DNA structure & Replication | Anthropology Optional for UPSC PDF Download

What is DNA

• DNA, or deoxyribonucleic acid, is a collection of molecules that serves as the carrier and transmitter of genetic information or hereditary instructions from parents to offspring. This applies not only to living organisms but also to viruses, which can have either RNA (ribonucleic acid) or DNA as their genetic material. For example, the Human Immunodeficiency Virus (HIV) contains RNA, which converts to DNA once it attaches to a host cell.
• In addition to its role in inheritance, DNA is crucial for protein production in all living beings. In eukaryotic organisms, nuclear DNA is the genetic material found within the cell nucleus, and it contains the majority of the organism's genetic information. Meanwhile, mitochondrial DNA and plastid DNA hold the remaining genetic information.
• Mitochondrial DNA is located in the cell's mitochondria and is passed down from mother to child. In humans, it consists of around 16,000 base pairs. Plastids also possess their own DNA, which is essential for 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

There are three distinct types of DNA conformations:

• A-DNA: This conformation is a right-handed double helix, similar to the more common B-DNA form. A-DNA is typically found in dehydrated DNA, as it serves as a protective structure during extreme conditions, such as dehydration. When proteins bind to DNA, they remove the solvent from it, causing the DNA to adopt the A-DNA form.
• B-DNA: This is the most prevalent DNA conformation and also has a right-handed helix structure. Under normal physiological conditions, the majority of DNA exists in the B-DNA conformation.
• Z-DNA: Unlike the other two forms, Z-DNA is a left-handed DNA conformation, where the double helix twists to the left in a zig-zag pattern. Discovered by Andres Wang and Alexander Rich, Z-DNA is typically found near the start site of a gene, and as such, it is believed to play a role in gene regulation.

Who Discovered DNA

In 1869, Swiss biologist Johannes Friedrich Miescher first discovered and recognized DNA while studying white blood cells. However, it wasn't until later when James Watson and Francis Crick gathered experimental data that the double helix structure of a DNA molecule was revealed. This groundbreaking discovery eventually led to the understanding that DNA is responsible for storing 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 can be visualized as a twisted ladder, known as a double helix. DNA is a type of nucleic acid, and all nucleic acids are composed of units called nucleotides. Each nucleotide consists of three distinct components: a sugar molecule, a phosphate group, and a nitrogen base.
• Nucleotides, the basic building blocks of DNA, are made up of a sugar group, a phosphate group, and a nitrogen base. The sugar and phosphate groups connect the nucleotides, forming the two strands of the DNA molecule. There are four different types of nitrogen bases: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C).
• These four nitrogenous bases pair together in a specific way: A with T, and C with G. These base pairs are crucial for maintaining the double helix structure of DNA, which looks like a twisted ladder.
• The sequence of the nitrogenous bases determines the genetic code, which serves as the DNA's instructions for various functions within an organism.

Components of DNA Structure

Among the three components of DNA structure, sugar is the one which forms the backbone of the DNA molecule. It is also called deoxyribose. The nitrogenous bases of the opposite strands form hydrogen bonds, forming a ladder-like structure.

DNA Structure Backbone

• DNA molecules consist of four nitrogen bases: adenine (A), thymine (T), cytosine (C), and guanine (G), which form the building blocks of a nucleotide. Among these bases, adenine and guanine are classified as purines, while cytosine and thymine are categorized as pyrimidines.
• The DNA structure comprises two strands that run in opposite directions and are held together by hydrogen bonds between complementary bases. These strands twist around each other in a helical shape, with each strand forming a right-handed coil. A single turn of this helix consists of ten nucleotides.
• The spacing between each helix turn, known as the pitch, measures 3.4 nanometers (nm). Consequently, the distance between two consecutive base pairs, which are the hydrogen-bonded bases from opposite strands, is 0.34 nm.

The DNA coils up, forming chromosomes, and each chromosome has a single molecule of DNA in it. Overall, human beings have around twenty-three pairs of chromosomes in the nucleus of cells. DNA also plays an essential role in the process of cell division.

Chargaff’s Rule

Biochemist Erwin Chargaff discovered that the presence of nitrogenous bases in DNA occurs in equal proportions. Specifically, the quantity of adenine (A) is equal to thymine (T), and the quantity of cytosine (C) is equal to guanine (G).

A=T; C=G

This means that in the DNA of any cell from any organism, there should be a balanced 1:1 ratio of purine (A and G) and pyrimidine (T and C) bases.

DNA Replication

DNA replication is an important process that occurs during cell division. It is also known as semi-conservative replication, during which DNA makes a copy of itself.

The process of DNA replication occurs in three main stages:

Stage 1: Initiation

• The first step of DNA replication starts at a specific point called the origin of replication. At this point, the two DNA strands are separated by an enzyme called DNA helicase, creating a structure known as the replication fork.

Stage 2: Elongation

• During this stage, an enzyme called DNA polymerase III reads the nucleotide sequence on the template strand of the DNA and builds a new complementary strand by adding matching nucleotides one by one. For example, if the template strand has an Adenine, the enzyme will add a Thymine to the complementary strand.
• While constructing the new strand, gaps called Okazaki fragments are formed on the lagging strand. These gaps are later sealed by an enzyme called ligase, ensuring a complete complementary strand is formed.

Stage 3: Termination

The final stage of DNA replication occurs when the termination sequence, located opposite the origin of replication, signals the end of the replication process. A protein called TUS (terminus utilization substance) binds to the terminator sequence, which stops the movement of DNA polymerase, effectively ending the replication process.

DNA Function

DNA is the genetic material that carries all the hereditary information for an organism. It is composed of smaller segments called genes, which mostly consist of 250 to 2 million base pairs. Each gene codes for a polypeptide molecule, with every sequence of three nitrogenous bases representing one amino acid.

Polypeptide chains fold into secondary, tertiary, and quaternary structures to form various proteins. Since each organism possesses numerous genes in its DNA, a wide range of proteins can be produced. Proteins serve as the primary functional and structural molecules in the majority of organisms.

In addition to storing genetic information, DNA plays a crucial role in several processes, such as:

• Replication: DNA ensures the transfer of genetic information from a parent cell to its daughter cells and from one generation to the next. It also ensures the equal distribution of DNA during cell division.
• Mutations: DNA sequences may undergo changes, called mutations, which can affect an organism's traits and characteristics.
• Transcription: DNA is involved in the process of transcription, which is the first step in gene expression, where a specific segment of DNA is copied into RNA.
• Cellular Metabolism: DNA plays a role in regulating cellular metabolism by coding for enzymes involved in various metabolic pathways.
• DNA Fingerprinting: DNA can be used to identify individuals based on their unique DNA patterns, a process called DNA fingerprinting.
• Gene Therapy: DNA is used in gene therapy, a technique that involves altering or replacing defective genes with healthy ones to treat or prevent genetic disorders.

Conclusion

DNA (Deoxyribonucleic Acid) is a vital molecule that carries genetic information and is responsible for the inheritance of traits in living organisms. It consists of a double helix structure made up of nucleotides, which comprise a sugar molecule, a phosphate group, and a nitrogen base. DNA plays a crucial role in replication, transcription, cellular metabolism, and the production of proteins. It is also involved in gene therapy and DNA fingerprinting, which are techniques used to treat genetic disorders and identify individuals based on their unique DNA patterns, respectively. Overall, DNA is essential for the proper functioning and survival of all living organisms.

Frequently Asked Questions (FAQs) for DNA structure & Replication

What are the three main stages of DNA replication?

The three main stages of DNA replication are Initiation, Elongation, and Termination.

What are the four nitrogenous bases in DNA, and how do they pair together?

The four nitrogenous bases in DNA are Adenine (A), Thymine (T), Cytosine (C), and Guanine (G). They pair together in a specific way: A with T, and C with G.

What is the role of DNA in protein production?

DNA carries the genetic information for coding polypeptide molecules, which fold into various proteins. These proteins serve as the primary functional and structural molecules in the majority of organisms.

What is Chargaff's Rule?

Chargaff's Rule states that the presence of nitrogenous bases in DNA occurs in equal proportions, specifically, the quantity of adenine (A) is equal to thymine (T), and the quantity of cytosine (C) is equal to guanine (G). This results in a balanced 1:1 ratio of purine (A and G) and pyrimidine (T and C) bases.