Structure And Functions Of DNA

Introduction to DNA

In the year 1869,  German biochemist  Frederich Miescher has observed the DNA for the first time. But many scientists were unclear about the complete picture of DNA as there was no clear evidence about the exact anatomy and physiology. In the year 1953, group of scientists;  James Watson, Francis Crick, Maurice Wilkins and Rosalind Franklin have identified the double helix. In the year 1962, Watson  Crick and Wilkins received the Nobel prize for the discovery of the DNA. DNA (deoxyribonucleic acid) is one of the macromolecules essential for cellular functions. DNA is the central aspect of all life processes as it carries the genetic material necessary. Each organism is unique in many ways. The uniqueness of individuals is driven by the central messengers called genes. In layman terms, DNA is an information molecule help in storing and distributing the instructions for making proteins. Protein is the lifeblood of the survival of all the organisms. The instructions are stored inside each individual cells within the chromosomes. There are 46 chromosomes in our body. Each chromosome is made up of thousands of  DNA in a segmental fashion. The fragments of DNA makes up genes. The terms genes, genome, nucleic acids, DNA and RNA are frequently used. Although they come from the same context but there is a slight difference between each. For more information on genetics and heredity Click here. This article is designed to brief the structure, functions and double-helical model of DNA.

 

Table Of Contents

1. Introduction to DNA

2. Structure and functions of DNA

   2.1 Structure of nucleotides

   2.2 Nitrogenous bases

   2.3 Bonding pattern of purines and pyrimidines

   2.4 Functions of Nucleotides

3. Types of DNA 

4. Double Helical model of DNA

5. Packaging of DNA Helix( Selenoid model of DNA packing)

 

Structure and functions of DNA

DNA is a  polymer having a long polymerase chain. The  DNA has a reverse spiral nucleotide chain. The 2 strands are called helices, run in a parallel direction. Each helix is made up of polynucleotides. An individual polynucleotide has multiple numbers of monomers piled together with many nucleotide sequences. 

 

Structure of nucleotides

A nucleotide is made up of 3 major elements; pentose sugar, a phosphate group, and a nitrogenous baseA pentose sugar is a simple monosaccharide containing 5 carbon atoms hence, the name pentose sugar. Pentose sugars tend to bound with the phosphate groups. You can see in the image below depicting how the pentose sugars are interlinked alongside the phosphate group throughout the chain. A phosphate binds with a sugar to produce the sequence called sugar-phosphate sequence. They are present throughout the wall of the double helix. In a DNA,sugar-phosphate complex acts as a backbone of the structural framework of the genetic material as the variety of combinations of phosphate with sugars gives a particular pattern. In between the 2 helices, there are nitrogenous bases. They form the supporting base that holds the polynucleotide chains. There are 2 types of nitrogenous bases; purines and pyrimidines.  The permutation and combination of the purines and pyrimidines results in 64 varieties of codons. 

 

Nitrogenous bases

Nitrogenous bases are the result of a combination of purines and pyrimidines. Both the purines and the pyrimidines form a variety of combinations of nitrogenous bases.  Purine bases are indicated as ATGC in DNA  ( In the case of DNA adenine (A) pairs with thymine (T) while cytosine (C) pairs with guanine (G). In the case of  RNA, uracil (U) takes the place of thymine (T. ). Purines and pyrimidines are otherwise known as the base pairs present between the 2 strands. The nitrogenous base is an information-carrying part of the nucleotide structure. For more information on genetic code Click here.

 

 

Bonding pattern of purines and pyrimidines

As you can see in the image, Adenine is a purine bonding with thymine ( in the case of  RNA, adenine bonds with uracil). Guanine is also a purine nucleotide bonding with cytosine ( same in both DNA and RNA). The formation of the bond is supported by triple (3) hydrogen resulting in a strong adherence of cytosine-guanine bond. On the other hand, the thymine-adenine bond has only 2 hydrogen bonds in the middle. One common aspect found in the nitrogenous bonds of DNA and RNA is the cytosine bonding with guanine in both the cases. Thymine bonds with adenine in DNA whereas in RNA has thiouracil instead of adenine. 

 

Functions of Nucleotides

Nucleotides assist in a range of functions but the key role is the catalyst for many biological reactions happening in our body. For example, the formation of ATP is catalyzed by nucleotides. Some of the nucleotides act as intermediaries in the cellular communication during normal physiology. Nucleotides are also found in coenzymes such as  NAD and NADP.

 

Types of DNA 

B-DNA is a complete double-helical model explained by James Watson. Later on, new DNA models have evolved. A-DNA is another right-handed duplex model containing 11 base pairs for each helix.A-DNA helix forms  20° against the perpendicular to the axis. Another form of DNA is the B-DNA   which is very active. C-DNA has 9 base pairs for each right-handed twist and the  D-DNA  has only 8 base pairs per twist.  ZIg-Zag -DNA or  Z-DNA  is the only left-handed double helix.

 

Double Helical model of DNA

In the year 1953, James Watson and Francis published a descriptive model for DNA, it is popularly known as ‘double helix’ DNA model.  The model has clearly explained the properties of polynucleotide chains. The double-helical model of DNA  postulated that the hydrogen bonds between the 2  nitrogenous bases help them in holding together.  The pairing formula of nitrogenous bases provides a unique double-helical formation. The double-helical model is characterized by right-handed coiling or twisting with each twist of the helix measuring 3.4nm or 34 Angstrom units. In a single twist, 10 nucleotides are embedded forming 2 groves- the minor groove and the major groove.  A minor groove occurs when the helix is being stretched apart. On the other hand, a major groove is formed when the strand is being compressed. The 2 grooves look like they are twisted around base pairs as shown in the picture. In a double-helical model, the 2 strands run in an antiparallel mode, in other words, the 2 strands are placed in a way that the 5' end (phosphate-bearing end) of one strand joins the 3' end (hydroxyl-bearing end) and vice versa. This is called an antiparallel polarity organization.

 

Packaging of DNA Helix( Selenoid model of DNA packing)

 

                                   Packaging of DNA Helix

 

The biological phenomenon of the formation of chromosome lies behind the packaging of DNA. DNA is packed in a compact manner to produce nucleosomes. The order for the formation of chromosomes is DNA → nucleosome → solenoid → chromatin fibre → chromatid → chromosome. The packaging system vary between different species. For instance, in Escherichia coli, the total length of its DNA is 1.36 mm whereas the human DNA  measures around  2.2 meters. Similarly, the average distance between the 2  adjacent bases in an E-coli is about 0.34nm when compared to that of the human`s  6 micrometres (µm). In a prokaryote like E. coli, the genetic material is double-coiled (coiled and recoiled) with the help of RNAs. However, in the case of eukaryotes, the packing of DNA is carried out by a positively-charged histone octamer ( 8 histone molecules) closely wrapped around a negatively-charged DNA forming a  nucleosome.  Multiple nucleosomes join together forming a long, beaded,thread-like structure; chromatin. The chromatin again recoils to form chromatin fibres. Many chromatin fibres coiled to form the final unit -chromosomes. Altogether, the process is known as the SELENOID model of DNA packing.

 

 





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