Download Higher Biology Unit 1: DNA and the Genome 5

Higher Biology
Unit 1: DNA and the Genome
5. Genomic sequencing
Notes
Genomic sequencing
Determining the sequence of the nucleotide base molecules all the way along an organism's DNA
and then relating the information about the genes to their functions.
Genomic sequencing involves the Genome shotgun approach which uses restriction endonucleases
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Restriction endonuclease: a type of enzyme that recognises a specific short sequence of DNA
nucleotides called a restriction site on a DNA strand.
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It ‘cuts’ the DNA at this exact site all the way along the DNA strand at different point
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Take each fragment produced and sequence it to establish the order of its bases. Many of
the fragments may overlap.
Bioinformatics:
The fusion of molecular biology, statistical analysis and computer technology. Has allowed
genomics to make major scientific advances.
Phylogenetics
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This is the study of evolutionary relatedness among different organisms.
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Uses information found from comparison of genome sequence data to deduce phylogenies
(sequences of events involved in eth group’s evolution) and to construct phylogenetic trees.
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The evolutionary distance between the two groups can be found by counting the number of
differences per unit length of DNA sequence between the two genomes.
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These distances can then be used to construct a phylogenetic tree.
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This shows the probable evolution of related groups of organisms and their phylogenetic
patterns of divergence.
Divergence
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Over time a group of closely related living things acquires its own set of mutations (such as
nucleotide substitutions) which gradually alter its genome.
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This group may give rise to two groups that become more and more different from one
another and eventually diverge.
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One they have diverged each group’s genome may undergo changes which are distinct to
that group's genome.
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Therefore the more different the base sequences of two genomes are found to be, the more
distantly related the two groups to which they belong and vice versa.
Molecular clocks
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When genetic sequences for two related groups of organisms are compared, the number of
nucleotide substitutions by which they differ is regarded as being proportional to the length
of time that has elapsed since the groups diverged, i.e. since they shared a common
ancestor.
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Therefore a molecule of nucleic acid (or a protein coded for by the nucleic acid) can be
regarded as a molecular clock.
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A molecular clock can be given an actual time scale by plotting a graph the number of
molecular differences it has evolved against a time scale based on fossil records.
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Molecular clocks are used as tools to try to date the origins of groups of living things and to
determine the sequence in which they evolved.
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When a stretch of DNA does indeed behave like a molecular clock, it becomes a powerful
tool for estimating the dates of lineage-splitting events. For example, imagine that a length
of DNA found in two species differs by four bases (as shown below) and we know that this
entire length of DNA changes at a rate of approximately one base per 25 million years. That
means that the two DNA versions differ by 100 million years of evolution and that their
common ancestor lived 50 million years ago. Since each lineage experienced its own
evolution, the two species must have descended from a common ancestor that lived at least
50 million years ago.
Personal genomics
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The branch of genomics involved in sequencing an individual's genome and analysing it using
bioinformatics tools.
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In the near future it may be possible to sequence a person’s entire genome early in life and
store that information for future consultation by doctors when required.
Personalised medicine (pharmogenetics)
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1 in 10 drugs (including the blood thinner warfarin) varies in effect depending on differences
such as SNPs in the person's DNA profile.
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In future it may be possible to prescribe the most suitable drug and the correct dosage as
indicated by personal genomic sequencing, reducing side effect and the increasing the
effectiveness of the drug.
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It may also be possible to scan an individual's genome for predisposition to a disease and
predict risk early enough to allow suitable action to be taken (for example, through
appropriate drug treatment combined with a healthy lifestyle).
Three domains of living things
This evidence supports the idea that living things are made up of three domains.
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the bacteria (prokaryotes)
the archaea (mostly prokaryotes that inhabit extreme environments such as hot
springs and slat lakes)
the eukaryotes (fungi, plants and animals)