Synthesis and Structure of Genetic Material | Agriculture Optional Notes for UPSC PDF Download

Introduction

Nucleic acids are organic compounds found in all living organisms, existing as DNA or RNA. They result from the amalgamation of nitrogenous bases, sugar molecules, and phosphate groups, interconnected by various types of bonds and arranged in specific sequences. The structure of DNA dictates the fundamental genetic composition of our bodies, and it indeed governs the genetic blueprint of almost all life forms on our planet.

What is DNA?

“DNA is a group of molecules that is responsible for carrying and transmitting the hereditary materials or the genetic instructions from parents to offsprings.”

• Viruses share this characteristic, with the majority of them possessing either RNA or DNA as their genetic material. Some viruses, for instance, have RNA as their genetic material, while others employ DNA. A notable example is the Human Immunodeficiency Virus (HIV), which initially contains RNA that gets converted into DNA upon binding to the host cell.
• Beyond its role in genetic inheritance for all life forms, DNA also assumes a critical function in protein synthesis. Nuclear DNA, situated within the cell nucleus of eukaryotic organisms, encodes most of the organism's genetic information. Mitochondrial DNA and plastid DNA, on the other hand, oversee the remainder.
• Mitochondrial DNA, found in the cell's mitochondria, is inherited from the mother to the child. In humans, it encompasses around 16,000 base pairs. Similarly, plastids possess their DNA and are integral to the process of photosynthesis.

Full-Form of DNA

DNA is known as Deoxyribonucleic Acid. It is an organic compound that has a unique molecular structure. It is found in all prokaryotic cells and eukaryotic cells. 

DNA Types

Three distinct types of DNA structures exist:

• A-DNA: A-DNA is a right-handed double helix akin to the B-DNA form. Dehydrated DNA adopts an A structure, which safeguards it under extreme conditions like desiccation. Protein interactions can also induce the DNA to assume an A structure by eliminating the solvent.
• B-DNA: The B-DNA structure is the most prevalent DNA conformation, representing a right-handed helix. In normal physiological conditions, the majority of DNA takes on a B-type conformation.
• Z-DNA: Z-DNA is characterized by a left-handed double helix, where the helix winds to the left in a zig-zag pattern. This DNA structure was discovered by Andres Wang and Alexander Rich. Z-DNA is typically located upstream of gene start sites and is believed to have a role in gene regulation.

Who Discovered DNA?

The initial discovery and identification of DNA were credited to the Swiss biologist Johannes Friedrich Miescher in 1869, as he conducted research on white blood cells.

Subsequently, the revelation of DNA's double helix structure was achieved through experiments, with James Watson and Francis Crick playing a pivotal role in this discovery. Eventually, it was firmly established that DNA serves as the repository for genetic information in living organisms.

DNA Diagram

The following diagram explains the DNA structure representing the different parts of the DNA. DNA comprises a sugar-phosphate backbone and the nucleotide bases (guanine, cytosine, adenine and thymine).

DNA Structure

The structure of DNA is often likened to a twisted ladder, forming a double helix, as depicted in the provided figure. DNA, classified as a nucleic acid, consists of nucleotides, which are the fundamental units of all nucleic acids. These nucleotides, in turn, comprise three key components: sugar, phosphate groups, and nitrogen bases.

Nucleotides, the fundamental building blocks of DNA, consist of a sugar group, a phosphate group, and a nitrogen base. The sugar and phosphate groups serve as the binding elements that link the nucleotides together to create each DNA strand. Four nitrogen bases, specifically Adenine (A), Thymine (T), Guanine (G), and Cytosine (C), play a crucial role in the structure and function of DNA.

These nitrogenous bases are arranged in pairs as follows: Adenine pairs with Thymine, and Cytosine pairs with Guanine. These base pairings are essential for the formation of DNA's double helix structure, which resembles a twisted ladder.

The sequence of these nitrogenous bases determines the genetic code, or the instructions encoded in the DNA.

The DNA structure's essential components include sugar, specifically deoxyribose, which forms the backbone of the DNA molecule. Hydrogen bonds form between the nitrogenous bases of complementary strands, creating a ladder-like structure.

The DNA molecule consists of four nitrogen bases: adenine (A), thymine (T), cytosine (C), and guanine (G), and these bases constitute the structure of a nucleotide. Adenine and guanine are classified as purines, while cytosine and thymine are categorized as pyrimidines.

The two DNA strands run in opposite directions and are held together by hydrogen bonds formed between complementary bases. These strands exhibit a helical twist, with each strand forming a right-handed coil, encompassing ten nucleotides in a single turn.

The pitch of each helix measures 3.4 nanometers, resulting in a distance of 0.34 nanometers between two adjacent base pairs, which represent hydrogen-bonded bases of opposite strands.

DNA assumes a coiled structure, ultimately forming chromosomes. Each chromosome contains a single DNA molecule. Humans typically possess twenty-three pairs of chromosomes within the nuclei of their cells. DNA also plays a critical role in the process of cell division.

Chargaff’s Rule

Erwin Chargaff, a biochemist, discovered that the number of nitrogenous bases in the DNA was present in equal quantities. The amount of A is equal to T, whereas the amount of C is equal to G.
A = T; C = G
In other words, the DNA of any cell from any organism should have a 1:1 ratio of purine and pyrimidine bases.

DNA Replication

DNA replication is a crucial process that occurs during cell division and is often referred to as semi-conservative replication because it entails the creation of a duplicate DNA strand.

DNA replication unfolds in three distinct stages:

Step 1: Initiation 

The commencement of DNA replication transpires at a specific location termed the origin of replication. Here, the DNA helicase plays a pivotal role in unwinding the two DNA strands, leading to the formation of a structure known as the replication fork.

Step 2: Elongation 

During the elongation phase, DNA polymerase III assumes the responsibility of reading the nucleotides on the template strand and sequentially constructing a new strand by incorporating complementary nucleotides. For example, if it encounters an Adenine on the template strand, it appends a Thymine to the corresponding strand.

While adding nucleotides to the lagging strand, interruptions arise in the form of gaps, known as Okazaki fragments. These gaps, or nicks, are subsequently sealed through the action of ligase.

Step 3: Termination 

The termination stage is triggered by the presence of a termination sequence situated opposite to the origin of replication. At this juncture, the TUS protein, signifying "terminus utilization substance," associates with the terminator sequence and brings about the cessation of DNA polymerase movement, effecting termination.

Why DNA is called a Polynucleotide Molecule?

DNA is referred to as a polynucleotide due to its composition. The DNA molecule is constructed from nucleotides, including deoxyadenylate (A), deoxyguanylate (G), deoxycytidylate (C), and deoxythymidylate (T), which are linked together to form extensive chains known as polynucleotides. In accordance with the structure of DNA, it comprises two sets of these polynucleotide chains.