AQA A2 BIOLOGY UNIT 5: Genetic Control of Protein Structure and Function

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  • Created on: 18-04-14 14:23
Preview of AQA A2 BIOLOGY UNIT 5: Genetic Control of Protein Structure and Function

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DNA is the genetic material of all living cells.
The sequence of nucleotides in DNA forms a code that determines the sequence of amino acids in
the proteins of an organism.
DNA is largely confined to the nucleus.
Sections of the DNA code are transcribed onto a single-stranded molecule called ribonucleic acid
mRNA transfers the DNA code from the nucleus to the cytoplasm and acts a messenger.
It is small enough to leave the nucleus through the nuclear pores and to enter the cytoplasm, where
the code that it contains is used to determine the sequence of amino acids in the proteins which are
synthesised there.
It is the sequence of nucleotide bases on mRNA that is referred to as the genetic code.
The mRNA code is derived from the DNA code and is complimentary to it.
The term codon refers to the sequence of three bases (triplet) on mRNA that code for a single amino
The main features of the genetic code:
Codon Each amino acid in a protein is coded for by a sequence of three
nucleotide bases on mRNA
Single codon A few amino acids only have a single codon
Degenerate The code is a degenerate code which means most amino acids have
more than one codon
Stop codons Three codons do not code for any amino acid and are called stop
codons and mark the end of a polypeptide chain
Non-overlapping The code is non-overlapping, that is, each base in the sequence is
read only once
Universal code The same codon codes for the same amino acids in all organisms

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Ribonucleic Acid (RNA) Structure
RNA is a polymer made up of repeating mononucleotide subunits.…read more

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DNA never leaves the nucleus, but proteins are synthesised in the cytoplasm, so a copy of each gene is
made to carry the "message" from the nucleus to the cytoplasm.
This copy is mRNA, and the process of copying is called transcription.
1. The enzyme DNA helicase acts on a specific region of the DNA molecule to break the hydrogen
bonds between the bases, causing the two strands to separate and expose the nucleotide bases in
that region
2.…read more

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DNA is made up of sections of introns and exons.
Exons Code for proteins
Introns Do not code for proteins and interfere with the synthesis of a polypeptide
In the pre-mRNA of eukaryotic cells, these non-functional introns are removed and the functional
exons are joined together in a process called splicing.
Once the introns have been removed, the remaining exon sections can be re-joined in a variety of
different combinations.
Mutations can affect the splicing of pre-mRNA.
1.…read more

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Once mRNA has passed out of the nuclear
pore it determines the synthesis of a
A ribosome becomes attached to the
starting codon at one end of the mRNA
The tRNA molecule with the
complimentary anti-codon sequence
moves to the ribosome and pairs up with
the sequence on the mRNA
This tRNA carries an amino acid
A tRNA molecule with a complimentary
anti-codon pairs with the next codon on
the mRNA
This tRNA molecule carries another amino
The ribosome moves along the…read more

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Any change to the base sequence of DNA is called a mutation.
Mutations arise in body cells are not passed on to the next generation.
Mutations occurring during the formation of gametes may be inherited, often producing sudden and
distinct differences between individuals.
They are therefore the basis of discontinuous variation.
Any change to one or more nucleotide bases, or any rearrangement of bases, in DNA is known as a
gene mutation.…read more

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Deletion of bases
A gene mutation by deletion arises when a nucleotide is lost from the normal DNA sequence.
The loss of a single nucleotide from the thousands in a typical gene may seem a minor change but
the consequences can be considerable.
Usually the amino acid sequence of the polypeptide is entirely different.
This is because the genetic code is read in units of three bases.…read more

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Cell division is controlled by genes.
Most cells divide at a fairly constant rate to ensure that dead or worn out cells are replaced.
In normal cells, this rate is tightly controlled by two genes:
Proto-oncogenes that stimulate cell division
Tumour suppressor genes that slow cell division
Proto-oncogenes stimulate cell division.
They encode proteins that function to stimulate cell division, inhibit cell differentiation and halt death.
All of these processes are important for normal human development and for the maintenance of tissues
and organs.…read more



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This is really helpful, thanks!

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