Counting Chromosomes: Accidents in Cell Division

A 50-year-long piece of scientific knowledge on the correct number of chromosomes was found to be inaccurate in April 1956, when genetic biologist Joe Hin Tjio published his findings on the true number of chromosomes (Barnes). For more than five decades, the magical number was 48, and Joe Hin Tjio also believed that there were 48 chromosomes until his fortunate accidental discovery; his serendipity changed the scientific world forever.

Dr. Tjio’s techniques were not new; for decades, biologists have been trying to count the number of chromosomes by observing testicular tissues on a glass slide under a microscope (Arney). Adjusting the procedure as needed, Dr. Tjio used an embryonic cell from lung tissue and produced a clear microscopic image, expecting the number of chromosomes to be 48, as believed to be (Barnes). Startled to find 46 chromosomes instead of 48, he began a massive counting experiment: using over more than 250 cells, he came to the conclusion of the definitive fact that humans have 46 chromosomes, not 48 (Arney). The impact of this discovery is astounding. Ever since humans discovered the normal number of chromosomes, what is considered genetically “abnormal” was slowly revealed.

Accidents happen without reasonable opponents to blame, but with definite consequences. When accidents happen in cell division, humans can end up having more or fewer chromosomes; sometimes, the chromosomes are merely in the wrong place. According to the National Human Genome Research Institute, chromosomal abnormalities are deviations from normal chromosome karyotypes. Chromosomal abnormalities are also separated into numerical and structural abnormalities (“Chromosome Abnormalities Fact Sheet.”). Numerical abnormalities occur when there are additional or missing chromosomes in a human body. Down syndrome is a common example of a numerical abnormality: there are three copies of chromosome 21 due to an additional chromosome. In contrast, structural abnormalities are alternations of the chromosomes through deletion, duplication, translocation, inversion, or formation of rings. Chromosomes with structural abnormalities may either have longer or shorter chromosomes, or shapes that deviate from the normal chromosomes. Distinguishing between these two types of chromosomal abnormalities has helped scientists and biologists narrow down the possible phases in cell division where these accidents happen.

Specifically, nondisjunction is the process that is responsible for many chromosome abnormalities, according to Connor Kerndt, who studied at the Michigan State University College of Osteopathic Medicine. Nondisjunction is the failure of separation of chromatids and can happen in mitosis or meiosis, the two processes of cell division. Ilona Miko, who has a bachelor's in biology from Barnard College, explained that mitosis happens in somatic cells, which are cells that are not involved in the production of reproductive cells. Mitotic nondisjunction can happen in anaphase, the third stage of mitosis. Normally, sister chromatids would separate during this stage by small structures called microtubules (Kerndt). They pull each sister chromatid to the opposite side, completely separating them and preparing them to be the chromosomes of the dividing cells.

Recent studies have revealed that a complex process involving condensin, separase, and topoisomerase II is responsible for the failure of the separation of sister chromatids (Kerndt). Tatsuya Hirano, who has a Ph.D. in molecular biology from Kyto University, further explains this discovery. Separase is a substance that triggers chromosome separation by hydrolyzing condensin, which is a protein that holds chromatids together after their synthesis (Hirano). When separase is not released and inhibited in the cell, failure of separation can happen. As mentioned earlier, topoisomerase also plays a significant role in chromosome separation, which is why its dysfunctionality can lead to nondisjunction, according to Natassja Bush, who works in the Department of Biological Chemistry at the Babraham Institute. Its major role in chromosome segregation explains why its absence or dysfunctionality also leads to nondisjunction (Bush). Mitosis occurs before meiosis, which is also why accidents in mitosis are crucial (Kerndt).

Chromosomal abnormalities are accidents. Humans can trace back to the substances responsible for these aberrations, but there is no specific reason for these phenomena. They are also irreversible, and currently, there are no treatments for chromosomal disorders. However, humanity took one step forward in alleviating the lives of many, many people when the scientific discovery was made. By pursuing more understanding, more depth, and more appreciation in the world of science, humanity is once again becoming closer to bringing a positive change for all.