Cell Cycle and Cancer
Investigation 2 – Concept Day
Cell Cycle and Cancer: Investigation 2
This Investigation will largely be a review of cell growth and division for you. Most of the material in this Investigation was first seen in the CELL Genes and Proteins.
The new and important information presented in Investigation 2 is the control of the process of cell division, that is, control of the cell cycle. In the CELL Genes and Proteins, you learned about one of the four phases of the cell cycle, mitosis. In this Investigation, you will see that mitosis actually occupies very little of an individual cell’s lifetime, perhaps only as small as a few percent.
While this appears to be a rather lengthy presentation, containing a total of 17 slides, most of these slides are introductory in nature or a review of slides that you likely have seen in the past.
- This is a simple introductory slide to cell division and replication. It shows an artist’s representation of the final step in cell division, the cytokinesis event of mitosis.
- This is another introductory slide to cell division and replication. It shows an artist’s representation of an overview of all of the steps in mitosis while eliminating most detail. It does, however, suggest the most important point of cell division, namely that one cell goes through several steps to produce two exact copies of itself.
- Cell division is essential for the growth of a multicellular organism. It is also essential for repairing tissue damage in higher organisms. These two aspects of cell division are essential for life. However, as we will see, uncontrolled cell division can cause a life-threatening disorder known as cancer.
- With this slide, we begin a several-slide walk through the cell cycle. Since it is a cycle, we could begin our discussion anywhere, but we will choose the G1 phase as our start here.
- At the beginning of G1, the cell has just come out of mitosis (M). As such, each cell contains two copies of each chromosome and gene (the products of mitosis).
- It is during G1 that the cell increases in size. This, of course, requires the synthesis of many new proteins and other metabolic products. Thus, as the cell increases in size, it is in one of its most metabolically active states.
- In order to make proteins to increase cell volume and size, RNA must be transcribed from the DNA molecule in the cell’s nucleus and proteins must be translated from the RNA templates in the cell cytoplasm. These two important biosynthetic processes were discussed in detail in the CELL Genes and Proteins but are nonetheless repeated in the next two slides as a refresher and review.
- Transcription is the process by which RNA messages are made from a DNA template. Remember that while proteins do all of the important work of cells, they cannot be made directly from DNA. An intermediate step is required in which the DNA molecule makes a molecule called RNA (ribonucleic acid) that retains all of the original base code information from the DNA molecule. The process by which RNA is made from DNA is called transcription. The process of transcription takes place in the cell nucleus.
- In essence, a specific area of a double-stranded DNA molecule is selected and its base pairs are broken. The new bases that bind to the unpaired DNA bases are adenine, guanine, cytosine, and uracil: there is no thymine in RNA molecules. Each location where thymine would be inserted, it is replaced in RNA as uracil. A new backbone will be formed to join the new sequence of bases into a new RNA molecule. The RNA backbone, like the DNA backbone, is composed of phosphate and sugar. However, in the case of RNA, the sugar is ribose instead of deoxyribose as in the case of DNA.
- Once transcription is complete, the DNA repairs and rewinds to form the original twisted helix. Many different areas of a single DNA molecule may undergo transcription at the same time. The stretch of DNA that codes for the transcription of an RNA molecule is a gene and will ultimately produce a specific protein through the process of translation shown on the following slide.
- This slide shows the process of protein translation. After they are made in the nucleus from a DNA template, newly formed RNA molecules (called transcripts) leave the nucleus through pores in the nuclear membrane (nuclear pores) and enter the cytoplasm of the cell where important components required for protein synthesis are located. These components are many and include amino acids and ribosomes.
- The sequence of each RNA molecule transcript is arranged into repeating groups of three bases. The RNA base triplets are called codons. It is this sequence of codons, dictated by the original DNA molecule and inherited through chromosomes from an individual’s parents, that directs the synthesis of thousands of different protein molecules.
- Ribosomes are very large complexes composed of many different proteins and other molecules. They bind to RNA transcripts in the cytoplasm. The ribosome recognizes the codons arranged along the RNA transcript, each of which codes for a specific amino acid (see code on the wheel on the lower left of this slide). There are 20 different amino acids. The ribosome moves along the RNA transcript and forms a chain of amino acids in a sequence dictated by the codon sequence. The newly formed chain of amino acids is a protein.
- Protein synthesis continues as the ribosome moves along the RNA transcript until a specific codon (the stop codon) is reached. This codon tells that ribosome that the amino acid sequence information for the protein being made is now complete. When reaching the stop codon, the ribosome detaches from the RNA transcript, and the synthesis of that particular protein is complete. The protein is then released from the ribosome.
- Once the cell grows in G1, it may stay at this point for an indefinite period of time, generally until it is time to divide again. When this time comes, the G1 cell enters the S (DNA synthesis) phase.
- In the S phase, the cell makes an exact copy of all of the DNA contained in its nucleus by the process of DNA replication. We will once again review this process on the next slide.
- As a result of DNA replication, at the end of S phase, each chromosome in the nucleus consists of 2 identical sister chromatids that contain all of the genetic information required for the next round of mitosis.
- The replicated chromosomes at the end of S phase on this slide are presented in condensed forms. Condensation does not take place until M phase. The condensed forms are used to help you understand more easily the replication process.
- DNA replication occurs during the S phase. This process must occur prior to any cell division so that both new cells each receive the exact same copy of the DNA molecule packaged into the chromosomes.
- The important final result of the DNA replication process is that the entire DNA molecule faithfully reproduces an exact copy of itself. This is the molecular basis of reproduction. Replication of DNA occurs in every DNA molecule in every chromosome during the S phase.
- After DNA replication occurs during the S phase, the cell enters the G2 phase. At this point, all of the genetic information required for cell division is present and the cell is large enough to successfully divide. Therefore, there is relatively little metabolic activity in G2.
- Upon signaling from specific cellular proteins, the G2 cell will enter the M phase and mitosis will proceed to completion.
- This slide highlights the M Phase (mitosis phase) of the cell cycle. In this phase, the sister chromatids produced during the S phase are separated into the two new cells. This provides all of the genetic information required for the two new cells to survive, grow and divide again in the future if required.
- At the end of mitosis, we finish one complete turn of the cell cycle and the newly divided cells enter the G1 phase of the cell cycle. There they will grow and await protein signals to leave G1 and proceed into the next phase of the cycle for another round of cell division.
- This slide provides an overview of the various steps of mitosis. Notice that interphase is at the top of the diagram. Interphase is essentially the state of the cell during G1 through G2. Once the cell receives protein signals to enter mitosis (M phase of the cell cycle), it moves directly into prophase.
- We have studied the steps and phases of mitosis in the CELL Genes and Proteins in considerable detail. The next several slides are simply meant to refresh your memory of the various phases of the mitotic sequence.
- This slide is a review of the prophase stage of mitosis: During prophase, the nuclear membrane dissolves and centriole fibers begin to form in the cytoplasm. Also at this stage, the DNA chromosomes become condensed and readily detectable by light microscopy.
- This slide is a review of the metaphase step of mitosis: During metaphase, the chromosomes line up on a plane in the middle of the cell. In addition, spindle fibers stretch from centrioles at opposite poles of the cell and attach to a central domain of each pair of sister chromatids called kinetochores (shown here as a dark spot on each of the chromosomes).
- This slide is a review of the anaphase step of mitosis: During anaphase, the spindle fibers pull the sister chromatids apart and away from each other, towards opposite poles of the cell. With the genetic information thus segregated, the cell and nuclear membrane can be reformed in the next step of mitosis.
- This slide is a review of the telophase step of mitosis: During telophase, the spindle fibers break down and a nuclear membrane forms around each of the two new groups of chromosomes. Once the chromosomes are safely secured in nuclear membranes at opposite ends of the cell, the cytoplasm of the cell is “pinched” into two new cells by a complex process known as cytokinesis.
- We now return to an overview of the entire cell cycle. As suggested on this slide, the movement from one phase of the cell cycle to the next: G1 to S, S to G2, and G2 to M is dependent upon cell cycle control proteins.
- When under perfect control, cells flow through the cell cycle as required for the growth of the organism or repair of localized tissue damage. Fortunately, this is the normal sequence of events that occurs by the vast majority of cells over the vast majority of their lifetimes. It is one of the most perfectly balanced occurrences in the biological world.
- Unfortunately, sometimes mutations occur in the cell cycle control proteins and the cell moves from phase to phase of the cell cycle, dividing and dividing with very little control. Such uncontrolled movement around and around the cycle can result in cancer.
- This final slide shows what can happen when normal cells in a tissue lose control of the cell cycle. The red cells pictured here represent normal cells. Under cell cycle control, they only divide when necessary and cell division is dictated by cell cycle control proteins produced by the cell. This results in normal, organized, and functional tissue.
- Loss of cell cycle control is seen in the blue cells in this drawing. Due to problems with the cell cycle control proteins, these cells divide and divide repeatedly regardless of the needs of the normal tissue. As a result, a tumor composed of cancer cells may develop. Unlike the normal cells they were derived from, cancer cells result in an unorganized, functionless mass of cells called a tumor. We will study tumors in greater detail in Investigation 3.