Transcript DNA - heredity2

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.
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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 recessive
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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
• 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
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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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