Maybe we can read anywhere that the human chromosomes were numbered according to their size , i.e. chromosome 1 is bigger than chromosome 2 and chromosome 2 is bigger than chromosome 3 and so on .
But when we see the actual size of each chromosome we find that the chromosome 20 is bigger than chromosome 19 , why is that ?
Maybe for historical or other reasons?
regards
There are three possible answers of this question. Fisrt there are more dupliacting regions in the chromosomes, high gene density with many splicing sites, and relatively high amount of junk DNA. All genes of human chromosome 19 have very high GC contents, but chromosome 20 regions contain large numbers of repeats and variants of known human DNA sequences. High length of kinetochore - a highly complex multiprotein structure in chromsome 20, it also contains large arrays of repetitive DNA (e.g. satellite DNA) where the sequence within individual repeat elements is similar but not identical. In humans, the primary centromeric repeat unit is called α-satellite (or alphoid), although a number of other sequence types are found in this region.
As far as I know the human chromosomes were numbered according their size and the only exception is chromosome 21, which is smaller than chr20. It has a historical story behind. About your question I think this question arise because of your colouring technique which employed to prepare your slides.
The chromosomes were numbered by "landmarks" in an International Meeting in Chicago in 1966, 19 became 19 then, only much later, after the Genome Project were the actual number of bases identified. By the mid 1960's it was apparent thet number 22 was larger than 21 but in 1962 Jerome Lejeune had identified the "21" as the cause of the trisomy. No one wanted to change the numbering for this historical reason. Remember this was still the prebanding era.
Karyotyping is done based on the perceived individuality of chromosomes. Two factors considered usually and as a matter of routine are size and position of the centromere. In recent time, molecular features are now being put into consideration. However it was done, human chromosome 19 remains human chromosome 19, which obviously had a tilt of opinion and consideration to be numbered before chromosome 20 at the time. It is possible that there could be a change in opinion about the relative sizes of the two chromosomes, but the identities of the two chromosomes cannot be swapped without serious consequences.
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