The amount of DNA (measured in ng per mg ventricle) decreased. One nucleus is present in each vertebrate myocardial cell (each cell is mononucleated). Explain how it is possible that a decrease in mass-specific DNA concentration indicates growth of ventricular tissue.
The decrease in mass-specific DNA concentration indicates growth of ventricular tissue due to two factors: cell division and cell enlargement.
First, let's break down the process. In the vertebrate myocardial cells (cells of the heart muscle), each cell contains only one nucleus and is therefore mononucleated. As the ventricular tissue grows, new cells are produced through cell division, known as mitosis. This means that the existing myocardial cells replicate themselves by dividing into two daughter cells, each with a single nucleus.
During cell division, the total amount of DNA is distributed equally between the two daughter cells. Therefore, the individual cells resulting from cell division will have a lower concentration of DNA compared to the original cell. This is because the same amount of DNA is now spread across a larger number of cells. Consequently, the mass-specific DNA concentration decreases.
Secondly, as the ventricular tissue grows, individual myocardial cells can also increase in size through a process called cell enlargement or hypertrophy. In hypertrophy, the size and volume of each cell increases due to an increase in protein synthesis and organelles. While the overall amount of DNA remains constant in an enlarged cell, the mass-specific DNA concentration decreases because the total mass of the enlarged cell has increased.
To summarize, the decrease in mass-specific DNA concentration indicates growth of ventricular tissue because it is a result of both cell division, which leads to more cells with a lower individual DNA concentration, and cell enlargement, which increases the overall mass of the cells while keeping the DNA amount constant.