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DNA Replication: The Basics
Deoxyribonucleic acid (DNA) is one of the most vital molecules in cells. It is basically the body's personal filing system, carrying all the genetic information just waiting to be expressed. DNA replication in eukaryotic (plant, animal, etc) organisms is the process through which a single DNA molecule is copied to produce a second identical molecule of DNA.
The whole point of DNA replication is to make more DNA, but why does it matter? Well, DNA replication is a prerequisite to cell division through mitosis and meiosis. Replication sets the foundation on which cells grow and reproduce.
Where it Takes Place:
DNA replication, as its name suggests, takes place on cell DNA. This tightly coiled DNA is stuffed into the nucleus (kind of like the brain) of a single cell.
Structure of DNA:
Before we move on to the actual process of replication, it's important to see how DNA is structured;
As you can see from the picture, DNA takes a helical shape, with two strands coiled around each other, which is what we call double stranded. Each strand has it's own components that constitute half of the DNA molecule before the strands bond to each other to create an entire molecule.
Sugar-Phosphate Backbone: These are the two ribbon-like strands of the DNA helix. Aptly named, it consists of sugar and phosphate groups attached to each other through covalent bonds (bonds created when two molecules share electrons). These strong covalent bonds between phosphate groups and five-carbon sugars are called phosphodiester bonds. The backbone provides structure and support for the molecule and acts as a protective barrier to prevent genetic information from being damaged. Each strand of the backbone has one 5' end and one 3' end (pronounced five prime and three prime respectively). 5' prime means that one end of the strand ends at the fifth carbon molecule on a phosphate group, and 3' prime means that the other end of the strand ends at the third carbon of a hydroxyl group (oxygen and hydrogen; OH).
Notice that purines only bond with pyrimadines and vice versa. Each nucleotide only bonds to one other nucleotide. Adenine bonds with thymine, and guanine bonds with cytosine.
Nucleotides: In the picture, these are the long sticks jutting out of each strand, and are where genetic information is actually stored. Think of nucleotides the body's personal genetic filing system. Nucleotides come in two types; purines and pyrimadines. We don't really need to go into the details at this point, but for now, just think of purines and pyrimadines as two sides to the same coin - they're a little different, but ultimately buy you the same thing. Nucleotides from one strand attach to those on another strand through hydrogen bonding (bonds created when hydrogen interacts with oxygen, nitrogen, or fluorine. Take a look at the table below for information on the four nucleotides present in DNA and how they bond to each other.
Antiparallel: The two strands of a DNA molecule are oriented antiparallel to each other. This means that the strands run directly next to each other, but at one end of the helix, one strand's 5' end is next to the other strand's 3' end, and vice versa. Take a look at the drawing below to get a feel for what I mean.
5'=================================3' (Strand 1)
3'=================================5' (Strand 2)
On to the Steps:
1. First, the enzyme (a protein that breaks substances down) helicase, acts as a zipper by breaking the hydrogen bonds between the nucleotides in order to unwind the DNA helix. The area where unwinding begins is known as the origin of replication, and the resulting split between the two strands is called the replication fork.
2. More proteins, which we'll just call single stranded binding proteins, bind to each strand to prevent them from re-annealing (re-binding) into a double strand.
3. DNA replication can only occur in the 5' to 3' direction. Since the two strands run in opposite directions, the replication process differs between the two strands, one becoming the leading strand, and the other becoming the lagging strand. We'll describe replication in each strand separately.
Replication in Action:
For example, if the template strand read
then the new strand would read
Notice that the new strand runs in the opposite direction, but if you place the strands next to each other, you see that adenine occurs wherever thymine occurred on the template strand, and guanine wherever cytosine occurred on the template (and vice versa).
Leading Strand: A group of enzymes collectivley known as DNA polymerase, binds to the strand running in the 5' to 3' direction. The polymerase then "reads" the sequence of nucleotides on the DNA strand, using it as a template strand that can be used to determine which nucleotides to add to the other strand. The new strand has to match its nucleotides to the template strand, doing so by adhering to the rules for bonding nucleotides as I described above. An enzyme called RNA primase begins this process by adding a few RNA nucleotides to the strand, as DNA polymerase cannot start a new strand, but only continue building an existing one. The leading strand is the strand where DNA polymerase can run from 5' to 3'. This allows DNA polymerase to continuously add nucleotides to the growing 3' end, resulting in a new DNA molecule with one strand from the original template and a newly synthesized daughter strand. Once replication of the leading strand is complete,
P.S. DNA replication is called "semiconservative" because the new DNA molecule conserves one template strand and adds one daughter strand.
Lagging Strand: The lagging strand runs from the 3' end to the 5' end, which means DNA polymerase has to work backwards. A small portion of the lagging strand is fed through helicase at a time, then a new group of proteins called run backwards through DNA polymerase so that new nucleotides can bond from 5' to 3'. Because DNA polymerase must synthesize the new strand in short segments, the strand is made of fragments called okazaki fragments. In addition, while RNA primase is only needed once to start the continuous leading strand, the lagging strand requires RNA primase to begin each okazaki fragment. Finally, a special enzyme called DNA ligase catalyzes the bonding between the okazaki fragments to create one continuous DNA molecule.