Tuesday, June 14, 2011

The Joy of Chromosomes (Part 1)

So, you have found yourself transported in time to the 1930s, and you have a strong desire to study mutations?  But those uncivilized brutes have yet to develop methods for examining changes in DNA?  Never fear!  You can do the second-best thing, and rediscover everyone’s favourite pastime: karyotyping!

What is a karyotype, you ask?  Let me guess, you learned about karyotypes in first year biology, but immediately forgot about it when you realized it had little to no relevance within a modern genetics laboratory.  A karyotype is simply the number and structure of chromosomes contained in an individual nucleus of an individual cell.  The chromosome is made up of DNA and little balls of proteins called histones – the DNA is wrapped around these histones, allowing the chromosomes to pack in a lot of genetic information without being huge.  Histones in turn regulate all sorts of processes that affect genes; for example, by ‘letting go’ of certain regions of DNA, the histones can turn a gene on. Chromosomes, then, are the large, visible structures of DNA within our cells, and the cells of every living thing.  Even in the early 1900s, scientists could see and study chromosomes, even if they didn’t know what they were made of.

The structure of a chromosome - The X-shape in chromosomes occurs before a cell splits; it indicates a doubling in the DNA

You have likely seen the human karyotype displayed as a karyogram, like this one:

File:NHGRI human male karyotype.png
Karyogram of human male

What do you notice about this karyogram?  First, you should see that there are 46 individual chromosomes in a normal human autosomal (body) cell.  If we were looking at a karyogram of a sperm or egg cell, there would only be 23 chromosomes.

The second thing you should notice is that these chromosomes differ from one another in shape and size.  For example, the two chromosome ones are much larger than the two chromosome nineteens.  Each chromosome carries an assortment of genes; we know from experiments that each chromosome type (ie X, Y, I, II, III, etc) carries a different set of genes, and some chromosome types carry more genes than others.  Some chromosomes have regions that are virtually gene deserts, carrying DNA that is apparently noncoding.

Thirdly, you should notice that most of these chromosomes come in nearly-identical pairs.  There are two chromosome ones, two chromosome twos, etc, making 23 pairs of chromosomes.  But wait!  This isn’t completely true – in the X/Y pair, chromosome X is much larger than chromosome Y.  This pair does not match – this individual would be a male.  A female human would have two X chromosomes (in other species, such as some butterflies and fish, the reverse would be true).  We say, then, that humans have 22 pairs of autosomal chromosomes, and one pair of sex chromosomes.

Now what would happen if we examined the karyotypes of other species?  Here’s a chimpanzee karyotype:

Karyogram of a female chimpanzee

It looks pretty similar to our own, with one major exception: chimpanzees have one more pair of chromosomes than us.  Orang-utans, bonobos, gorillas and chimpanzees all have 24 pairs of chromosomes; humans have 23.


Here’s another karyogram, again belonging to a human, but this time instead of pairs, each chromosome type exists in triplicate. 

Karyotype of 69,XXY (triploidy), common finding in spontaneous abortion. Risk for chromosomal anomaly in subsequent pregnancy is not increased significantly.
Karyogram of a human male, with two X chromosomes and an extra of each autosomal chromosome.  This lethal karyotype is referred to as 'triploidy'

A karyotype such as this is generally lethal in humans.  There are other, less dramatic karyotypes that I could show, in which a human is missing an entire chromosome pair, or has triple of only one chromosome type.  Clearly, there is variation within a species when it comes to karyotypes.

How could something like this develop?  Might it be related to the differences we see between human karyotypes and that of the great apes?

Or take a look at the number of chromosomes that are in different species of an Iranian plant within the genus Bromus.  Pay special attention to the first four entries, which designate chromosome number within different populations of the same species of plant.

Chromosome number (referred to as '2n' for 'diploid number') for several species and populations of Bromus plants.  Note how different the species are, but especially note that the number 14 seems to play an important role (28 = 14 x 2, 42 = 14 x 3, 56 = 14 x 4, 70 = 14 x 5, 84 = 14 x 6).  Also note that one species, B. tomentellus, differs wildly in chromosome number with population.
Clearly, there are a lot of interesting things going on above the gene level.  Just as Dobzhansky argued that differences between species are due to mutations at the gene level, he argues in chapter four of Genetics and the Origin of Species that this does not explain all variation – much of the world’s diversity can be explained by higher-order mutations in the karyotype, what today we refer to as chromosomal mutations.  Unlike genetic mutations, in 1937 chromosomal mutations were well documented, and their mechanisms were becoming understood.  Evolution by natural selection, argues Dobzhansky, is a theory that can both explain the diversity we see among karyotypes, and be explained by a knowledge of karyotypes.

Hopefully your curiosity has been aroused.  Come back later this week for the lessons Dobzhansky learned from studying karyotypes.

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1 comment:

Keith Shields said...

Nice explanation. Your post stirs the imagination as I think about possible mechanisms for large evolutionary changes.