 In this video I will describe the location and structure of DNA, list the steps involved in DNA replication including the enzymes involved in their functions, and compare the replication of the leading strand versus the lagging strand. DNA, or deoxyribonucleic acid, is the chemical that functions as the genetic instructions for cells. DNA is found inside of the nucleus. During interphase of the cell cycle, DNA is loosely wrapped around histone proteins, forming the structure known as chromatin, which is uncondensed and spread out through the nucleus. However, during m-phase of the cell cycle, the chromatin will condense to form linear strands known as chromosomes. This figure shows us the structure of DNA. DNA is double-stranded. There are two nucleic acid polymers that are held together by complementary base pairing. The complementary base pairing always pairs thymine with adenine, and thyminess held together to adenine by two hydrogen bonds. The complementary base pairing will always pair guanine with cytosine, and guanine is held to cytosine with three hydrogen bonds. While the hydrogen bonding between base pairs holds two strands together to form a double-stranded, double helix molecule of DNA, within each nucleic acid polymer, the single strand is formed from nucleotides that are held together by a sugar phosphate backbone. The sugar phosphate backbone is a phosphodiester bond formed between the phosphate group of one nucleotide and the five-carbon sugar of an adjacent nucleotide. The free end of a nucleic acid polymer that has a phosphate group unbound is known as the five prime end. The opposite end of that polymer, where there is a free five-carbon sugar that is unbound, is known as the three prime end. The two strands of DNA are anti-parallel to each other, so that the five prime end of one strand is forming hydrogen bonding to create the complementary base pairing with the nucleotide at the three prime end of the other strand. This structure of DNA as a double helix with complementary base pairing enables the semi-conservative mechanism of DNA replication, where two-parent strands serve as a template for the synthesis of the new complementary strands. When DNA replication occurs, there will be a replication bubble formed, and that replication bubble has two replication forks at either end, where DNA replication can proceed in two opposite directions, producing both a leading and a lagging strand. DNA replication will always proceed in the five prime to three prime direction, adding nucleotides onto the three prime end. Therefore, the leading strand will be the strand where nucleotides will continuously be added at the three prime end to form one continuous copy. In contrast, the lagging strand will have to be initiated multiple times, and then the fragments produced from replicating smaller pieces of DNA, known as okazaki fragments, will be joined together. Multiple replication bubbles will form simultaneously in one double helix DNA molecule, so there can be hundreds or thousands of replication forks copying DNA simultaneously. The first step of DNA replication is known as initiation. During initiation, the two complementary strands of DNA are separated, and this creates what's known as a replication bubble. There are thousands of origins of replication throughout the chromosome, and at each of these origins, a DNA replication bubble is formed, and on either end of the replication bubble is a replication fork. The illustration here shows us a replication fork at one end of a replication bubble. At the end of DNA replication, all of the replication bubbles will have joined together, and then the chromosome will have been completely replicated. So in the beginning of initiation, the first step of DNA replication, an enzyme known as helicase, breaks the hydrogen bonds that hold together complementary base pairs. Following helicase, the enzyme primase constructs an RNA primer complementary to the DNA template. The second step of DNA replication is called elongation. During elongation, the enzyme DNA polymerase adds new DNA nucleotides on to the 3'n, creating a longer DNA polymer that grows from the 3'n of the RNA primer. DNA polymerase can only move in the 5'-3' direction, but there are two template strands of the DNA that are simultaneously being replicated. On the leading strand, DNA polymerase can follow along in the 5'-3' direction right behind helicase as the replication fork continues to open up. However, on the lagging strand, a new RNA primer will have to be formed after helicase opens up a new segment of the DNA. Then DNA polymerase can move in the 5'-3' direction, adding new DNA nucleotides on to the 3'n of that RNA primer, creating a nucleic acid sequence that's a short fragment known as an Okazaki fragment. During DNA replication of both the lagging strand and the leading strand, the RNA primers will have to be removed and replaced with DNA nucleotides. This process is carried out by an enzyme known as RNAase H. RNAase H is the enzyme that replaces the RNA primer with DNA. Then finally, the enzyme DNA ligase joins the fragments together, forming a single DNA strand. There are numerous fragments of DNA formed during the elongation step of DNA replication. At the end of the elongation step in DNA replication, DNA ligase catalyzes the formation of phosphodiester bonds, uniting a long sugar phosphate backbone through the entire chromosome as one long DNA molecule. As all of the replication bubbles are joining together at the end of DNA replication, this is the third and final step of DNA replication known as termination. During termination, the enzymes, the DNA polymerase, primase, and finally DNA ligase are released from the DNA, and the process of DNA replication is completed with the creation of two identical DNA molecules that are continuously sealed with a sugar phosphate backbone connecting throughout from the 5' to the 3' end of the entire chromosome.