DNA Replication
DNA replication is an essential part of cell division and the growth of organisms. The process of DNA replication uses strands of DNA as templates to create new strands of DNA.
The replication of DNA is an incredibly fast and accurate process. On average, around one mistake is made for every 10 billion nucleotides that are replicated. The process includes over a dozen different types of enzymes and other proteins to run correctly.
Structure of DNA
A DNA molecule is made from a series of smaller molecules called ‘nucleotides’. Nucleotides link together to form a linear strand of DNA. Each nucleotide consists of a sugar, a base and a phosphate group.
A nucleotide contains one of four different nitrogenous bases – adenine, thymine, guanine or cytosine. The bases of each nucleotide from one strand of DNA bond to the bases of another strand of DNA to form the DNA double helix.
The four different bases each have a complementary base that they bind with. Adenine and thymine only ever bond with each other, while guanine and cytosine will only bond with each other. The two strands of a double helix are therefore the exact opposite to each other in terms of their sequence of bases.
Starting DNA replication
The process of DNA replication begins at a specific site along a strand of DNA called the ‘origins of replication’. The origins of replication are short sections on a DNA molecule that contain a specific set of nucleotides.
Prokaryotic cells will often have only one origin of replication for their ring of DNA. Eukaryotic cells on the other hand can have hundreds to thousands of origins.
The process is started by a set of proteins that recognise the set of nucleotides at the origins of replication. These proteins are able to separate the two strands of the DNA double helix and create a ‘bubble’ between the two strands.
DNA replication moves in both directions along the two strands of DNA. The bubble increases in size as several other proteins continue to unwind, straighten and separate the two strands of DNA.
As the two strands are separated, binding proteins latch on to the single strands of DNA and prevent them from bonding back together. Both strands are then able to be used as templates for building two new strands of DNA.
The new strand of DNA begins with a short segment of a molecule called RNA. The short segment is known as an RNA primer and it is usually around 5-10 nucleotides long. The new DNA strand begins by attaching a DNA nucleotide to the RNA primer.
Building a DNA molecule
An enzyme called ‘DNA polymerase’ drives the process of building a new strand of DNA. DNA polymerase controls the addition of DNA nucleotides to the new strand of DNA. The polymerase is responsible for adding the correct nucleotides with complementary bases to the template DNA strand. For example, a nucleotide with a thymine base needs to be added to a nucleotide with a complementary adenine base.
Eukaryotic cells have a variety of different DNA polymerase enzymes. Currently more than 10 different DNA polymerases have been discovered. There is, however, only one known DNA polymerase in prokaryotic cells.
DNA polymerases are able to add nucleotides at very impressive rates. In the bacteria E. coli, new strands of DNA are built at a rate of around 500 new nucleotides per second. Human cells aren’t quite that quick but can still add around 50 nucleotides per second to a growing DNA strand.
Leading strand and lagging strand
The two ends of the RNA primer are different and nucleotides are only able to be added to one end. A new strand of DNA can therefore only be grown in one direction.
As the bubble in the double helix grows the two new strands of DNA are built in opposite directions. Each new strand is built towards one of the two forks at the edge of the bubble.
From the RNA primer, the DNA polymerase enzyme can build the new strand of DNA continuously towards the fork. This strand of replicated DNA that grows continuously towards the fork is known as the ‘leading strand’.
As the bubble grows however, new nucleotides are exposed behind the RNA primer at the origin of replication which also need to be replicated. As nucleotides can only be added to one end of an RNA primer, new nucleotides can’t be added in the opposite direction from the origin of replication.
DNA polymerase must replicate the template strand behind the origin of replication. This is achieved by adding short segments of nucleotides to the newly exposed sections from the fork towards the origin of replication. The section of the DNA strand behind the origin of replication is known as the ‘lagging strand’.
Lagging strands are split into multiple short segments. These short segments are known as Okazaki fragments, named after the one of the scientists who discovered them. The lagging strand is split into Okazaki fragments because they cannot continue to grow once they reach the origin of replication or the start of the previous Okazaki fragment.
Each Okazaki fragment is started by its own RNA primer. Two enzymes, a polymerase and a DNA ligase, replace the RNA primer at the start of each Okazaki fragment. This converts the lagging strand into a continuous strand of DNA.
Errors in DNA replication
In the initial pairing of bases with the template DNA strand there is around one error for every 100,000 nucleotides paired. Polymerase enzymes proofread the new strand of DNA against the template strand and fix errors. This fixing reduces errors to around one error for every 10 billion nucleotides. An extremely accurate process.
The one in 10 billion errors exist when a polymerase incorrectly replaces the error with the another incorrect nucleotide. These rare errors are the cause of genetic mutations and cancer.
Telomeres
Telomeres are short sections at the end of DNA strands that get shorter and shorter with each replication of a DNA strand. Telomeres don’t contain information for specific genes but are a safety net for a slight problem with DNA replication in eukaryotic cells.
Remember that DNA replication begins with the attachment of an RNA primer and DNA polymerase can only add nucleotides to one end of the RNA primer. Every time a strand of DNA is replicated the section of DNA behind the RNA primer cannot be replicated.
This is an issue only for eukaryotic cells that have linear strands of DNA. Prokaryotic cells have a single ring of DNA so all their DNA is able to be replicated.
Telomeres provide a solution to this problem. They are short sections at the end of a DNA strand that usually contain one repeating sequence of bases. The sequence is repeated between 100-1000 times and contains no genetic information. Having telomeres at the end of strands of DNA prevents the loss of genetic information through imperfect replication of DNA.
A telomere become shorter each time DNA is replicated. The shortening of telomeres is thought to be involved in the process of aging for both cells and whole organisms. As an individual grows older, the DNA of all of their cells will have been through many replications. If a cell has been through too many replications it is possible for the entire telomere to be lost and the cell is likely to be killed.
Mutations
An error in DNA replication is known as a mutation. If an error is not corrected and remains present in the new DNA strand then every time that strand of DNA is replicated the error will be replicated. If an error occurs in sperm or egg cells the mutation can be passed to the next generation.
Most mutations are harmful but some can be beneficial. Many mutations can affect how a cell performs and often mutant cells will die before they can replicate again.
Mutations are the only way new genetic material is produced. Over billions of years the rare beneficial mutations have taken life from simple, single-celled organisms to diverse array of complex and spectacular species. Mutations are a key part of evolution.
Mutation rate in humans
The DNA of humans has a total of six billion base pairs. With an error rate of around one error for every 10 billions bases, around 0.6 errors will occur for every replication of a cell’s DNA.
A fully grown human has around 37 trillion cells in their body. By the time a human is fully grown their cells will have been replicated over 37 trillion times.
With around 0.6 errors per replication, a fully grown human will have had around 22 trillion mutations to their DNA. Fortunately, the vast majority of these are lost and never have an impact on our lives. Very rarely do mutations become a problem. Needless to say however, we are all mutants!
Last edited: 15 March 2016
