DNA - the carrier of genetic information

Introduction

DNA molecules are carriers of an organism's genetic information. A cell's chromosomes are made from DNA molecules and associated proteins. This chapter looks at the discovery and structure of DNA - the molecule of life.

Discovery of DNA

Johann Friedrich Miescher was the first scientist who proposed the existence of a molecule that carries genetic information. He called it nuclein. But it was only in the 1950s that two young scientists, James Watson and Francis Crick, discovered the true genetic information carriers - molecules of DNA and RNA. They were the first to describe the DNA structure as a double helix (double spiral). This double helix model of DNA is referred to as the Watson-Crick model of DNA. Today, the DNA molecule is probably the most famous molecule in the world. It is sometimes called the molecule of life.

The Watson-Crick model of DNA

DNA is an abbreviation of deoxyribonucleic acid. DNA is a polymer, which means it is a large molecule composed of smaller units called monomers. The monomer units of DNA are called nucleotides, which makes the DNA molecule a polynucleotide. It qualifies as an acid because when dissolved it gives a solution a pH (the measure of the activity of hydrogen ions (H+) in a solution) of less than 7.

Nucleotides are made of one nitrogenous base, one phosphate molecule and one sugar molecule (deoxyribose in DNA and ribose in RNA) There are four different types of nucleotides found in DNA. When drawing the DNA molecule model these nucleotides are referred to by their 'initials':

• A is for adenine
• G is for guanine
• C is for cytosine
• T is for thymine

Nucleotides are joined in a specific pattern, forming polynucleotide chains. Two of these chains are twisted into a spiral and held together by complementary nucleotides on opposing strands, forming a DNA molecule. So, DNA is a double-stranded macromolecule that is also referred to as a double helix because it resembles a spiral ladder. The side rails of this 'ladder' are made of sugar and phosphate units and the rungs are made of nitrogen bases. Because of their chemical structure, nitrogen bases can pair only in a specific pattern, forming complementary pairs which are the following:

• A - T
• C - G

Each base within a rung of the DNA ladder is always paired with the same complementary base. The sequences of these bases form genes. Genes are sections of DNA that determine an organism's characteristics and function.

DNA molecules are quite long. For example, human DNA is about two metres long. In order to fit inside the cell nucleus, DNA has to be coiled and twisted upon itself many times. Different enzymes are responsible for DNA packaging. See image 1.

DNA replication

During cell division, the DNA molecule is copied for future generations of cells. This process of DNA copying is called replication. See image 2.

During replication, two DNA strands unwind and then separate like two sides of a zipper. After the DNA is 'unzipped' each strand becomes a matrix for the other new half. That way DNA can reproduce itself without changing its structure. Once the DNA strands have separated, they must be held apart for better nucleotide sequence 'reading' by the enzyme called DNA polymerase. DNA polymerase moves along the exposed DNA strand, binding together new nucleotides into a new DNA strand that is complementary to the template. DNA polymerase cannot start a strand from scratch because it can only bond new nucleotides to a free sugar end of a nucleotide chain. A primer is needed to start the process. A primer is a short strand of previously existing DNA or RNA that is complementary to the first part of the DNA segment being copied. Primers are formed from free cellular nucleotides by an enzyme called DNA primase. After the new DNA chain is created, it is stitched together with the pre-existing one by the enzyme called DNA ligase. The double helix of the new molecule of DNA is formed as a result. The accurate replication of DNA is ensured by about 30 different cell enzymes.

The precise sequence of nucleotides (sugar, phosphate and base) in the DNA molecule directs protein synthesis. Because different organisms and tissues are made from different proteins, protein structure determines the identity of an organism.

RNA

RNA is an abbreviation of ribonucleic acid. RNA is a single-stranded polymer. Its structure is similar to DNA except it has sugar ribose in place of deoxyribose and base uracil in place of thymine. During protein synthesis RNA molecules transfer genetic information from DNA molecules.

They also maintain the structure of a cell's ribosomes - protein-making 'factories'. Unlike DNA, RNA can leave the cell's nucleus. See image 3.