DNA is our source of genetic information, the material that allows both our genotype and phenotype to be passed on to the next generation. It is contained in every cell of our body and has caused major debate in scientific history.
Structure of DNA
DNA is the vital code for life in almost all complex life. It is store of genetic information that can be inherited from one generation to the next. DNA is found in the nucleus and has a complex structure which was only discovered in relatively recent times, most notably by scientists such as Rosalind Franklin, Maurice Wilkins, James Watson, and Francis Crick. The collective works of these scientists allowed us to begin to decipher the DNA code.
DNA is a two stranded double helical structure. The two strands are made up of nucleotides connected by phosphodiester bonds that run anti-parallel to one another. Nucleotides, the subunit of DNA include a pentose sugar which in this case it is deoxyribose, a phosphate group and a base.
The bases are hydrophobic, or repelled by water and so face inwards in the helix, away from the aqueous solution surrounding DNA in the nucleus. The strands are made up of the sugar-phosphate backbone which are hydrophilic and face outwards.
In DNA there are four types of bases, A-Adenine, T-Thymine, G-Guanine, and C-Cytosine. These bases are split into complimentary base pair that are attracted to one another, and biologically fit together. Therefore, adenine bases are always attracted to thymine bases, and guanine bases are always attracted to cytosine. This is because of hydrogen bonding between bases.
A and G are both purine bases and C and T are both pyrimdine bases. Chargaff’s Rule states that two purines, and two pyrimdines cannot bond, bases of the same type are not found paired.
The success of DNA relies on it’s ability to be replicated, repaired and it’s expression must be regulated. Mathew Meselson, and Franklin Stahl were the first to prove that DNA replication was semi- conservative. This means that both strands of a parent molecule of DNA are used as templates, resulting in each daughter DNA molecule to be made up on one new and one old strand of DNA.
When replication of DNA occurs in Eukaryotic cells there are two main stages, Transcription and Translation. As DNA is a complex and large molecule it cannot leave the nucleus and must create a small copy of itself to reach the rest of the the cell. This occurs in transcription, where DNA in the nucleus is unwound and unzipped so that one strand can attract the complimentary nucleotides of mRNA.
mRNA is a transport molecule similar to DNA accept that it is single stranded and therefore able to leave the nucleus and it contains a U-Uracil base instead of thymine.
So if the bases for DNA were; A T G T C, the complimentary mRNA bases would be;U A C U G. Before the mRNA leaves the nucleus all non-coding DNA or introns are removed.
The next stage of DNA replication is translation. The single stranded mRNA travels out on the nucleus to the surface of a ribosome where, here it will be translated to protein form. The mRNA binds to a start codon or promoter found on the ribosome. This process of protein synthesis is integral in replicating DNA.
tRNA another type of RNA contains complimentary anti-codons, to the codons carried by the mRNA. These are coded for by amino acids found within the tRNA structure. tRNA carries anti-codons to codons all along the mRNA strand until it reaches the stop, or terminator codon. This process produces a complete polypeptide.
Expression and Regulation of Genes
Error is DNA occur frequently and must be regulated to prevent major damage to the organism. These mistakes in coding can be created by common replication errors, such as deletion, substitution, or addition, that can cause shift frames if unregulated. This kind of mistake occurs at a level of 1 in every 10 to the -5. However another more sinister cause of DNA error is anthropogenic and can be caused by mutagens such as mustard gas, UV light and nitrous gas.
Luckily some proof reading or regulation occurs, for example if DNA polymerase inserts an incorrect base into the DNA sequence then the reaction of the polymerase will be stopped because the orientation of the complimentary anti parallel strand will be mismatched.
There are also some enzymes that exist to complete error after replication, particularly if the error distorts the secondary structure of DNA. Other method include; base-excision repair, and damage reversal.