From this structure of DNA, which was worked out after analysing the x-ray crystallography patterns obtained by Rosalind Franklin, (who died of cancer in 1958 age 37, four years before Maurice Wilkins was awarded the Nobel Prize for the discovery of the double helix) (3), they concluded that the bases always pair in the same way with adenine pairing with thymine and cytosine pairing with guanine. The bases will pair up differently in RNA as there is no thymine, as it is replaced with uracil.
DNA is only found in the nucleus and makes up only 15% of each chromosome; the other 85% are made up of protein. It is made up of two chains a bit like a ladder, the sides of the ladder comprise of alternating sugar and phosphate groups while the rungs of the ladder are made up of the bases. These bases are held together by weak hydrogen bonds, two bonds holding adenine to thymine and three bonds holding cytosine and gunaine together. This structure is twisted into a helix and because of the ladder shape was given the name of ‘the double helix’. The hydrogen atoms on one base are attracted to oxygen and nitrogen atoms on another. The shapes and sizes of the bases mean that correct distance between the two sugar-phosphate backbones can only be maintained by adenine-thymine and cytosine-guanine bonding which produces a regular and stable DNA molecule.
DNA molecules are large in comparison to other molecules and give an advantage to an organism because it means that a vast number of different genes are confined to a fairly small number of DNA molecules. This helps in the passing on of information from one generation to the next during cell division.
(2)
The twisting of the strands ensures that the weak hydrogen bonds linking the bases together are protected in the centre of the molecule, which prevents the code being corrupted by other chemicals present in the nucleus. When a cell replicates to enable the organism to grow, an exact copy is made of each chromosome. The hydrogen bonds holding the two strands of the helix together break, so the strands are no longer joined. As the bonds break the two halves of the molecule separate. This is described to be like a zip unzipping and each half of the molecule acts as a template on which a new strand of DNA can be built. The free nucleotides enter the nucleus from the cytoplasm and convene on the template of the DNA according to the pattern of base pairing. The nucleotides containing adenine join up with thymine, while cytosine pair up with guanine. The enzyme DNA polymerase joins the new nucleotides together, forming a new sugar phosphate spine.
With DNA being able to copy itself, it is also a store of genetic information. The coiled strand of DNA, which makes up each chromosome, can be divided up into shorter sections known as genes. The length of DNA that makes up a particular gene carries information needed to make a certain protein known as genetic coding.
RNA plays an important role in replication and there are three different types of RNA:
(m RNA) messenger RNA, (r RNA)0 Ribosomal RNA, and (t RNA) Transfer RNA. RNA carries out three main jobs in the cell, which is the reason for the three different types. It carries out instructions about which proteins are made, picks up specific amino acids and makes up the bulk of the ribosomes. This makes master copies of the original DNA and then uses these copies to produce enough protein that a cell needs.
Every strand of DNA has exons (pieces that contain genetic information) and introns (no relevant genetic information). Although the introns hold no genetic information, they do contain repeated sequences of base pairs called Variable Number Tandem Repeats (VNTR) and can contain from twenty to one hundred base pairs. VNTR patterns are inherited genetically, so the more VTNR probes used the more distinctive and individual the DNA will be.
In genetic fingerprinting, which was developed by Professor Alec Jeffreys (5), DNA was examined from a person or body fluid sample. Enzymes (restriction enzymes) identify sections of DNA with highly repetitive sequences (VNTR) of bases cut the DNA at these points. These DNA samples are separated according to size by agarose gel electrophoresis. This exposes the fragments of DNA to an electric current in a trough of gel. The DNA is then negatively charged and will move towards the anode (positive terminal) when an electric field is used. The DNA fragments then become a single strand and are transferred to a nylon membrane by a process called Southern Blotting (named after Edward Southern, 1975) (4). Radioactive DNA probes are used to attach to specific parts of the fragments. Any probe sequences that have not bound to the DNA fragments are washed off and the membrane is dried. The nylon sheet is then placed under x-ray film and the radioactive probes expose the fragments, which produces a pattern of light and dark similar to a barcode.
(1)
All this information is used by isolating DNA from blood, hair, skin cell or any other genetic evidence left at the scene of a crime, through VNTR patterns from a criminal suspect to determine their innocence or guilt. This information could also be used in identifying a body found that is decomposed and cannot be identified.
This does not give a definite assured explanation, as DNA fingerprinting is not 100% accurate. The VNTR pattern gives a probability that the person in question is indeed the person whom the VNTR pattern belongs. With the patterns varying in each individual the probability could be 1 in 20 million, which could be said that it is easily matched, the probability could only be 1 in 20 which in that case would leave a large amount of doubt to the owner of that specific VNTR.
This gives argument on the implications and ethics of the reliability of using this method along with technical difficulties that can occur. When the DNA sample is very small this can be unreliable, as there is not much room for error. If the analysis of the sample involves amplification of the sample (creating a larger sample of genetically identical DNA from what little material is available), and when the wrong DNA is amplified the consequences can be profoundly harmful. Error in the hybridization and probing process must also be taken into consideration as if the sample of DNA is not collated in the correct manner and sealed at the scene then it can be said that it is not reliable.
References:
- Collins Advanced Molecular Sciences
Biology AS Second Edition Mike Bailey and Keith Hurst
(2) Advanced Biology for You Gareth Williams
(3) Heinmann Advanced Science Biology Ann Fullick
- Notes taken in class
(5) Internet
