Anjali Mishra Content Writer-SME
Content Writer-SME
Key Highlights
- DNA or Deoxyribonucleic acid exists in three forms: Primary, Secondary, and Tertiary form.
- The linear structure of DNA is considered as its primary structure.
- The nucleotides in the secondary structure of DNA are attached by a phosphodiester bond.
- DNA is the molecular basis of genetic mutation that is often found as a right-handed double helix form.
- Structural modifications in the primary form of DNA result in the formation of a secondary structure of DNA.
The secondary structure of DNA, also known as the Watson and Crick model of DNA is responsible for the shape of DNA and RNA.
- The fundamental components of DNA's secondary structure are the interactions that occur between nitrogenous bases, the phosphate backbone, and pentose sugar.
- The hydrogen bonds that hold DNA's two strands together are responsible for the double helical structure of DNA.
| Table of Content |
Key Terms: Secondary Structure of DNA, DNA, Deoxyribonucleic Acid, Nitrogenous Base
Primary structure of DNA
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The primary structure of DNA is the simplest structure of Deoxyribonucleic acid (DNA) arranged in a linear sequence.
- It is formed when the 3` carbon end of one nucleotide is attached to the 5` carbon end of other nucleotides through phosphodiester bonds.
- The sequences of DNA are represented by the repeating units of nitrogenous bases.
Secondary Structure of DNA
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It is a complex structure of DNA formed due to the winding of two complementary strands of DNA.
- This results in the formation of a double helical structure of DNA.
- In comparison to the primary structure, the secondary structure of DNA shows high stability.
- The stabilising bonds are mainly hydrogen bonds and Vanderwall force of interaction.
- The secondary structure of DNA is based on the double-helical structure of DNA which was proposed by Watson and Crick in the early 20th century.
- Deoxyribonucleic acid, also called DNA, is a hereditary material that is composed of nucleotides.
- The nitrogenous bases that form the backbone of nucleotides are purines and pyrimidines.
- Adenine and guanine are called purine bases whereas cytosine and thymine are called pyrimidine bases.
- In the secondary structure of DNA, adenine is bonded to thymine with three hydrogen bonds.
- On the other hand, guanine is attached to cytosine with two hydrogen bonds.
- Minor and major grooves are also part of double helix DNA.
Tertiary structure of DNA
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The tertiary structure of DNA is a more complex form in which strands of DNA are arranged in a three-dimensional structure.
- Usually, the DNA exists in its tertiary form.
- It is useful in molecular recognition of binding sites of proteins and catalysts.
DNA Packaging
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Once the structural modifications in DNA are done, the next step is DNA packaging. It involves the wrapping of DNA around the proteins called histones.
- It is divided into three levels: Nucleosome level, Solenoid level, and Scaffold level of packaging.
- The primary purpose of DNA packaging is to separate the chromosomes during the time of cell division.
- It is also essential for the synthesis of ribonucleic acid which is further involved in protein synthesis.
Sample Questions
Ques. What is transcription and translation? (5 marks)
Ans. The synthesis of proteins is a two-step process that involves transcription and translation.
Transcription
- The process of conversion of (Deoxyribonucleic acid) DNA into (Ribonucleic acid) RNA by the action of enzymes is called transcription.
- The process involves modifications in the secondary structure of DNA.
Translation
- In this, the RNA synthesized in the process of transcription further undergoes structural modification and results in the synthesis of protein.
- The protein is further utilized in biochemical reactions.
Ques. How does DNA play a role in genetic mutation? (2 marks)
Ans. Genetic mutation is a biological process that results from sudden changes in DNA sequences.
- This causes genetic diversity.
- In some cases, genetic mutations resulting from errors in DNA sequence are linked with genetic disorders.
Ques. What do you understand about Watson and Crick's Model of DNA? (2 marks)
Ans. Watson and Crick, the two scientists, proposed the double helical structure of DNA in 1953, which was later called the Watson and Crick model of DNA.
- According to them, DNA is composed of two complementary strands in which nitrogenous bases are connected by hydrogen bonds.
- The two strands are arranged in helical form.
Ques. What are histone proteins? (2 marks)
- DNA wrapped with eight histone proteins forms a structure called a nucleosome.
- It is composed of various charged amino acids such as lysine, arginine, etc.
Ques. What are the major differences between the structure of DNA and RNA? (5 marks)
Ans. The major differences between the structure of DNA and RNA are as follows:
| DNA | RNA |
|---|---|
| It is called Deoxyribonucleic acid. | It is called Ribonucleic acid. |
| Presence of deoxyribose sugar. | Presence of ribose sugar. |
| Cytosine and thymine are the pyrimidine bases in DNA. | Cytosine and uracil are the pyrimidine bases in the case of RNA. |
| It is present in both double and single-stranded forms. | It is only present in single-stranded form. |
| It is responsible for carrying genes from parents to offspring. | It is responsible for coding amino acids and further synthesis of proteins. |
Ques. What are the bases that form the complete structure of DNA? (2 marks)
Ans. DNA is composed of nitrogenous bases that are divided into two categories: Purines and Pyrimidines.
- Adenine and guanine are considered purine bases having two ring structures.
- The two pyrimidine bases having a single-ring structure are cytosine and thymine.
Ques. How do nucleotides differ from nucleosides? (2 marks)
Ans. The nucleotides and nucleosides differ from each other in terms of their composition.
- A nucleotide is made up of nitrogenous bases, pentose sugar, and a phosphate group.
- A nucleoside is composed of only nitrogenous bases and a pentose sugar.
Ques. What do you understand about DNA supercooling? (3 marks)
Ans. The twisting of two complementary strands of DNA around a helical axis is called DNA supercoiling.
- An important enzyme required for DNA supercoiling is DNA topoisomerase I.
- It is essential for the maintenance of chromosome structure.
- Positive and negative supercoiling are the two types of DNA supercoiling.
Ques. What are the characteristics of DNA? (2 marks)
Ans. DNA or Deoxyribonucleic acid is the hereditary material of a living organism that is passed from parents to their children.
- It is composed of nucleotides.
- It is more stable than RNA (ribonucleic acid) due to the presence of deoxyribose sugar.
Ques. What are the primary functions of the secondary structure of DNA? (2 marks)
Ans. The important functions of DNA are:
- The shape of nucleic acid is contributed by the secondary structure of DNA.
- Small local motifs in the DNA indicate the unique binding sites for proteins.
Ques. What are the stabilising bonds present in the secondary structure of DNA? (2 marks)
Ans. All the nitrogenous bases are strongly bonded to each other by hydrogen bonds.
- Two hydrogen bonds are present between adenine and thymine whereas three hydrogen bonds are present between cytosine and guanine.
- Apart from hydrogen bonding, the secondary structure of DNA is stabilised by Vanderwall force of interaction.
Ques. What is the binding force between the two DNA strands? (1 mark)
Ans. Hydrogen bonds hold the DNA molecule's double-stranded structure intact. The nitrogenous bases in the two DNA strands form a hydrogen bond with each other.
Ques. What constitutes a protein's secondary structures? (1 mark)
Ans. The α helix and the β pleated sheet are the two most prevalent kinds of secondary structures. The stabilising bonds of both structures are hydrogen bonds.
Ques. What is the secondary structure? (2 marks)
Ans. The consistent folding of regions into particular structural forms within a single polypeptide chain is known as a secondary structure. It represents the next level up from the primary structure.
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