14. Sometimes a mistake occurs when cells divide, causing errors in the number of chromosomes a person has
"A person" probably means that we're discussing humans. "When cells divide" means we're talking about mitosis and/or meiosis. These are the names for two processes of cell division. Mitosis is the process by which one cell divides into two. You might be familiar with this process because it produced you. When you were a just-fertilized egg (a one-cell embryo), that cell had to divide to produce two cells. And two cells divided to produce four. And, pretty soon, you were you. Meiosis also involves cell division, but doesn't have the task of producing two genetically identical daughter cells. Instead, meiosis happens to a select few of our cells: those cell divisions produce our gametes (sperm from males, eggs from females). In general, meiosis differs from mitosis because meiosis involves two serial rounds of cell division.
Either way, cell division is critical to the study of genetics, because it is essential to ensure that the cells produced by division have the same number and composition of chromosomes. For example, humans have 23 different chromosomes, and our cells typically have two of each (we get one of each of the 23 chromosomes from our father at fertilization; one each of the 23 from our mother) = 46 chromosomes. Even seemingly minor changes in the number of chromosomes we inherit can have significant effects. Individuals that do not have two copies of each chromosome are referred to as "aneuploid."* For example, if you happen to get an extra copy of chromosome 21, then you probably have Down syndrome. For most of our 23 chromosomes, having one extra (3) or one too few (1) is not even compatible with life. It could also be that a common cause of spontaneous abortions is imbalance in chromosome number in a developing fetus.
So, I already knew that errors in the way chromosomes are distributed at cell division could cause such errors. The Fact is definitely true. But, that's my opinion. I just "know" this because all of my textbooks say so. Should I trust those sources? Are they, themselves, based on data and facts? Would undergraduate genetics students find research-based sources to support this Fact? Absolutely.
Student Responses
Of all responses, I'll focus on two. One showed that a problem in the equal division of chromosomes at cell division leads to aneuploidy in one cell type (Yang et al. 2003). A more recent study (Holubcová et al. 2015) made the striking discovery that a cellular mechanism overseeing chromosome sorting into human eggs is itself prone to errors.
Student Decision: Fact or Fiction?
Fact (overwhelmingly)
Nota bene
Amazingly, a particularly relevant research paper was published days after I assigned this Fact to my students for analysis. So, I'll briefly describe what we all just learned from this new publication.
For context: many of us have probably heard that there is a positive correlation between the age of a woman and the probability that she has a child with a genetic disorder. In other words, the older a woman is, the more likely she might be to miscarry or to have a child with aneuploidy (like Down syndrome).
Well, Gruhn et al. (2019) just showed that this pattern exists not just for older women but also for young women! They describe that "aneuploidy follows a U-curve." In other words, the eggs produced by both young and old women are more likely to have chromosome imbalances, with the mid- to late-20s appearing to be the ideal time to have children with the lowest probability of this type of chromosome imbalance. Moreover, they find that problems in different cellular mechanisms result in higher rates of aneuploid eggs in young women compared to aneuploid eggs of older women. That is, the Fact is true: not only do cell division mistakes cause errors in the number of chromosomes inherited by a person, but also the rate of such mistakes depends on the age of the mother. In all fairness, I'm not aware (yet) of studies that also look at the same issue in terms of the quality of sperm production by males, but there are biological reasons to think that cell divisions in males that produce sperm would not be subject to the same issues that might face females and the production of eggs.
In sum, the ability of our cells to produce eggs (and maybe sperm, too) that contain the correct number of chromosomes is not perfect.
* the sex chromosomes (X and Y) do not meet this strict definition of aneuploidy, because male humans typically have one X and one Y chromosome (hence they have one fewer X than females, who have two X chromosomes, and they have one more Y than females, who have zero).
Literature Cited
- Gruhn et al. (2019) "Chromosome errors in human eggs shape natural fertility over reproductive life span" Science 365:1466-1469.
- Holubcová et al. (2015) "Error-prone chromosome-mediated spindle assembly favors chromosome segregation defects in human oocytes." Science 348(6239):1143-1147.
- Yang et al. (2003) "Chromosome Segregation Defects Contribute to Aneuploidy in Normal Neural Progenitor Cells" J. Neurosci. 23(32):10454-10462
No comments:
Post a Comment