Short Notes on DNA, RNA and The Central Dogma for CBSE & NEET

Understanding DNA, RNA and protein is the first step towards understanding Molecular Biology. This article discusses the important topics on DNA, RNA and protein. Read on to find crisp notes on the Central Dogma of molecular biology.

DNA : deoxyribonucleic acid

the building block of the life.

DNA is a long polymer made of repeating units called nucleotides. It contains the information the cell requires to synthesize protein and to replicate itself.  It was first observed by  Friedrich Miescher in 1869, and its molecular structure was first identified by Watson-Crick in 1953DNA is double stranded (dsDNA) and has a double helix structure. Sometimes DNA can also be single stranded (ssDNA), but these are mostly transitory phases and rare.

The DNA is made of the sugar backbone, the nucleotide bases and the phosphate group. The sugar of one nucleoside binds to the sugar of the adjacent nucleoside through a phosphodiester bond. There are basically four nucleotide bases, Adenine (A), Guanine (G), Thymine (T) and Cytosine(C).  Adenine and guanine are the Purines, and Thymine and Cytosine are the Pyrimidines.  

A DNA sequence looks some thing like this “ATTGCTGAAGGTGCGG”.  It is measured according to the number of base pairs it consists of, usually in kBp or mBp(Kilo/Mega base pairs).  Each base has its complementary base to which it binds through Hydrogen bonds. A is complementary to T, and G is complementary to C (A-T, G-C).  

The DNA of an organism is collectively called its genome. The size of the human genome is 3000 Mb.

The two strands run opposite to each other and are thus Antiparallel.  One is the sense strand, which is  also the template strand, and the other is called the anti-sense strand, which is the coding strand. 

During the production of RNA, the strand from which genes are transcribed, is called the template strand.

DNA has major and minor grooves due to its twisting helical structure. The major groove, is 22 Å wide and the minor groove, is 12 Å wide. These grooves can act as a binding site on the DNA.  Mostly DNA is a right-handed helix with 10-10.5 bases per turn. This is the B-form of the DNA. But alternative structures are also found, such as the A-form and the Z-form.

Supercoiling of DNA:

DNA can be twisted like a rope in a process called DNA supercoiling. With DNA in its “relaxed” state, a strand usually circles the axis of the double helix once every 10.4 base pairs, but if the DNA is twisted the strands become more tightly or more loosely wound.

If the DNA is twisted in the direction of the helix, this is positive supercoiling, and the bases are held more tightly together. If they are twisted in the opposite direction, this is negative supercoiling, and the bases come apart more easily.

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RNA: Ribonucleic acid

RNA was the first genetic material. It is different from DNA because it is single stranded and has the nucleotide Uracil instead of Thymine. This makes it relatively unstable than DNA, because Ribose (in RNA) has a hydroxyl group (OH) attached to the pentose ring in the 2′ position, whereas deoxyribose (in DNA) does not. This OH group makes the RNA to mostly take the A form geometry. 

In RNA, A is complementary to U, instead of T. U is also a pyrimidine. This means that during the process of production of RNA from DNA, the A’s are transcribes to U’s. RNA is broadly of three types: messenger RNA (mRNA), ribosomal RNA (rRNA) and transfer RNA (tRNA). We will discuss more about them in detail during transcription and translation.

The Central Dogma:

Production of RNA from DNA, by Transcription, and production of protein from RNA, by Translation, is termed as the Central Dogma.

The central dogma of molecular biology explains the flow of genetic information, from DNA to RNA, to make a functional product, a protein.  

First, DNA replication occurs, in which the helix separates and each strand is replicated to form daughter strands with the help of DNA polymerase enzyme.

In transcription, the template strand is read and an mRNA is produced with the help of RNA Polymerase.

During translation, the mRNA is ‘read’ by the RNA Polymerase, and amino acids polypeptide chain is formed with the help of the tRNA.

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