Super mice and species

Several newspapers today carry a story about genetic changes in mice. The basic issue is that species of mice common in europe have cross-bred with a species of Algerian mouse. This is of interest because the Algerian mice have a genetic resistance to many types of poison.

A commonly used poison on rodents is warfarin. You may remember it being mentioned in GCSE – it is an anticoagulant; this means it can prevent blood clotting. In high concentrations it kills mice and rats by causing excessive bleeding, but some populations are becoming resistant to it. This was identified back in the 1960s, and is a good example of how rapidly animals can evolve.

Returning to the Algerian mice, the warfarin resistance (and other rodenticides) is common in this species. The interesting part is that two different species of mice have potentially interbred to produce viable offspring. If you remember how we defined species as organisms that can breed to produce fertile offspring, how is it possible that two different species can interbreed? The answer is that our definition of ‘species’ is too simple (for example, what about species that don’t reproduce sexually?) and that genes can be passed between different species quite regularly, known as horizontal gene transfer. It is however more commonly observed in plants and bacteria rather than animals. Some scientists suggest that it may be more common in animals than we suspect, particularly in marine animals. Scientists have identified genes crossing between phyla (e.g. fungi to animals) as well.

What does all this mean for you? Apart from showing evolution in action, it is a nice reminder that very often in biology things are a little more complicated than they may first appear, and things don’t always fit into neat boxes as we may like.

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And eventually…alcohol dehydrogenase

I said I’d do this sometime back in response to a question from, um…someone. Now we are into the glorious long summer and I have more time, some words on alcohol dehydrogenase.

Its role on humans is to convert ethanol to less harmful substances. It does this by oxidising ethanol CH3CH2OH to the toxic chemical acetaldeyde CH3CHO (notice what has happened in the oxidation reaction, look at the enzyme name as well). This is then oxidised further into harmless acetic acid (bonus points for working out the name of that enzyme).

However, the place we first encountered alcohol dehydrogenase was in the anaerobic respiration pathway of yeast, where pyruvate was decarboxylated to ethanal, which was then reduced further to ethanol with hydrogen from reduced NAD. Remember that the advantage of this step is to re-oxidise NAD so that more H can be accepted during glycolysis. This presents a few questions.

1) what is the difference between acetaldehyde and ethanal?

Nothing, it is the same chemical. Ethanal is an internationally accepted name, whereas acetaldehyde is a name in common usage by some people. I’m afraid it’s an example of scientists from different areas using different names for the same thing. I’d suggest sticking with ethanal (if in doubt, go with the syllabus).

2) If enzymes are specific, how come it is doing two different things in two different places?

This is really the same reaction in reverse (although there are actually quite a lot of different versions of alcohol dehydrogenase, we’ll leave that aside). Enzymes speed up the rate of a reaction, but they do not alter the equlibrium. In simple terms, in the case of yeast and human liver we have different substrate concentrations, leading the equilibrium in one direction more than the other.

Ethanol production in yeast gives the fungus an advantage because it is toxic to other organisms. What is the advantage of alcohol dehydrogenase in humans? Ethanol occurs naturally when fruit begins to ferment. Our ancestors diets included fruits, so anyone with this enzyme would have had a natural advantage in removing the toxin.

You asked part 2

Jack raised the point about theories in the lesson this week. In science, a theory has a different meaning to how we use the word in everyday language. Theory can be used to mean a conjecture, idea, speculation or simply a guess about why something happens. This use is fine as long as you realise that words can have multiple meanings, and the context the word is used in is important.

In science, a theory denotes something else. It used to describe statements or principles that not only describe what is happening, they also provide an explanation. It has predictive power, in other words a theory can be used to predict an event or observation that has not yet happened. Theories are well tested and have evidence to support the statements. If you wanted to show that a theory is incorrect, you could show that the predictions it makes do not happen under experimental conditions.

To give an example, let’s take the good old theory of evolution. Broadly speaking, it would predict that organisms closely related would share more genetic similarities than more distantly related organisms. If you found a monkey more closely related to a fish than a gorilla, you could start questioning the theory. Older fossils should be found in older rocks – if you found a human remains mixed in with a T rex’s bones then you may have a problem. So far these things have not occurred, but if they did and were shown to be valid (e.g. not faked) then the theory would be changed. Theories are changed in reponse to new evidence, that’s how science works.

In the example Jack brought up (ATP synthase mechanism) the theory is incomplete because it doesn’t provide an explanation for all observations. Although there is no hierachy of theories, some have been tested more than others so we often consider them to be more robust. Many accepted theories are used simply because they work well enough for the time being, in other words they hold up to testing so far.

Scientific theories are often described as falsifiable. This means that you could potentially cause a theory to be changed or abandoned by evidence that shows its predictions do not hold up in experiment or observation. Knocking down established theories is a good way to get a Nobel prize. But there’s a reason why the big theories are rarely abandoned…

You asked part 1

Since Charlotte asked about the proton pumps, I thought I’d go into a bit more detail.

The pumps are referred to as complexes, given Roman numerals I, II, III and IV. Complex I is really an enzyme, called NADH dehydrogenase (why is it called a complex? There are 45 separate polypetide chains). 2 electrons are passed onto a substance called ubiquinone (or just Q), which is reduced to ubiquinol (QH2). This is lipid soluble, and moves easily through the membrane. 4 protons are pumped through the membrane.

Complex II (aka succinate dehydrogenase)  gives additional electrons to Q, which is then passed to Complex III (aka cytochrome bc complex). Here the electrons are passed on to another molecule, cytochrome C. 6 protons in total are translocated (moved across the membrane) at this point. Finally cytochrome C (whgich is a water soluble, integral protein membrane) electrons on to Complex IV, where the electrons are removed and used in reducing oxygen to water. Cytochrome C is inhibited by cyanide – a poison which will stop aerobic respiration.

Here’s a nice summary from wikipedia:

In eukaryotes, NADH is the most important electron donor. The associated electron transport chain is

NADHComplex IQComplex IIIcytochrome cComplex IVO2 where Complexes I, III and IV are proton pumps, while Q and cytochrome c are mobile electron carriers. The electron acceptor is molecular oxygen.

Remember, you will be tested on what is in the specification, but reading further in your subject will always help.