h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I need as much information for my end of year biology exams as possible, and there are three things I don't really get from my textbook. Can't contact the profs because they're all gone researching.

1: What is the difference between mitosis and binary fission?

2: What is the difference between the alpha and beta chains in haemoglobin?

3: What makes one allele dominant over another?

I can answer question 2.

Alpha chains use a helix binding for strength.
Beta chains use a concertina binding (like when you fold a piece of paper up). Not as strong but allows for more surface area.

HF

an allele will be more dominant if it gives a characteristic which is more suitabe for the environment so that will be expressed and the individual carrying it will be more likely to reproduce and therefore pass it on

1. Mitosis is when a eukaryotic cell (e.g. a human cheek cell) divides to produce two identical copies. Binary fission is when a prokaryotic organism (e.g. a bacterium) divides to reproduce itself.

2. 'Normal' haemoglobin consists of two alpha chains and two beta chains, which, as IHF said, have different structures. It's the combination of the alpha and beta chains, holding an iron atom, which give haemoglobin its oxygen carrying properties.

3. There are several mechanisms by which a dominant allele expresses itself. For example, a dominant allele may make a protein that a recessive allele does not (or produces an inactive protein). Therefore the dominant allele will always show up against the recessive one. For example, in flower colour, a dominant allele may be one that produces a pigment, whereas the recessive allele does not. Therefore, even a single dominant allele will lead to coloured flowers. Alternatively, a dominant allele may prevent something happening that is allowed by a recessive allele; again, only one copy would be needed to show activity.

Hope that helps!

YEH. aGREED.

From a university lecturer friend of mine who knows his genes (and his semantics!):

What makes one allele dominant over another?

The question is wrong (in the strictest sense of the words). Dominance is a character (blue eyes, colour blindness). So a character is dominant if it is manifest in a heterozygote (you have two genes for each protein (too simple but lets go with it) which are identical. If they are identical this is termed homozygous. But if the genes are slightly different - one may have a different amino acid at a certain position - but essentially they are the same protein. In this case they are termed heterozygous.



In both cases each gene is called an allele



So if a CHARACTER is dominant if manifest in the heterozygote, and recessive if not. In this case the normal gene is responsible for a phenotype and even though the other gene is faulty, the correct one , well corrects it.



A recessive character is ONLY manifest in the organism in a HOMOZYGOTE, thus two faulty genes



Complicated area this one but the take home message - genes are not dominant the character resulting from the gene IS !!!

Fair comment!

OK, so the mechanisms I was describing above explain how certain alleles can lead to dominant characteristics. Another example (in this case, of codominance) can be found in the Entry on blood groups: A2116621

There's never any Chemistry questions posted on Ask...

B

As my chemistry exam gets nearer, I can make some for you

Erm...if it's degree-level, I'll be struggling with Inorganic or Organic Chem...

B

Tsk tsk, you've got a degree in it. It is just first year uni chemistry. I'm sure you can handle it

My degree is in Metallurgy, and people NEVER ask questions about that.

B

When phenol is reacted with conc HNO3 and H2SO4 what product and at what rate is it produced. What will be produced in phenol is reated with dilute nitric and sulpuric?

Some homework from work from our 'Chemistry for Chemical engineers course'.

I believe the question is about electrophilic addition and ortho, meta, para positions.

They asked about that smell-removing metal egg, and you couldn't answer that

At least one of the answers will be something to do with trinitrophenol, which was a forerunner to TNT. Why a chemical engineer would want to blow something up is beyond me, though.

B

You do mean that ironically I assume. Why would anyone become a chemical engineer if they don't get to blow stuff up?

They're not supposed to blow it up _deliberately_.

B

Ah, I remember my 1st year chem exams, a few days ago...

Not nearly as bad as I expected, though physics, which I looked forward to, turned out to be harder than expected. Murphy's Law of Exams, I guess.

I'd like to clarify a few things about my original three questions.

1: What is the difference between mitosis and binary fission?

"Mitosis is when a eukaryotic cell (e.g. a human cheek cell) divides to produce two identical copies. Binary fission is when a prokaryotic organism (e.g. a bacterium) divides to reproduce itself"

I know that, but I want to know how the *processes* are different.

2: What is the difference between the alpha and beta chains in haemoglobin?

"Alpha chains use a helix binding for strength.
Beta chains use a concertina binding (like when you fold a piece of paper up). Not as strong but allows for more surface area"

OK I wasn't very clear, though this is a somewhat tricky area. I know all about alpha and beta protein chains. However, haemoglobin consists of four subunits, two termed alpha and two termed beta, which both contain a mixture of alpha and beta chains. I'm asking about the difference between the subunits.

3: What makes one allele dominant over another?

"There are several mechanisms by which a dominant allele expresses itself. For example, a dominant allele may make a protein that a recessive allele does not (or produces an inactive protein). Therefore the dominant allele will always show up against the recessive one. For example, in flower colour, a dominant allele may be one that produces a pigment, whereas the recessive allele does not. Therefore, even a single dominant allele will lead to coloured flowers. Alternatively, a dominant allele may prevent something happening that is allowed by a recessive allele; again, only one copy would be needed to show activity."

Makes sense, but is there a general biochemical reason for one character being dominant, or is it just the way DNA amd nature happen to be?

On haemoglobin: I don't think there is a great deal of difference between the normal subunits. They are of very slightly different chain lengths and have very slightly different 3D structures. Each of the 4 subunits binds a haem group and each haem group binds an oxygen molecule (so 4 O2's per haemoglobin tetramer).

Alpha and beta subunits are coded for by different genes. Actually 2 different gene families: the alpha globin and the beta globin clusters. There are slightly different forms of alpha and beta unit that are used in different conbinations at different times in development, each one having different oxygen-binding properties. For example different subunits are expressed in a foetus compared to an adult (and some of the foetal ones aren't called alpha or beta...). By the time you're out of babyhood, you have almost 100% of your haemoglobin with the same two alpha and beta chains - just what does the job best I suppose.

If there is not a 1:1 production of alpha and beta globin chains (e.g. in thalassaemia, which involves mutations in either the alpha or beta genes) then unusual types of haemoglobin are produced, red cell manufacture is disrupted and the cells may burst open. In sickle cell disease there is a mutation in the beta chain which makes the haemoglobin polymerise. So the subunit arrangement is obviously important...

1: What is the difference between mitosis and binary fission? [...] I want to know how the *processes* are different.

Aha... slightly more involved then! Don't know the ins and outs, but the main difference I imagine is that a eukaryotic cell undergoing mitosis must also replicate its organelles (mitochondria, chloroplasts etc.) as well as dividing. It also needs to make sure its chromosomes end up in pairs and split one of each pair to each daughter cell, hence the use of the spindle etc. A prokaryotic cell has no organelles, and bacteria only have one giant chromosome (and possibly some plasmids), so the whole process is a lot simpler. Beyond that, we're getting out of my area so I can't give you much more detail, I'm afraid!

3. Dominance etc.

I don't believe there is a general biochemical reason for dominance. It occurs by numerous different mechanisms, some of which I imagine are yet to be discovered. It all depends on what the protein in question is and what it does, how much of it is needed for it to function and, in a heterozygote, the relationship between the 'dominant' and 'recessive' alleles.

For example, if a protein exists as a dimer (two copies of the same protein), then having one defective copy of the gene may act as a recessive character, as there is still enough wild-type protein to form working subunits. If, however, the mutant protein can still form dimers, then it may show up as a dominant characteristic, as it can block wild-type protein dimers.

If you see what I mean... (this would be so much easier with diagrams!)