Remember that deoxyribonucleic acid (DNA) is the material within the nucleus of a cell that carries genetic information. This information is passed on from parents to offspring. Two long chains of molecules form a double helix to make the DNA structure. The chains are linked together in a pattern like a twisted ladder spiraling around its long axis. What is DNA replication? DNA replication is the process by which the DNA double helix unzips and makes a new DNA molecule.
Fast and efficient, the replication of around five million base pairs of a bacterial chromosome occurs in twenty minutes. Our forty-six chromosomes consisting of approximately three billion base pairs can be replicated within hours. Each chromosome, made up of one DNA molecule serves as a template for creating a new DNA structure.
Before DNA replication can be explained, let’s first look at the structure of the DNA molecule. The nucleotide, the basic unit of the DNA molecule is made up of a five carbon sugar called deoxyribose, a phosphate group and a purine or pyrimidine nitrogenous base. The DNA molecule has four nitrogenous bases: adenine, guanine, cytosine, and thymine. Hydrogen bonding between the nitrogenous bases holds the two chains of the double helix together, like the “rungs" of a ladder.
DNA Replication and Enzymes
The first step of DNA replication is the unwinding of the double helix at specific points, called origins of replication. There may be hundreds or thousands of origins of replication in our DNA; however, there is usually only one in bacteria. The hydrogen bonds, mentioned earlier separate as enzymes catalyze the replication process. Adenine separates from thymine, and guanine separates from cytosine.
DNA primase, DNA polymerase and helicases are the main enzymes involved in DNA replication. The original DNA structure is unwound by helicases, at the origin of replication and each original single-stranded DNA molecule is used as a template. Single-stranded binding proteins then bind to the template to prevent degradation of the original DNA molecule.
DNA polymerase copies from the template and adds nucleotides from the 5’ end to the free 3’ end. As both ends of the double helix separate, Y-shaped regions result called replication forks. The two chains of the double helix are antiparallel so as nucleotides are added, there is a leading strand and a lagging strand – the two prongs of the fork.
The leading strand continually adds in the 5’ to 3’ direction along the template as DNA polymerase moves toward the replication fork. The lagging strand discontinually adds nucleotides to the template in small pieces called Okazaki fragments as DNA polymerase moves away from the replication fork.