2023-10-01 08:00:00
In studying the mystery of DNA, researchers at Thomas Jefferson University in Philadelphia are revealing fascinating intricacies. Their discovery, published in Nature Genetics, sheds light on the complex mysteries of genetic transmission. Thus, they offer a captivating insight into the mechanisms that guarantee the genetic integrity of our offspring.
Our genetic heritage
In our genetic heritage, mitochondrial DNA comes exclusively from the mother, unlike the rest of our genetic information. The chromosomes, present in the cell nucleus, come equally from our two biological parents, forming 23 distinct pairs.
Each pair includes one maternal and one paternal chromosome. However, mitochondrial DNA is only maternal. For what ? A true DNA mystery.
Mitochondrial DNA, source of energy
Mitochondrial DNA, also known as mtDNA, is of vital importance. It codes for proteins responsible for energy production. Evolution decided to transmit only maternal mitochondrial DNA despite, or perhaps because of, its importance.
This raises a fundamental question: how does the embryo eliminate mitochondrial DNA from paternal mitochondria? It’s a true DNA mystery!
The researchers’ discovery
Researchers at Thomas Jefferson University in Philadelphia recently provided an answer to this DNA mystery. They published it in the journal Nature Genetics on September 18, 2023. The process is fascinating: Sperm lose their mitochondrial DNA during maturation.
At the beginning of this process, the sperm-producing stem cells, called spermatogonia, contain a large amount of mitochondrial DNA. However, as they differentiate into spermatocytes, these cells contain less and less mitochondrial DNA.
At the final stage, the sperm no longer have any detectable mitochondrial DNA at all. This phenomenon, discovered by researchers, explains why sperm do not transmit mitochondrial DNA to their offspring.
The role of mitochondrial transcription factor A (TFAM)
The mystery of the disappearance of mitochondrial DNA from sperm mitochondria is due to the inability of these cells to renew their DNA. In fact, they no longer produce some of the proteins necessary for mitochondrial DNA replication.
Researchers have identified an essential protein, mitochondrial transcription factor A (TFAM), as being involved in this process. Spermatozoa have a TFAM different from that usually observed.
This modified TFAM does not concentrate in mitochondria as it usually does in other cells. Instead of that, it remains in the head of the spermatozoon, in the cell nucleus. This concentration in the nucleus is linked to the disappearance of mitochondrial DNA. Thus, without this protein, the mitochondria are unable to renew their DNA.
The evolutionary reasons for this process
Now that the mystery of precise mitochondrial DNA is understood, scientists are looking into the evolutionary reasons for this process. One hypothesis is that the elimination of paternal mitochondrial DNA avoids heteroplasmy problems.
This phenomenon occurs when different types of DNA coexist in the mitochondrion, causing genetic instability. Another theory suggests that this elimination guarantees fresh mitochondrial DNA in the embryo.
The sperm unfold a lot of energy to reach the oocyte, intensely soliciting their mitochondria. This effort can lead to the accumulation of mutations in the mitochondrial DNA of spermatozoa.
These mutations could be passed on to the embryo, potentially affecting its energy production. On the other hand, the mitochondrial DNA of the oocyte is less stressed, because it draws its energy from surrounding cells. As a result, it remains healthier and provides the embryo with a “brand new engine under the hood.”
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