Heredity: Topic 3 Chromosomes, Mitosis and Meiosis, Questions 1-6

Study Questions for Friday's Quiz

Explain how mitosis fits into the human life cycle, and some reasons that cells divide by mitosis in multicellular organisms.

Mitosis is a process of cell division that results in two genetically identical daughter cells. In the human life cycle, it facilitates growth and tissue repair. Mitosis is the name for a very general type of cell division that takes place throughout the life cycle of humans and all multicellular organisms. The way a human zygote (fertilized egg) begins to develop into a multicellular embryo is by going through many cycles of mitosis. The embryo develops into a fetus, child, and eventually a full grown adult by continuing to go through cell growth and mitosis over a period of years. Even in the body of an adult, mitosis continues to occur. It is used to regenerate cells that are lost from tissues on a continuous basis (for example skin cells and those lining your digestive tract.) Mitosis can also replace cells that are lost due to injury in certain tissues. For example, if you get a cut in the lining of your mouth, those cells will be replaced via mitosis in surrounding cells.

List examples of animal tissues whose cell are "in the cell cycle" as well as examples of animal tissues whose cells have pulled out of the cell cycle.

Examples of tissues in the cell cycle include skin and intestinal epithelium, while neurons and cardiac muscle cells typically exit the cell cycle. In some of the tissues of your body cells are "in the cell cycle" that means they can undergo mitosis. Cells in any epithelial tissue  are in the cell cycle. These include cells in your epidermis (upper skin layer), and cells lining your digestive, respiratory, and reproductive tracts. Cells in your bone marrow and lining your blood vessels are in the cell cycle as well.

 

By contrast some cells in your body have pulled out of the cell cycle, meaning they are no longer capable of dividing. These include specialized neurons (information conducting cells of the nervous system) and some types of muscle fibers.

Explain how DNA molecules are packaged in both dividing and non-dividing cells

In cells that are not actively dividing, the DNA fibers are somewhat loosely coiled around organizing proteins in the nucleus. When the DNA fibers and their organizing proteins are in this state, we refer to them as "chromatin fibers" The entire collection of chromatin fibers within the nucleus are sometimes referred to as that cell's "chromatin."

 

By contrast. in a dividing cell the DNA fibers are tightly coiled around their organizing proteins. When the DNA fibers and their organizing proteins are in this highly condensed state we refer to them as "chromosomes." A cell's chromosomes appear as sausage-like structures when viewed through a microscope.

Name all the stages in the cell cycle in the order in which they occur. Describe the major events that occur in each stage.

The cell cycle consists of two major phases, interphase and the cell division phase (mitotic phase.) Interphase refers to the time between cell divisions and can be subdivided into three stages: G1, S, and G2. A small newly formed cell begins its life in Gl. During G1 it takes up nutrients and grows larger. During the middle stage S, all the cell's DNA is replicated. During the last stage G2, enzymes and other proteins are produced that will help guide the cell through mitosis.

 

The cell division phase includes two major events, mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm.)                                                               

Mitosis occurs in four distinct stages: prophase, metaphase, anaphase and telophase. During prophase chromatin fibers condense into chromosomes, the nuclear envelope breaks down, and the spindle apparatus begins to form. At metaphase the chromosomes are lined up along the equator of the cell. At anaphase sister chromatids within a chromosome separate and migrate toward opposite poles. (Note: once the sister chromatids separate and begin their journey towards opposite poles, we refer to each one as an "unreplicated chromosome.") Telophase begins when the unreplicated chromosomes reach their respective poles. As telophase progresses, chromosomes uncoil back into chromatin fibers, and a nuclear envelope forms around each set of fibers. Cytokinesis overlaps in time with telophase. It involves a ring of contractile fibers forming beneath the plasma membrane which pinches the cell into two smaller cells.

Draw a chromosome as it appears in a cell that is just beginning mitosis. Label the sister chromatids, centromeres and kinetochore. B. Define sister chromatids, centromeres, and kinetochore.

A chromosome in early mitosis appears as two identical strands, called sister chromatids, joined at a region known as the centromere. The kinetochore is a protein structure that forms at the centromere, facilitating the attachment of spindle fibers for chromosome movement during cell division.

Below are illustrations of cells in various stages of the cell cycle. Identify which stage of the cell cycle each one is in. Label the following structures wherever you see them: chromatin, chromosomes, sister chromatids, spindle fibers.

The images depict different phases of the cell cycle, which include interphase, prophase, metaphase, anaphase, and telophase. Structures such as chromatin are seen during interphase, while chromosomes and sister chromatids are visible during mitosis.