Replication
DNA replication is the copying of DNA in order to create more cells with identical DNA. In eukaryotes (organisms whose cells have nuclei) the DNA is copied into the chromosomal formation while in prokaryotes (those without) the DNA has a circular structure. The process of replication occurs during the S phase just before cell division, or mitosis, in which the cell will split into two cells. It is not an integrated part of the normal cell cycle but rather provides the needed new copy of DNA that will belong to the daughter cells after division.
DNA replication uses a semiconservative model in which two parent strands are unwound by DNA helicase and serve as a template for a new strand built by the enzyme DNA polymerase, an enzyme that assembles complementary nucleotide chains, building in the 5' to 3' direction. Hydrolysis of the bond between 2 phosphates provides energy for the bond of the new base. A primer made of RNA begins the 5' end of the new complementary strand, commencing the replication process. DNA polymerase adds individual nucleotides from the cytoplasm onto the complementary strand, creating matching base pairs by putting in G, C, A, or T according to the corresponding protein on the original strand. Another DNA polymerase does the same work on the antiparallel strand of the original DNA, the side from which the other strand was separated at the replication fork. Because they are running antiparallel to each other, replication must go in the opposite direction on the two strands.
Replication begins at an origin of replication, a stretch of DNA with a specific nucleotide sequence that proteins recognize and respond to by forming a replication fork. The leading strand is the new complementary strand assembled in the direction in which the replication fork is unwinding. The lagging strand is built in fragments called Okazaki fragments because it is copying in the opposite direction, so the fork must keep reopening and thus restarting the process on a new fragment. After the entire original strand of DNA has been opened up and replicated, DNA ligase closes the gap between the fragments and the result is the two new double helices. Replication occurs in all parts of the individual's DNA while the cell is preparing to divide into two new daughter cells, so the cell is now ready to give one copy of DNA to each daughter cell.
DNA replication uses a semiconservative model in which two parent strands are unwound by DNA helicase and serve as a template for a new strand built by the enzyme DNA polymerase, an enzyme that assembles complementary nucleotide chains, building in the 5' to 3' direction. Hydrolysis of the bond between 2 phosphates provides energy for the bond of the new base. A primer made of RNA begins the 5' end of the new complementary strand, commencing the replication process. DNA polymerase adds individual nucleotides from the cytoplasm onto the complementary strand, creating matching base pairs by putting in G, C, A, or T according to the corresponding protein on the original strand. Another DNA polymerase does the same work on the antiparallel strand of the original DNA, the side from which the other strand was separated at the replication fork. Because they are running antiparallel to each other, replication must go in the opposite direction on the two strands.
Replication begins at an origin of replication, a stretch of DNA with a specific nucleotide sequence that proteins recognize and respond to by forming a replication fork. The leading strand is the new complementary strand assembled in the direction in which the replication fork is unwinding. The lagging strand is built in fragments called Okazaki fragments because it is copying in the opposite direction, so the fork must keep reopening and thus restarting the process on a new fragment. After the entire original strand of DNA has been opened up and replicated, DNA ligase closes the gap between the fragments and the result is the two new double helices. Replication occurs in all parts of the individual's DNA while the cell is preparing to divide into two new daughter cells, so the cell is now ready to give one copy of DNA to each daughter cell.