Q.(a) What is a genetic code ?
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Start your 14-day free trial to unlock the full solution →The genetic code is the set of rules by which information in DNA (nucleotide triplets) is translated into proteins (amino acid sequences); scientists proposed it to explain how four bases specify twenty amino acids, and it is called universal because nearly all organisms use the same code.
What is a genetic code?
The genetic code is essentially the dictionary that cells use to translate genetic information into functional proteins. DNA and RNA are written in a language of four nucleotide bases—adenine (A), guanine (G), cytosine (C), and thymine (T) in DNA, with uracil (U) replacing thymine in RNA. Proteins, on the other hand, are built from twenty different amino acids. The genetic code is the rulebook that specifies which sequence of nucleotides corresponds to which amino acid.
The code operates through triplets of nucleotides called codons. Each codon—a sequence of three consecutive bases—codes for one specific amino acid or acts as a signal to stop protein synthesis. For instance, the codon AUG codes for the amino acid methionine and also serves as the start signal for translation. With four bases taken three at a time, there are 64 possible codons (4 × 4 × 4), more than enough to specify the twenty amino acids. This redundancy means that most amino acids are encoded by more than one codon, a feature called degeneracy.
Why did scientists feel the need to propose a genetic code?
By the mid-twentieth century, scientists had established that DNA carries hereditary information and that proteins are the workhorses of the cell, performing virtually all biological functions. The central puzzle was this: how does a molecule made of just four types of nucleotides direct the synthesis of proteins made from twenty different amino acids?
The mathematical problem was clear. If one nucleotide specified one amino acid, you could code for only four amino acids. If pairs of nucleotides (doublets) were used, you could specify 16 amino acids (4 × 4)—still not enough. A triplet code, however, yields 64 combinations, comfortably covering all twenty amino acids. George Gamow was among the first to propose a triplet code in 1954, reasoning from this numerical necessity.
Experimental evidence soon followed. Marshall Nirenberg and Heinrich Matthaei cracked the first codon in 1961 by synthesizing an artificial RNA strand made entirely of uracil (poly-U) and showing that it produced a protein chain of only phenylalanine. This proved that UUU codes for phenylalanine. Over the next few years, the entire genetic code was deciphered, codon by codon, confirming that the triplet hypothesis was correct.
The genetic code is also described as non-overlapping (each nucleotide is part of only one codon) and comma-less (no punctuation between codons; the sequence is read continuously in groups of three from a fixed starting point).
Why is the genetic code said to be 'universal'? …
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