AQA Biology Unit 5 Notes

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Unit 5
Gene Expression and Control
Structure of RNA- Ribonucleic acid:
Made up of ribose nucleotides instead of deoxyribose like in DNA
Uracil replaces Thymine
RNA is single stranded
RNA is shorter than DNA
3 types of RNA:
mRNA- Made up of thousands of mononucleotides- It is a long stand
that is arranged in a single helix.
mRNA carries the "message" that codes for a particular protein from
the nucleus. After it is made, the mRNA leaves the nucleus via the
pores in the nuclear envelope and enters the cytoplasm where it
associates with the ribosomes. It then acts as a template where
proteins are built.
It has a short lifetime and it is easily broken down so only exists when it is needed to make a
Structure related to function because it has the correct sequences of the many triplets of
organic bases that code for a specific polypeptide.
tRNA-Transfer RNA - tRNA is a small molecule that is made up of
around 80 nucleotides- it is a single stranded chain which folds up by
complementary base pairing to form a looped-clover leaf structure.
At the end of the molecule there is an ACC base sequence where the
amino acid binds to. On the middle loop there is a triplet nucleotide
sequence- the anticodon- there are 64 different tRNA molecules each
with a different anticodon sequence complementary to the 64
different codons.
The amino acids are attached to their tRNA molecule by specific
aminoacyl tRNA synthase enzymes. These are highly specific, so that
each amino acid is attached to a tRNA adapter with the appropriate
rRNA- Ribosomal RNA - together with proteins forms ribosomes,
which are the site of mRNA translation and protein synthesis.
Ribosomes have two subunits, small and large, and are assembled in the nucleolus of the
nucleus and exported into the cytoplasm. rRNA is coded for by numerous genes in many
different chromosomes.
Ribosomes free in the cytoplasm make proteins for use in the cell, while those attached to
the RER make proteins for export.
Double polynucleotide chain Single polynucleotide chain Single polynucleotide chain
Largest out of the three Smaller than DNA but bigger than Smallest out of three
Double-helix Single-helix- straight Clover ­shaped- folded
Pentose sugar is deoxyribose Ribose Ribose
Adenine or Guanine Purine base Adenine or Guanine Purine base Adenine or Guanine Purine base

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Cytosine or Thymine pyrimidine base Cytosine or Uracil pyrimidine base Cytosine or Uracil pyrimidine base
Found mostly in nucleus Manufactured in the nucleus but Manufactured in the nucleus but
found throughout the cell found throughout the cell
Hydrogen bonding between strands No hydrogen bonding Present in some parts of the
molecule giving it the clover shape
Very stable chemically- won't break Easily broken down More stable than mRNA but less than
down easily DNA
Contains codons Contains codons Contains anti-codons
The Genetic Code (mRNA codons)…read more

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The non-coding, non-functional, sequences within genes are introns and the coding
sequences are exons. The introns need to be removed before the mRNA can be translated
into protein. This removal is called post-transcriptional modification/splicing.
The Pre-mRNA is an exact copy of the gene on the DNA, so it contains exons and introns. The
introns are cut out using catalytic RNA, and the exons are joined together by enzymes. Some
of this splicing is done by the RNA intron itself, acting as an RNA enzyme.…read more

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Any change to one or more nucleotide bases or arrangement of bases in DNA during DNA
replication is a mutation. There are three types of mutations- Substitution, deletion and
addition. Only deletion and addition case a frame shift- mutation to occur.
Substitution mutations only affect one amino acid- little or no effects. If the substitution is on
the third base of a codon it may have no effect because the third base doesn't affect the
amino acid coded for.…read more

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For a cancer to occur, one or more tumour suppressor genes must also be
mutated, so their control is removed and cells divide out of control. So a cancer is usually the
result of several different mutations building up.:
Homeostasis- The maintenance of a constant internal environment.
Usually achieved by negative feedback- deviation from the norm (set point). Positive
feedback occurs when the change causes an increase in the same direction
(hyperthermia/hypothermia i.e.…read more

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Ectotherms use less energy than endotherms so they can
survive for weeks without eating. However, when it is really cold for example, they have to
move slowly to prevent a loss of heat so prey can easily catch them.
Regulation in endotherms
The thermoregulatory centres receive input from two sets of thermo receptors: receptors in
the hypothalamus which monitor the temperature of the blood as it passes through the brain
(the core temperature), and receptors in the skin which monitor the external temperature.…read more

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Control of blood glucose concentration
The main source of blood glucose is the digestion and absorption of carbohydrate, mostly
starch. Blood from the intestine goes directly to the liver in the hepatic portal vein before
being carried to the rest of the body.
If the concentration of glucose becomes too high, hyperglycaemia, it will lower the water
potential of the blood and cells will dehydrate. If it becomes too low, the cells will also die
because they cannot respire.…read more

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Animals don't normally synthesise glucose, but when dietary glucose is scarce proteins and
nucleic acids can be used to synthesise glucose (gluconeogenesis).
The Pancreas
The pancreas is a large pale-coloured gland which is situated in the upper abdomen behind
the stomach.
It produces protease, amylase and lipase which are used in digestion and insulin and glucagon
which are used in regulating blood glucose concentration.
The islets of Langerhans produce the hormones.…read more

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This results in a lower blood glucose concentration because the rate of absorption of glucose
in the cells, especially muscle cells, increases. The rate of respiration also increases so more
glucose is used up decreasing the amount in blood. The conversion of glucose to glycogen in
the liver and muscles increase (glycogenesis) and the rate of conversion of glucose to fat
increases.…read more

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Secondly, IRS actives the enzyme glycogen synthase, which synthesises glycogen from
cytoplasmic glucose. Here: IRS activates one enzyme, which activates another, which
activates another, which activates glycogen synthase. This cascade amplifies the effect, so
each molecule of insulin can activate thousands of molecules of glycogen synthase. If present
in the blood, glucagon and adrenaline each bind to their specific membrane receptor
proteins.…read more


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