The cell theory was developed over time and contributed to by a number of scientists to explain the relationship between cells and living things. There are three components of the cell theory:
1. All living organisms are made up of one or more cells.
2. The cell is the smallest unit of life.
3. All cells arise from existing cells.
Cellular reproduction, which you will learn about later, is the production of new cells from existing cells. To ensure each new cell is capable of all the correct functions of the parent cell, the DNA must be copied before the cell divides. The copying process is different in eukaryotic and prokaryotic cells. This course only looks at replication in eukaryotic cells.
DNA replication is the process of making an exact copy of all DNA within the cell. The main steps in the process are as follows.
1. DNA replication begins when the enzyme DNA helicase unwinds the double helix and breaks the hydrogen bonds between the nitrogen bases, separating the two strands.
2. Single-strand binding proteins attach to each of the single strands of DNA and prevent them from reannealing, or re-joining. The point at which the strands separate is called the replication fork. There may be multiple replication forks on a single chromosome and replication occurring in multiple places at once to speed up the process.
The two strands with their unpaired nitrogen bases are now templates for the new strands of DNA to be built upon.
3. A short sequence of ribonucleic acid (RNA) nucleotides (you will learn more about RNA later) is added to the DNA by an enzyme called RNA primase. This short sequence is called a primer and helps initiate the process of replication.
4. DNA polymerase, another enzyme, adds complementary nucleotides to the template strand one at a time, starting from the 3’ end of the RNA primer. An associated protein, called the sliding clamp, helps the DNA polymerase slide along the template strand.
5. Another enzyme, nuclease, removes the primers from the template DNA stands and DNA polymerase fills in the gaps with complementary DNA nucleotides.
6. Finally, yet another enzyme, DNA ligase, joins the sections of replicated DNA into a single long strand.