Dear Biologist,
Recently i was reading supplementary article of Kalhor nature biotechnology 2012 paper. And there in one line where they said centromere in p-arm and q-arm of chromosome. Till now i was thinking that centromere are located in junction of p-arm and q-arm and only one centromere exist per chromosome. I am totally confused now what is true? One or two centromere per chromosome?
Hi Ankit,
I haven't read the paper, but it depends on the context you were reading:
1. Normal chromosomes should look like those in attached picture #1.
2. There are some called 'dicentric' chromosomes. "A dicentric chromosome is an abnormal chromosome with two centromeres. It is formed through the fusion of two chromosome segments, each with a centromere, resulting in the loss of acentric fragments" (see attached pictures #2).
3. A centromere can also be on chromosomes with two p-arms or two q-arms (picture #3).
Yes, you are correct Ankit, chromosomes typically have only one centromere, which appears as a primary constriction on metaphase chromosomes. Humans and other complex eukaryotes have large regional centromeres. These consist of a kinetochore flanked by large arrays of satellite DNA. The satellite DNA sits within the proximal short (p) and long (q) arms of each chromosome.
Dear Agrawal,
I haven't read the Kalhor nature biotechnology 2012 paper.
Such as those described above, there are different positions Centromere or different chromosome types. Holocentric chromosome is one of these. The holocentric or holokinetic chromosomes are characterized with lack a localized centromere. In contrast, monocentric chromosomes are characterized with a single localized centromere in many organisms.
Thanks everyone for your answers. I was trying to understand this table from Kalhor paper. Could you explain us what they are trying to show here.
I couldn't find the paper you are referring to but this table appears to be showing the genomic coordinates from around the human centromeres (excluding the Y chromosome). The centromere region itself is intractable to sequencing because of its highly repetitive sequence. Hence the 'centromere' is designated as a gap which is arbitrarily assigned several Mb of sequence. This is the gap in the sequence coordinates between the p and q arms for each chromosome in the table. The segments listed in the table appear to be low copy repeats and satellite DNA flanking the gap region. There are large segments of heterochromatin in the proximal long arms of chromosomes 1,9 and 16 (hence the much larger gap regions here). The short arms of acrocentric chromosomes (13,14,15,21,22) also consists of highly repetitive DNA hence there is no coordinates listed in the proximal p arm. I'm unsure what the 'two segments' comment refers to without seeing the paper.
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