Why Are We Here? Part II: Karyotyping

When studying genetics, it’s important to know how DNA is organized in your species.  For the Elkhorn coral Acropora palmata, we’re still missing an important piece of data: just how many chromosomes are found in the cell nucleus?

A chromosome is a densely packed string of DNA–the genetic material that codes for life processes.  Humans have 23 pairs of them, cabbages have 9, and corals, well, it depends.  Across one lineage, the number of pairs is conserved at 14.  However, within the genus Acropora, the variation can be quite extreme.  The only way to find out for sure is to test your particular species, so that’s what we’re going to do with our larvae.

Chromosomes can be visualized and counted, but it’s not an easy process.  This type of analysis, called karyotyping, involves arresting cell development during replication, when the chromosomes are at their densest.  This requires the use of a nasty chemical called colchicine; over-exposure is similar to the effect of arsenic poisoning.  Don’t worry, we’ll be careful!  After the larvae are exposed, we stain their DNA and look at them under a powerful microscope.

 karotype

Karyotype of a human female (Image: http://scistroh.wordpress.com/2011/02/28/karyotype/)

Since the larvae lack symbionts just after fertilization, there’s no concern about cross-contamination; we won’t accidentally count algae chromosomes.  We’ll be able to detect if there’s anything strange going on, such as polyploidy.  And, importantly for genome sequencing, we’ll have the basis for physical mapping of genes onto different chromosomes.  This will be a valuable piece of information for further molecular studies.

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