Types of DNA: Structure, Properties, Types and Functions

DNA is responsible for carrying the genetic information required for inheriting, cell function, and reproduction in all living organisms. Types of DNA are studied in the Class 12 chapter Molecular Basis of Inheritance in Biology. It is discussed in detail about different types of DNA, primarily A-DNA, B-DNA, and Z-DNA, which are different in structure and function. This topic is important for competitive exams like NEET and AIIMS BSc Nursing exams, as it shows the conditions for the storage and transmission of genetic information from one generation to the next.

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This Story also Contains

1. What is DNA?
2. DNA Structure
3. Types of DNA
4. Types of DNA Sequences
5. Applications of Different DNA Types
6. Recommended Video on Types of DNA

What is DNA?

DNA, or deoxyribonucleic acid, is the genetic material found in practically all living organisms except viruses. It carries crucial information required for the growth, development, functioning, and reproductive processes of current forms of life. The working unit of DNA guides cellular processes and conveys hereditary information across generations.

DNA can be broadly categorised into two main types: nuclear DNA and mitochondrial DNA. Nuclear DNA is located within a cell's nucleus, in the form of chromosomes, and contains the bulk of the genetic material of any organism. On the other hand, mitochondrial DNA (mtDNA) is located in the energy-producing structures in a cell, called mitochondria. Unlike nuclear DNA, mtDNA is maternally inherited and used for studies on the maternal lineage and energy metabolism.

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DNA Structure

• The double helix structure was a postulation or model by Waston and Crick.

• In this model, two strands appear like a helix twisted around each other.

• Each strand is also a chain of nucleotides composed of three parts; a sugar (deoxyribose), phosphate group, and nitrogenous base of one of four types: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G).

• The bases pair specifically (A with T and C with G) through hydrogen bonds, maintaining the stability of DNA and fidelity in its replication.

Types of DNA

• Each of these DNA forms: A-DNA, B-DNA, and Z-DNA exhibits different structures and functions, thus representing the flexibility and versatility of DNA as a molecule that can interact and perform different biological functions.

A-DNA

• A-DNA is a right-handed helix, like B-DNA, but varies substantially in its physical structure.

• It is shorter and wider compared to B-DNA, with the groove containing approximately 11 base pairs per turn, compared to 10.5 base pairs per turn present in B-DNA.

• Under dehydrated conditions or by the presence of some chemical agents that take out the water from the DNA molecule, the formation of A-DNA occurs.

• This form of DNA is relatively rare in living cells but can be found or induced by other laboratory conditions.

• The more dense and compact structure makes it play another function in cellular processes where it is involved in DNA-RNA hybrid structures as well as certain enzyme-DNA interactions.

B-DNA

• B-DNA is the most common kind of DNA found in cells under physiologic conditions.

• It is best differentiated by its double-strand helix that is right-handed with about 10.5 base pairs in every turn.

• This is further stabilised by hydrogen bonds between the nitrogenous bases and by the hydrophobic interactions between the stacked bases.

• The major and minor grooves of B-DNA present unique sites for protein binding, which is an essential requirement in processes like transcription, replication, and DNA repair.

• The regular repeating structure makes B-DNA capable of easily storing and permitting the retrieval of genetic information and, therefore, these are widely regarded as the reference DNA in all standard biological contexts.

Z-DNA

• A left-handed helix Z-DNA is a unique form of helix, different from the more common right-handed A-DNA and B-DNA.

• It has a zigzag backbone, a defined helical twist, and about 12 base pairs per turn. Z-DNA can occur physiologically when a DNA sequence is rich in alternating purines and pyrimidines, for example, CG-repeats—or when exposed to stress, for example, high salt concentration or negative supercoiling.

• This form of DNA is believed to play a role in the regulation of genes and the organisation of chromatin. Z-DNA regions form signals for protein binding and allow for the transcriptional activity of flanking genes, and these contribute to the dynamical behaviour that lies within the chromosomal architecture.

Types of DNA Sequences

The types of DNA sequences are mentioned below:

Coding vs. Non-Coding DNA

• Coding DNA sequences are transcribed to mRNA and translated to proteins, and therefore, are the basis of the genetic material.

• Non-coding DNA, although not codified into proteins, plays a relevant regulatory role, including gene expression control, and structural defining roles, such as those in chromosomes.

• Examples of non-coding DNA include introns, promoters, and enhancers.

Repetitive DNA

• It consists of DNA sequences that are repeated numerous times in the genome. Tandem repeats, represented by microsatellites and minisatellites, and interspersed repeats, which comprise SINEs (Short Interspersed Nuclear Elements) and LINEs (Long Interspersed Nuclear Elements), play roles in genome evolution and regulation.

Applications of Different DNA Types

• DNA profiling can be identified as one of the most important tools of forensic science with which people can be identified through their unique genetic composition. This will have vast applications in criminal investigations, paternity disputes, and identifying people's remains.

• DNA analysis is very important for an understanding of genetic diseases that will allow a clinician to identify a disease-causing mutation, possibly leading to the development of a directed therapeutic approach. Genetic research also probes into the function and regulation of genes.

• Gene clones are produced by biotechnology with the help of plasmids, which are used as vectors, to enable scientists to introduce genes into cells from hosts that are used to synthesise proteins, study their functions, and genetically modified organisms.

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Recommended Video on Types of DNA