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The secret of aging! The secret of aging! Inside the center or nucleus of a cell, our genes are located on twisted, double–stranded molecules of DNA called chromosomes. At the ends of the chromosomes are stretches of DNA called telomeres, which protect our genetic data, make it possible for cells to divide, and hold some secrets to how we age. Over time, due to each cell division, the telomere ends become shorter. When they get too short, the cell no longer can divide and becomes inactive or "senescent" or dies. This process is associated with aging. Researchers can use the length of a cell's telomeres to determine the cell's age. This is important in anti–aging research. Thus, cell division plays several important roles in the life of an organism, which you are going to study here extensively.

Learning Objectives

After completing the topic, the student will be able to:

  • Predict what happens if the cell does not undergo cell division and appreciate the importance of cell division.
  • Define cell division and understand the key roles of cell division.
  • List the phases of cell cycle and explore the significance of each phase.
  • Define mitosis and list the different phases involved in it and examine how cytokinesis is different for animal cells and plant cells.
  • List and describe the factors that influence the rate of cell division.
  • Define binary fission and evaluate the steps involved in it and investigate how researchers use bacterial binary fission method to produce gene of interest.
  • Understand how complete cell cycle is controlled by molecular control system.
  • Define check points and understand how these are helpful to regulate the cell cycle.
  • Investigate what happens if cell cycle loses its control over its regulation.
Cell divisionCell division is the process where a parent cell divides into two/more daughter cells. Cell division is considered as a small segment of a cell cycle. This process of cell division in prokaryotes is known as binary fission and the cell division in eukaryotes is known as mitosis. Another type of cell division in eukaryotes is called meiosis, here a cell is permanently transformed into a gamete and does not divide again until fertilization, it undergoes DNA replication. Cell division help to function in renewal and repair, replacing the dead cells from normal wear and tear or accidents in a fully grown organism.
Cell division

Every cell that is capable of undergoing division passes through a cyclic sequenceof events involving growth and division and is called as cell division or cell cycle. It encompasses the entire sequence of events that occur in a cell from the time it is formed from its parent cell till the time of its own division into daughter cells.

The key roles of cell division:
The ability of organisms to reproduce their own kind is the one characteristic that best distinguishes living things from nonliving matter. This unique capacity to procreate, like all biological functions, has a cellular basis.

Rudolf Virchow, a German physician, put it this way in 1855: Where a cell exists, there must have a preexisting cell, just as the animal arises only from an animal and the plant only from plant. The continuity of life is based on the reproduction of cells, or cell division. Cell division plays several important roles in the life of an organism. When a unicellular organism, such as an amoeba, divides and forms duplicate offspring, the division of one cell reproduces an entire organism.

Cell division also enables sexually reproducing organisms to develop from a single cell- the fertilized egg, or zygote. And after an organism is fully grown, cell division continues to function in renewal and repair, replacing cells that die from normal wear and tear or accidents. For example, dividing cells in your bone marrow continuously make new blood cells. Passing identical genetic material to cellular offsprings is a crucial function of cell division.

Cell division results in genetically identical daughter cells A cell is not like a soap bubble that simply enlarges and splits in two. Cell division involves the distribution of identical genetic material - DNA ‐ to two daughter cells. What is most remarkable about cell division is the fidelity with which the DNA is passed along from one generation of cells to the next. A dividing cell duplicates its DNA, allocates the two copies to opposite ends of the cell, and only then splits into daughter cells.

Distribution of chromosomes during cell division Assortment of chromosomes during cell division Cell division is an important process in every organism's life. Cell division also enables sexually reproducing organisms to develop from the single-celled zygote, where it was produced by cell division from gametes. After its growth, cell division helps to function in renewal and repair, replacing the dead cells from normal wear and tear or accidents in a fully grown organism.
Cellular organization of the genetic material

A cell's endowment of DNA, its genetic information, is called its genome. Although a prokaryotic genome is often a single long DNA molecule, eukaryotic genomes usually consist of a number of DNA molecules. The overall length of DNA in a eukaryotic cell is enormous. A typical human cell, for example, has about 2m of DNA- a length about 250,000 times greater than the cell's diameter. Before the cell divides, all of this DNA must be copied and then the two copies separated so that each daughter cell ends up with a complete genome. The replication and distribution of so much DNA is manageable because the DNA molecules are packaged into chromosomes, so named because they take up certain dyes used in microscopy. Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus.

When a cell is not dividing, and even as it duplicates, its DNA is in preparation for cell division. Each chromosome is in the form of a long, thin chromatin fiber becomes densely coiled and folded, making the chromosomes much shorter and so thick, that we can see them with a light microscope.

Each duplicated chromosome has two sister chromatids. The two chromatids, each containing an identical DNA molecule, are initially attached by adhesive proteins all along their lengths. In its condensed form, the duplicated chromosome has narrow “waist” at a specialized region called the centromere where the two chromatids are most closely attached.

Later in the cell division process, the two sister chromatids of each duplicated chromosome separate and move into two new nuclei, one at each end of the cell. Once the sister chromatids separate, they are considered individual chromosomes. Thus, each new nucleus receives a group of chromosomes identical to the original group in the parent cell.