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DNA replication, also known as semi-moderate replication, is the interaction by which DNA is multiplied. It is a significant interaction that happens inside the separating cell.
| Table of Content |
Keyterms: DNA replication, DNA, Cell, Replication, Molecule, Protein, RNA
Read Also: DNA Fingerprinting
DNA Replication
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In molecular biology, DNA replication is the natural cycle of creating two indistinguishable imitations of DNA from one unique DNA molecule. DNA replication happens to take all things together living creatures going about as the most fundamental part for organic legacy. This is fundamental for cell division during the development and fix of harmed tissues, while it likewise guarantees that every one of the new cells gets its duplicate of the DNA. The phone has the unmistakable property of division, which makes replication of DNA fundamental.
In conservative replication, the parental DNA stays together, and the recently shaped daughter strands are together. The semi-conservative strategy recommends that every one of the two parental DNA strands goes about as a layout for new DNA to be integrated; after replication, each two-fold abandoned DNA incorporates one parental or "old" strand and one "new" strand.
Read more: Central dogma
Steps of DNA Replication
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Step 1: The initial phase in DNA replication is to 'unfasten' the two-fold helix construction of the DNA.
Step 2: This is completed by a protein called helicase that breaks the hydrogen bonds holding the correlative bases of DNA together (A with T, C with G).
Step 3: The partition of the two single strands of DNA makes a 'Y' shape called a replication 'fork'. The two isolated strands will go about as formats for making the new strands of DNA.
Step 4: One of the strands is situated in the 3' to 5' bearing (towards the replication fork), this is the main strand. The other strand is arranged in the 5' to 3' course (away from the replication fork), this is the slacking strand. Because of their various directions, the two strands are imitated unexpectedly.
Further Steps:
| Step | Leading Strand | Lagging Strand |
|---|---|---|
| Step 5 | A short piece of RNA called a preliminary (delivered by a protein called primase) tags along and ties to the furthest limit of the main strand. The groundwork goes about as the beginning stage for DNA blend. | Various RNA preliminaries are made by the primase compound and tie at different focuses along the slacking strand. |
| Step 6 | DNA polymerase ties to the main strand and afterward 'strolls' along with it, adding new reciprocal? Nucleotide bases (A, C, G, and T) to the strand of DNA in the 5' to 3' course. | Pieces of DNA, called Okazaki parts, are then added to the slacking strand additionally in the 5' to 3' bearing. |
| Step 7 | Such replication is called consistent. | This kind of replication is called irregular as the Okazaki pieces should be signed up later. |
Step 8: When the entirety of the bases are coordinated up (A with T, C with G), a catalyst summoned exonuclease strips the primer(s). The holes where the primer(s) are then filled by yet more reciprocal nucleotides.
Step 9: The new strand is edited to ensure there are no errors in the new DNA succession.
Step 10: At last, a catalyst called DNA ligase seals up the grouping of DNA into two constant two-fold strands.
Step 11: The consequence of DNA replication is two DNA atoms comprising one new and one old chain of nucleotides. This is the reason DNA replication is portrayed as semi-moderate, half of the chain is important for the first DNA particle, half is fresh out of the plastic new.
Step 12: Following replication, the new DNA naturally ends up into a two-fold helix.
Note:
- DNA replication is semiconservative in the process. Each strand of DNA in the double helix acts as a single template for the synthesis of a new and complementary strand.
- New DNA is made by participation of the enzymes called DNA polymerases, which require a template, a primer, and a synthesized DNA in the 5' to 3' direction.
- During the process of DNA replication, one new strand (that is leading strand) is made as a continuous piece. The other strand (that is the lagging strand) is made in small pieces.
- DNA replication also requires some other enzymes in addition to DNA polymerase, DNA primase, DNA helicase, DNA ligase, and topoisomerase.
Read More: DNA Packaging
Stages of DNA Replication
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Initiation
DNA synthesis is initiated at the specific points within the DNA strand and is known as ‘origins’, which are present within the replication complex. The enzyme DNA Helicase, which unwinds the double-helical structure and exposes each of the two strands, so that they can be used as a template for the process of replication. It takes place by hydrolyzing. The ATP is used to form the bonds between the nitrogen bases, therefore breaking the bonds which are holding the two strands together.
DNA Primase is another enzyme that plays an important role in DNA replication. It synthesizes a small RNA primer, which acts as a kick-starter for the enzyme DNA polymerase. DNA polymerase is the enzyme that is completely responsible for the creation and expansion of the new strands of the DNA-specific coding regions.
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Elongation
Once the DNA polymerase gets attached to the original and unzipped two strands of DNA (i.e. the template strands), it can start the synthesis of the new DNA to match the templates. It is important to note that DNA polymerase is only able to extend the primer by adding some free nucleotides to the 3’ end, not to the 5’ end. RNA primers are added to the newly exposed bases on the lagging strand during replication and DNA synthesis takes place in fragments, but still in the 5′ to 3′ direction as before. These fragments are called Okazaki fragments.
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Termination
The process of expanding the new DNA strands continues until there is either no more DNA template left for the replication (i.e. at the end of the chromosome), or two replication forks meet and subsequently terminate. The meeting of two replication forks is not regulated and occurred randomly along the course of the chromosome.
Once the DNA synthesis has finished, the newly synthesized strands must be bound and get stabilized. With regards to the lagging strand, two enzymes are needed to achieve this stabilized stage. The RNAase enzyme removes the RNA primer that is at the beginning of each Okazaki fragment, and the DNA Ligase enzyme joins the fragments together to create a complete strand.
Read More: Genome
Enzymes Involved In DNA Replication
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DNA replication is a highly enzyme-dependent process. There are many enzymes involved in the DNA replication which includes the enzymes DNA-dependent DNA polymerase, helicase, ligase, etc. Among them, DNA-dependent DNA polymerase is the main enzyme.
Read More: Structure of DNA
DNA dependent DNA polymerase
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It helps in the polymerization and catalyzes and regularises the whole process of DNA replication with the support of other enzymes. Deoxyribonucleoside triphosphates are the substrate as well as the energy provider for the replication process. DNA polymerase is of three types:
DNA Polymerase I
It is a DNA repair enzyme. It is involved in three activities:
- 5′-3′ polymerase activity
- 5′-3′ exonuclease activity
- 3′-5′ exonuclease activity
DNA Polymerase II
It is responsible for primer extension and proofreading.
DNA Polymerase III
It is responsible for in vivo DNA replication.
Helicase
Helicase is the enzyme which unzips the DNA strands by breaking the hydrogen bonds between them. Thus, it helps in the formation of the replication fork.
Ligase
Ligase is the enzyme which glues the discontinuous DNA strands.
Primase
This enzyme helps in the synthesis of RNA primer complementary to the DNA template strand.
Endonucleases
These produce a single-stranded or a double-stranded cut in a DNA molecule.
Single-stranded Binding Proteins
It binds to single-stranded DNA and protects it from forming secondary structures.
Also Read:
| Related Articles | ||
|---|---|---|
| Human Genome Project | Lac Operon | Molecular Basis of Inheritance |
| Structure of RNA | Transcription | Translation |
DNA Replication in Prokaryotes
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The DNA replication in prokaryotes takes place in the following place:
- The two strands of DNA unwind at the origin of replication.
- Helicase opens the DNA and replication forks are formed.
- The DNA is coated by the single-strand binding proteins around the replication fork to prevent rewinding of DNA.
- Topoisomerase prevents the supercoiling of DNA.
- RNA primers are synthesised by primase. These primers are complementary to the DNA strand.
- DNA polymerase III starts adding nucleotides at the end of the primers.
- The leading and lagging strands continue to elongate.
- The primers are removed and the gaps are filled with DNA Polymerase I and sealed by ligase.
DNA Replication in Eukaryotes
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The DNA replication in eukaryotes is similar to the DNA replication in prokaryotes. However, the initiation process is more complex in eukaryotes than prokaryotes. In eukaryotes, there are multiple origins of replication present. A pre-replication complex is made with other initiator proteins. The process is entirely the same but the enzymes used are different. E.g. in eukaryotes, the polymerization process is carried out by the enzyme Pol δ, whereas in prokaryotes it is done by DNA Pol III.
Read Also: Important Difference Between DNA and RNA
Sample Questions
Ques. Which property of DNA double helix led Watson and Crick to hypothesise semi-conservative mode of DNA replication? Explain (2 marks)
Ans. The antiparallel, double-stranded nature of the DNA molecule led Watson and Crick to hypothesise semi-conservative mode of DNA replication. They suggested that the two strands of DNA molecule uncoil and separate, and each strand serves as a template for the synthesis of a new (complementary) strand alongside it. The template and its complement, then form a new DNA double strand, identical to the original DNA molecule. The sequence of bases which should be present in the new strands can be easily predicted because these would be complementary to the bases present in the old strands. A will pair with T, T with A, C with G, and G with C. Thus, two daughter DNA molecules identical to the parent molecule are formed and each daughter DNA molecule consists of one old (parent) strand and one new strand. Since only one parent strand is conserved in each daughter molecule, this mode of replication is said to be semiconservative. Meselson and Stahl and Joseph Taylor, later proved it by experiments.
Ques. What is DNA fingerprinting? Mention its application.(2 marks)
Ans. DNA fingerprinting or DNA typing is a technique of determining nucleotide sequences of certain areas (VNTRs) of DNA which are unique to each individual. Each person has a unique DNA fingerprint. Unlike a conventional fingerprint that occurs only on the fingertips and can be altered by surgery, a DNA fingerprint is the same for every cell, tissue and organ of a person. It cannot be changed by any known treatment. Applications of DNA fingerprinting are as follows:
- Paternity disputes can be solved by DNA fingerprinting.
- DNA fingerprinting technique is being used to identify genes connected with hereditary diseases.
- It is useful in detection of crime and legal pursuits.
- It can identify racial groups, their origin, historical migrations and invasions.
Ques. Give two reasons why both the strands of DNA are not copied during DNA transcription?(1 mark)
Ans. If both the strands code for RNA two different RNA molecules & two different proteins would be formed hence genetic machinery would become complicated.
Since the two RNA molecules would be complementary to each other, they would wind together to form dsRNA without carrying out translation which means process of transcription would be futile.
Ques. What is DNA Replication?(2 marks)
Ans. DNA Replication is a process of producing two identical copies of DNA from a single DNA molecule. It is a process of biological inheritance. DNA is a double helix in which two strands are complementary to each other. These two strands of a helix separate at the time of replication to form two new DNA molecules. Out of the two strands of DNA formed, one is identical to one of the strand and another strand is complementary to the parent strand. This form of replication is known as semi-conservative replication. Before the cell enters the mitosis, the DNA is replicated in S phase of interphase.
DNA polymerase in the most important enzyme involved in DNA replication.
Ques. What is Transcription?(2 marks)
Ans. It is a process of formation of RNA such as messenger RNA from DNA before gene expression or protein synthesis occurs. During transcription, one of the strands of DNA acts as a template for mRNA formation. The synthesis of mRNA occurs via RNA polymerase enzyme. Transcription usually occurs for a particular DNA segment which is required further for gene expression. Other than the messenger RNA, other forms of RNA such as ribosomal RNA, micro RNA, small nuclear RNA can also be transcribed in the similar manner.
Some viruses have a property of reverse transcription. They are able to convert RNA template into DNA. The enzyme used is known as reverse transcriptase.
For Example: Human immunodeficiency virus that causes “AIDS”.
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