Mitosis is the process by which one cell divides and becomes two. During mitosis, a chemical reaction called transcription disrupts the meta stable hydrogen bonds that hold the two single strands of the DNA molecule in its helical embrace. This transcription process is promoted by an enzyme produced by RNA, called polymerase.
The DNA molecule itself looks like a spiral staircase, each step representing two nucleotides joined at the middle with a hydrogen bond. At the opposite ends of the nucleotide pair, peptide bonds join the step to the one above and below it. These peptide bonds involve a sugar-phosphate molecule called deoxyribose (the D in DNA). This deoxyribose sugar forms a lattice structure which molecular biologist refer to as the backbone of the DNA macromolecule. The nucleotides can be one of four amino acid molecules, Adenine, Guanine, Cytosine, and Thymine. Each step is comprised of two of these, but there is a strict chemical protocol that defines how these amino acids can be combined. Cytosine (abbreviated "C") can be paired with Thymine (T) to form a pyridine base pair and Adenine (A) can be paired with Guanine (G) to form a purine base pair. This permits four possible permutations of nucleotide pairs (AT, TA, CG, and GC).
As the polymerase enzyme works its way down the DNA double helix, it parses the nucleotide pairs in the middle and then attaches a new nucleotide of the corresponding, chemically correct, base pair counterpart. The new amino acids are drawn from the cytoplasm soup in the cells nucleus which is rich in free amino acid molecules and polypeptide chains. When the Polymerase gets to the end of the DNA macromolecule, one DNA has essentially be made into two identical copies of the original. Well, that is not quite true.
It turns out that there is another amino acid molecule called Uracil, also floating around in the nucleic cytoplasm which is very similar in chemical construct to Thymine. On an almost regular basis, the polymerase enzyme will latch onto a Uracil molecule and attach it to a DNA macromolecule undergoing transcription. On rarer occasions altered configurations of the other amino acids are also attached to a DNA macromolecule, resulting in a copying error or mutation which molecular biologist refer to as polymorphisms.
Most of the time, polymorphisms have no effect on the function of the DNA molecule which manifests them. The human genome (the aggregate of 46 molecules of DNA called chromosomes) consists of about 3 billion nucleotide pairs, but only about one in a hundred thousand of the nucleotides is associated with an active gene. Genes, are sequences of nucleotides arranged vertically on the DNA helix that code for proteins or amino acids. Active genes, or the proteins and amino acids they code for, actually influence metabolic functions of the being or organism which they are located in. Sequences of nucleotides in DNA that are not active genes are pretty much innocuous stuff. Much of this inconsequential matter is representative of dormant genes which may have had some function thousands, millions, or even billions of years ago, but have sense been rendered obsolete by mutation and the process it drives, evolution.
So, mutations are happening every time DNA macromolecules are copied. The odds that a mutation will effect an active gene are actually pretty low, and depending on the importance of the gene to metabolic function, may have little or no obvious effect on the organism. I should point out also, that in some cases, the effect of other enzymes will actually correct nucleotide missteps. This is particularly true in the case of the Uracil-Thymine mutation.
Mutations can cause harmful as well as beneficial changes in the host organism. Simple organisms like viruses and bacteria, can actually experience genetic mutation driven variations quite quickly. This is in part due to a short term reproductive cycle. Since mutational variation is expressed from one generation to the next, species with longer life spans and reproductive cycles evolve more slowly. But what is important, is that all of the change we can see, the differences between species and even subspecies, are all the result of random mutations which have occurred when DNA macromolecules are copied.
Mitosis is a multiphase process, and I have focused here on a very small aspect of it. Indeed, the chemical interactions going on within the nucleus of any living cell represent a staggering degree of chemical complexity for sure, but the science of molecular biology is slowly sorting it all out. I have tried to convey here but a snippet of information to summarize with some specificity the intrinsic mechanisms which underlay instances of genetic mutation.
James D. Watson, DNA the secret of life,