DNA

Deoxyribonucleic acid, or DNA, is the molecule that contains all the information necessary for the growth, repair and function of all living organisms, from complex animals like humans and dinosaurs, to single-celled organism such as planktons. DNA contains all an organism’s genetic information, which is passed on from one generation to the next. This is why it is aptly named ‘the molecule of life’ or ‘the molecule of inheritance’. When a cell reproduces for growth by mitosis, it copies its DNA almost exactly.

DNA molecules are made of two twisting, paired strands, joined by rungs like a ladder, as shown in the accompanying model I have made, a photo of which is shown below.

As you can see from the model, each rung of the ladder is made up by a pair of two of four chemical units, called nucleotide bases. The bases are adenine (A), thymine (T), guanine (G) and cytosine (C). Bases on opposite strands pair specifically; an A always pairs with a T, and a C always with a G. They pair with hydrogen bonds between the nucleotide bases, there being three bonds between G and C, and two bonds between A and T. This is the reason for the specific pairing. The ‘uprights’ of the ladder are a sugar-phosphate backbone.

DNA twists once every 10.4 rungs of base pairs, and this creates the famous double-helix. The structure of DNA was first discovered by Watson and Crick (with the aid of Rosalind Franklin) in 1953. Nine years later, Watson and Crick were awarded the Nobel prize for Physiology or Medicine for their efforts. The photograph below left, taken using an electron microscope, is the first actual photograph of the helical structure, and was taken in 2012! The photograph below right (again taken through an electron microscope) is of a strand of DNA.

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DNA’s coiled structure allows an incredible length to be packed into a small space. In fact, these coils are so effective that, if unwound, the DNA from a single chromosome would stretch about 5cm. That is amazing, given that the nucleus, in which the chromosomes reside in which it is carried requires a microscope to be seen. As the cell prepares to divide, the DNA coils even tighter, into a supercoil, which is shorter and denser. These are visible as the typical chromosome X shapes, which are clearly visible in the photograph below.

The human genome (the total DNA in a human) contains approximately 3 billion of the base pairs mentioned earlier, which reside in the 23 pairs of chromosomes within the nucleus of all our cells. However, only 22 pairs are equivalent and are numbered from 1(largest) to 22 (smallest). The twenty-third is the sex pair, with XX signifying a female, and XY signifying a male.

A chromosome is a string-like thread, located in the nucleus of a cell (see the photograph on the previous page), which carries the DNA. Each chromosome contains hundreds to thousands of genes (the unit of inheritance, and is the smallest segment of a chromosome that is responsible for the production of a specific product), which carry the instructions for making proteins. Each of the estimated 30,000 genes in the human genome makes an average of three different sorts of proteins.

With two exceptions, every cell of an organism contains the complete genome of that organism – all the DNA required to create an entire organism. The exceptions are red blood cells that do not have a nucleus, and reproductive cells, which contain only half the total genome. Although much of the DNA within the nucleus is known as non-coding and ‘junk’ DNA (in other words, that part of the strand is not a gene), these sections may still regulate gene function. Junk DNA, though, is different from non-coding DNA; its structure does not resemble that of genes.

Every single species on earth, and every variety within that species, has a slightly different DNA – that’s what makes them different! My DNA differs very slightly from your DNA. A small change in the genome can manifest as a massive change in the final organism. The genes of organisms that look very different are surprisingly similar. For example, human DNA sequences are over 95% identical to chimpanzee sequences and around 50% identical to banana sequences. And look at the difference.

This is a very important fact to remember, as I will discuss further in the section ‘Completing the Genome’. Genes vary enormously in size – size being measured in numbers of base pairs. Some genes, such as that for creating the protein beta-globin, (part of the haemoglobin molecule) are ‘only’ 2000 base pairs long. Others are measured in millions of base pairs. The Gene for blood clotting factor VIII is 200,000 base pairs long.

My model of DNA

First image of DNA double-helix taken 2012 through an electron microscope

Image of DNA strand

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Chromosomes through an electron microscope.