Collegedunia Team Content Curator
Content Curator
Double helix structure of DNA looks like a twisted stairway. A DNA molecule consists of two strands that are wound around one another like a twisted ladder, and each strand has a backbone with alternate groups of phosphate and sugar groups. These two strands are held together by the nitrogenous bases, which are positioned in the middle.
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Key Terms: DNA, Replication, Polymerase, Division, Phosphate, Adenine, DNA Strand, Initiation, Termination, Elongation, DNA Double Helix Model
Structure of DNA
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The structure of DNA can be shown as:
- Each strand of a DNA molecule comprises a backbone with alternate groups of phosphate and sugar groups. DNA molecules are composed of two strands coiled around one another like a twisted ladder. The nitrogenous bases, which are situated in the centre, connect these two strands.
- Each sugar has one of the four bases— (A) adenine, (G) guanine, (C) cytosine, or (T) thymine—attached to it. Adenine bases pair with thymine while cytosine bases pair with guanine to generate bonds that bind both strands together.
DNA Structure
- Since the DNA strands are complementary to one another, this pairing is known as the base complementary rule.
- Therefore, if a strand has the sequence TTGGCCAA, the complementary strand should have the sequence AACCGGTT.
- DNA strands are antiparallel, complementary to one another, and moving in opposite directions as a result of this base pairing. The 3′ carbon end of the complementing strand confronts the 5′ carbon end of the other strand. This antiparallel orientation is vital for DNA replication and a number of nucleic acid interactions.
- Each DNA strand in the double helix structure of DNA is long and linear. It is composed of chains of nucleotides, which are smaller units. The phosphate and sugar molecules combine to produce the chemical linkages known as the sugar-phosphate backbones.
DNA Replication
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In the Double Helix Structure of DNA, DNA unwinding is carried out by an enzyme called a DNA helicase, and the creation of new DNA strands is completed by an enzyme known as a polymerase during DNA replication in both prokaryotes and eukaryotes.
DNA Replication
Both animals practise semi-conservative replication, in which individual DNA strands are created in various directions. And they both start fresh DNA strands with a small RNA primer. However, prokaryotic and eukaryotic DNA replication differ from one another.
Steps of DNA Replication
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There are three steps of DNA Replication, which are:
Initiation
- DNA sequences identify the initial location, where DNA replication begins, as the point of origin.
- Helicase starts unravelling and unwinding the DNA right at the origin. The strands are now divided, and single-stranded binding proteins (SSB) allow for the preservation of this separation. A DNA Replication fork develops as a result.
- The DNA is prevented from supercoiling by topoisomerase (that is, over-winding of the DNA at the same time as we need DNA to be separated for Replication to arise).
- Primase is now accessible and performs its function in creating RNA primers on both strands. The ability of DNA polymerase to determine where to begin its movement is very useful.
Elongation
- DNA Polymerase III binds to the primer and creates the new strand in the 5' to 3' course, i.e., it adds new bases to the three' region on the new strand, because DNA strands are antiparallel to one another, i.e., one can be 3' to 5' and the other 5' to 3'.
- The strand that it constantly develops is known as the main strand, while the other strand is known as the lagging strand since it has to wait for the original strand to unravel in order to start replication before adding bases inside the 3' end.
Termination
At this point in the replication process, one original DNA molecule has been split into two identical double-helix DNA molecules. Because each of the two copies has one original and one newly created strand, the entire process is semi-conservative.
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Enzymes in DNA Replication
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DNA replication enzymes have the power to accelerate processes and either strengthen or weaken the substances they interact with. The enzymes involved in DNA replication are mentioned below:
- DNA Polymerase III: By replicating DNA molecules, this enzyme creates a fresh DNA strand. Additionally, it has the ability to proofread, matching the proper DNA nucleotides to code the right gene and produce the right protein.
- Helicase: As it unwinds the two strands of DNA, the helicase enzyme is also referred to as the "unzipping" or "unwinding" enzyme. It dismantles the hydrogen bonds binding the DNA bases together as it opens the zipper.
- Primase: Without it, DNA polymerase finds it challenging to determine where to begin. This component is known as the initializer. To make it simpler for DNA polymerase to choose where to begin the work, primase creates the primer. And RNA is what makes up this primer.
- RNase H (DNA Polymerase I): Vital for lagging strands because it removes primer when DNA polymerase III approaches it.
- DNA Ligase: It is referred to as the gluer. because it aids in the fusion of DNA strands.
Previous Year Questions
- The process of translation is … [NEET 1993]
- The tumor-inducing capacity of Agrobacterium … [KCET 2007]
- Experiments involving use of radioactive thymidine … [KCET 2021]
- DNA polymerase enzyme is required …
- What is plasmid? … [UPSEE 2017]
Things to Remember
- Each DNA strand in the double helix is long and linear. It is made up of chains of nucleotides, which are smaller units.
- The phosphate and sugar molecules combine to produce the chemical linkages known as the sugar-phosphate backbones.
- During DNA replication, each DNA strand is copied, resulting in the production of a daughter DNA double helix.
- There are mainly three steps of DNA Replication, namely Initiation, Elongation and Termination.
Sample Questions
Ques. What are the DNA Watson and Crick model? (2 marks)
Ans. The "double helix" that Watson and Crick called the molecular structure of DNA was their idea. The Watson and Crick model of DNA is distinguished by the precise pairing of nucleotides. Each DNA strand in this instance is long and linear. It is made up of chains of nucleotides, which are smaller units.
Ques. What is the Chargaff Rule? (2 marks)
Ans. According to Chargaff’s rule:
- The amount of A is equal to T, whereas the amount of C is equal to G.
- The DNA of any cell from any organism should have a 1:1 ratio of purine (A and G) and pyrimidine (C and T) bases.
Ques. What do you mean by DNA Replication? (3 marks)
Ans. A new complementary daughter strand is created by using each parental strand as a template during the semiconservative process of DNA replication. Deoxyribonucleoside five′-triphosphates (dNTPs), which are joined to form the growing DNA chain, are joined by the enzyme DNA polymerase.But a single enzymatic process cannot compare to the complexity of DNA replication. Proteins that are involved and mechanisms for error checking are necessary to make sure that the accuracy of replication is compatible with the low frequency of errors required for cellular replication. It is also required for specific DNA sequences and other proteins to both initiate replication and replicate the ends of eukaryotic chromosomes.
Ques. Write about DNA Replication in Prokaryotes. (3 marks)
Ans. Prokaryotic cells may have one fundamental component, and replication takes place in the cytoplasm of the living thing. Prokaryotic cells have one or two types of polymerases at this location, and replication occurs simultaneously in two opposite orientations. Prokaryotes replicate significantly more quickly than eukaryotes do.
A few microbes can finish it off in 40 minutes, and because they have circular chromosomes, they don't need to create telomeres like eukaryotes do. When replication takes place in the cytoplasm of prokaryotic cells, there is only one point of origin. Prokaryotic cells have one or two different types of polymerases, and replication is happening here simultaneously in two opposite directions. Prokaryotes replicate significantly more quickly than eukaryotes do. Some bacteria can finish it off in 40 minutes, and because they have circular chromosomes, they don't need to create telomeres as eukaryotes do.
Ques. Write about DNA Replication in Eukaryotes. (3 marks)
Ans. A typical eukaryotic cell has DNA that is 25 times bigger than a prokaryotic cell. Eukaryotic cells use unidirectional replication inside the cellular nucleus and derive from a variety of origins. These have four or more polymerase enzymes to help with DNA replication all the way through. Eukaryotes have a unique process for reproducing the telomeres found at the ends of their chromosomes, which can take up to 400 hours. During this stage of the S-segment of the cell cycle, the cell replicates its DNA.
The average eukaryotic cell contains DNA that is 25 times larger than that of a prokaryotic cell. Eukaryotic cells use unidirectional replication inside the cell nucleus and have numerous origins. These aid in DNA replication with four or more polymerase enzymes. Eukaryotes have a unique technique for duplicating the telomeres found at the ends of their chromosomes, which can take up to 400 hours. During this stage of the cell cycle, the S-phase, the cell replicates its DNA.
Ques. When Does DNA Replication Occur? (2 marks)
Ans. When a cell needs DNA before dividing so that the new daughter cells can also receive a copy of the DNA, DNA replication takes place. In a eukaryotic cell, it specifically takes place in interphase, which comes before mitosis or meiosis.
Ques. Write the importance of DNA Replication. (5 marks)
Ans. The ability to inhibit DNA replication in dangerous cells like DNA has led to the development of numerous life-saving medicinal treatments. A double-stranded DNA molecule is copied during DNA replication (using a variety of different proteins) to create an identical double-stranded DNA molecule (so whereas the cell began with one copy, it ended up with equal copies).
DNA replication is essential because, without it, a mobile department would be unable to develop. that is, either one cell may receive all the DNA while the other received none (there could have been no genuine mobile department), or each cell could receive the best half of the DNA it wishes (in which case, the daughter cells could not continue to exist), or any combination of the three (also main to dying of both daughter cells). The DNA of a cell can be copied through DNA replication so that each cell that is produced after the division has its own complete set of DNA. In theory, this allows the mobile department to last indefinitely.
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