Not exactly. The phosphate ‘backbone’ is added last. Each subunit of a DNA molecule is made up of three parts: a base (A, G, T or C), a sugar (deoxyribose), and a phosphate group. (All of this is easier to explain with pictures; I’ll leave it to you to search for some, as there are many sites that explain DNA structure and replication.) The base and the sugar together are called a nucleoside; a nucleoside with a phosphate group (at the 5’ position) is a nucleotide. (The nucleosides are built first, then the phosphate groups are added.) When DNA is copied, the two strands are pulled apart, and each single strand is copied by adding nucleotides to a growing chain. This can be done because the bases are complementary; that is, A only pairs with T and C with G. So, when a strand is being copied, the appropriate nucleotide comes in and binds first with the complementary base. Then, its phosphate group is bonded to the 3’ position of the growing strand.
When properly processed, each gene will yield one protein, but there is much more to a gene than the sequence of letters that tell the ribosomes which amino acids to use in building the protein. (As has been said, 3 ‘letters’ of DNA – a codon – corresponds with one amino acid in a protein. This is because there are only four possible bases in DNA, but 20 possible amino acids in proteins.) Genes contain several types of other information:
- codes that allow the enzymes that make RNA copies of DNA to locate the correct gene
- codes that indicate to the enzyme that the gene is about to begin
- codes that indicate the beginning of the gene
- codes that indicate which amino acid should be added to a growing protein (‘codons’, the most important part of a gene; sections of the gene that are used to indicate the protein sequence are collectively called ‘exons’)
- codes that indicate the end of the gene and tell the enzyme to stop copying (‘stop codons’)
- long sections of ‘junk DNA’ (introns) that are not part of the protein sequence
After an RNA copy of the DNA is made (transcription), the resulting ‘messenger RNA’ is modified by removing the sections that won’t form part of the protein and by adding and removing some other sections. Then the messenger RNA is delivered to a ribosome, a complex of RNA and proteins that makes new proteins. The RNA is read, three ‘letters’ at a time, and the appropriate amino acids are strung together (translation). The protein then undergoes some further modification (folding, changing some of the amino acids, cutting off parts, adding metal atoms, etc.) before it is ready for use.
Hopefully that explains a bit better what genes actually are. Mutations generally affect one gene at a time, although some very serious mutations might affect many. Mutations might change one base into another (which changes one amino acid in the protein sequence, which can make the difference between a diabetic and a non-diabetic). Or they might add or subtract a base from the sequence, resulting in a change to every amino acid in the sequence after the mutation (resulting in a totally different and probably non-functional protein). Sometimes either of these changes causes also premature or delayed termination of the protein.
One more thing I want to add, because I often thought about it and didn’t learn the answer until very late in the process of studying this kind of thing. The really crucial difference between DNA and RNA is that there is a quick, easy way for RNA molecules to break down chemically (into nucleotides), while there is no easy way for DNA to break down. Thus, RNA is used in applications where the genetic information is used briefly and then recycled, while DNA is used to store genetic information throughout an organism’s lifetime.