For those that have been following my blog, I have discussed how life originated from the chemical soup, a process that culminated in the emergence of the first cell from which more complex life gradually evolved. I also discussed the different types of cells that exist, the functions of the different organelles each cell type possesses, and how the cell theory came into existence through the efforts of several researchers. In today's post, I will be taking a critical look at the cell theory. As a form of revision, the cell theory has three major components, which include;
- The cell is the basic unit of life.
- All organisms are made up of cells.
- Cells arose from preexisting cells.
The cell as the basic unit of life
The smallest, indivisible unit of life is the cell. This means that in order to consider an object living, it must be made up of, at least, a cell. Living organisms that are made of a single cell, whether prokaryotic or eukaryotic, are said to be unicellular. Those that are made of 2 or more cells are said to be multicellular.
In most cases, multicellular organisms have their body organized in such a way as to encourage the division of labour and promote efficiency in terms of metabolic activities. Thus, cells that are similar both in structure and functions are organized as tissue. Similar tissues are also organized to form organs while organs come together to form organ systems. However, not all multicellular organisms have their cells organized into tissues, organs, or systems. For example, sponges are made up of multiple cells, but the cells only interact minimally. Hence, sponges are not even at the tissue level.
All living things are made up of cells
As pointed out earlier, the smallest living organism must be made of, at least, a cell. This is one of the reasons viruses are not considered to be living organisms, except when they get into a living host. When they are not in a living host, they are only made up of DNA/RNA enclosed in a protein shell and are referred to as particles, rather than cells. As such, viruses are often referred to as acellular organisms.
Cells arose from preexisting cells
This is the part of the cell theory that completely puts the theory of spontaneous generation to rest. Cells, and life, as a matter of fact, cannot just come from nowhere, except an already existing cell. After the first cell - or group of cells - has been formed from the chemical soup according to the chemosynthetic origin of life, the only way other cells can arise is by emerging from already existing cells and this can only happen by cell division.
A cell can divide and give rise to two (or more) cells. While the old cell has now lost its identity, it cannot be said to have died but only transform to take up new lives. The division of cells happens through a process known as mitosis for vegetative cells and meiosis for sex cells. Vegetative cells are cells that are concerned with the growth and development of the body of organisms while sex cells are concerned with the sexual reproduction of organisms.
During mitosis, the first thing that happens to the cell is the breaking down of the nuclear membrane and the condensation of the genetic materials within the nucleus. Thus, the genetic materials which have condensed and become visible under the microscope as chromosomes now lie freely within the cytoplasm while other organelles move to the periphery of the cell. This stage of the mitotic division is referred to as the prophase.
While all the above is happening, the centrosomes move to the opposite poles of the cell and give off spindle fibers that start gradually to grow and extend towards the center of the cell. The chromosomes then align at the center of the cell known as the metaphase plate. Each chromosome is made of two chromatids, otherwise known as sister chromatids, joined together by a structure known as the centromere. The spindle fibers engage the aligned chromosomes at their centromeres from opposite sides. This completes the stage known as metaphase.
Thereafter, the spindle fibers start retracting back to the centrosomes at the opposite poles, pulling sister chromatids of the same chromosome apart and along with them. Thus, chromatids of the same chromosomes start moving towards opposite poles. This marks a stage known as the anaphase.
The chromatids complete their journey to the poles through the shortening of the spindle fibers at a stage known as telophase. Once this happens, each group of chromatids at the two poles become the genetic material for new cells, and the cytoplasm of the old cell divides (cytokinesis) for the two cells to be independent. This is how cells are able to arise from preexisting cells.
Reproductive cells divide in a way that is a bit different from vegetative cells. The cells first undergo halving and recombination of their genetic materials culminating in the formation of two initial daughter cells. Each daughter cell then undergoes mitosis to give rise to two daughter cells each. Thus, meiosis results in the production of four daughter cells instead of two that result from mitosis.
The cell is the basic unit of life, all organisms are made of cells, and cells can only come from already existing cells. All living organisms are cellular. Viruses, however, exist between the living and nonliving boundaries and are described as acellular organisms. Cells are only able to arise from an existing cell through cell division which happens in several stages. Eventually, the mother cell gives rise to two daughter cells and lost its own identity but perpetuates its existence through the daughter cells. More about meiosis can be found [here[(https://www.yourgenome.org/facts/what-is-meiosis)
Till we meet again in my next post.
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