Biological Molecules: Structure and Function DNA

Structure:
DNA or deoxyribonucleic acid is a chain of nucleotide units consisting of a nitrogenous base, a deoxyribose sugar and a phosphate. There are 2 types of nitrogenous bases, namely, pyramidine and purine. In DNA, the pyramidines are cytosine and thymine and the purines are guanine and adenine. Each nitrogenous base is connected to a molecule of deoxyribose sugar to form a nucleoside. Each nucleoside is joined to a phosphate group to form a nucleotide. Repeating units of nucleotides form DNA. The structure of DNA is double helix. The edges on the outer side are formed by alternating phosphate groups and deoxyribose sugar molecules. The two strands of the helix run in opposite directions (1 "up" and 1 "down"). The inner edges of the helices are formed by the nitrogenous bases and they run in pairs. Adenine pairs with thymine with 2 hydrogen bonds and cytosine pairs with guanine with 3 hydrogen bonds (Murray et al, 2007).
Functions:
DNA is present in the chromosomes of most of the living beings. The human genome approximately has 3 billion base pairs of DNA which are arranged in the 23 pairs of chromosomes. Sequence pieces of DNA form genes which carry vital information. The genetic information is achieved by complementary base pairing. Transmission, transcription and translation are the steps involved in transmission of genetic information. Currently, DNA is used in the field of genetic engineering for the treatment of many diseases (Murray et al, 2007).
RNA Structure:
RNA or ribonucleic acid is similar to DNA except that it is single stranded and the sugar present in it is ribose sugar. RNA consists of repeating units of nucleotides which are made up of a nitrogenous base, a ribose sugar and a phosphate. The pyramidines are guanine and cytosine and the purines are adenine and uracil. RNA is transcribed from DNA by the enzyme called RNA polymerase. There are basically 2 types of RNA: coding RNAs and non-coding RNAs. Messenger RNA (mRNA) is a coding RNA. There are many non-coding RNAs. These include ribosomal RNA (rRNA), transfer RNA (tRNA) and others.
Functions:
mRNA carries information from DNA to the ribosome which is the site of synthesis for proteins. The amino acid sequence in the protein is based on the coding sequence of the mRNA. tRNA and rRNA are involved in the process of translation. Many other non-coding RNAs are involved in RNA processing, gene regulation, catalysis of chemical reactions, cutting and ligation of other RNA molecules, catalysis of peptide bond formation in the ribosomes and other roles (Murray et al, 2007).
Carbohydrates Structure:
Carbohydrates are organic compounds that are either ketones or aldehydes with many hydroxyl groups added. These hydroxyl groups are added one on each carbon atom that is not a part of either the aldehyde or the ketone group. Thus the general formula of a carbohydrate can be designated as (OH20)n. Here n is any number greater than 3. There are many carbohydrates like uronic acids and fucose which don't have this formula in their structure. Also, all chemicals which have this structure do not fall into the category of carbohydrates. The basic carbohydrate units are monosaccharides. These are further classified based on the number of carbon atoms, the placement of the carbonyl group and the chiral handedness. Monosaccharides with the carbonyl group aldehyde are known as aldoses, those with carbonyl group ketone are known as ketoses, those with three carbon atoms are known as trioses, those with four are called tetroses, five are called pentoses and six are hexoses. Each carbon atom with a hydroxyl group (except the first and last carbon atoms) is assymmetric making them stereocenters with either right-side configuration or left-side configuration. Thus many isomers can exist for any given monosaccharide formula. Glucose, galactose and fructose are monosaccharides. Monosaccharides can link together in many ways to form disaccharides and polysaccharides. One or more of the monosaccharide units in a polysaccharide can be modified by replacing or adding certain groups. Disaccharides have two monosaccharides. Sucrose and lactose are disaccharides. The monosaccharide units in these carbohydrates are bound together by a covalent bond known as glycosidic linkage. Oligisaccharides (between two to nine monosccharide units) and polysaccharides (more than nine monosaccharide units) are composed of many number of monosaccharide units bound together by glycosidic bonds. Disaccharides, trisaccharide raffinose and the tetrasaccharide stachyose are oligosaccharides. Amylose, amylopectin, glycogen, cellulose, chitin, callose, laminarin, xylan, galactomannan, fucoidan and mannan are examples of polysaccharides.
Function:
Carbohydrates have many roles in the living organisms. They store and transport energy, they form the structure of various organs, they also take up major roles in other functions like fertilization, blood clotting, development and immunity.
Lipids Structure:
Any fat-soluble, naturally occuring molecules are known as lipids. There are basically eight groups of lipids and each group has its own structure. These groups are: glycerolipids, fatty acyls, glycerophospholipids, saccharolipids, sphingolipids, polyketides, sterol lipids and prenol lipids. In fatty acyls, the carbon chain may be saturated or unsaturated and may be attached to certain functional groups containing nitrogen, oxygen, halogens and sulphur. Thromboxanes, leukotrienes and prostaglandins are examples of fatty acyls. Glycerolipids are composed of mono-, di- and tri-substituted glycerols. Triglycerides are good examples of these lipids. Phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine are examples of glycerophospholipids. In sphingolipids, a sphingoid base forms the backbone of the molecule. Cerebroside and ganglioside are examples of these lipids. Sterol lipids have fused four- ring core structure. Cholesterols fall into this category of lipids. Prenol lipids are synthesized fromthe 5-carbon precursors isopentenyl diphosphate and dimethylallyl diphosphate. Carotenoids are important prenol lipids (Murray et al, 2007).
Functions:
Lipids have many key biological functions. They are structural components of cells and cell membranes. They are sources of energy storage and behave as intermediates in the pathways of various biological signals.
Proteins
Structure:
Organic compounds which are made up of aminoacids in a linear chain are known as proteins. These aminoacids are joined together by peptide bond between the carboxyl and amino groups of adjacent amino acid residues. The sequence of amino acids in any given protein is based on the genetic coding of the gene which codes the protein. Most of the proteins have 3- dimensional structures. There are 4 aspects of protein structure. The primary structure is that of amino acid sequence. The secondary structure consists of regularly repeating local structures which are stabilized by hydrogen bonds. The tertiary structure is the spatial relationship of the secondary structures to one another and this structure is stabilized by non-local interactions like hydrophobic core, hydrogen bonds, salt bridges and hydrogen bonds. The quaternary structure results from interaction between protein subunits.
Functions: Proteins participate in almost all processes in the body. They catalyze various biological reactions which are vital for metabolism. They have structural and mechanical functions. They take part in immune responses, cell signaling, cell adhesion and also cell cycle.

References
Murray, R.K., Granner, D.K., Mayes, P.A., and Rodwell, V.W. (2007). Harper's Biochemistry. McGraw Hill: Singapore.

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