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Meselson and Stahl (1958) raised Escherichia coli on ammonium chloride containing heavy isotope of nitrogen (¹⁵N) for several generations to study semi-conservative replication of DNA.

1. Central dogma of protein synthesis explains a unidirectional or one way flow of information from DNA to RNA (transcription) and from RNA to polypeptide (translation).
2. Genetic code is triplet coding for one amino acid in a polypeptide chain. There are four different types of nucleotides - A, U, G, and C. Since a code is a triplet thus 64 (4×4×4) distinct triplet of bases determine the 20 amino acids. But out of 64, only 61 code for 20 amino acids. Three codons that do not code for any amino acids are terminator or non sense codons (UAG, UGA and UAA). Still in 61 triplet codons there is an excess 41 codons and therefore more than one codon codes for same amino acid.
3. Nucleosomes are oblate spherical structure having an octamer of four histone proteins (constituting 2 molecules of each type).
4. Genetic code is the relationship of amino acid sequence in a polypeptide chain and base sequence of mRNA. It includes adenine, guanine, cytosine and uracil.

Correct answer: Genetic code is triplet coding for one amino acid in a polypeptide chain. There are four different types of nucleotides - A, U, G, and C. Since a code is a triplet thus 64 (4×4×4) distinct triplet of bases determine the 20 amino acids. But out of 64, only 61 code for 20 amino acids. Three codons that do not code for any amino acids are terminator or non sense codons (UAG, UGA and UAA). Still in 61 triplet codons there is an excess 41 codons and therefore more than one codon codes for same amino acid.

Solution

The genetic code is a triplet code, meaning that each codon consists of three nucleotides. This results in 64 possible codons, but only 61 of them code for amino acids. The remaining three codons are stop codons that signal the end of a polypeptide chain.

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