DNA Essay, Research Paper

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-diester bond to form a sugar

phosphate backbone. ? Diagram

of a nucleotide showing the tri-phosphate group attached to C5 and the

nitrogenous base attached to C1The

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

pyrimidenes: 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 polymerisation of nucleotides by condensation and b) the

formation of a double stranded DNA?

moleculeDNA 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 3D 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 synthesise 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 Gyrase, 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 polymerised 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 DNAIn 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