Structure of DNA and RNA molecules

We know that all organisms produce offsprings of their own kind whether it is a single celled animal like Amoeba or a multicellular animal like a horse. Amoeba produces a daughter amoeba, and a horse produces a baby horse. All this is possible just because a very special molecule that is termed as deoxyribonucleic acid or DNA. The DNA contains the hereditary material which makes every individual unique and this material is transferred from the parents to the offsprings. The DNA is present in a special organelle of the cell called the nucleus. As the size of the cell is very small and each organism has many molecules of DNA so the DNA must be tightly packed inside the nucleus and this packed form of DNA is called as chromosome. DNA spends it most of the time inside the cell in the form of chromosome. During cell division, the DNA unwinds so that it can be copied and transferred to the daughter cells. DNA also carries instructions for protein synthesis so that other biological processes can be regulated normally. The DNA present inside the nucleus is termed as nuclear DNA and the complete set of nuclear DNA is designated as genome. Apart from its occurrence inside the nucleus, DNA is also present in the cell organelle named as mitochondria which are the power houses of the cells. During sexual reproduction the offsprings inherit half of the nuclear DNA from the father and half from the mother but the mitochondrial DNA is inherited completely from the mother as the sperm cells do not bear mitochondria after fertilization.

The DNA molecule was first observed in the late 1800s by a German biochemist Frederich Miescher. But nearly a century passed after that and the scientists couldn't succeed in unraveling the mystery of the DNA molecule. The mystery of the DNA was solved in 1953 by the eminent works of James Watson, Francis Crick, Maurice Wilkins and Rosalind Franklin. By using X-ray diffraction technique the scientists pointed out the double helical structure of DNA that encodes the genetic information of every organism living on this earth.

The chemical building blocks of DNA are called as nucleotides. The nucleotides are formed of three components: a phosphate group, a sugar and one of the four types of nitrogenous bases. To form a complete strand of DNA nucleotides are linked in the form of chains with alternating arrangement of phosphate groups and sugars. The four types of nucleotide bases that form DNA are adenine (A), guanine (G), cytosine (C) and Thymine (T). The arrangement of these nitrogenous bases within a DNA molecule is very specific. The adenine can always pair with thymine on one side of the DNA helix and cytosine can also pair with guanine on one side of the DNA helix. This specific arrangement of base pairs in a DNA strand follows a rule called as Chargaff's rule which plays a very important role in the replication of the DNA molecule.

The process of DNA replication proceeds after the breaking of the weak chemical bonds between the two poly nucleotide chains by an enzyme. The DNA strand breaks in the middle separating the base pairs. These newly separated strands now work as templates from which the new strands of DNA will be obtained. Inside the nucleus many extra nucleotides are present. The bases first bond with the bases present on the template which will match just according to the Chargaff's rule. When the base pairing is completed the phosphate groups and the sugar is added to form other poly nucleotide chain. This procedure is repeated with both the template strands of DNA. The whole process is repeated thousands of time in order to form the two molecules of DNA which are exactly the replicates of the original DNA molecule and all this happens during mitosis so the daughter cells receive the exact similar character of the DNA. When an error occurs during the process of DNA replication mutation occurs. The mutation causes either deletion or addition of base pairs and the proteins also get defected by having wrong pairs of amino acids.

One of the important functions of DNA is protein synthesis. The process of protein synthesis is completed in two steps. The first step is transcription and the second step is translation. In transcription the cell uses the information from a gene in order to form a protein. Both the DNA and RNA molecules are similar in structure except the fact that RNA is shorter than DNA and bears the sugar ribose instead of deoxyribose that is present in DNA. RNA also differs from DNA in having a base uracil in place of thymine. During transcription, the type of RNA that is created is called mRNA or messenger RNA because it is used as a "messenger" to send information from a gene on DNA to a ribosome so that protein can be created. RNA polymerase recognizes and attaches to a DNA nucleotide chain at the beginning of the gene, at a place called the promoter. The promoter positions the RNA polymerase on the right strand of DNA and guides it to the right direction. As the RNA polymerase moves, it creates a new chain from the extra nucleotides. The RNA polymerase continues until it reaches a stop signal at the end of the gene. The RNA polymerase then detaches itself from the DNA and the RNA chain is released, creating mRNA.

When the mRNA sends the information from the DNA to the ribosomes it is converted into the language of amino acids. When amino acids are formed a protein is created. The mRNA transfers the information from the DNA to the ribosomes in the language of nucleotides. The ribosomes attach on a particular place on the mRNA which is called start codon which is made up of three nucleotide bases indicating that it is ready to read a message. The amino acids that will later form protein come across the transfer RNA or tRNA while they are attached to the ribosomes. The tRNA moves the amino acids along the mRNAs so that the message can feed across the ribosome. There the amino acids are all linked together to form a protein chain. Every organism on uses this process to make proteins.

Thus, it can be concluded that DNA is very essential component of an organism's life.