Unraveling the Process of DNA Replication
As organisms grow and change over time, cells need to be able to multiply in order to facilitate these changes. A key component of cells is DNA, or deoxyribonucleic acid, and this large molecule is made of building blocks that provide our cells coded information so that they can produce proteins that are key to life. When cells replicate, the DNA has to replicate as well, so that the new cell also has a copy of DNA. The DNA replication process, though complicated, is a crucial part of any organism’s life cycle.
What is DNA?
DNA is a large chemical compound, composed of carbon, hydrogen, oxygen, nitrogen and phosphorous. These elements make up the four DNA building blocks that are linked into a very long strand. These building blocks are adenine, guanine, thymine, and cytosine, and their individual molecular structures allows for the DNA double helix shape to form from two individual strands of DNA. The order of these building blocks on a DNA strand reflects specific code that through a series of steps tells the cell what types of proteins to make. Not every cell makes all of the proteins coded by its DNA, but based on the location of the cell, specific proteins are made to carry out specific functions.
The structure of DNA is a double helix, which is commonly represented by two strands wrapping around each other. The double helix is composed of two distinct regions, the sugar-phosphate backbone and the nucleotide bases.The bases of the DNA strand, also known as nucleotides, are paired specifically, either adenine and thymine or cytosine and guanine, and these specific pairs allow for the two strands of DNA to form the double helix structure.
These two strands are important in the DNA replication process, as one strand is called the coding strand while the other is called the template strand. During DNA replication, the DNA double helix is unwound, and a new strand of DNA is created using the template strand.
DNA is replicated semi-conservatively which means that, when DNA is replicated (from one to two double helices) each of the new double helices has one strand of DNA from the old double helix and one strand of newly created DNA.
Note: This is a very abbreviated overview, DNA replication is a complicated process and I encourage you to view this video here after reading this short summary.
Step 1: Opening the Double Helix
The first step of DNA replication is unwinding the double helix structure. The two DNA strands in a double helix are complementary to each other, and the ends of the strands are labeled either 3’ or 5’. This means that on one end of the double helix, one end of the DNA strand is always 3’ and the other is always 5’. This is important because the enzymes that are involved in DNA replication move in specific directions on the DNA strand (either 3’ to 5’ or 5’ to 3’).
The enzyme DNA helicase splits the two complementary DNA strands that make the DNA helix in order to prepare for new subunits to be linked to the exposed strands, which leads to the creation of two new DNA helices.
Step 2: Priming DNA
Proteins called single stranded binding proteins attach to the individual DNA strands to stabilize them prior to nucleotide subunits being attached to the individual strands. This is important because it is necessary that the nucleotides pair correctly (remember A and T, and C and G). Pairing incorrectly can result in a DNA replication error.
Step 3: Creating New DNA
The leading strand is the strand of DNA that the enzyme DNA polymerase attaches to. DNA polymerase can only add DNA bases in the 5’-3’ direction, and therefore can only help with adding nucleotide bases to one of the two strands in the unwound double helix. This enzyme adds nucleotide bases one by one, pairing them with the original strand of DNA. But remember, there are two strands of DNA that need nucleotides (see the semiconservative model). The other strand of DNA is called the lagging strand and due to the direction of this strand, DNA bases are added by the attachment of fragments of DNA strands that are bound together by another enzyme called DNA ligase. These DNA fragments that add nucleotides to the lagging strand are called Okazaki fragments.
Though this may be confusing at first, but important takeaways from this article are:
- DNA is made up of nucleotide bases and a sugar phosphate backbone
- The four nucleotide bases are adenine, guanine, thymine, and cytosine, and they are paired between strands, specifically A with T and C with G
- DNA replication is semiconservative
- DNA replication is a series of specific steps that need to work perfectly in order for DNA to be replicated without errors
Click here to watch a cool video of what scientists think DNA replication looks like.
Author: Sydni Britton