In 10 seconds? A new theory trying to explain why most human pregnancies are lost in the first weeks proposes that the reason could be ’selfish chromosomes’ fighting to increase their transmission to the next generation.
Hold on, I’m a bit confused here... Ok, let me explain the basics. While human males produce four equal sperm from one stem cell, females produce only one egg (the others degenerate). This process is called meiosis. During meiosis gametes (aka reproductive cells) acquire only one set of chromosomes (at this stage they are called haploid). And when fertilization happens, the number of chromosomes doubles (representing each parent), and the fertilized egg, the zygote becomes a 'diploid'. Animals have different numbers of chromosomes. For example, mice have 40, monkeys have 48, and humans normally have 46. But often it's not the case with one extra or one missing chromosome. Which results in miscarriage and distress for the parents.
OK… and what’s the discovery about? Well, it is a new theory that has to be proven. When meiosis goes wrong during gamete formation because a chromosome "sneaks" in (or out), the gamete becomes vulnerable. If it degenerates and is lost, nothing more happens. But, if this altered gamete is fertilized, the resulting embryo will be endangered because of the "selfish" behavior of the chromosome that has sneaked in. From the point of view of the selfish chromosome, it is a win-win situation: either the embryo succeeds and develops letting the chromosome be passed on, or it doesn’t which causes the loss of the chromosome’s ‘adversary’.
What are these selfish chromosomes again? OK, let’s go back in time. Since the beginning of the 19th century, selfish genetic elements were described in arthropods, insects, and plants. Genes were treated as the central unit of evolution, whereas individuals became simple vehicles for gene inheritance. In this scenario, when meiosis occurs in the egg stem cells, a specific part of the chromosomes called centromeres tries to win its place in the egg (but contrary to its goal, it will be destroyed).
And when does this phenomenon happen? Well, most human embryos have the wrong number of chromosomes, and one more or less chromosome most likely will lead to the death of the embryo (sadly). This is called aneuploidy and, although the sperm could have a wrong number of chromosomes, the main cause is an error in egg formation, more than 40% of a woman's eggs are aneuploid. So, when an egg with a wrong number of chromosomes is fertilized, the aneuploid embryo could be lost and this happens in the first weeks of development. |
