Btec Level 3 Unit 25 D2

The structure of DNA and the effect of point mutations (25 MARKS)
Deoxyribose Nucleic Acid (DNA) is a polynucleotide molecule that encodes the genetic instructions used in the development and functioning of all known living organisms and many viruses. Most DNA molecules are double stranded helices, consisting of two polynucleotide strands made up of simpler molecules known as nucleotides. A nucleotide is made up of an organic nitrogenous base, a deoxyribose sugar and phosphate groups. It is order of these bases which make up the genetic code; a set of rules, by which information is encoded within genetic material.
The organic bases that can form a nucleotide include Adenine, Thymine, Cytosine and Guanine, which
In this case, only one DNA triplet may be altered and so a frameshift does not occur. This substitution may result in a nonsense, mis-sense or silent mutation. A nonsense mutation will occur if the substitution results in the formation of one out of three of the stop codons. This will cause the synthesis of the polypeptide to be stopped prematurely and therefore the final protein will be very different from the protein that was originally intended to be synthesised. This new polypeptide will be shorter and is unlikely to function. An example of this is the non functional protein that causes albinism; a disease that means the organism cannot produce melanin, a characteristic selected against in animals due to highlighting the organism to predators. A mis-sense mutation is when a DNA base change results in a new mRNA codon that codes for a different amino acid. The new polypeptide chain will have one amino acid different to the normal form, which may not have a serious effect on the polypeptide but as the primary sequence of the amino acid is important in determining the tertiary and quaternary 3D structure of the protein, the shape of the protein may change. This is particularly important in enzymes, where the protein may not function normally as the active site shape may no longer be complementary to the substrate. An example of a mis-sense mutation can be seen in sickle cell anaemia. Sickle Cell Anaemia is an example of co-dominance as both alleles are expressed in the phenotype of heterozygote. A substitution of the T in base CTC causes the new codon to become CAC. This one amino acid is changed on the beta globulin chain of haemoglobin causing glutanic acid to become valine acid. This causes abnormal haemoglobin molecules to be produced, producing red blood cells that are sickle shaped. These red blood cells cannot carry as much oxygen to cells for respiration, resulting in less ATP being