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