Core Concepts of DNA Replication
In this tutorial, you will learn about DNA replication, why it is important in cells, and its process.
Topics Covered in Other Articles
- DNA Transcription Explained
- DNA Translation Explained
- The DNA Tautomer
- Central Dogma of Biology
What is DNA Replication?
DNA is the genetic material that defines every cell in our body. DNA replication is the process of DNA duplication that a cell undergoes before a cell duplicates into daughter cells. The process of replication occurs in several steps that involve proteins (replication enzymes and RNA). In eukaryotic cells, DNA replication occurs during interphase (S phase) in the cell cycle. This process is vital for cell growth, repair, and reproduction in organisms. This process also has industrial and research applications, such as with molecular cloning and PCR.
Basic Overview of DNA Replication Process
The process of DNA replication is semiconservative, this means that each strand in the DNA double helix acts as a template for the synthesis of the new strands. The process takes on starting molecule that has two “daughter” molecules, each forming a new double helix having a new and old strand. Cells are able to copy their DNA very quickly.
The key molecule in DNA replication is the enzyme DNA polymerase. It is responsible for synthesizing DNA by adding nucleotides to grow the DNA chain. There are several key features of DNA polymerase which as listed below.
- Always need a template
- Can only add nucleotides to the 3′ end of a DNA strand
- Cannot start making a DNA strand from scratch, they have to have a pre-existing chain called a primer
- Proofread by removing a vast majority of “wrong” nucleotides that are accidentally in the chain.
However, the addition of nucleotides requires energy. The energy required comes from the nucleotides themselves, they have three phosphates attached to them. When the bond breaks, the energy released provides energy for the new incoming bonds.
Other Replication Enzymes
Besides DNA polymerase, there are several other enzymes in use during the process of replication.
- DNA helicase unwinds and separates the double-stranded DNA as it moves along the DNA. It allows the formation of a replication fork by breaking hydrogen bonds between nucleotide pairs.
- DNA primase (type of RNA polymerase) that generates RNA primers.
- DNA gyrase or Topoisomerase unwinds and rewinds DNA strands to prevent DNA from becoming supercoiled (tangled).
- Exonucleases is a group of enzymes that remove nucleotide base from the end of a DNA chain.
- DNA ligase joins DNA fragments together by forming phosphodiester bonds between nucleotides.
The DNA Replication Process
The process has four main stages: replication fork formation, primer binding, elongation, and termination.
Replication Form Formation
Before replication can take place, the double-stranded DNA must be “unzipped” into two separate single strands. For DNA to unwind, the interactions between base pairs break. DNA helicase aids in this by disrupting the hydrogen bonding between base pairs. As a result, this action separates the strands into a Y shape called a replication fork. Therefore, this area will be the template for replication.
DNA has directionality on both stands which are signified by a 5′ and 3′ end. The end with the phosphate group is the 5′ end while the end with the hydroxyl group is the 3′ end. However, the replication fork is bi-directional. The strand orientated in the 3′ to 5′ direction is the leading strand while the strand oriented 5′ to 3′ is the lagging strand.
The leading strand is the simplest to replicate. After the DNA strands separate, a primer binds to the 3′ end of the strand. It is important to note that the primer always binds as the starting point for the process of replication. Primers are generated from DNA primase.
DNA polymerases are responsible for creating the new strands during the elongation phase. Polymerase III is the main replication enzyme and is responsible for repair and error checking. It binds to the strand at the site of the primer and begins adding new complementary Watson-Crick base pairs. Due to replication proceeding in the 5′ to 3′ direction on the leading strand, the newly formed strand is considered continuous.
However, the lagging strand is a discontinuous process. This is because each primer is only several bases apart, and the DNA polymerase adds pieces of DNA called Okazaki fragments. These new fragments are not connected to one another.
Once the continuous and discontinuous strands are created, the exonuclease removed all the RNA primers from the original strands. The primers are replaced with the appropriate bases. Another exonuclease reads the newly formed DNA to remove and replace any errors. As a result of the discontinuous strand having Okazaki fragments, an enzyme called DNA ligase joins these together to form a unified strand. The ends of the parent DNA contain a repeated DNA sequence called telomeres (act as a protective cap at the end of chromosomes). A telomerase catalyzes the synthesis of the telomere sequences.
After completion, the parent strand and its complementary DNA strand coil into a double helix shape. As a result, replication produces two DNA molecules.