DNA

Biochemical Processes and Forensic Applications.

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Revision of DNA

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Biochemical Processes

• Sequences of nucleotides code for specific traits/characteristics

• These individual traits are at the same position on a chromosome in everyone (unless a large mutation has occurred)

• The different traits for a specific gene are called alleles– e.g. Blue, green and brown eyes are different

alleles for eye colour.3

Biochemical Processes (cont.)

• It is alterations to these alleles that are most often the cause of genetic diseases

• DNA structure/sequence determines protein structure

• Proteins are determined by the sequence of amino acids

• Amino acids are determined by the sequence of bases (also called codons or triplet codes)

• Changing just one of these bases in a codon can change what amino acid is produced

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Genes and Inheritance

• A normal persons genome contains two of each of the 22 chromosomes as well as a pair of sex chromosomes which will be either XX of XY

• Your genes are a combination of your parents genes

• Their gametes contain half of their genome• Many genetic traits are referred to as either

dominant or recessive6

Genes and Inheritance (cont.)

• Recessive trait – a person needs to have the gene on both of the chromosomes to express the characteristic

• Dominant trait – a person requires only one copy of the gene to express the characteristic

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Sickle Cell Anaemia

• Affects the haemoglobin• Most common form is caused by a recessive

trait on chromosome 11, a single base change (T A) which makes the 6th codon Val not Glu on the β-globin polypeptide

• Causes RBCs to form a sickle shape when the concentration of oxygen is low

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Sickle Cell Anaemia (cont.)

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Sickle Cell Anaemia (cont.)

• This occurs most often near active tissues as they produce more energy

• The shape of the RBC’s causes clots to form more easily in narrow blood vessels due their shape

• Once the RBC has formed the sickle shape its lifespan is greatly reduced

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Cystic Fibrosis

• A recessive disease• Caused by the omission of one amino acid

(Phe) on chromosome 7• Protein that is coded for by this gene is

responsible for the transport of Cl- across the cell membrane

• Defect alters the conformation of the protein so that it folds incorrectly

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Cystic Fibrosis (cont.)

• An effected person is unable to effectively remove Cl- from the lungs, intestines, pancreas and sweat glands

• Sufferers also prone to lung infections and impaired digestion

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Other Genetic Disorders

• Monosomy – where a person has only one copy of a chromosome– e.g. Turner’s syndrome – only one X chromosome,

leads to sterility, poor health and often mental impairment

• Trisomy – When a person has 3 copies of a chromosome– e.g. Down’s syndrome, causes physical

deformities and mental impairment

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Other Genetic Disorders (cont.)

• Trinucleotide repeat mutation – occurs when an amino acid is repeated too many times– e.g. Fragile X syndrome – repeat of CGG, a normal

person has 29-31 repeats, 31-55 repeats are debateable as to if it is normal or not, 55-200 repeats are termed pre-mutation (will most likely pass it on to next generation), 200+ is full mutation causing physical mental and emotional disability the severity of which increases with the number of repeats present.

– Most children with excessive repetition die before they reach adulthood.

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Forensic Applications

• Sample selection– Need to have the samples you are testing and

samples that you want to compare them with• e.g blood, tissue, hair, fluid

• Restriction Enzymes– Target specific sequences of DNA (often a stop

codon or a repeated sequence of amino acids)– Cut the chromosome into fragments which can

then be analysed by their mass and electronegativity

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Polymerase Chain Reaction (PCR)

• DNA produces an identical daughter strand every time it replicates

• PCR enhances this characteristic so that there is enough DNA to perform multiple analyses

• As little as 1 nanogram of material is needed at the beginning of this process

• PCR produces copies of DNA exponentially with each cycle

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PCR (cont.)

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Electrophoresis

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Electrophoresis (cont.)

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DNA Profiling

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• Approximately 5% of your DNA codes for proteins

• The other ~95% is non-coding or ‘junk’ DNA which varies greatly between individuals

• In this ‘junk’ there are sections which have repeated patterns

• These repeated patterns are what is used to identify an individual when doing DNA profiling

• a match of 10 sites across multiple chromosomes are required for positive identification

DNA Profiling (cont.)

• This information can be used for paternity, suspect identification and clearing, identification of remains, identification of species of animals, etc

• The exception to these rules is that it is not possible to tell the difference between identical twins as they have the exact same DNA (unless it is mutated by an external source)

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Electrophoresis plate

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