DNA: Structure, Types, Functions and Diagram

DNA or deoxyribonucleic acid is the genetic material present in all living organisms. It is a nucleic acid that has two different strands that wind into a double helical structure. 

• Each of the strands are made up of long nucleotide chains consisting of sugar, phosphate group, and nitrogen.
• They are also composed of nitrogenous bases namely guanine, cytosine, thymine, and adenine.
• DNA is a common cell component and transfers instructions to produce proteins and certain molecules required for the growth and development of organisms. 
• This genetic material is present inside the nucleus or sometimes found in mitochondria. 

In this article, we will learn more about what is DNA, types of DNA, structure of DNA, DNA replication steps, etc.   

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What is DNA?

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DNA is a nucleic acid and is defined as a group of molecules that carries the genetic information and transmits it from parents to offspring. It is present in the cell nucleus of all organisms. 

• Nucleic acid is the organic material present in all organisms in the form of DNA or RNA.
• The combination of nitrogenous bases, sugar molecules, and phosphate groups are linked by different bonds in a series of sequences leading to the formation of nucleic acids.
• Their DNA structure defines the basic genetic makeup of our body. It defines the genetic makeup of nearly all lives on Earth.
• It is the fundamental unit of heredity, when an organism reproduces a part of the genetic material is passed on to the offspring producing genetically diverse species.

Composition of DNA

As stated earlier, these nucleic acids are made up of nucleotides and each nucleotide is made up of three basic components: nitrogenous bases, and sugar molecules linked with a phosphate group. and base pairs.

• Adenine (A), Thymine (T), Guanine (G), and Cytosine (C) are the four distinct DNA nucleotides, each characterized by a unique nitrogenous base. 
• Nucleotides get their names from the nitrogenous bases they contain, although their deoxyribose molecule is responsible for most of their structure and bonding abilities.
• Each carbon in the ring is referred to by a number, followed by the prime sign ('). The phosphate group is linked to the nucleotides. 

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Discovery of DNA

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Friedrich Miescher, a Swiss researcher, discovered DNA in 1869 while researching the makeup of lymphocytes (white blood cells). He extracted a novel molecule from a cell nucleus that he named nuclein (DNA with related proteins).

• While Miescher was the first to recognize DNA as a separate molecule, numerous scientists and researchers have contributed to the current knowledge of DNA.
• The significance of this molecule in genetic heredity did not begin to be investigated and recognized until the early 1940s.
• In the later years, the double helical structure was introduced by James Watson and Francis Crick.
• Watson and Crick made significant contributions in terms of genetic inheritance, while Friedrich Miescher made significant advances and contributions to the area even before their work.

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Types of DNA

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DNA is an organic molecule with a unique molecular structure. They are present in the cells of all organisms including eukaryotes and prokaryotes. On the basis of structure there are three types of DNA namely:

• B-DNA
• A-DNA
• Z-DNA

B-DNA

B-DNA is the most common form of DNA that exists in normal environmental conditions.

• It is a double helical structure where the nitrogenous base pairs are linked together by hydrogen bonds.
• One turn of this type of DNA has 10 base pairs with a length of 3.4 nm.
• Similar to the A-DNA, these types are also made from sugar phosphates linked by phosphodiester bonds.

A- DNA

This type of DNA appears as a double helical structure and is right-handed.

• One turn of this type of DNA has 11 base pairs with a length of 2.86 nm.
• A-DNA is rarely found in normal conditions and is present in environments with less than 75% relative humidity.
• Generally forms when the DNA is dehydrated.

Z-DNA

Z-DNA is a left-handed double helical structure that winds in a zigzag pattern to the left.

• One turn on this type of DNA has 12 base pairs with a length of 4.56 nm.
• This type of DNA was found by Andres Wang and Alexander Rich.
• Many claim that Z-DNA helps in gene regulation and genetic recombination.

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DNA Diagram

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The DNA diagram is given below which highlights various components like nitrogenous bases, sugar-phosphate backbone, and base pairs. 

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Structure of DNA

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In the earlier section, we came to know that the structure of DNA was discovered in 1953 by James Watson and Francis Crick. We can assume its appearance is like a twisted ladder, hence the name, double Helix. 

• The ladder slides are made up of alternating sugar and phosphate molecules, while the ladder steps are made up of two nitrogen bases.
• Nitrogen bases may be divided into Adenine (A), Guanine (G), Thiamine (T), and Cytosine (C).
• There is a particular matching pattern for nitrogen bases where A pair with T, and C pairs with G
• This is because the quantity of Adenine equals the amount of Thymine, and the amount of Guanine equals the amount of Cytosine.
• The strands of DNA are antiparallel which means one runs in a 5’ to 3’ direction and the other from 3’ to 5’.
• Hydrogen bonds connect the two stands, which are complementary to one another.

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Chargaff’s rule of DNA

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According to Chargaff’s rule, the number of purines and pyrimidines in DNA is in a 1:1 ratio. This rule was introduced by scientist Erwin Chargaff. It serves as the foundation for base pairing.

• This means that the quantity of adenine is equal to thiamine and that of guanine is equal to cytosine

A = T ; C = G

• This rule may be used to identify the existence of a base in DNA as well as the length of the strand. 
• Erwin Chargaff studied the base makeup of DNA from diverse animals. As a result, he came up with two basic guidelines, which he dubbed Chargaff’s rules.
• Chargaff discovered the content of DNA differed between species in terms of the relative quantities of the A, C, G, and T bases.
• This molecular diversity supported the theory that DNA is a genetic substance.

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DNA Replication Steps

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The process by which DNA produces two duplicated copies of itself is called DNA replication. It is an important step to ensure that each daughter cell produced during cell division obtains equal amounts of genetic information. This process involves the following three steps:

1. Initiation
2. Elongation
3. Termination

Initiation

To initiate the process of replication, the double-stranded molecule needs to unzip into two single strands. This process takes place at a site called the origin of replication.

• An enzyme called DNA helicase binds to this site of origin disrupting the hydrogen bonds between the bases (A=T, G=C).
• A Y-shaped structure called the replication fork forms after the separation of the DNA strands. 

Elongation

This is the second step of replication where the role of the DNA polymerase III enzyme takes place. As the two strands run in opposite directions the replication occurs in different ways.

• One strand runs from 3’ to 5’ direction towards the replication fork and is termed the leading strand.
• The other strand is the lagging strand as it runs from 5’ to 3’ direction away from the fork. 
• Primase enzyme produces a short piece of RNA called primer which is attached to the 3’ end of the leading strand for it to replicate.
• DNA polymerase III enzyme reads the sequence of DNA and keeps adding the complementary bases.
• For example- if the enzyme reads guanine on the template strand it will add cytosine to the complementary strand.
• Nucleotides when added to the lagging strands lead to gap formation called the Okazaki fragments.

Termination

The last step towards the end of replication is the termination stage where the role of exonuclease takes place.

• This enzyme eliminates all the RNA primers from the original strands once all the bases have been paired.
• DNA polymerase checks, removes, and replaces any mistakes in the freshly produced DNA.
• Okazaki Fragments are joined together by another enzyme called DNA ligase to create a single strand. 
• Finally terminus utilisation substrate binds to the terminal site to end the process.

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DNA Cloning 

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The technique of generating numerous, identical copies of a specific portion of DNA is known as DNA cloning.

• A gene or other DNA fragment of interest is first placed into a cellular piece of DNA called a plasma in a conventional cloning method.
• Insertion is carried out by enzymes that cut and paste DNA, into a molecule of recombinant DNA. 
• After that, the recombinant plasmid is introduced in cell of bacteria that contain the plasmid are chosen and cultivated.
• The plasmid is replicated and transmitted to their progeny when they reproduce, creating copies of the DNA.

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Functions of DNA

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There are various functions of DNA, a few of them are as follows:

• DNA is a genetic substance that contains all the information passed down through generations. The genetic information is encoded in the nitrogen base arrangement.
• Reproduction is a part of replication, for generating carbon copies. In this the genetic information is transferred from one cell to its daughters and from one generation to the next.
• The genetic material is found within chromosomes. This is necessary to ensure that DNA is distributed evenly during cell division.
• DNA functions in recombination, this means that two DNA molecules share the genetic material with each other during meiosis.
• Mutations are caused by changes in the sequence of nitrogen bases caused by addition, deletion, or incorrect replication. Mutations are at the root of all variation and evolution.
• It is a technique used in labs to identify a person’s identity based on their nucleotide sequence. 
• Most nucleotides are used in replacing a damaged gene with a healthy one to cure any genetic related disorder.

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Why is DNA called a Polynucleotide Molecule?

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Deoxyribonucleic acid is termed a polynucleotide molecule as it is made up of nucleotides linked together to form a lengthy chain.

• It is made up of four basic nucleotides namely Deoxyadenylate (A), Deoxyguanylate (G), Deoxycytidylate (C), and Deoxythymidylate (T).
• As DNA is a long-chain polymer of nucleotide molecules, therefore, it is called a polynucleotide molecule.
• These nucleotides are linked together by phosphodiester bonds between adjacent sugar phosphate molecules and hydrogen bonds between nitrogen bases on the opposite strand. 

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Things to Remember

• DNA is a part of unit 7 Genetics and Evolution, chapter 6 Molecular Basis of Inheritance, and carries a total of 5 to 6 marks. 
• The nitrogenous bases that make up the nucleotides are called purines and pyrimidines.
• There are two types of DNA: mitochondrial and nuclear.
• A nucleotide is made up of three parts: a phosphate group, sugar, and a nitrogen base.
• The ability of DNA to make copies of itself during cell division is called replication.
• Nucleotides are the basic building components of the double-helical structure.