Answer:
B (Cytosine)
Explanation:
DNA is a molecule that stores genetic material. It contains all of the instructions a cell needs to sustain itself. These instructions are found within genes, which are segments of DNA made up of specific sequences of nucleotides. The stored information in this DNA needs to be expressed and this is done by two major processes; transcription and translation.
Transcription is the passing on of the genetic instruction in a DNA molecule to a RNA molecule. It involves making a complementary strand of mRNA from DNA template. Transcription occurs in steps which generally occurs when an enzyme called RNA polymerase binds to a region of a gene (DNA) called PROMOTER region. This signals the DNA molecule to unwind allowing RNA polymerase to add nucleotides to the mRNA strand. RNA polymerase reads the unwind DNA strand and builds the mRNA molecule using complementary base pairs.
Complementary base pairing is the phenomenon where purines (Adenine,A and Guanine,G) forms hydrogen bonds and binds to pyrimidines (Thymine,T and Cytosine, C) respectively i.e. A-T, G-C. In the newly made RNA molecule, all the T nucleotides are replaced with U nucleotides, as Uracil is present in RNA not thymine.
During transcription, the DNA serves as a template for complementary base pairing, in which RNA polymerase synthesizes a complementary strand of the nuceotide it reads i.e. if it reads Adenine in the DNA template, it synthesizes Uracil nuceotide in the new mRNA growing strand while it synthesizes Cytosine nucleotide if it reads Guanine nucleotide in the DNA template molecule and vice versa.
