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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…

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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…

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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…

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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…

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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. These sugars can then be used for respiration,…

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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. Each pair…

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A2 Biology Revision Notes Cellular

Chromatids of each chromosome are randomly assorted (arranged)

Anaphase 2

Centromeres divide and chromatids are pulled to opposite
poles by the spindle fibres. Chromatids randomly

Telophase 2

Nuclear envelopes reform around haploid daughter nuclei
In animals, the two cells now divide to give…

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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. Holding the homologous pairs on the spindle equator ensures that when

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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…

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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…


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