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Describe the logic behind the system for storage, expression and transmission of genetic information?
Compare organisation of the genome in microorganisms and eukaryote. DNA is the key molecule that is responsible for the storage, expression and transmission of genetic information. A single DNA molecule consists of two long polypeptide chains known as DNA strands and each strand is made up of four different nucleotide subunits. Nucleotides found in DNA consist of a five carbon sugar (deoxyribose) which is covalently attached to one phosphate group and a nitrogen base. The covalent bonds that occur are essential as it ensures stability in the two strands. The base that is attached to the deoxyribose may be adenine, cytosine, guanine or thymine. Adjacent nucleotides on the DNA strand form phosphodiester bonds between the phosphate group on the 5 th carbon on one dexyribose sugar and the hydroxyl group on the 3rd carbon on the adjacent nucleotide. This forms a backbone of alternating sugar and phosphate. The two DNA strands found in a DNA molecule are held together by hydrogen bonding that occurs between the bases on the two different strands. In each strand nucleotides with a base T form two hydrogen bonds with nucleotides with base A on the opposite strand whereas nucleotides with base G form three hydrogen bonds with nucletodies with base C. As a result of this complementary base pairing the two sugar phosphate backbones twist around each other to form a double helix as this is the energetically most favourable arrangement. This structure results in the sugar-phosphate backbones being on the outise and the bases being protected in the inside. Another key point about the DNA structure is that the two DNA strands which make up a DNA molecule are antiparallel to each other. The main reason why DNA is suited to its role of storage is because it is a very stable molecule which can be passed from generation to generation. Like RNA it has a phosphodiester bridge which is negatively charged and this repels nucleophilic species such as hydroxide ions but a key difference between the two molecules is that DNA does not have a hydroxyl group attached to the second carbon atom. This absence gives the DNA molecule increased stability and resistance to hydrolysis which makes it more suitable at storing the genetic information than RNA. This structure of the DNA is stabilised by two different bonds; covalent bonds that occur within the nucleotide and hydrogen bonding that occurs between the complementary bases pairs. Individually the hydrogen bonds are relatively weak but the combined action of many hydrogen bonds makes a significant contribution to the stability of the structure and is also important in maintaining the double helix. As hydrogen bonds are easily broken and remade at physiological temperature they also have an important functional role in the replication of DNA as it allows the strands to be separated. The double helical structure also provides protection because the sugar phosphate backbone protects the DNA bases which contain the gentic information from being attacked. In eukaryotes the DNA is stored within the nucleus and this allows the nuclear and cytosolic enzymes to be kept separate which is vital for the functioning of eukaryotic cell as it prevents other enzymes interfering. The compartmentalisation of the DNA within the nucleus also increases the efficiency of reactions that are involved with the DNA because all the enzymes and substrates are concentrated within one place. The double stranded DNA is coiled around a central core of eight histone proteins to form a nucleosome. Adjacent nucleosomes are then attached to each other to form a chain which twists and coils to form a structure known as the chromosome. This packaging allows a large amount of genetic information to be stored in a relatively small space in the nucleus. The sequence of bases found within a DNA molecule encodes for the genetic information which is vital for the development and functioning of living organisms. The genetic information is stored in sequence of bases which forms a gene and the gene codes for proteins. The genetic code stored in the DNA is interpreted by gene expression. Within a cell the instructions stored within the DNA are read and
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