By Vaishnavi Kumar, Allan Kalapura, and Elaine WangDNA, short for deoxyribonucleic acid is an extremely complex molecule that contains all the information necessary to build and maintain an organism. It serves to pass on genetic information from parent to offspring, a characteristic known as heredity. DNA is composed of smaller subunits called nucleotides, of which there are four varieties. Nucleotides are made up of three main components, a 5-carbon sugar molecule called deoxyribose, a phosphate group, and a nitrogenous base. The nitrogenous base is responsible for the identity of the nucleotide. DNA is composed of four types of nucleotides: adenine (A), cytosine (C), guanine (G), and thymine (T). These nucleotides bond in pairs, with A and T only bonding to each other, and G and C bonding to each other. In total, there are over 246 million nucleotides in one chromosome, and over 6 billion in one cell. The picture below shows the structure of DNA in detail. In order for DNA to be passed on from parent to offspring, it must be replicate. ![]() The process of DNA replication starts with helicase, an enzyme that pulls apart the double stranded DNA. It stars in an area rich with A-T pairs since these are easier to separate because they contain two hydrogen bonds as opposed to C-G pairs that contain three. Once the double stranded structure is split, two DNA polymerase enzymes collaborate to copy the leading strand and the lagging strand. On the leading strand, DNA polymerase binds the nucleotides in 5’-3’ direction, while RNA primase inserts starter RNA primer at the initial point, giving the DNA polymerase a signal to start adding the corresponding nucleotides to the strand. On the lagging strand however, RNA starts the binding process. DNA polymerase has to work backwards in 3’-5’ direction, resulting in okazaki fragments, which are short DNA segments that make up the lagging strand of the newly synthesized DNA. After finishing one okazaki fragment, the “clamp” that secures DNA polymerase to the lagging strand dissociates and lets DNA polymerase release the lagging strand temporarily. As the double stranded structure keeps getting split by the helicase, the RNA primase is initiated and inserts a short RNA primer. Then DNA polymerase clamps back to the lagging strand again and begins working from where the primers are. The polymerase stops when it reaches the point where the preceding okazaki fragment is and releases the lagging strand, waiting for RNA primase to signal. Once the copying work is done, an enzyme called exonuclease takes away the RNA primer, and DNA polymerase replaces the gap with DNA nucleotides. At the end of the process, ligase fill the gaps left in the sugar-phosphate backbone. ![]() The structure of DNA easily facilitates the function of replication. Despite the strong double helix that stabilizes the base pairs inside the structure, the weak hydrogen bonds that hold the base pairs together allow the molecule to untwist easily for replication. The DNA molecule is split down the middle into two strands and is now able to create two copies of DNA. Various enzymes are present to stimulate the reaction as the base pairs dislodge from each other. Since the nucleotides are exposed, corresponding base pairs match up to recreate the ladder structure. Once this is complete, this newly generated DNA strand coils back up into the double helix. DNA replication is a crucial part of cell reproduction in all living organisms. In fact, life is dependent on this because without replicating DNA, our information would not pass down to future generations and life would no longer exist. Works Cited
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