

  1. Three nucleotides/bases encode an amino acid, there are 20 different amino acids which are the building blocks for proteins.
  2. The genetic code is non-overlapping, for example a sequence UGGAUCGAU is read UGG AUC GAU rather than UGG GGA GAU etc.
  3. The code has no punctuation, so no base serves as a "comma" between groups of bases, therefore the code is read sequencially three bases at a time.
  4. The code is degenerate, meaning more than one codon encodes for the same amino acid. There are 64 possible triplets yet only 20 amino acids so most amino acids are encoded by 2 or more codons. Triplets that code for the same amino acid are known as synonyms.
  5.  AUG has two functions.It acts as an initiator codon and codes for Methionine(met).[2]

The genetic code is a set of rules for translating information encoded in DNA into proteins through RNA in genes. The code is read 5' to 3' direction in a fixed reading frame beginning from the start codon (AUG)

The code has the following features:

  1. Codes for a single amino acid whereby the bases are read in sets or groups of 3s called a TRIPLET CODE or CODON. Example is UUU for Phenylalanine, CGC for Arginine.
  2. Is non-overlapping meaning that the triplets (group of 3s) are read separately. A deletion or insertion of bases can cause a frameshift mutation[3].
  3. Is degenerate- more than one triplet can code for a particular amino acid. This occurs due to redundancy; there are four different bases read in groups of three to give 64 possible combinations but only 20 amino acids (43 = 64). Examples are; serine has the following (UCU, UCC, UCA, UCG, AGU, AGC), glycine which has (GGU, GGC, GGA, GGG). These codons are called synonymous codon [4]. However, some amino acid have only one codon which specifies it on the genetic code, example is tryptophan (UGG) and methionine (AUG), also, the stop codon has three codons which specifies it (UGA, UAA, UAG)
  4. According Berg et al. (2012), "the code lacks punctuation (comma)" [5].

As well as being degenerate, the genetic code is also referred to as 'unambiguous' which means that each possible codon can code for one amino acid only.[6]

The genetic code is almost universal (i.e. it is the same for all living organisms and in all types of DNA/RNA) however there are some exceptions. An example is in human mitochondrial genomes whereby UGA codes for tryptophan, AGA and; AGG code for stop codon; this occurs because mitochondrial DNA encodes a distinct set of tRNAs[7].

References

  1. Berg, Tymoczko and Stryer (2012) Biochemistry, 7th edition, New York: W. H. Freeman and Company
  2. http://ncertbooks.prashanthellina.com/class_12.Biology.Biology/
  3. Hartl,and7637-7216-1
  4. Hartl and Jones,. Genetics: analysis of genes and genomes, 7th Edition(2009) p.g 370. Jones and Bartlett. ISBN 978-0-7637-7216-1
  5. Berg JM, Tymoczko JL, and Stryer L. Biochemistry, 7th Edition (2012) p.g 133. Pearson International. ISBN 1-4292-7635-5
  6. 1. William S. Klug (2009) Concepts of Genetics, 9th edition, San Fransisco: Pearson/Benjamin Cummings.
  7. Berg JM, Tymoczko JL, and Stryer L. Biochemistry, 7th Edition (2012). Pearson International. ISBN 1-4292-7635-5

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