Introduction
Scientists use models to represent natural structures and processes that are too small, too large, or too complex to investigate directly. Scientists develop their models from observations and experimental data usually accumulated from a variety of sources. Building a model can represent the culmination of a body of scientific work, but most models represent a well-developed hypothesis that can then be tested against the natural system and modified.
Linus Pauling's novel and successful technique of building a physical model of hemoglobin was based on available chemical data. This technique was later adopted by Francis Crick and James Watson to elucidate the nature of the hereditary material, DNA. Watson and Crick built a wire model utilizing evidence collected by many scientists. They presented their conclusions about the structure of the DNA helix in the journal Nature in April 1953 and were awarded the Nobel Prize for their discovery in 1962.
Today in lab you will work with a partner to build models of cell division: mitosis and meiosis. Using these models will enhance your understanding of the behavior of chromosomes, centrioles, membranes, and microtubules during the cell cycle. After completing your model, you will consider ways in which it is and is not an appropriate model for the cell cycle. You and your partner should discuss activities in each stage of the cell cycle as you build your model. After going through the exercise once together, you will demonstrate the model to each other to reinforce your understanding.
In the model of mitosis that you will build, your cell will be a diploid cell (2n) with four chromosomes. This means that you will have two homologous pairs of chromosomes. One pair will be long chromosomes, the other pair, short chromosomes. (Haploid cells have only one of each homologous pair of chromosomes, denoted n.)
During interphase, a cell performs its specific functions. Liver cells produce bile; intestinal cells absorb nutrients; pancreatic cells secrete enzymes; skin cells produce keratin. Interphase consists of three subphases, Gl, S and G2, which begin as a cell division ends. As interphase begins, there is approximately half as much cytoplasm in each cell as there was before division. Each new cell has a nucleus that is surrounded by a nuclear envelope and contains chromosomes in an uncoiled or decondensed, state. In this uncoiled state, the mass of DNA and protein is called chromatin.
Procedure
In the Gl phase, the cytoplasmic mass increases and will continue to do so throughout interphase. Proteins are synthesized, new organelles are formed, and some organelles such as mitochondria and chloroplasts grow and divide in two. Throughout interphase one or more dark, round bodies, called nucleoli (singular nucleolus), are visible in the nucleus.
Unique activities taking place during the S phase of the cell cycle are the replication of chromosomal DNA and the synthesis of chromosomal proteins. DNA synthesis continues until chromosomes have been duplicated. Each chromosome is now described as double-stranded, and each strand is called a sister chromatid.
Centriole duplication begins in late G1 or early S phase.
During the G2 phase, in addition to continuing cell activities, cells prepare for mitosis. Enzymes and other proteins necessary for cell division are synthesized during this phase.
How many pairs of homologous chromosomes are present in your cell during this stage of the cell cycle?
In the M phase, the nucleus and cytoplasm divide. Nuclear division is called mitosis. Cytoplasmic division is called cytokinesis. Mitosis is divided into five subphases: prophase, prometaphase, metaphase, anaphase, and telophase.
Procedure
Prophase begins when chromosomes begin to coil and condense. At this time they become visible in the light microscope. Centrioles continue to move to opposite poles of the nucleus, and as they do so, a fibrous, rounded structure tapering toward each end, called a spindle, begins to form between them. Nucleoli begin to disappear.
During prometaphase chromosomes continue to condense. The nuclear envelope breaks down as the spindle continues to form. Some spindle fibers become associated with chromosomes, and the push and pull of spindle fibers on the chromosomes ultimately leads to their movement to the equator. When the centromeres lie on the equator, prometaphase ends and the next phase begins.
How many double-stranded chromosomes are present in your prophase/ prometaphase nucleus?
Note: Students often find it confusing to
distinguish between chromosome number and chromatid number. To
simplify this problem, count the number of centromeres. The number
of centromeres represents the number of chromosomes.
In metaphase, double-stranded chromosomes lie on the equator (also called the metaphase plate). The two sister chromatids are held together by the centromere. Metaphase ends as the centromere splits.
After the centromere splits, sister chromatids separate and begin to move toward opposite poles. Chromatids are now called chromosomes. Anaphase ends as the chromosomes reach the poles.
Note the movement of the chromosome arms as you move the centromeres to the poles.
As chromosomes reach the poles, anaphase ends and telophase begins. The spindle begins to break down. Chromosomes begin to uncoil and nucleoli reappear. A nuclear envelope forms around each new cluster of chromosomes. Telophase ends when the nuclear envelopes are complete.
How many chromosomes are in each new nucleus?
How many chromosomes were present in the nucleus when the process began?
The end of telophase marks the end of nuclear division, or mitosis. Sometime during telophase, the division of the cytoplasm, or cytokinesis, results in the formation of two separate cells. In cytokinesis in animal cells, a cleavage furrow forms at the equator and eventually pinches the parent cell cytoplasm in two. In plant cells, a cell plate begins to form in the center of the equatorial plane and grows until it eventually extends across the cell, dividing the cytoplasm in two. Cell wall materials are secreted into the space between the membranes of the cell plate.