Fluffy The Cat's New Writeups

recessive gene (idea)

2001-12-13T13:33:11Z

Recessiveness (and, to some extent, dominance) of genes was first noted by Gregor Mendel in his pea plant experiments. It's a gross oversimplification, but entirely understandable given the lack of knowledge about the inherent basis for genetic traits and biochemistry. In most cases, a recessive gene will encode the absence of a protein (well, more usually it'll be the reduction in functionality of the gene rather than its complete absence - imagine an enzyme with a malformed active site, for instance). If (and this is a fairly big if) the level of the activity of the enzyme is not critical (so only a relatively small amount of functional enzyme needs to be present compared to the amount usually present) then carrying one copy that fails to encode a functional enzyme will still leave you with 50% of the normal level and everything will still work. With two copies, the level will drop further, the pathway will no longer work properly and a new phenotype will be…

Biological robustness (idea)

2001-11-23T19:24:36Z

Living organisms are not precision devices. Most biological processes are inherently stochastic - whether or not a gene is transcribed is non-deterministic. As a result, the number of molecules of any given type (for instance) is not usually tightly regulated (you could postulate the existence of mechanisms for counting the number of molecules produced, but remember that for every extra mechanism you're sticking in a cell you're increasing the amount of work that that cell has to do). At the same time, the environment in which the organism is living is unpredictable. Faced with these problems, a pathway that is highly dependent on the number of molecules of something present or which depends on a particular precise set of conditions is unlikely to be a great deal of use. As a result of this, robustness is a hallmark of many biological systems - that is, they are capable of functioning even when quite badly perturbed by things such as molecular concentration or…

chemotaxis (thing)

2001-11-23T19:15:32Z

E. Coli (and many other bacteria) rely heavily upon chemotaxis in order to find areas of food and keep out of areas of harmful substances. One of the important tricks involved in this is the ability to "compare" the current concentration of attractant/repellent to the previous concentration. This is achieved by adapting to the response - as the concentration outside the cell increases (in the case of attractants) or decreases (in the case of repellents), the receptors are methylated in order to return them to their normal level of activity. Therefore, a signal only occurs if the bacterium is swimming either up or down a gradient. If the concentration is not going in the direction that the bacterium "wants" it to go in, it tumbles around randomly until it happens to be going in the right direction again (towards attractants, away from repellents) which triggers the signal again and causes the bacterium to swim in a straight line.

Survival of the fittest (idea)

2001-11-23T18:44:02Z

As far as evolutionary biology goes, fitness is entirely a measure of your ability to reproduce and pass on your genes. An organism that is incredibly strong, lives for years, slaughters all that stand in its way and in the process produces two offspring is less fit than another member of the same species that's unable to fight at all, dies when it's half the age of the other but produces four offspring. The fittest do not necessarily survive, but their genes do.

Hardy-Weinberg Equilibrium (thing)

2001-11-23T18:37:27Z

The Hardy-Weinberg equilibrium model tries to explain why genotype frequencies may remain constant within a population. Hardy-Weinberg equilibrium in its simplest form is based on the existence of two alleles at a single genetic locus. These are generally called p and q. If we make the assumption that the population is large (effectively infinite), mating is random and that there is no difference in fitness between the genotypes, given sufficient time the genotype frequencies will be:

Genotype      pp     pq      qq
Frequency     p²     2pq     q²

This should (assuming the above assumptions are true) be stable - although the genotype frequencies may be different to the starting point, the allele frequencies should be the same as before. Equilibrium has therefore been reached. Sadly for the Hardy-Weinberg equilibrium model, the above assumptions are rarely true. In most cases, genetic equilibrium is reached by strong selection pressure in opposite directions stabalising the population.

mitochondria (thing)

2001-11-23T17:50:09Z

The purpose of the Krebs cycle is to generate nadh in order to power the electron transport chain. Up until this point, respiration has produced relatively small amounts of ATP. Now we finally get the big payoff. Hydrogen ions from NADH are transferred across from the interior of the mitochondria to the external matrix, generating a potential difference across the internal membrane. Why is this good? Potential difference means electrical current. We get a flow of electrons across the internal membrane which are used to power ATP synthase. Eventually the electrons become reunited with hydrogen ions and we use oxygen as an acceptor (the reason for the requirement of oxygen in aerobic respiration) which produces water which is nice and easy to deal with.

The increased ability to produce energy that mitochondria provide is probably one of the reasons for us eukaryotes being up here and prokaryotes never getting round to discovering fire, inventing the wheel, forming civilisation, that sort of thing.