First of all, lets talk about fertilisation n a way that is known to all of us; all cells have two copies of 23 chromosomes – one copy is inherited from the mother and the other copy is inherited from the father. This must mean that when an egg or sperm is created, each copy of the 23 chromosomes have an equal chance at being passed on. However, a team from the University of Pennsylvania seem to have disproven this universally-accepted theory.
Scientists have been aware for a long time that genetic elements ‘compete’ during meiosis (which can be seen from the differences in transmission rate to the gamete), however the team of scientists from the university of Pennsylvania now think that some chromosomes can, infact, ‘cheat’ and increase their chance of making it into a sex cell by creating asymmetry in meiosis and moving themselves to the right side of the cell during anaphase in order to be in the egg.
The scientists hope to use this discovery to lead to a more confident and better understanding of meiosis and why mistakes / cheating occurs. As said by Michael Lampson, associate professor of biology and senior author of this study…
“If we understand how these selfish elements are exploiting the mechanics of meiosis, then we’ll understand more deeply how that process works in the first place. Usually we think about selfish genes at the level of natural selection and selection of the fittest, that might mean a gene that makes you live longer or reproduce more or kill your enemies is more likely to be passed on. But we can also think about selfishness at the level of the gene itself. In that context, genes are competing with each other to get into the gamete. And while we had evidence that this could happen, we didn’t really understand how it did happen.”
This finding was discovered through experimentation of microtubules in mouse gametes, where they found that the asymmetry in the cell was only present at the anaphase stage, when the spindle movers towards the cortex from the middle of the cell. This means that some sort of signal is being transmitted from the cortex, telling the signal to move towards it in an asymmetrical fashion. But what is that signal? Scientists are yet to figure that out.
Lampson summarised the finding of the study quite nicely:
“If you’re a selfish centromere and you’re facing the wrong way, you need to let go so you can face the other way, that’s how you ‘win.’ This work gave us some good information about biased transmission of centromeres, but it also brings up a ton of other questions: Why do our centromeres look the way they do, and how do they evolve to win these competitions? These are fundamental biological questions that we still don’t know a lot about.”
Hopefully, scientists will find out the answers to these questions very soon and be able to distinguish between the characteristics of strong centromeres and weak centromeres.
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