DNA (Deoxyribonucleic acid)

Structure of DNA

Replication

Mutation

Structure of DNA

DNA is a nucleic acid - a complex molecule composed of nucleotide chains - found in the nucleus of cells. It carries the genetic instructions for the biological development of all cellular life forms, instructions that tell each cell what role it will play in the building, growing and functioning of a body.

These genetic instructions define all aspects of a cell, such as its structure, function, behaviour, growth, development and replication. DNA is sometimes referred to as the 'molecule of heredity' since it is inherited (transmitted to offspring) after it is replicated during reproduction, thereby allowing traits to be propagated.

Within the nucleus of a cell the DNA molecule is packaged into a compact structure called a chromosome. Within this chromosome the DNA molecule is tightly coiled many times around proteins, called histones, that support its structure and presents an efficient storage unit for the somewhat long DNA molecule. All somatic cells (non-sex cells) in humans have forty six chromosomes divided into two sets of twenty three chromosomes.

But how do cells divide and make copies of themselves? This was a question many Scientists puzzled over and in the early 1950s James Watson and Francis Crick in Cambridge, and Maurice Wilkins and Rosalind Franklin in Kings College London made tremendous progress in uncovering this copying (replication) and division (mitosis) process and thereby discovering the secret of life itself! Each of them knew there had to be some sort of script or instructions to follow for the cell copying and division part of the cell cycle, and it had to be in the chromosomes.

Watson and Crick's research indicated that DNA was composed of only four ingredients. They thought that if they could work out how the atoms of these four ingredients were arranged, they might work out what they did - the 3D picture might reveal the function.

After considerable research throughout the years we now know that the Structure of DNA consists of two long strands of nucleotides twisted around each other into a double helical structure (like a spiral staircase). The strands are held together by hydrogen bonds (the 'steps' or 'building blocks'). Each 'step' is made up of a combination of just two of the four of these nucleotides (chemical bases). Each nucleotide consists of a sugar molecule (deoxyribose) linked to a phosphate group to form the helical backbone. Different nucleotides are distinguished only by the identity of the nitrogen-base unit (called a nucleotide base) bonded to the sugar molecule.

The four bases are adenine (A), cytosine (C), guanine (G) and thymine (T) and lie in the central region of the double helix. Each base is linked by hydrogen bonds to a specific complementary base on the partner strand.

Watson and Crick did learn that the relative amounts of the four basic ingredients of DNA remained the same. The amount of A equalled the amount of T and the amount of C equalled the amount of G. This suggested that the chemicals somehow went together in pairs - the first clue in identifying the structure for Watson and Crick. This is what is called a base-pair rule. 'A' pairs only with 'T' and 'C' with 'G'. And the order matters, A+T is not the same as T+A.

The second clue for Watson and Crick for determining the structure of DNA was the image of an 'X', which was the diffraction pattern produced by passing X-rays through DNA. They obtained this from Franklin in Kings college and it strongly suggested that DNA was a helix type structure.

DNA molecules will typically consist of millions of base pairs. In fact the human DNA molecule contains about 3 billion base pairs. Since there are only four possible combinations: A+T, T+A, C+G, G+A, naming only one base on the conventionally chosen side of the strand is enough to describe the sequence.

The order of the bases along the length of the DNA (the sequence) is what's important. It's this sequence of bases which determines hereditary characteristics.