A2 OCR Biology Cellular Control

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A2 Biology Revision Notes Cellular
How DNA codes for proteins
Genes code for polypeptides, including enzymes.
Genetic code ­ the biochemical instructions that translate the genetic information present as a
sequence of codons in messenger RNA into the correct sequence of amino acids for the synthesis of
a particular peptide chain or protein.
How does the nucleotide sequence code for the amino acid sequence in a polypeptide?
Genes are found on chromosomes in the cell nucleus but proteins are assembled in the cytoplasm, at
ribosomes. A copy of the genetic code has to be made which can pass through a pore in the nuclear
envelope to the cytoplasm. Messenger RNA is this copy
This is the first stage of protein synthesis. An mRNA molecule is made. For this, one strand (template
strand) of the length of DNA is used as a template. There are free nucleotides in the nucleoplasm and
free RNA nucleotides in the nucleolus.
A gene to be transcribed unwinds and unzips. To do this the length of DNA that makes up the
gene dips into the nucleolus. Hydrogen bonds between the bases break
Activated RNA nucleotides bind, with hydrogen bonds, to their exposed complementary
bases. U binds with A, G with C and A with T on the template strand. This is catalysed by the
enzyme RNA polymerase
Two extra phosphate groups are released. This releases energy for bonding adjacent
mRNA produced is complementary to the nucleotide base sequence on the template strand
of the DNA and is therefore a copy of the base sequence on the coding strand of the length
of DNA
The mRNA is released from the DNA and passes out of the nucleus, through a pore in the
nuclear envelope, to a ribosome.
Translation ­ the assembly of polypeptides (proteins) at ribosomes
Ribosomes are assembled in the nucleolus of eukaryotic cells, from ribosomal RNA (rRNA) and
protein. Each is made of two subunits and there is a groove into which the length of mRNA, with the
code for the sequence of amino acids, can fit. The ribosome can then move along the mRNA, which

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A2 Biology Revision Notes Cellular
can slide through the ribosomal groove, reading the code and assembling the amino acids in the
correct order to make a functioning protein.
Transfer RNA
Transfer RNA is made in the nucleus and passes into the cytoplasm.
These are lengths of RNA that fold into hairpin shapes and have
three exposed bases at one end where a particular amino acid can
bind. At the other end of the molecule are three unpaired
nucleotide bases, known as the anticodon.…read more

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A2 Biology Revision Notes Cellular
It is a silent mutation. Although the base triplet has changed, it still codes for the same amino
acid, so the protein is unchanged
If the mutation does cause a change to the structure of the protein, and therefore a different
characteristic, but the changes characteristic gives no particular advantage or disadvantage to the
organism, then the effect is also neutral.…read more

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A2 Biology Revision Notes Cellular
The lac operon is a section of DNA within the bacterium's DNA. It consists of a number of parts;
The structural genes ­ Z codes for the enzyme -galactosidase and Y codes for the enzyme
lactose permease. Each consists of a sequence of base pairs that can be transcribed into a
length of mRNA
The operator region ­ O, is a length of DNA next to the structural genes.…read more

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A2 Biology Revision Notes Cellular
4. As a result, E.coli bacteria can use the lactose permease enzyme to take up lactose from the
medium, into their cells. They can then convert the lactose to glucose and galactose using the
-galactosidase enzyme.…read more

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A2 Biology Revision Notes Cellular
Chromatin condenses, undergoes supercoiling, chromosomes shorten and thicken. They can
take up stains and be seen under light microscope
Chromosomes come together in homologous pairs to form a bivalent. Each member of the
pair has the same genes at the same loci.…read more

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Chromatids of each chromosome are randomly assorted (arranged)
Anaphase 2
Centromeres divide and chromatids are pulled to opposite
poles by the spindle fibres.…read more

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A2 Biology Revision Notes Cellular
It produces new combinations of alleles on the chromatids. Chiasmata remain in place during
metaphase and they hold maternal and paternal homologues together on the spindle, facing the way
they will migrate.…read more

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A2 Biology Revision Notes Cellular
Recessive ­ an allele is said to be recessive if it is only expressed, in the phenotype, in the presence
of another identical recessive allele for the same gene.
Codominant ­ two alleles of the same gene are called codominant if they are both expressed in the
phenotype of a heterozygote. For example, in cattle, one of the genes for coat colour has two
alleles. Cr codes for red hairs and Cw codes for white hairs.…read more

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A2 Biology Revision Notes Cellular
Epistasis ­ the interaction of different gene loci so that one gene locus masks or suppresses the
expression of another gene locus
The genes involved may control the expression of one phenotypic characteristic in one of the
following ways;
They may work against each other (antagonistically) resulting in masking
They may work together in a complementary fashion
Working antagonistically
The homozygous presence of a recessive allele may prevent the expression of another allele at a
second locus.…read more


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