Transcription, which is catalyzed by an RNA polymerase, proceeds in three steps: initiation, elongation, and termination. The genetic code relates the information in mRNA (as a linear sequence of codons) to protein (a linear sequence of amino acids).

Question 1

If the code were just single letters (A or T or G or C), each “letter” would translate to one amino acid (41). But there are 20 amino acids, so a single-letter code could not unambiguously identify all amino acids. A doublet code would create 16 possible codons (42), not enough to uniquely identify all 20 amino acids. A triplet code using four letters has 64 unique possibilities (43), more than enough for 20 amino acids and stop codons. The code is redundant (there is more than one codon per amino acid) but not ambiguous (a codon does not stand for more than one amino acid).

Question 2

RNA polymerase binds to DNA at the promoter. The DNA is unwound to expose the bases. The enzyme has binding sites for substrates, the ribonucleoside triphosphates. The enzyme then adds nucleotides to a growing chain by complementary base pairing to template DNA.

Question 3

DNA must be replicated exactly. Any error in DNA in a gene that encodes a protein will result in an error in the RNA that is transcribed from that DNA region. This could result in a different codon and therefore a different amino acid at that location in the protein. The protein’s function may change. RNA is made in many copies. So an error in an RNA could result in an error in the protein translated from it, but since there are many more normal copies of that RNA, there would be plenty of the normal protein for normal function.