Chromosomes

The characteristic elongated 'X' shape of a chromosome is the most familiar (perhaps with the exception of the double-helix) image of inside our nucleus and what makes us who we are. But it is a common misconception that this is the normal form of DNA. DNA only turns into this super-condensed form during the mitotic phase of the cell cycle. As we can see, this phase lasts for a short time when compared to the much longer growth and synthesis phases. Nevertheless, these short-lived gangly bundles of nucleic acid play a vital role in our everyday lives and in organising our DNA.

Another problem I've always had with pictures of chromosomes (like the diagram on the left) is that this shows a chromosome with every gene, every allele and every nucleotide doubled: it's constituent DNA has undergone replication. Therefore, whenever you see a picture like this, you need to remember that each arm (one on the left and one on the right) is absolutely identical (or thereabouts, seeing as even the best DNA synthase enzymes occasionally make a mistake) so you can think of this like two chromosomes, called sister chromatids. Each chromatid is held together by a centromere. At the time of writing, I was not sure what a centromere was, so after some googling I found that it is part of the DNA. At the centromere is a region rich in adenine and thymine which causes the two chromatids to be drawn together and causes kinetochores to form. What are kinetochores? Kinetochores are the binding sites for spindle fibres to attach to the centromere for mitosis/meiosis. Incidentally, the shorter pair of arms on the diagram are called 'p' because of the French for 'small': 'petit'.

Just one more bit on chromosome structure! Chromosomes form around bundles of proteins called 'histones'. Imagine, if you like, a cotton reel surrounded by thread. In this analogy you can imagine that the thread is like the DNA, winding around the cotton reel (the histones). Now if we extend this further, imagine that more reels are joining our original reel and the thread is continuing between the reels to make one continuous strand of DNA wrapped around many histones. This is just like the formation of a chromosome.

So, now chromosome anatomy is done it's time to explain their purpose in genetics. We humans have a grand total of 46 chromosomes (which 23 pairs, obviously). These pairs can be arranged into  a karyotype. To the left is the human karyotype, which is basically all of the chromosomes paired up into their homologous pairs (one from each parent, unless you're the result of parthenogenesis... which you probably aren't) and put onto a chart. As you move from pair 1 to pair 22 (the autosomes), the chromosomes get smaller. This suggests to me that as the pair number gets higher, the more recently that chromosome emerged in our evolutionary history - as it has had less time to accumulate more DNA as introns or mutations. Interestingly though, the sex chromosomes (pair 23, Y and/or X) are larger and look roughly the same size as pair 10. This suggests that the male mutation (how insulting, I know) arose at about the same time as pair 10 did. Therefore, find when pair 10(ish) turned up, and you're within a few million years of finding Adam.

After enough waffle to feed a small village, I've decided to move onto genes and alleles. Each chromosome carries a specific set of genes. Another important point to remember is all people have the same genes, hence same species, but have different alleles - which results in variation. NEVER say we all have different genes. But anyway, to exemplify my point the gene SERPINA1 is found on chromosome 14 and is responsible for the production of alpha-1 antitrypsin which controls the action of elastase in the lungs. An absence of the dominant allele can lead to hereditary emphysema. Each gene is also only ever found in one position on a chromosome, called a locus. Remember, each chromosome has a homologous pair, so each gene will be found twice (but maybe with different alleles) in each karyotype - but on the same locus on each chromosome.

One last thought, each DNA molecule is about 2nm wide and 50mm long. If we scaled that up to 5mm string, then it would be 125km long. So next time you are struggling to pack your suitcase, spare a thought for the size of the job for the histones.