As can be seen from the above table, the name of the nucleotide depends on which of the nucleosides it contains and where the phosphate is attached to the ring of the sugar. For example, a nucleotide which contains adenine and ribose, with the phosphate attached to the C-5′ of the ribose is adenosine-5′-monophosphate or AMP, where the 5′ indicates where on the sugar the phosphate is attached. In a polymer of nucleic acid, which is also known as a polynucleotide, covalent bonds called phosphodiester linkages join together nucleotides.
The name of the bonds refers to the position of the bond, with it occurring between the phosphate of one nucleotide and the sugar of another. In DNA, the nucleotides are joined by 3’5′ phosphodiester bonds. The 3’5′ is significant as it details that the bond is between the C-3′ of one sugar and the C-5′ of the other. Every nucleotide of DNA carries a negative charge. Joining the nucleotides together in this fashion produces the DNA “backbone” of alternating sugar and phosphate, and it is to this backbone the nitrogenous bases are attached.
With the backbone assuming this form, in the finished polymer, at one end of the chain is a free 3′-hydroxyl group and the other has a free 5′-hydroxyl group, thus giving the DNA strand polarity. The sequence of nitrogenous bases attached to the backbone always starts from the free 5′ end towards the other. While the actual structure of the backbone is consistent in all DNA, the sequence of the bases is unique to each and every gene, and all the information the cell needs from the DNA is contained in these bases. DNA actually consists of two of these strands intertwined with the bases on the inside.
The bases pair up, with adenine paired with thymine and guanine paired with cytosine. The result is a double helix. This helix is important in cell division, as when this time comes it separate into two strands, with each strand ending up in a different cell. With these single strands, another complementary strand can be replicated, thus forming two finished macromolecules. RNA is very similar to DNA in its structure, with the only real differences being that the sugar used is ribose and the base thymine is replaced by uracil.
This base uracil still pairs with adenine as thymine does. There are three different types of RNA found in a cell: Ribosomal (rRNA), Transfer (tRNA) and Messenger (mRNA) RNA. Ribosomal RNA is most abundant in the cell, accounting for 70-80% of the total RNA in the cell, and is found in the ribosomes. Ribosomes are where protein is synthesised. Messenger RNA carries the coded information from DNA to the ribosomes and effectively determines what the sequence of amino acids will be in a protein. Messenger RNA only accounts for a very small percentage of the total RNA, only about 3-4%.
The size of the mRNA polymers varies widely, depending on what information is to be carried and it is fairly short lived. Transfer RNA is the smallest of the three RNA molecules. It makes up around 20% of a cell’s total RNA and works as an adaptor in protein synthesis. Carbohydrates have three basic functions within a cell. They are biological fuel, they assume a storage form of food reserves and they are the structural components of cell walls. Carbohydrates can be subdivided into three groups: Monosaccharides, Disaccharides and Polysaccharides.
Monosaccharides are made up of single units and are used in metabolism and as the basis from which the other two groups are produced. They are polyhydroxyl compounds and contain either an aldehide or keto group. Which o these two groups it contains determines its name. An aldose, e. g. glucose, is a compound which contains aldehide, and a ketose e. g. fructose, contains a keto group. The second of the three types of carbohydrate, Disaccharides, are produced when two monosaccharides join together by a dehydration reaction.
The resulting bond is called a glycosidic bond. The two monosaccharides may be the same, or they may be entirely different. Finally, Polysaccharides are long chains of monosaccharides, which may be all the same, in the case of homopolysaccharides, or they may be different. Polysaccharides account for the majority of carbohydrates which are found in nature, act as a stored energy source, e. g. starch in plants and glycogen in animals, or take on a structural role, such as cellulose. Lipids are hydrophobic molecules, meaning that they do not mix with water.
Within the cell they are responsible for the structure of cell membranes, the storage and transport of food reserves, protection and play a regulatory role by vitamins and hormones. One particular form of lipids are phospholipids. These are found almost always in cell membranes, and are in fact the most important aspect of the membrane. Phospholipids are quite similar to fats,. Differing in that they only have two fatty acids chains joined to a phosphate group, rather than three as found in a fat. A variety of phospholipids can be formed via the linking of other smaller molecules to the phosphate group.
As previously mentioned, lipids are generally hydrophobic, but phospholipids are a bit different. The chains of fatty acids are indeed hydrophobic, but the phosphate “head” is not. This quality is particularly effective in the formation of cell membranes. When a phospholipid is added to water, the phosphate heads rearrange themselves to protect their hydrophobic fatty acid chains from the water. At a cell’s surface the phospholipids are set out in a bilayer with the phosphate heads towards the outside and the fatty acid chains pointing into the centre of the layer.
The type of phospholipid used varies between different types of cells, for example, in a nerve cell the phospholipids found in the cell membrane are usually Plasmalogens. It is clear that there is a hugely diverse population of molecules and macromolecules in a cell, each performing its own function. However, it would seem that frequently the success of one type of molecule or macromolecule is directly or indirectly determined by another, such is the equilibrium of a cell.