Genes: DNA's instructions
Transcription: copying the code
The copy of the sequence to be read is made of RNA, a chemical similar in structure and properties to DNA. RNA has bases attached to a chemical 'backbone' but, unlike DNA, tends to only have a single strand of bases.
More about RNA
RNA has a different sugar in its sugar-phosphate backbone; it uses ribose, rather than deoxyribose. RNA also uses a different base from the thymine (T) that DNA uses; the RNA base is called uracil (U). Although they are different bases, RNA's 'U' pairs with 'A' just as DNA's 'T' does.
The enzyme RNA polymerase makes the RNA copy, recognizing the 'start here' and 'stop here' signals that appear in the DNA code. It uses available bases, sugars and phosphate molecules from the nucleus to form an RNA molecule that is 'complementary' to the DNA strand. This means that the base A always binds to T (or U in RNA), and C always binds to G. For the codon GTC, the complementary triplet would be CAG.
The RNA molecule that is made - called 'messenger RNA' (mRNA) - then carries its 'message' out of the nucleus to the outer part of the cell (the cytoplasm). The mRNA passes through the pores in the nuclear membrane, and makes its way to cellular components called the rough endoplasmic reticulum (ER), where proteins are made. It is called 'rough' ER because, under the microscope, has a bumpy, blobby appearance. The 'blobby' structures are ribosomes: the factories of the cell.
