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Nucleic Acids

Nucleic acids are large organic compounds found in the chromosomes of living cells and viruses. They are strong acids found in the nucleus of the cells. The nucleic acid polymers are with high molecular weights as high as 100,000,000 grams per mole. With proteins, nucleic acids are most important biological macromolecules. They are found in abundance in all living cells. 

History

In 1869, Friedrich Miescher isolated nuclei from pus cell and found that they contained phosphate-rich substance, he named it nuclein.

In 1899, Altmann, introduced the term nuclei acid. Fischer in the 1880s, discovered purine and pyramidine bases in nucleic acids. Zacharis in the year 1881, identified nuclein with chromatin. In 1884, Hertwig claimed that nuclein is responsible for the transmission of hereditary characters. In 1941, Caspersson and Brachet, related that nucleic acids were connected to protein synthesis. In 1944, Oswald T. Avery, Colin M. MacLeod and Maclyn McCarty, experimented that DNA is directly involved in inheritance. In 1953, James D. Watson and Francis H.C. Crick constructed the double helical model for the DNA molecule.

 

Nucleic Acid Definition

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Nucleic acids are essential large biological molecules for all forms of life. The nucleic acids include the DNA and the RNA. They are the hereditary determinants of living organisms. They are present in most living cells either in free state or bound to proteins as nucleoproteins. The nucleic acids are biopolymers with mononucleotides ad their repeating units. The monomers are known as nucleotides, they are made up of three units: a sugar, an amine and a phosphate group.

Structure of Nucleotide

Properties of Nucleic Acids

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Properties of nucleic acids are as follows:
  • Nucleic acids (DNA and RNA) are long polymers made of repeating units of nucleotides. 
  • Nucleotide units are made of phosphate-sugar-nitrogenous base units.
  • The nitrogenous bases found in DNA are adenine, guanine, cytosine and thymine. 
  • Adenine and guanine are purine bases while cytosine and thymine are pyrimidnes.
  • In RNA, the thymine bases id replaced by uracil which is also a pyrimidine. 
  • The nucleotides are linked with phosphodiester bonds. 
  • They are linked by a phosphate group on the 5th position of sugar residue becomes linked to 3' hydroxyl group of the preceeding sugar molecule.
  • The double stranded model of the DNA was worked out by Watson and Crick in 1953.
  • The double helix model consists of two strands wound around a central axis with the bases stacked inside. 
  • The order of the strand is in opposite directions, i.e., from 5' to 3' direction in one and 3'to 5' direction in the other. 
  • The bases stacked in the center of the helix as they interact with each other through weak hydrogen bonds.
  • Hydrogen bonds are weaker than covalent bonds, they are continually forming and disassociating. 
  • In the double stranded nucleic acids, the adenine form hydrogen bonds only with thymine (or uracil) molecule. While cytidine will only form hydrogen bonds with guanine. 
  • Hence, in a a given strand of DNA, the amount of adenine is always equal to the amount of thymine, and the amount of cytidine always equals the amount of guanine, in a given species.
  • The per cent of the G-T and the A-T is variable from species to species. 
  • The base pairs form a flat plain in the helix, the adenine forming two hydrogen bonds with thymine, and the cytidine forming three bonds with guanine. 
  • Using the concept of base pairing, all the enzymes and substrates necessary, the two DNA strands when copied separately, wherever there is adenine in the original strand, the duplicated strand will have thymidine, and guanine would be matched to cytosine. 
  • After replication, each original strand (parent strand), is paired with a new (daughter) strand. This type of replication is known as semi-conservative mode of duplication. 
  • The double stranded DNA can be denatured by alkaline conditions or heat.
  • Small stretches of polynucleotides will anneal to larger single stranded DNA molecule if the DNA sequence matches, by base pairing. 

Types of Nucleic Acids

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Nucleic acids are of two types DNA and RNA

Nucleic Acid Types

DNA (deoxyribonucleic acid)
  • DNA is one of the macromolecules, they are essential to all living forms. 
  • Deoxyribonucleic acid contains the genetic information, it is used in the development and functioning of all living organisms. 
  • The DNA segments carry genetic information are called the genes. 
  • Other DNA segments have structural functions or regulate the genetic information. 
  • DNA are made of two chains made of polymer units of nucleotides.
  •  The backbones of DNA are made of sugar and phosphate groups which are joined by ester bonds.
  • The two strands of DNA are anti-parallel, they run in opposite directions.
  • Each sugar molecule is attached to one of the four nucleobases. 
  • The nucleobases encode genetic information, that is read using the genetic code. 
  • Inside the cell, the DNA are arranged in long structures called chromosomes.
  • The chromosomes are duplicated in the process of DNA replication, during cell division.
  •  Each cell has its own one complete set of chromosomes. 
  • In eukaryotic organisms, most DNA is stored in the nucleus of the cell, and also some of it in cellular organelles like mitochondria or chloroplast. 
  • The prokaryotes store the DNA in the cytoplasm.
  •  Chromatin proteins like histones compact and organize the DNA
RNA (ribonucleic acid)
  • The functions of ribonucleic acid is to convert genetic information from genes into amino acid sequences of protein. 
  • In some viruses, RNA contains the genetic information. 
  • RNA is of three types, they are tRNA (transfer RNA), mRNA (messenger RNA) and rRNA (ribosomal RNA).
  • Messenger RNA, as the name suggests acts a messenger. It carries genetic information sequences between DNA and ribosomes, and it also directs protein synthesis.
  • rRNA is a major component of the ribososmes, they catalyze the formation of peptide bond.
  • The tRNA act as a carrier molecule for the amino acids that are used in protein synthesis. The tRNA are also responsible for decoding the mRNA.

Structure of Nucleic Acids

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Structure of nucleic acids DNA and RNA are similar. The structure is divided into four different levels, primary, secondary, tertiary and quaternary. 

Components of Nucleic Acids

Primary Structure

Primary structure of nucleic acids is a linear sequence of nucleotides, which are linked to each other by phosphodiester linkages. Nucleotides are made up of three components - Nitrogenous base, 5-carbon sugar and phosphate groups. 
Nitrogenous base are purines(adenine, guanine) and pyrimidines {cytosine, thymine (present in DNA only), uracil (present in RNA only)}. The 5-carbon sugar is deoxyribose for DNA and and ribose sugar in RNA. The purine bases, form glycosidic bond between their 9' nitrogen and the 9' - OH group of the sugar molecule. The pyrimidine bases, they form glycosidic bond between 1' nitrogen and the 9' -OH of the deoxyribose. In both purine and pyrimidine bases the phosphate group forms a bond with the sugar molecule between one of its negatively charged oxygen groups and the 5' -OH of the sugar. Nucleotides forms phosphodiester linkages between the 5' and 3' carbon atoms, these form the nucleic acids. Nucleotides sequences are complementary to one another.
 
Example of complementary sequence AGCT is TCGA. 

Primary Structure Nucleic Acid
Secondary Structure

Secondary structure is the interaction between the bases. This structure shows parts of which strands are bound to each other. The two strands of DNA in the double helix of the DNA are bound to each other by hydrogen bounds. The nucleotides on one strand base pairs with the nucleotides of the other strand. The secondary structure of the DNA is predominantly the base pairing of the two polynucleotide strands forming a double helix. 

Secondary Structure Nucleic Acid

Tertiary Structure

Tertiary structure is the three dimensional shape into which the entire chain is folded. Tertiary structure arrangement differs in four structural forms:
  1. Left or right handedness.
  2. Length of the turn of the helix.
  3. Number of base pairs per turn.
  4. The difference in size between major and the minor groove. 

Quaternary Structure


Quaternary structure is the higher-level of organization of the nucleic acids. This structure refers to the interactions of the nucleic acids with the other molecules. The most commonly seen organization is the form of chromatin which shows interaction with small proteins histones. 

Functions of Nucleic Acids

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Functions of nucleic acids are:
  • The main functions is store and transfer genetic information. 
  • To use the genetic information to direct the synthesis of new protein. 
  • The deoxyribonucleic acid is the storage for place for genetic information in the cell. 
  • DNA controls the synthesis of RNA in the cell. 
  • The genetic information is transmitted from DNA to the protein synthesizers in the cell.
  • RNA also directs the production of new protein by transmitting genetic information to the protein building structures.
  • The function of the nitrogenous base sequences in the DNA backbone determines the proteins being synthesized.
  • The function of the double helix of the DNA is that no disorders occur in the genetic information if it is lost or damaged. 
  • RNA directs synthesis of proteins. 
  • m-RNA takes genetic message from RNA.
  • t-RNA transfers activated amino acid, to the site of protein synthesis.
  • r-RNA are mostly present in the ribosomes, and responsible for stability of m-RNA.
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