REPLICATION OF DNA

-

 

REPLICATION OF DNA

  • This is a complex process that takes place during cell division, (interphase, and S phase) whereby DNA makes copies (duplicates) before the cell divides through mitosis and meiosis.
  • DNA replication is a semi conservative process where a parental strand (template) is used to synthesize a new complementary daughter strand using several protein elements which include enzymes and RNA molecules.
    • DNA replication process uses DNA polymerase as the main enzyme for catalyzing the joining of deoxyribonucleoside 5′-triphosphates (dNTPs) forming a growing chain of DNA.
    • Other proteins are also involved for initiation of the process and copying of DNA, along with proofreading capabilities to ensure the replication process takes place accurately.
    • Therefore DNA replication is a process that produces identical helices of DNA from a single strand of the DNA molecule.
    • DNA replication is an essential mechanism in enhancing cell growth, repair, and reproduction of an organism.

    MECHANISM OF REPLICATION

    DNA replication takes place in three major steps

    1.      Unwinding of double strand helical structure of DNA

    2.      Preparing the template strands

    3.      Assembly of the newly synthesized DNA segment

    ·         During the separation of DNA, the two strands uncoil at a specific site known as the origin. With the involvement of several enzymes and proteins, they prepare the strands for duplication.

    ·         At the end of the process, DNA polymerase enzyme starts to organize the assembly of the new DNA strands.

    ·         These are the general steps of DNA replication for all cells but they may vary specifically, depending on the organism and cell type.

    ·         Enzymes play a major role in DNA replication because they catalyze several important stages of the entire process.

    ·         DNA replication is one of the most essential mechanisms of a cell’s function and therefore intensive research has been done to understand its processes.

    ·         The mechanism of DNA replication is well understood in Escherichia coli, which is also similar to that in eukaryotic cells.

     

    STEPS INVOLVED IN DNA REPLICATION

    1.      Initiation

    2.      Elongation

    3.      Termination

    INITIATION

    ·         DNA synthesis is initiated at particular points within the DNA strand known as origin of replication.

    ·         These origins are targeted by initiator proteins, which go on to recruit more proteins that help aid the replication process, forming a replication complex around the DNA origin.

    ·         The first step in DNA replication is to unzip the double helix structure of the DNA molecule.

    ·         This is carried out by an enzyme called helicase which breaks the hydrogen bonds holding the complementary bases of DNA together (A with T, C with G).

    ·         The mechanism of DNA helicase enzyme is by hydrolyzing the ATP that is used to form the bonds between the nucleobases, thus breaking the bond that holds the two strands.

    ·         The separation of the two single strands of DNA creates a Y shape called a replication fork. The two separated strands will act as templates for making the new strands of DNA.

    ·         DNA polymerase enzyme functions by growing the new DNA daughter strand

     





Replication fork requires five basic compounds:

  •  Helicase to unwind the DNA,
    • Single-strand-binding proteins to protect the single nucleotide strands and prevent secondary structures,
  •      The topoisomerase gyrase to remove strain ahead of the replication fork,
  •      Primase to synthesize primers with a 3′-OH group at the beginning of each DNA fragment, and
  •      DNA polymerase to synthesize the leading and lagging nucleotide strands.

ELONGATION

·         Once the DNA polymerase has attached to the original, unzipped two strands of DNA it is able to start synthesizing the new DNA to match the templates.

·         As the DNA unwinds, the template strand that is exposed in the 3′→5′ direction allows the new strand to be synthesized continuously, in the 5′→3′ direction. This new strand, which undergoes continuous replication, is called the leading strand.

·         The other template strand is exposed in the 5′→3′ direction. After a short length of the DNA has been unwound, synthesis must proceed 5′→3′; that is, in the direction opposite that of unwinding. The newly made strand that undergoes discontinuous replication is called the lagging strand

·         The short lengths of DNA produced by discontinuous replication of the lagging strand are called Okazaki fragments, after Reiji Okazaki, who discovered them. In bacterial cells, each Okazaki fragment ranges in length from about 1000 to 2000 nucleotides; in eukaryotic cells, they are about 100 to 200 nucleotides long. Okazaki fragments on the lagging strand are linked together to create a continuous new DNA molecule.

·         The Okasaki fragments are then joined by the action of DNA ligase, which forms an intact new DNA strand known as the lagging strand.

·         The leading strand is the simplest to replicate. Once the DNA strands have been separated, a short piece of RNA called a primer binds to the 3' end of the strand. The primer always binds as the starting point for replication. Primers are generated by the enzyme DNA primase.

·         The RNA nucleotides from the short RNA primers must be removed and replaced by DNA nucleotides, which are then joined by the DNA ligase enzyme.


TERMINATION

·         After the synthesis and extension of both the continuous and discontinued stands, an enzyme knows as exonuclease removes all RNA primers from the original strands.

·         The primers are replaced with the right nucleotide bases.

·         While removing the primers, another type of exonuclease proofread the new stands, checking, removing, and replacing any errors formed during synthesis.

·         DNA ligase enzyme joins the Okazaki fragments to form a single integrated strand.

·         The ends of the parent strand consist of a repetition of DNA sequences known as telomeres which act as protective caps at the ends of chromosomes preventing the fusion of nearby chromosomes.

·         The telomeres are synthesized by a special type of DNA polymerase enzyme known as telomerase.

·         It catalyzes the telomere sequences at the end of the DNA.

·         On completion, the parent and complementary strand coil into a double helical shape, producing two DNA molecules each passing one strand from the parent molecule and one new strand.

ENZYMES INVOLVED IN DNA REPLICATION

DNA POLYMERASE

  • DNA polymerases are enzymes used for the synthesis of DNA by adding nucleotide one by one to the growing DNA chain. The enzyme incorporates complementary amino acids to the template strand.
  • DNA polymerase is found in both prokaryotic and eukaryotic cells. They both contain several different DNA polymerases responsible for different functions in DNA replication and DNA repair mechanisms.

DNA HELICASE ENZYME

  • This is the enzyme that is involved in unwinding the double-helical structure of DNA allowing DNA replication to commence.
  • It uses energy that is released during ATP hydrolysis, to break the hydrogen bond between the DNA bases and separate the strands.
  • This forms two replication forks on each separated strand opening up in opposite directions.
  • At each replication fork, the parental DNA strand must unwind exposing new sections of single-stranded templates.
  • The helicase enzyme accurately unwinds the strands while maintaining the topography on the DNA molecule.

DNA PRIMASE ENZYME

  • This is a type of RNA polymerase enzyme that is used to synthesize or generate RNA primers, which are short RNA molecules that act as templates for the initiation of DNA replication.

DNA LIGASE ENZYME

  • This is the enzyme that joins DNA fragments together by forming phosphodiester bonds between nucleotides.

EXONUCLEASE

  • These are a group of enzymes that remove nucleotide bases from the end of a DNA chain.

TOPOISOMERASE

  • This is the enzyme that solves the problem of the topological stress caused during unwinding.
  • They cut one or both strands of the DNA allowing the strand to move around each other to release tension before it rejoins the ends.
  • And therefore, the enzyme catalysts the reversible breakage it causes by joining the broken strands.
  • Topoisomerase is also known as DNA gyrase in E. coli

TELOMERASE

  • This is an enzyme found in eukaryotic cells that adds a specific sequence of DNA to the telomeres of chromosomes after they divide, stabilizing the chromosomes over time.



Comments

Post a Comment

Popular posts from this blog

Staphylokinase (Sak) and Streptokinase (SK)

-
Staphylokinase (Sak)  and  Streptokinase (SK)  are fascinating enzymes with a dual identity—used both in medicine and by bacteria as virulence factors. Medically, streptokinase (produced by  Streptococcus pyogenes ) has long been used as a clot-dissolving drug in conditions like heart attacks, deep vein thrombosis, and pulmonary embolism. It is cost-effective but has notable drawbacks, including a high risk of systemic bleeding due to its non-specific action and the potential to trigger immune reactions, especially upon repeated use. In contrast, staphylokinase (from  Staphylococcus aureus ) is still under clinical investigation but shows promise as a more clot-specific alternative, acting mainly at the site of clots and posing a lower bleeding risk. However, its clinical application is limited so far due to slower activation and ongoing trials. From a pathogenic standpoint, both enzymes help bac...
Read more

Activation of Yellow marrow into Red marrow

-
 When the body senses a shortage of blood cells , it sends signals to reactivate the yellow bone marrow. 1. Detection of Blood Cell Shortage In conditions like anemia , injury , or infection , there are not enough red blood cells (RBCs) or other blood cells. The body detects low oxygen levels or low cell counts in the blood.  2. Signal Release The kidneys release a hormone called erythropoietin (EPO) . Cytokines (immune signaling proteins) are also released. These signals reach the bone marrow through the blood.  3. Stem Cell Activation in Yellow Marrow The yellow marrow contains mesenchymal stem cells (fat-storing stem cells). Under normal conditions, these produce fat. But in response to signals (like EPO and cytokines), they start converting into hematopoietic cells (blood-producing cells). 4. Fat Cells Decrease Fat cells in yellow marrow shrink or die off . More space opens up for blood-forming cells to grow. 5. Re...
Read more

Complement system

-
  Complement system is significant part of innate immune system.it contains 30 proteins in blood plasma and on the surface of the cell which will activate as a chain reaction (one will activate the next) Following are the major functions of complement system. Opsonization- label the microbes so that the phagocytic cells will get activated Chemotaxis-Attract the immune cells to the infected site Cell lysis- Forms a membrane attack complex (MAC) that can directly kill bacteria Clearance of Immune complexes- Helps to remove  antibody antigen complex from the body Pathways 1. Classical pathway classical pathway will be triggered by the antibodies and it comes under the part of adaptive immunity. Example If a person got infected with the measles virus . The body will start produced IgG antibodies against the virus. These antibodies bind to the virus. The classical pathway is activated to destroy the virus . classical pathway needs antibodies to induce the immune response . 2. Lect...
Read more