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Higher Biology – Chapter 29

This week we have been learning about the Lac Operon, phenylalanine metabolism and differentiation in cells.

Lac Operon

This is also known as the Jacob-Monod Hypothesis and is based on the idea that genes can be switched on or off depending on whether or not the protein they code for is needed at that time.  Looking at the images below you should make sure that you can explain the steps involved in the switching on and off of the β-galactosidase structural gene.

Absence of Lactose



Presence of Lactose


There will be many essay questions on these topics and I would strongly suggest you all try a number of them without your books open.


Phenylalanine Metabolism

The metabolism of phenylalanine can be summarised in the image below;

Phenylalanine metabolism


If a mutation occurs in the gene for enzyme 1, this will lead to a condition known as phenylketonuria (PKU).  This is an inborn error of metabolism.

Phenylalanine will not be converted into tyrosine, leading to a build up of toxic chemicals and limited mental development.

This can be detected at birth when babies are screened for high levels of phenylalanine.

Treatment: A diet low in phenylalanine.

Tyrosine can still be obtained from the diet.

If a mutation occurs in the gene for enzyme 3, this will lead to a condition known as albinism since the organism cannot produce melanin.


When a cell is differentiated some genes are switched on and some are switched off.  When Dolly the sheep was cloned an udder cell was taken and placed into an egg cell of another sheep.  This shows that a differentiated cell can be reversed to become a stem cell again and then re-differentiate. For an up to date example have a look at Katie: The Science of Seeing on 4OD;






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