Nucleic acid

Definition

noun
plural: nucleic acid
nu·cle·ic ac·id, nuˈkleɪ.ɪk ˈæsɪd
Any of the group of complex compounds consisting of linear chains of monomeric nucleotides whereby each monomeric unit is composed of phosphoric acid, sugar and nitrogenous base, and involved in the preservation, replication, and expression of hereditary information in every living cell

Details

Overview

A biomolecule refers to any molecule that is produced by living organisms. As such, most of them are organic molecules. The four major groups of biomolecules include amino acids and proteins, carbohydrates (especially, polysaccharides), lipids, and nucleic acids. A nucleic acid refers to any of the group of complex compounds made up of linear chains of monomeric nucleotides. Each nucleotide component, in turn, is made up of phosphoric acid, sugar, and nitrogenous base. Nucleic acids are involved in the preservation, replication, and expression of hereditary information. Two major types of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

History and terminology

The discovery of nucleic acids was credited to the Swiss physician and biologist, Friedrich Miescher 1844 –1895, in 1868. He was able to isolate a biological molecule that was neither a protein, nor a carbohydrate, nor a lipid from the nuclei of white blood cells. He named the compound nuclein based on where he derived it from.¹ The acidic properties of the compound were discovered by the German chemist, Albrecht Kossel 1853 –1927. He was also known to be the first to identify the nucleobases: adenine, cytosine, guanine, thymine, and uracil. Later, nuclein was replaced with nucleic acid; the term was coined in 1889by the German pathologist, Richard Altmann 1852 –1900.² The nuclein discovered by Miescher was later particularly identified as DNA. The double helical model of DNA was attributed to the molecular biologists James Watson (American) and Francis Crick (British) in 1953. Their double-helical model of DNA was based largely on the X-ray diffraction image (referred to as Photo 51) by Rosalind Franklin 1920 – 1958 and Raymond Gosling in1952.

Structure

A nucleic acid is a biopolymer composed of monomeric units of nucleotides. Each nucleotide that makes up a nucleic acid is comprised of phosphoric acid, sugar (5-carbon), and nitrogenous base. The chains of nucleotides in a nucleic acid are linked by 3′, 5′ phosphodiester linkages. This means that the 5′-phosphoric group of one nucleotide is esterified with the 3′-hydroxyl of the adjoining nucleotide.

Types

Two major types of nucleic acids are DNA and RNA. DNA is a double-stranded nucleic acid containing the genetic information of a living thing. It is essential for the cell growth, division, and function of an organism. RNA is a single-stranded nucleic acid except for some viral RNAs and siRNA that are double-stranded.

	DNA	RNA
Structure	DNA is composed of two strands that twist together to form a helix, forming a ladder-like structure. Each strand consists of alternating phosphate (PO4) and pentose sugar (2-deoxyribose), and attached on the sugar is a nitrogenous base, which can be adenine, thymine, guanine, or cytosine. In DNA, adenine pairs with thymine and guanine with cytosine. Not all DNAs are double-stranded. For instance, a group of viruses have single-stranded DNA genome.	RNA consists of a long linear chain of nucleotides. Each nucleotide unit is comprised of a sugar, a phosphate group and a nitrogenous base. It differs from DNA in having ribose as its sugar, (deoxyribose in DNA) and the bases are adenine, guanine, cytosine, and uracil. In RNA, adenine pairs with uracil and guanine with cytosine. RNAs are single-stranded except for certain viruses whose genome consists of double-stranded RNA.
Location	In eukaryotes, most DNAs are located in the nucleoli and chromosomes in the nucleus. A small fraction of the total DNA is present in mitochondria, chloroplasts, and cytoplasm. In prokaryotes and viruses, DNA is found in the cytoplasm.	In eukaryotes, RNA is found in the nucleus and in the cytoplasm. In prokaryotes and viruses, it is found in the cytoplasm.
Function	DNA is a long polymer of nucleotides to code for the sequence of amino acid during protein synthesis. DNA carries the genetic ‘blueprint’ since it contains the instructions or information (called genes) needed to construct cellular components like proteins and RNAs.	In some viruses, RNA is the genetic material. For most organisms, RNAs are involved in: protein synthesis (e.g. mRNA, tRNA, rRNA, etc.), post-transcriptional modification or DNA replication (e.g. snRNA, snoRNA, etc.), and gene regulation (e.g. miRNA, siRNA, tasiRNA, etc.).

Common biological reactions

Common biological reactions

DNA replication is a process whereby the original (parent) strands of DNA in the double helix are separated and each one is copied to produce a new (daughter) strand. This process is said to be semi-conservative since one of each parent strand is conserved and remains intact after replication has taken place. Several enzymes, e.g. DNA polymerases, are involved in DNA replication. One of the parental strands of the DNA molecule is replicated by base pairing so that the newly synthesized strand would be complementary to the original or parent strand. That is the purine nucleobase (i.e. adenine and guanine) is paired with the pyrimidine nucleobase (i.e. cytosine and thymine). In particular, the adenine will be paired with thymine while guanine with cytosine. DNA replication is necessary in cell division. In the early stages of mitosis (prophase) and meiosis (prophase I), DNA is replicated in preparation for the late stages where the cell divides to give rise to two cells containing identical copies of DNA. After replication, copies of DNA molecule are checked by proofreading mechanisms. DNA replication can be carried out artificially through a laboratory technique called polymerase chain reaction that can amplify the target DNA fragment from the genome.

Common biological reactions

DNA carries the genetic information that codes for a particular protein. Thus, during protein translation, the genetic code for a protein is first copied into the RNA (specifically, mRNA). This process of creating a copy of DNA into mRNA through the help of the enzyme RNA polymerase is called transcription. Although RNA polymerase traverses the DNA template strand from 3′ → 5′, the coding (non-template) strand is usually used as the reference point. Hence, the process proceeds in the 5′ → 3′ direction, like in DNA replication. However, unlike DNA replication, transcription does not need a primer to start and it uses base pairing to create an RNA copy containing uracil instead of thymine.
In prokaryotes transcription occurs in the cytoplasm whereas in eukaryotes it takes place primarily in the nucleus before the mRNAis transported into the cytoplasm for translation or for protein synthesis.

Common biological reactions

The degradation of nucleic acids yields purines, pyrimidines, phosphoric acid, and a pentose, either D-ribose or D-deoxyribose.

Biological importance

Nucleic acids contain the genetic information crucial for all cellular functions and heredity. Mutation in the genetic code could lead to metabolic disorders and diseases. Many of such disorders are due to a supposedly functional protein that apparently is insufficiently produced or has turned dysfunctional due to a mutation in the gene(s) coding for it. Many metabolic disorders and diseases are heritable since genes are passed on across generations. On the other hand, mutations are also necessary, evolutionary speaking. They increase variability of living things, enabling them to better adapt to the similarly changing environment.

Supplementary

Derived term(s)

• deoxyribonucleic acid
• ribonucleic acid
• Minus-strand nucleic acid
• Repetitive sequences nucleic acid
• In situ nucleic acid hybridization

Further reading

See also

• biomolecule
• nucleotide
• nucleoside
• nucleobase
• gene
• chromosome
• nucleoprotein

Reference

1. “nucleic acid”. (2014). Retrieved from ://www.nature.com/scitable/definition/nucleic-acid-274 Link
2. Gribbin, J. (2002). The Scientists: A History of Science Told Through the Lives of Its Greatest Inventors. New York: Random House. p. 546. ISBN 0812967887.

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