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DNA, Genes, Chromosomes
Eukaryotic DNA is associated with proteins called histones
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Genetic Code – Sequences of bases along an organism’s DNA
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Triplet Code
Degenerate – each amino acid is coded for by more than one triplet
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This change of tertiary structure changes the active site shape resulting into no ES
complexes being formed
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This can happen
if a base is paired wrongly, due to radiation damage or chemical damage
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The incorrect base will be paired with
its complementary nucleotide to complete the mutation
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The mutation only affects a maximum of one amino acid
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This causes it to add one or more extra nucleotides to the new strand
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The extra base pair has a knock-on effect on the base triplets along the rest of
the strand
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Deletion – As DNA is replicated, the DNA Polymerase ‘slips’, overlooking a repeating base or
sequence of bases
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When the
gene containing the error is next replicated, one of the daughter strands will be missing one or more
base pairs
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The primary structure of the gene’s protein will be changed, which may be fatal
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This will prevent the rest
of the polypeptide chain being coded
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This could affect
the structure and function of the polypeptide chain
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There is no change in the polypeptide chain produced
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It will cause a different sequence of amino acids to be coded for
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This causes the two strands to separate
RNA Polymerase binds to a base sequence called the promoter region
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RNA Polymerase moves along the DNA strand in the 5’ to 3’ direction as it passes over the
bases it forms complementary mRNA strand from free nucleotides in the nucleus
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When a terminator region is reached the DNA is no longer
copied
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Processing of mRNA
Eukaryotic DNA contains some regions that don’t code for proteins
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To produce functional proteins these introns, need to be spliced out
of the mRNA, leaving only the regions that code for proteins, called
exons
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The mRNA
is attached at the start codon (AUG), at the 3’ end of the strand
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The
tRNA molecule will bind with a complementary codon on the mRNA strand
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The two amino acids are then joined together with a peptide bond by an
enzyme
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The first
tRNA molecule is released and is free to collect another amino acid from the cytoplasm
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8cm long
Nucleosome fibre – The DNA is wrapped around histone proteins, forming chromatin
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Solenoid Fibre – more histones help the nucleosome fibre form a tightly – called structure called a
solenoid fibre
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Super coiling – to reduce the stress on the molecule, the solenoid fibre twists and writhes, like a
curly telephone cable
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Chromosome – the DNA molecule is packaged so much that the average human chromosome is just
6 micrometres long
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It is loose
in the cytoplasm – no histones or chromosomes
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The adaptations the species have allow for them
to survive better than other species allowing them to pass on genes compared to animals who don’t
fit the requirements, survival of the fittest
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Continuous – range or graduation of characteristics, controlled by many genes, environment
has significant effect
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Homologous classification – this is done by grouping organisms according to evolutionary
relationships
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Species
Members of the same species have:
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Similar physical characteristics
Similar genetic information
Similar developmental patterns
Similar immunological responses
Similar ecological niches
The ability to produce viable offspring with members of the same species

Complications
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One species can become another
There can be a lot of variation within a species
Lots of species are extinct and have no fossil record, so will never be known
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Courtship Behaviour
Ensures successful reproduction as it allows individuals to:
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Recognise members of the same species – ensures that mating occurs only between
members of the same species
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Identify a mate that is capable of breeding – both partners need to be sexually
mature, fertile and receptive to mating
Form a pair bond – leading to successful mating and raising of offspring
Synchronise mating – ensuring that mating occurs when there is the maximum
probability of the sperm and egg mating
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Other animals prefer to
form a temporary pair bond

Courtship and mating in fruit flies can occur equally well in the light or dark
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Biodiversity within a community
species diversity – a measure of how many different species are present in an area, and how many
individuals of each species there are in a community
Genetic diversity – refers to the variety of genes possessed by the individuals that make up any one
species
Ecological diversity – is a measure of how many different habitats are present in an area
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Human Impact to Biodiversity
Increase:
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DNA sequence differences/ allele differences
Diverse range of niches

→Gene Pool Increase
Decrease:
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Reduced niches/individuals in a species

→Gene pool decrease/ genetic bottleneck

Investigating Diversity
Species A – TCGACGTGGGTAATCGAGC
Species B – TCCACGTGTGTAATCGAGT
Species C – ACGCCGAGTGTTATGGAGT
Species A and species B base sequences differ 3 times
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Species A is more closely related to species B as there is less differences in the base
sequence compared to species C
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DNA Hybridisation
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Technique which determines the similarities between DNA sequences of different
organisms
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All strands within a DNA mixture will pair up with their partners eventually

How hybridisation works
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DNA from 2 species is extracted, purified and cut into small pieces
DNA of one species is labelled with a radioactive starter
Both sets of DNAs are mixed and heated to separate their strands
On cooling, the strands combine with others that have complementary base sequence
Hybrid strand is 50% radioactive labelled DNA so can be identified
Hybrid strands are separated out and the temperature is increased in stages
At each temperature stage the degree that each strand is bonded is recorded
If each species is closely related, then they will have more shared base sequences
Therefore, there will be more hydrogen bonds linking the hybrid strand together

Immunological comparisons
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Serum from species A injected in species B
Species B produces antibodies to the antigens in species A
Serum is extracted from B (with antibodies to A)
Serum B mixed with blood of species C
Antibodies respond to
antigens in species C
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The greater the number of
similar antigens the greater
the precipitate
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A SNP is a change of one DNA base pair into another
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These are
mutations in which one or more nucleotide pairs in a DNA strand is lost or gained
Before comparing sequences, you should make sure that you’re comparing stretches of DNA that are
evolutionary related
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There are several different computer programs that can generate an alignment
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An SNP in an alignment is a position where the letters in a column don’t match
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The place where two branches split apart is called a branch
point and represents the most recent common ancestor of all
species on branches
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There is no
common ancestor

Molecular Phylogeny predicts biology
Phylogenetic trees made using DNA sequences are usually
very similar to trees made using other evidence, such as
comparative anatomy
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DNA sequence comparison augments and sometimes clarifies
anatomical comparison
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