DNA Replication
Biology is simply amazing, especially once you start getting deeper into it. Once you learn about the processes that happen within your cells every second, you start to feel truly amazed and fascinated at the sheer complexity of it all.
One biological process that life depends on is the process of DNA replication. I mentioned in the Mitosis and Meiosis page that before either of the processes can occur, the cell must first replicate its DNA during the S phase of Interphase.
But how exactly does the cell do that? The process is more incredible and mindblowing than you think.
But before we get into the process of DNA replication, we should talk about how we discovered all of this information anyway and how we made these advancements in the field of molecular biology (the study of heredity at the molecular level).
It all begins (or somewhat begins) at a scientist named Fredrick Griffith. Griffith was experimenting with rats in his lab. He was injecting them with two strains of a pneumonia-causing virus: a nonpathogenic R strain and a pathogenic (disease-causing) S strain.
The rats he injected with the live R strain stayed alive, while the ones he injected with the live S strain died. He then took some of the S strain and killed the bacteria with heat before putting it into the mice. These mice lived. However, when he mixed live R strain and heat-killed S strain, the mouse still died. This was quite strange, so Griffith theorized that there was some sort of transforming agent causing the R strain to become pathogenic.
Later on in 1928, two scientists, Alfred Hershey and Martha Chase were attempting to figure out whether proteins or nucleic acids carry genetic information. To do this, they raised two strains of bacteriophages (virus that only infect bacteria). One strain was raised in radioactive sulfur (its proteins were radioactive) while the other was raised in radioactive phosphorus (its nucleic acids were radioactive). They then let the phages infect cells in some test tubes. Hershey and Chase used a blender as a centrifuge and rotated the test tubes. The cells of the bacteria rested on the bottom while the much lighter virus was floating in the liquid above. Now, all they had to do was measure the radioactivity in each tube. The tube that was being tested for proteins showed the radioactivity in the liquid. This meant that the proteins had not entered the bacteria while the phages were infecting them. The tube that was being tested for nucleotides, however, displayed radioactivity in the cell region. This meant that the nucleic acids had actually been the agent carrying the genetic information required to take control of the cell. This allowed Hershey and Chase to conclude that DNA carries genetic information.
A few years later, a woman named Rosalind Franklin and a man named Maurice Wilkins took an x-ray image of DNA. James Watson and Francis Crick, who were over visiting, saw the x-ray and used it to create the first ever 3-D model of DNA, which also confirmed Erwin Chargaff's discovery that the amount of thymine is equal to the amount of adenine and the amount of cytosine is equal to the amount of guanine in a cell.
But with that, let's get into the actual process of DNA replication, now that you know the basic history of the discoveries of DNA.
This image depicts the initiation stage of the process of DNA replication, starting with helicase, topoisomerase, and the single-stranded binding proteins.
Initiation
DNA replication has three steps, which I will go over. The first step is initiation. During this step, an enzyme called helicase unzips and breaks apart the two strands of DNA. The location that helicase is unzipping is called the replication fork. Single stranded binding proteins then hold the two DNA strands apart so they don't rejoin. In front of the replication fork, an enzyme called topoisomerase relieves the tension put on DNA from being held apart by breaking, twisting, and rewinding it together. Now actual DNA construction can begin.
Elongation
The second step of DNA replication is elongation. During this step, an enzyme known as DNA polymerase adds new nucleotides to each DNA strand. A primer protein binds to DNA, telling DNA polymerase where to start. DNA polymerase hardly ever makes a mistake, as it has an encoded double check feature. However, DNA polymerase can only construct from the 5' to 3' end of DNA (each strand of DNA ends with a phosphate group sticking out of one of the ends, as it is part of the sugar-phosphate backbone of DNA. The end where the phosphate group extends is known as the 5' end while the other end is known as the 3' end). On one of the strands, known as the leading strand, DNA polymerase can just keep constructing, bringing new nucleotides in to finish the strand. However, on the other strand, DNA polymerase builds awkwardly. It builds in the wrong direction, so it must build, jump back up to where the primer is telling it to go, and continue adding nucleotides, jumping further up the DNA each time. This however, creates fragments in the DNA called Okazaki fragments after the scientist who discovered them. These fragments are spaced apart by single nucleotides with no covalent bonds between the sugar-phosphate backbone. However, our body is ingenius, so we have an enzyme called DNA Ligase that patches up this little gap. In this way, one strand become the template for another, creating two copies of DNA out of one.
Here is a picture of what DNA polymerase is doing on both strands. On the leading strand, it builds towards the replication fork, assembling the new DNA.
The lagging strand is little bit more complicated, so this picture should help. DNA polymerase constructs away from the replication fork, creating Okazaki fragments that must be patched together using DNA Ligase.
The replication bubbles, overall process, and finished product of DNA replication are shown by this picture.
Termination
DNA strands are massive (compared to DNA polymerase). That way, to speed up elongation, multiple DNA polymerases construct at the same time. The polymerases start contructing in opposite directions, forming a replication bubble in the DNA. Often times, you can find multiple bubbles in replicating DNA. This type of replication is semi-conservative - each strand of DNA is made of one old strand one new strand of DNA. Either way, after elongation is finished and the DNA ligase has filled in all the gaps, DNA polymerase detaches from the DNA and we are left with two new strands, making the termination of DNA replication.
Unfortunately, this is all I have for you. I went through the basics of DNA replication, which is just so cool! All these enzymes working together.. wow! I tried my best to go easy so it wasn't difficult for you guys to understand. I also went through a few scientists with their major discoveries in molecular biology.
I hope you enjoyed this page and I hope to see you in the next one! If you have any questions at all please please please reach out to me at twisha.sharma30@gmail.com. I would be so honored to help you! Also, all the images I have linked above are from articles you can use to help understand the content if you didn't understand.