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DNA replication is the process by which genetic information is accurately duplicated within living cells. DNA is made up of two strands of nucleotides, held together by hydrogen bonds between complementary base pairs. Enzymes work to split the strands and unwind the DNA, allowing for the addition of new nucleotides to form a new double-stranded molecule. The replication process is highly accurate, but errors can occur, which can lead to mutations and diseases. However, replication errors also allow for evolution and adaptation to changing environments.
The DNA molecule forms the basis of all known life because its structure allows it to be easily copied within living cells, allowing them to reproduce. An organism’s genetic information is contained in its DNA, and accurate duplication is required to pass this information on to subsequent generations. The copying of genetic material inside the cell nucleus is called DNA replication. The mechanism by which it occurs is known as semi-conservative replication and involves the splitting of the molecule into two parts, each of which forms a template for a complete new molecule. The materials available within the cell are then added to these models to complete the process.
The structure of DNA
Each DNA molecule is made up of two strands, made up of sugars and phosphate groups, with molecules known as bases forming bonds between them. There are four different bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Each base, along with the sugar and phosphate groups it is attached to, is known as a nucleotide. The two strands are held together by hydrogen bonds between the bases; A bonds to T and C to G, forming pairs known as complementary base pairs.
The strands form a double helix, or two parallel spiral structures, while the base pairs span the space between the strands. DNA molecules are normally tightly coiled, many times, and form structures known as chromosomes. The complete genetic information, or genome of an organism, is contained within a set of chromosomes; the human genome contains about three billion base pairs. DNA replication forms a new set of chromosomes, before cell division. The replication process can be divided into a number of steps, each controlled by enzymes.
Split
To replicate, the DNA strands must be separated. The hydrogen bonds between the base pairs are strong enough to hold the strands together under normal circumstances, but weak enough to allow them to easily separate when needed. Since the molecule is normally in a highly coiled state, the two strands will not split without help. Enzymes called gyrases work to relax, or unwind, the DNA, while enzymes called helicases begin to unzip it, breaking the hydrogen bonds between base pairs. Special proteins then bind to the separated strands to keep them apart and allow for replication.
Duplication
Nucleotides exist independently of DNA in the nucleus of a cell or, in the case of bacteria, within the cell fluid. When a DNA molecule has been cleaved, these free nucleotides bind to the unpaired complementary bases of each strand – A to T and C to G – forming a new double-stranded molecule. This process is made possible by enzymes known as DNA polymerases. The two resulting copies each have one new strand and one from the original molecule. This is why DNA replication is called semi-conservative: half of each molecule is new and half is saved from its parent.
The processes of splitting and duplicating overlap. As the strands separate, new complementary strands are built as cleavage continues along the double helix. The DNA molecules in most organisms are very long, so it is more efficient for cleavage and duplication to occur in multiple places at the same time. These points are known as replica origins. When two of these origins meet, enzymes called ligases bring the new strands together.
Error checking
The replication process is highly accurate, but errors occur. Sometimes, a bond can form between the wrong combination of bases. For example, G can occasionally bond with T instead of A. Bases can also exist in slightly different forms which can bond in other mismatches.
Typically, there is about one error for every 100 million base pair bonds. In a human, this would result in about 30 errors for every complete replica. There are, however, a number of error checking and correction mechanisms that detect and repair errors very effectively. For example, the bonds between mismatched base pairs are relatively unstable, and the polymerase enzymes that assist the duplication process can even cleave an incorrect nucleotide, allowing for the addition of a new, correct one. These reduce the average number of errors per replica to around three.
Replication errors: mutations, cancer and evolution
Errors in DNA replication are usually a bad thing on an individual level. They can lead to mutations, generally unfavourable; can cause cancer or other life-threatening diseases. On the other hand, without these errors, humans and other organisms as they are known today would not be here. Occasionally, a mutation can give an advantage, increasing an organism’s chances of surviving long enough to reproduce and pass on the favorable change, which will then become more common. This is how evolution works: replication errors allow organisms to adapt to changing environments and evolve into new forms of life.
