Explain the process of transcription.

Transcription is the process by which a complementary mRNA copy is made of the specific region of the DNA molecule which codes for a polypeptide (about 17 base pairs). A specific region of the DNA molecule, called a cistron, unwinds. This unwinding is the result of hydrogen bonds between base pairs in the DNA double helix being broken, this breaking of hydrogen bonds is brought about by an enzyme called helicase. This exposes the bases along each strand. Each base along one strand attracts its complementary RNA nucleotide, i.e. a free guanine base on the DNA will attract an RNA nucleotide with a cytosine base. It should be remembered, however, that uracil, and not Thymine is attracted to adenine.The enzyme RNA polymerase moves along the DNA adding one complementary RNA nucleotide at a time to the newly unwound portion of DNA. The region of base pairing between the DNA and the RNA is only around 12-based pairs at any one time as the DNA helix reforms behind the RNA polymerase. The DNA thus acts as a template against which mRNA is constructed. A number of MRNA molecules may be formed before the RNA polymerase leaves the DNA, which closes up reforming its double helix.Being too large to diffuse across the nuclear membrane, MRNA leaves instead through the nuclear pores. Tin the cytoplasm it is attracted t o the ribosome. Along the MRNA is a sequence of triplet codes that have been determined by the DNA. Each triplet is called a codon.