DNA Replication - Class XII

Arthur Kornberg purified and characterized DNA polymerase from E. coli cells which is encoded by the polA gene. It catalyzes phosphoryl group transfer. The 3-hydroxyl group of the nucleotide at the 3' end of the growing strand serve as nucleophile for nucleophilic attack at the $\alpha$ phosphorus of the incoming deoxynucleoside triphosphate. Inorganic pyrophosphate is released in the reaction. 

DNA ligase is a specific type of enzyme, a ligase, that facilitates the joining of DNA strands together by catalyzing the formation of a phosphodiester bond. It plays a role in repairing single-strand breaks in duplex DNA in living organisms, but some forms may specifically repair double-strand breaks, i.e., a break in both complementary strands of DNA. Single-strand breaks are repaired by DNA ligase using the complementary strand of the double helix as a template, with DNA ligase creating the final phosphodiester bond to fully repair the DNA. DNA ligase has applications in both DNA repair and DNA replication.

DNA polymerase and ligase as well as RNA polymerase and ligase have an important role in DNA replication. DNA replication, like all biological polymerization processes, proceeds in three enzymatically catalyzed and coordinated steps : initiation, elongation and termination. All enzymes mentioned play important roles in these steps of replication.

DNA polymerase enzymes are strictly unidirectional, synthesizing new DNA strands only in the 5' to 3' direction by adding nucleotides to the 3' hydroxyl group.

DNA polymerase has a high degree of accuracy due to its proofreading capability, which allows it to remove incorrectly paired nucleotides during replication.

The DNA polymerases are enzymes that create DNA molecules by assembling nucleotides, the building blocks of DNA. These enzymes are essential to DNA replication and usually work in pairs to create two identical DNA strands from a single original DNA molecule. During this process, DNA polymerase reads the existing DNA strands to create two new strands that match the existing ones. Every time a cell divides, DNA polymerase is required to help duplicate the cells DNA, so that a copy of the original DNA molecule can be passed to each of the daughter cells. In this way, genetic information is transmitted from generation to generation.

DNA polymerase is the primary enzyme responsible for the replication, synthesis, and elongation of DNA strands.

Helicases move along the double-stranded DNA and separate the strands by breaking hydrogen bonds at the expense of energy from ATP. DNA unwinding by helicases creates topological stress/supercoils in the helical DNA structure which is relieved by the action of topoisomerases. These enzymes transiently break both DNA strands of one chromosome thereby making the other chromosome to pass through the break and relieve the stress. DNA polymerase enzyme synthesizes DNA strands by adding deoxyribonucleotides to 3' end of the primer via phosphodiester bonds; it uses parental DNA as template. Hydrogen bonding between base pairs is facilitated by the presence of hydrogen bond donor (primary and secondary amine groups or hydroxyl groups) and complementary hydrogen bond acceptor (carbonyls and tertiary amines) in bases; it is not enzymatic process. The correct option is C.

DNA polymerase I serve clean-up functions during replication, recombination, and repair. It has 3' --> 5' proofreading exonuclease activity and 5' --> 3' exonuclease activity. The 5' --> 3' exonuclease activity of DNA polymerase I can replace an error containing the segment of DNA/RNA, paired to the template strand which is then simultaneously replaced by polymerase activity of the same enzyme. The process is called as nick translation. DNA polymerase III with its ten types of subunits has 5' --> 3' polymerization and 3' --> 5' proofreading activities in its $\alpha$ and $\epsilon$ subunits, respectively. 

During DNA replication, two hexamers of helicases are loaded onto each DNA strand. Helicases move along the double stranded DNA and separate the strands by breaking hydrogen bonds between base pairs. ATP hydrolysis provides the required energy for breaking of hydrogen bonds; options B and C are correct. Thereby, DNA is unwound bidirectionally and two potential replication forks are created. The point at which two DNA strands are separated to facilitate the replication is called replication fork; option A is correct. Correct answer is D.

Two classes of enzymes are most important in generating recombinant DNA molecule. First, restriction endonucleases (also called restriction enzymes) recognize and cleave DNA at specific DNA sequences (recognition sequences or restriction sites) to generate a set of smaller fragments. Second, the DNA fragment to be cloned can be joined to a suitable cloning vector by using DNA ligases to link the DNA molecules together. DNA ligase catalyzes the formation of new phosphodiester bonds in a reaction that uses ATP or a similar cofactor.

DNA polymerase is an enzyme used in the replication of DNA. It ads the DNA primers to the parent chain. This enzyme acts in the 5'-3 direction and thus forms lading stand on the chain that has 3'5' direction.

RNA polymerase serves as primase during DNA replication and synthesizes RNA/DNA primer which is a stranded segment (complementary to the template) and has a free 3’ hydroxyl group to facilitate the addition of deoxyribonucleotides by DNA polymerase enzyme. DNA polymerase is polymerizing enzyme that has 5’ to 3’ polymerization and 3’ to 5’ exonuclease proofreading activities; it adds deoxyribonucleotides to free 3’ hydroxyl group of primer to synthesize new DNA strands. DNA ligase enzyme joins two Okazaki fragments together via phosphodiester bond between 3' hydroxyl at the end of one fragment and a 5' phosphate at the end of another fragment. Thus, option D is the correct answer. 

In molecular biology, DNA ligase is a specific type of enzyme, a ligase, that facilitates the joining of DNA strands together by catalyzing the formation of a phosphodiester bond.

DNA polymerase is an enzyme that synthesizes DNA molecules from deoxynucleotides, the building blocks of DNA. It helps in DNA replication (DNA dependent DNA synthesis). This enzyme works in pairs to create two complementary DNA strands from a single original DNA molecule.

The Meselson-Stahl experiment confirmed that DNA replication is semiconservative, meaning each new DNA molecule consists of one original strand and one newly synthesized strand.

DNA polymerase III is the primary enzyme for DNA replication and is structurally constrained to add nucleotides only in the 5' to 3' direction.

Escherichia coli is the classic model organism used by Meselson and Stahl to demonstrate the semiconservative nature of DNA replication.

DNA polymerase I has 5' to 3' exonuclease activity, which allows it to remove the RNA primers used to initiate DNA synthesis and replace them with DNA nucleotides.

DNA polymerase I is primarily responsible for DNA repair and the removal of RNA primers, whereas DNA polymerase III is the main replicative enzyme.

DNA replication is semiconservative, and because the two strands are antiparallel, one strand (leading) is synthesized continuously while the other (lagging) is synthesized discontinuously in Okazaki fragments.

DNA polymerase cannot initiate a new DNA strand from scratch; it requires a free 3' OH group provided by an RNA primer synthesized by primase.

Meselson and Stahl used heavy nitrogen (15N) and light nitrogen (14N) isotopes to distinguish between old and new DNA strands in their experiment.

DNA replication is a complex process requiring multiple enzymes, including DNA polymerase for synthesis, DNA ligase for joining fragments, and RNA polymerase (primase) for initiation.

In circular bacterial DNA, replication begins at a single origin and proceeds in both directions simultaneously around the circle, known as bidirectional replication.

DNA polymerase is an enzyme that synthesis DNA molecules from deoxyribonucleotides which are the building blocks of DNA. Each nucleotide contains a sugar molecule (deoxyribose sugar), a phosphate molecule, and any one of the nucleotides (adenine, guanine, cytosine and thymine). They basically help in DNA replication by working in pairs to form a new DNA molecule. 

DNA ligase is an enzyme that facilitates the joining of the strands of the DNA by assisting the formation of phosphodiester bonds between two nucleotides.

• RNA polymerase helps in DNA dependent RNA synthesis. This process is known as transcription. DNA strands are used as a template to synthesize RNA molecules.
  
• DNA ligase is used to join DNA segments during DNA repair or during replication (to join Okazaki fragments of the lagging strands).
  
• DNA polymerase works in the pair to synthesis new strand for DNA molecule from deoxyribonucleotides. In other words, it helps in DNA dependent DNA synthesis.
  
• Topoisomerase enzyme participates in overwinding or unwinding of DNA. During DNA replication or transcription, DNA becomes overwound ahead of the replication fork. It binds to DNA and cut the phosphate backbone of either one or both the DNA strands.
  

Thermophilic bacteria that live in hot springs and hydrothermal vents. Taq polymerase and Pfu polymerase are enzymes isolated from thermophilic bacteria. These enzymes are used in polymerase chain reaction as these enzymes are able to withstand the protein-denaturing conditions (high temperature) required during polymerase chain reaction ( PCR ). 

DNA polymerase is the enzyme required for the addition of nucleotides into a DNA strand being synthesized from an existing strand of DNA. Helicase refers to another enzyme that functions in unwinding of the double helix structure of DNA wound around one another to result in a melted region of DNA strand to begin the replication of DNA. The joining of the short segments of DNA created during replication of the lagging strand (3' to 5') happens due to the enzyme ligase that acts as a molecular glue.

In molecular biology, DNA ligase is a specific type of enzyme that facilitates the joining of DNA strands together by catalyzing the formation of a phosphodiester bond. It plays a role in repairing single-strand breaks in duplex DNA in living organisms. Single-strand breaks are repaired by DNA ligase using the complementary strand of the double helix as a template, with DNA ligase creating the final phosphodiester bond to fully repair the DNA.

DNA polymerase enzyme helps in DNA dependent DNA synthesis by addition of deoxyribonucleotides, building blocks of DNA. This enzyme uses DNA as the template and synthesizes DNA. This process is also known as replication.

The human genome is made of 3 billion DNA base pairs and out of them 2% only are responsible for coding for proteins and the rest may have regulatory functions. In an inducible operon such as the Lac Operon the inducer binds to the repressor to inactivate it such that the repressor can no longer bind to the operator and the gene expression can take place. DNA polymerase I (or Pol I) is an enzyme that participates in the process of prokaryotic DNA replication. This is responsible for proof reading and excising or removing off the RNA primers required for initiation of replication. DNA polymerase, beta, also known as POLB, is an enzyme present in eukaryotes involved in base excision and repair, also called gap-filling DNA synthesis.

In eukaryotes, the replication of DNA takes place at S-phase of the cell cycle. The replication of DNA and cell division cycle should be coordinated. A failure in cell division after DNA replication results into chromosomal anomaly.

Process of DNA synthesis whereby a parent DNA molecule is faithfully copied, giving rise to two identical daughter molecules is called DNA replication. In DNA synthesis, DNA polymerase plays an important role having the capability to elongate an existing DNA strand but cannot initiate the synthesis. So, the synthesis is initiated with the help of RNA primer formed by RNA primase RNA primase synthesises the short RNA primer of about 10 nucleotides that is elongated by DNA polymerase to form an Okazaki fragment of DNA during DNA replication. Helicase unzips the two strands of DNA and topoisomerase reduces the coiling tension developed due to the unwinding of the two strands.