An Introduction to DNA Structure
You must have heard this many times “It is in my DNA” and wondered what does it actually imply? DNA is a hereditary material that is present in every living organism on our planet Earth. The genetic instructions are carried in these structures which are passed from one generation to another and are responsible for how that organism develops, grows, reproduces, and functions. So let us dig into some of the basics and details of this material.
What is DNA?
DNA is a self-replicating genetic material that occurs in every living organism. It is a carrier of all genetic information. It carries instructions for organisms to grow, survive, develop and reproduce. It’s a long molecule that contains a genetic “code”. This genetic code is a starting point of human development. However, it can be affected due to external factors like lifestyle, nutrition, and the environment leading to human evolution.
Who discovered DNA – Contrary to the belief that DNA was discovered in the 1950s by Biologist duo James Watson from the US and Francis Crick from England, DNA was discovered as early as 1869 when the Swiss Biologist Johannes Friedrich Miescher was researching white blood cells. He was doing a series of research in which he found the existence of DNA, their main chemical components, and how they joined each other.
What is the full form of DNA – DNA stands for Deoxyribonucleic acid. If we bifurcate the word:
Deoxy means a missing oxygen atom
Ribo stands for Ribose sugar
Nucleic is present in the nucleus of cells
Acid – phosphate ion which is negatively charged and acidic
Components of the DNA molecule – DNA is made up of molecules known as nucleotides. The 3 components of a DNA molecule are the carbon-sugar group, phosphate group, and nitrogen base. The nitrogen bases are further divided into four- Adenine (A), Cytosine (C), Guanosine (G), and Thymine (T).
DNA Structure – Human DNA is unique and is made up of 3 billion base pairs. However, 99% of them are the same in all humans. The sequence of these bases determines what information needs to be coded that is necessary for the organism's growth and development. During the process of transcription (when DNA is replicated), a product is formed known as RNA (Ribonucleic Acid). This RNA is responsible for translating genetic information from DNA to protein followed by reading through ribosomes.
DNA molecules are polymers that are built by linking together smaller molecules called monomers. The smallest units of DNA are called nucleotides, which is made of monomers. These nucleotide monomers bunch together to form a DNA molecule.
Nucleotides are composed of a nitrogen base, sugar group, and phosphate group. Sugar is the backbone of this structure. Nitrogen bases are of 4 types: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C). The nitrogen bases form a unit called “base pair” with each other, A pairs with T and C pairs with G. Then these base pairs attach to a sugar molecule and a nitrogen molecule to form a nucleotide.
Each helix has a pitch of 3.4 nm that’s why the distance between any 2 base pairs is 0.34 nm. These nucleotides attach to each other forming 2 long strands, which run opposite to each other in a spiral shape to form a structure termed as a double helix. A double helix looks like a ladder whose rungs are made of nitrogen base and the 2 sides contain the phosphate group and sugar group.
Where is DNA found – DNAs are mostly located in the nucleus of a cell (where they are termed as nuclear DNA) but a small amount of it is also present in mitochondria and known as mitochondrial DNA. Mitochondria are powerhouses of our body that convert the food we eat into the form cells can use to produce energy.
How does the DNA work?
DNA is the most important material for any living organism. It contains vital information about an organism's trait that is passed from one generation to another. In the nucleus of a cell, DNA molecules are tightly held together to form chromosomes that secure DNA and keep it intact in one place. It stores the necessary information in the form of genes to determine an individual's genetic information.
The DNA works by copying itself into RNA ( a single-stranded molecule). If DNA is the blueprint, one can assume that RNA is the translator of instructions written in the blueprint. During the replication process, the DNA unwinds itself into two single strands. RNA is somewhat similar to DNA but it contains some significant molecular differences that set it apart from DNA. RNA acts as a messenger and carries important genetic information. This information is translated (the process of protein formation through RNA) to form proteins by ribosomes which then evolve living organisms.
Chargaff's Rule
Erwin Chargaff, a Biochemist, found that the number of nitrogenous bases is present in equal amounts in the DNA. The amount of adenine (A) is equal to the amount of thymine (T). Whereas, the amount of guanosine (G) is equal to the amount of Cytosine (C). Adenine is linked with Thymine by a double bond and Guanosine is linked with Cytosine by a triple bond.
The DNA of a cell contains equal amounts of purine and pyrimidine to keep the ratio 1:1. Purine contains the nitrogenous bases adenine and guanine while pyrimidine contains cytosine and thymine in DNA and cytosine and uracil in RNA.
DNA Functions- This genetic information is carried by genes. Genes are small pieces of DNA containing approximately two million base pairs. A gene code for a polypeptide molecule counts three nitrogenous bases on a single amino acid.
Polypeptide chains are further coiled into secondary, tertiary, and quaternary structures to form different proteins. As different organisms contain numerous genes, different proteins can be formed.
Proteins are the significant structural and functional units in most organisms. Apart from conserving genetic information, The most important function of DNA is to carry genetic information from generation to generation coded in the way the nitrogen bases are arranged in the nucleotides. Some of the other functions of DNA include:
Cellular Metabolism- The metabolic reactions of the cells is regulated by DNA which uses enzymes, hormones, and specific RNAs (Ribonucleic acid which is responsible for the regulation and expression of genes) for this purpose.
Transcription- RNAs are produced from DNAs through the process of transcription
DNA Replication- Genetic information is passed from one cell to daughter cells and from one generation to the next. It produces carbon copies through replication.
Development of Organisms- By the mechanism of the internal genetic clock, the development of organisms is controlled by the DNA molecules.
DNA FingerPrinting- The DNA sequence is unique for each individual and cannot match that of another individual. This property is useful in DNA fingerprinting, which is carried out to identify an individual through his or her DNA.
Mutations- The alterations that occur in the DNA sequencing. Mutations occur due to errors during DNA replication. However, the errors can be caused due to exposure of DNA to ultraviolet radiation, deletion or insertion of DNA segments, etc.
Gene Therapy- It is a technique in which an individual's gene is modified to cure any disease. Gene therapy can work by different mechanisms like
Replacing a disease-causing gene with a healthy copy of a gene.
Inactivating a disease-causing gene that is not working properly.
Introducing a modified or new gene in the body to treat diseases.
Types of DNA– There are 4 major forms of DNA that are double-stranded. They are connected by the interaction of complementary base pairs.
B-form DNA– This is the most common form of DNA in which 2 strands of DNA, each in a right-hand helix, are coiled around the same axis. The H-bonding between the bases holds the 2 strands together.
A-form DNA– This is similar to B-form DNA in the sense it is also a right-handed double-helical structure but is thicker and has a shorter distance between its base pairs. When DNA is dehydrated, it takes the A form so that it can be protected from extreme conditions like desiccation. It can also be formed when protein binding removes the solvent from DNA.
Z-form DNA– This is the 3rd form of duplex DNA but is a left-handed helical structure. A sequence of alternating purines and pyrimidines form a zigzag structure in a Z-form DNA. It is present at the starting of a gene site hence it is assumed to play some role in gene regulation and transcription.
The Future of DNA
DNA has a great future as researchers are trying their best to know the complexities associated with it. If the studies continue, we can expect a world with fewer and well-managed diseases, better treatment options, and a longer life span.
FAQs on DNA Structure and It's Details
1. What is the relationship of Chromosomes and DNA?
DNAs are very long structures, so long that they have to be packaged correctly to fit into cells. For this, they are coiled tightly into structures called chromosomes. Each chromosome has 1 DNA molecule and there are 23 pairs of chromosomes inside each cell’s nucleus of the human body. These chromosomes are made of chromatin. Chromatin is the DNA threads that further coiled up to form chromosomes. Genes are the DNA segments that code for protein. Each chromosome is made of DNA and consists of many genes.
2. How many bases do humans have?
There are close to 3 billion bases in humans out of which 99% are the same in all humans. The sequence or the order of the basis determines the genetic information available for maintaining and building an organism, similar to a way in which letters combine to form meaningful words and sentences. DNA bases combine, Adenine combines with Thymine and Guanine combines with Cytosine to form base units. Further, they are combined with a phosphate moiety and a sugar molecule to form a nucleotide.
3. What are genes?
The way nitrogen bases are arranged in a DNA sequence forms genes. Genes are responsible for protein creation by the cells.1. They are the basic unit of heredity that codes for the synthesis of a gene product may be RNA or protein. These genes get transferred from parent to offspring carrying their traits. Also, they form different DNA sequences called genotypes that get transferred during the Biological processes. Genes can also face changes or mutations due to factors like diseases, exposure to ultraviolet radiation, etc.
4. What is DNA sequencing?
It is a technology to determine the order of bases in a DNA sequence. The DNA base carries important information that a cell needs to code for making RNA and protein. DNA sequencing is crucial for scientists to understand the functioning of genes. This knowledge can bring a revolution in molecular Biology that can be helpful to treat diseases like cancer and provide better treatment facilities. However, An important breakthrough was made in 2017 when the DNA sequencing of Egyptian mummies was done to find their genome-wide data.
5. What is Chargaff’s rule?
This defines the grammar of DNA i.e. both strands of the nucleotide have a 1:1 ratio of pyrimidine and purine bases and amount of A = T; C = G. This rule was first discovered by Erwin Chargaff, an American Biochemist. The Chargaff rule provides the basis of base pairing. With this rule, one can determine the presence of a base in the DNA molecule. Moreover, it also helps to know the length of the DNA strand. For a particular species, the amount of Cytosine is equal to the amount of Guanosine connected with a triple hydrogen bond. Whereas, the amount of Adenine is equal to Thymine connected with a double hydrogen bond.
