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Cellular Control and Variation
A gene determines the sequence of amino acids in a polypeptide, for example hormones.
Many proteins are composed of several polypeptides and have quaternary structures such

A molecule of catalase is made of four identical polypeptides
a molecule of adult haemoglobin is made of two…

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6) During this, a new polypeptide grows by the attachment of amino acids.
7) When an amino acid has joined the polypeptide chain, its tRNA molecule moves onto
another amino acid to attach to.
8) When the ribosome reaches a STOP codon, the polypeptide breaks away and
begins to fold…

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the bacterium E.coli. The production of the enzyme is controlled along with two other
proteins that are required for the absorption and metabolism of lactose. Three genes
are transcribed together giving one mRNA transcript.

lac operon

Low Lactose concentration, High Glucose concentration
When there is a low concentration of Lactose,…

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Genes in Development
Homeobox genes code for transcription factors that bind to DNA. The homeobox
sequences of organisms are almost identical because they all have the same function ­
coding for transcription factors.

Apoptosis is programmed cell death. Cells respond to external and internal signals that
trigger an ordered sequence…


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