Information Strand Of The Dna Molecule example essay topic

DNA, deoxyribonucleic acid, is the genetic molecule. It carries all the genetic information of a living organism and is what distinguishes a human being from another species, after all we are all made up out of the same types of chemicals: proteins, lipids and carbohydrates, which form cells. It is the information carried in the nuclei of these cells which instruct these cells how to behave and grow together to form a complete living organism and which type of reaction to carry out. DNA carries the information in a code which describes the organism and allows this information to be passed on to the next generation of cells so each cell can carry an exact replica of the information code. DNA is a macro-molecule composed of many smaller molecules to form a polymer.

These molecules are called nucleotides which are made from a phosphate group, a pentose sugar (in the case of DNA deoxyribose) and a nitrogenous base. The sugar and phosphate group bond by a phospho-dieter bond to form a sugar phosphate backbone. Diagram of a nucleotide showing the tri-phosphate group attached to C 5 and the nitrogenous base attached to C 1 The bases are what the code is "written" in, there are 4 different bases which are divided into two groups, the purines: adenine and guanine and the pyrimidines: thymine and cytosine. DNA is a double alpha helix held together by hydrogen bases, adenine always bonding with thymine and guanine always bonding with cytosine. Diagram showing structure of nitrogenous bases and how they bond together DNA can be made from many million nucleotides, in the case of mammalian cells up to 2.5 billion units. This means the sequence of bases is almost infinite, providing an immense store of genetic information.

Diagrams showing a) the polymerization o nucleotides by condensation and b) the formation of a double stranded DNA molecule DNA is involved in the synthesis of proteins, the human body manufactures 20 amino acids which together with the essential amino acids obtained through diet are synthesised to form proteins. DNA controls the chemical reactions in the cells which are known to be activated by enzymes, Enzymes are proteins and DNA is a code for exactly which polypeptides and proteins are manufactured. DNA controls protein structure by determining the exact order in which the amino acids join together when proteins are synthesised. As we know it is the interactions between the R groups of amino acids which determine the protein's 3 D structure, DNA controls the exact order of the amino acids. The code is carried in the information strand of the DNA molecule, the other strand is made from the complimentary bases to the information strand bonded to an identical sugar phosphate backbone and contains no code. The information has to code for 20 amino acids so cannot be made from single bases as it would only code for 4 amino acids, nor can it be from paired bases as it would code for 16 amino acids.

Instead the code is made from triplets of bases which have enough combinations to code for 64 amino acids, as the body only needs to manufacture 20 amino acids several codes can form one amino acid so the code is said to be degenerate. Each triplet of the code is known as a "codon" The importance of this code is that it can be replicated time and time again so the information can be passed on to new generations of cells as well as being used to synthesis proteins which carry out and control reactions that occur in the cell. The replica of the information must be an exact copy of the original DNA otherwise the information passed on will be inaccurate and the cell will not carry out the correct tasks and will be a mutation. To form an exact copy of itself DNA is unwound by an enzyme, DNA Gyrose, into two strands with unpaired bases. Each chain acts as a template to form a new complimentary strand along side it. As the bases are specific about with base can hydrogen bond with each other, T in the original strand will only lie along side A in the new complimentary strand and so on.

As the appropriate nucleotide is brought into place it is joined to the growing molecule by DNA Polymerase which also proof reads the growing strand to make sure it is accurate. DNA is replicated always in the same direction, from 5 to 3, this means that the anti-parallel strand of DNA cannot be replicated from 3 to 5 and so replication occurs in short strips which are then polymerized together to form the new complimentary stand by another enzyme known as ligase. Diagram of DNA replication The final result is two DNA molecules, each made up of one newly synthesised chain and one chain which has been conserved from the original molecule. The process of replication is therefore known as semi-conservative replication. Protein synthesis requires the information in DNA to be transferred into the code for proteins to be manufactured. Protein synthesis cannot occur in the nucleus of the cell as the proteins made can be too big and the ribosomes where the synthesis occurs are found in the cytoplasm of the cell.

DNA is found in the nucleus of a cell (with the exception being that DNA is found in mitochondria and chloroplasts) and not in the cytoplasm, so for protein synthesis to occur DNA transcribes a single complimentary strand known as messenger RNA. The main differences with mRNA and DNA is that RNA has ribose sugar instead of deoxyribose sugar, is single stranded and contains the base uracil instead of thymine. To form mRNA the DNA unwinds as in replication but only one strand is copied. The enzyme RNA polymerase moves along the DNA adding the complimentary RNA nucleotides to the DNA template. mRNA then leaves the nucleus through a nuclear pore into the cytoplasm.

The mRNA contains the correct sequence of codons to manufacture proteins. Diagram showing transcription of mRNA from DNA In the ribosome the mRNA is translated into the correct protein, a specific sequence of amino acids are formed to compliment the codons on the strand of mRNA. The amino acids are combined with a transfer RNA molecule which bind with a specific amino acid. The ribosome acts as a framework which holds the mRNA and tRNA together until the two amino acids form a peptide bond between each other. Once combined the ribosome will move along to the next codon written on the mRNA and the next amino acid is bonded to the growing peptide chain. Eventually the mRNA will contain a code that does not have an appropriate amino acid, this is one of the "stop" codes which terminates the protein synthesis and the polypeptide chain is released into the cytoplasm where it is assembled into a protein.

The mRNA is easily broken down as it is unstable due to it's single stranded structure. Could the genetic information of an organism be contained in RNA rather than DNA In theory, yes. RNA is a copy of DNA containing the genetic code which is used for the synthesis of proteins, if only RNA existed proteins would still be manufactured. Yet RNA is a much less stable molecule to DNA, this is because of DNA's double helix structure, the single stranded RNA is much easily broken and would suffer considerable damage by the time it would be repaired by RNA polymerase where as DNA would have to have both of it's strands broken at the same time to suffer that amount of damage. As DNA has two strands the one strand would effectively hold the complimentary strand until it was repaired. Due to RNA's lack of stability the molecule would not be able to be as large as the DNA molecule which can contain many million nucleotide units.

Replication of DNA is accurate and effective due to its semi-conservative nature and can occur in small sections down the DNA molecule. RNA replication is smaller and less efficient and would have to occur twice, once to generate the RNA template strand from RNA nucleotides, then again to form an exact replica. DNA's stability also makes it a stronger, larger molecule.