DNA: Full Form, Definition Structure, Function, Diagram, Facts, Discovery

DNA Defination

DNA full form: The full form of DNA is Deoxyribonucleic Acid.

DNA meaning: DNA is a molecule that contains the information required for the synthesis of proteins, their functioning, and reproduction. It is made of two long chains spiralled into a double helix in a linear formation made up of nucleotides, which are comprised of sugar-phosphate molecules and nitrogenous bases, namely adenine, thymine, cytosine, and guanine.

DNA carries the information that gives an organism its features. In replication of DNA and gene expression, DNA is responsible for carrying hereditary traits to future generations. Therefore, DNA is an important biological molecule contributing to the genetics and molecular biology concepts of population and species variation and the evolution of these species on earth.

Structure of a DNA

DNA has a classic double helical structure, having major and minor grooves. They are of different types, such as A-DNA, B-DNA, and Z-DNA. The structure of DNA in detail is listed below:

Double Helix Model

The main shape of the DNA molecule is in two chains that are coiled into a helix, a spiral staircase. It is with the help of such a structure that the DNA can compactly store all the genetic information found in it and remain stable.

Contributions of Watson and Crick:

The application of X-ray diffraction came through Rosalind Franklin and Maurice Wilkins, whose data supported James Watson and Francis Crick in formulating the DNA double helix in the year 1953. Their model was beautiful in that it was unambiguous in illustrating how genetic information is both coded and copied.

DNA double helix:

The DNA double helix has two antiparallel strands that are connected through hydrogen bonds present between two related nitrogenous bases. In the first strand, adenine is paired with thymine, while in the second strand, cytosine is paired with guanine, thus making pairing of the base complete.

Given below is the structure of a DNA double helix model, showing its components

Components of DNA Nucleotides

The nucleotides, also known as the units of DNA, make up the DNA molecule. Each nucleotide contains a phosphate group, a deoxyribose sugar molecule, and one of four nitrogenous bases: adenine, thymine, cytosine or guanine.

Structure of a nucleotide

A nucleotide has three components:

Sugar: In nucleotides, the sugar part can include deoxyribose in the case of DNA or ribose in the case of RNA. This is because; deoxyribose contains one less oxygen atom as compared with ribose, thus making DNA more stable.

Phosphate Group: This group is made of just one phosphorus atom and there, it links four oxygen atoms. It links the sugar moiety of the two adjacent nucleotides forming the backbone of the nucleic acid polymer.

Nitrogenous Base: In nucleotides, nitrogenous bases form four types. In DNA, these are adenine (A), thymine (T), cytosine (C) and guanine (G). The bases paired together are A with T and C with G. In RNA, thymine is replaced by uracil; hence RNA has uracil (U) instead of T. These bases are paired particularly (A with T/U, and C with G) to transcribe genetic information.

Hydrogen bonds between bases: Complementary nitrogenous base pairs in DNA hydrogen bond with each other. Adenine has two hydrogen bonds with the molecules of thymine, and cytosine has three hydrogen bonds with guanine. These bonds assist in maintaining the double helix formation of the DNA molecule and also in the unwinding and replication of the DNA molecule during the process of cell division.

Function of DNA

DNA performs many functions, from carrying the genetic material to expression of those genes in the form of protein. The functions of DNA are discussed below:

Genetic Information Storage

Information in DNA is stored in the form of nucleotide base pairs, namely adenine, thymine, cytosine and guanine. These bases pair only (AT and CG), and they create a language or a code that determines how large protein structures and other actions in a cell are to be constructed.

DNA Replication

Semiconservative replication: In the process of DNA replication, one strand of the parent DNA molecule acts as the model against which a new strand is synthesised. This process ensures that in every newly synthesised DNA molecule, one is the old strand and the other a new one.

Enzymes involved:

Key enzymes in DNA replication include:

• DNA polymerase: replicates and duplicates DNA and can attach a new nucleotide to the existing chain.

• Helicase: negatively impacts the various functions in that it unwinds the double helix of DNA, splitting the two strands. It is one of the enzymes used in DNA replication.

• Ligase: joins the newly synthesised Okazaki fragments on the lagging strand.

Transcription and Translation

Process of transcription: Transcription therefore generally means using DNA as a mould to produce RNA. RNA polymerase has an affinity for a segment of the DNA (promoter) to bind to and release two threads of DNA and cause the synthesis of an RNA molecule from one of the DNA threads. There is also the RNA molecule called messenger RNA or mRNA; it can transport the DNA’s genetic information towards the ribosomes.

Process of translation

The process of translation in biology is the process in which information in mRNA is read to form proteins. It happens in the ribosomes and is the process by which tRNA molecules bring an amino acid to the form and position it according to the nucleotide sequence of mRNA specifying the protein. The ribosome moves along the mRNA sequence reading it in sets of three symbols called codons, which correlate with different amino acids. The building block of a protein is a polypeptide chain that is composed of amino acids, and its conformation makes up a functional protein.

DNA Discovery

The distinct double helix structure of DNA was not discovered until 1953 and was formulated by James Watson and Francis Crick. The authors of the model used XRD photographs of DNA provided by Rosalind Franklin and Maurice Wilkins. Franklin’s work proved helpful in the characterisation of the molecular conformation of DNA, its helical, as well as its dimensions.

Discoveries made by Watson, Crick, Franklin, and Wilkins are considered one of the key events in the biological sciences. They identified the structure of DNA molecules, the storage of genetic information and how it can be passed from one generation to another, hence revolutionising genetics, heredity and molecular biology.

Different forms of DNA

DNA has the potential to exist in different conformations, mainly in terms of how the double helix is coiled and the relative positioning of its building blocks.

Here are the main types of DNA structures:

1. A-DNA:

A-DNA has a right-handed coil and is more compact than B-DNA, though it has a longer pitch than B-DNA. It occurs under low humidity conditions or some specific sequences of DNA and RNA. A-DNA has approximately 11 base pairs per turn and is less frequently found in organisms; however, its importance is seen in molecular biology and biotechnology.

2. B- DNA:

This type of DNA is the most common and stable in solutions that mimic the physiological state of the organism. B-DNA is right-handed with a twist of about 10.5. This is the form of DNA seen in most biological processes and interactions.

3. Z-DNA:

Z-DNA is left-handed and is less common than right-handed B-DNA and A-DNA. It forms under special physiological conditions or when the sequence of bases is separated by purines and pyrimidines in turns. Z-DNA has a zigzag structure of the backbone and is more destabilised than B-DNA and A-DNA. It is involved in gene regulation, and it maps in regions of the genome being transcribed.

MCQs on DNA

Q1. Backbone in a polynucleotide chain of DNA is

Option 1: Sugar only

Option 2: Phosphate group only

Option 3: Sugar and phosphate group

Option 4: None of these

Correct answer: 4) Sugar and phosphate group

Explanation:

Backbone in a polynucleotide - The backbone in a polynucleotide chain is formed due to sugar and phosphate. The nitrogenous bases linked to sugar moiety project from the backbone. These bonds form between the 3’-hydroxyl (-OH) group of one sugar (deoxyribose in DNA or ribose in RNA) and the 5’-phosphate group of the next sugar, creating a strong, stable structure.

The nitrogenous bases (adenine, thymine, guanine, cytosine in DNA, and uracil replacing thymine in RNA) are attached to the sugar moiety at the 1' position. These bases extend outward from the backbone, allowing them to interact with complementary bases on another strand through hydrogen bonding, forming the double-helix structure in DNA. This backbone provides structural integrity and orientation to the nucleic acid chain, while the sequence of bases encodes genetic information.

Hence, the correct answer is option 4) Sugar and phosphate group

Q2. A Nucleoside is defined as:

Option 1: Pentose Sugar + Nitrogenous base

Option 2: Nucleotide – Phosphate group

Option 3: Pentose sugar joined with nitrogenous base via N-glycosidic linkage

Option 4: All of these

Correct answer: 4) All of these

Explanation:

Nucleoside -A nitrogenous base is limited to the pentose sugar through a N-glycosidic linkage to form a nucleoside. Eg: adenosine or deoxyadenosine, guanosine or deoxyguanosine. A nucleoside consists of a nitrogenous base (purine or pyrimidine) attached to a pentose sugar (ribose in RNA or deoxyribose in DNA). The base is linked to the sugar through a N-glycosidic bond, specifically at the 1' carbon of the sugar. When a phosphate group is added to a nucleoside, it forms a nucleotide, the building block of nucleic acids like DNA and RNA.

Hence, the correct option is 4) All of these

Q3. In a DNA molecule, the phosphate group is attached to carbon __________ of the sugar residue of its nucleotide and carbon __________ of the sugar residue of the next nucleotide by __________ bonds

Option 1: 5’, 3’, phosphodiester

Option 2: 3’, 5’, phosphodiester

Option 3: 5’, 3’, glycosidic

Option 4: 3’, 5’, glycosidic

Correct answer: 1) 5’, 3’, phosphodiester

Explanation:

Dinucleotide and polynucleotide - Two nucleotides are linked through 3' -5' a phosphodiester linkage to form a dinucleotide. More nucleotides can be joined in such a manner to form a polynucleotide.

Hence, the correct option is 1) 5’, 3’, phosphodiester

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