Honors Biology: Unit 10 - Cell Cycle Learning Targets

1. A. Explain how DNA differs and how it is similar among different organisms (prokaryotes and eukaryotes).

DNA differs among different organisms, namely between prokaryotes and eukaryotes. In prokaryotes, the DNA is smaller in size (single circular chromosome), circular, and located in the cytoplasm because there is no nucleus. In eukaryotes, DNA is larger in size (multiple chromosomes), arranged on the chromosomes, and located in the nucleus of the cell. DNA is similar between prokaryotes and eukaryotes because it is both double stranded, has a double helix, consists of purine (A and G) and pyrimidine (T and C) nitrogen bases, follow Chargaff’s rule, the strands are antiparallel for both, and more.

              B. Compare the processes of cell division in prokaryotes and eukaryotes.( 2pts )

There are some similarities in the process of cell division in prokaryotes and eukaryotes. Both processes involve creating two daughter cells from the parent cell that are genetically identical, replicating DNA, segregating the chromosomes, and cytokinesis. The processes of cell division in prokaryotes and eukaryotes are also different. In prokaryotes, binary fission, or asexual reproduction, is utilized, at least for most prokaryotes. The steps are DNA replication, chromosome segregation, and cytokinesis, or separation. In eukaryotes, reproduction must occur through either mitosis or meiosis. The two major steps of cell division in eukaryotes are mitosis (includes multiple phases) and cytokinesis.

2. Describe the structure and function of DNA. Draw a diagram.( 3 pts)

The structure of DNA consists of its monomers, known as nucleotides. Each nucleotide includes a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases (Adenine, Guanine, Cytosine, or Thymine). DNA is a “double helix” shape, with two nucleotide chains forming a twisted ladder. The two strands of DNA are antiparallel, meaning the sides run in opposite directions. Hydrogen bonds hold nucleotide chains together. The function of DNA is that the sequence of nucleotides determine the amino acid sequence that makes up proteins.

3. A. Explain the process of mitosis. Draw a diagram of each stage.(3pts)

Mitosis (PMAT), or nuclear division, is a series of events that produces two daughter cells with one copy of each chromosome of the parent cell. It is the asexual reproduction of somatic (body) cells. In prophase, chromosomes condense and become visible, the nuclear membrane breaks down and is stored in the vesicles, the centrioles move to opposite ends of the cells, and microtubules form spindle fibers. In metaphase, the replicated chromosomes line up in the middle of the cell (the metaphase plate is located in the middle, or “equator”). In anaphase, an enzyme breaks down the centromere joining the chromatids, and the spindle fibers (kinetochore) pull the sister chromatids apart. In telophase, two nuclear membranes form, the spindle fibers disappear, the chromosomes uncoil, and the cell begins to split, forming a cleavage furrow. In cytokinesis, the division of the nucleus occurs, or the cell completely splits into two. It is technically not mitosis, but the second step in the process of mitosis.

              B. Compare and contrast mitosis in plants and animals.( 1pts)

Mitosis in plants and animals can be similar. In plant cells, the first part of mitosis is relatively similar to animal cells in the fact that they both undergo interphase, prophase, metaphase, anaphase, and telophase with small differences. Mitosis in plants and animals are also very different. For instance, in the prophase stage of animal cells, centrioles move to opposite sides of the cell. Plant cells, however, do not have centrioles, but they do have other structures similar to centrioles. Also, in telophase, plant cells form a cell plate in the middle, creating two new cells. The cell plate later becomes the cell wall when cell division is complete in plants. Meanwhile, in animal cells, cytokinesis just divides the cytoplasm; in plant cells, during the cytokinesis stage, it forms and inserts a cell wall that separates the two new nuclei after mitosis.

4. Explain why cell division is important to living organisms. (1pts)

Cell division is important to living organisms. It allows surface area to keep up with the growing volume of an organism (surface area increases). Cell division also replaces cells that wear out or are damaged, helps with reproduction, and produces many types of cells that serve a variety of functions, or specialization. Cell division requires accurate replication and equal division of genetic information encoded in the cell’s DNA. 

5. Describe the four phases of the cell cycle and what controls it. Draw a diagram. (3 pts)

There are four phases of the cell cycle. They are G1 (Gap 1 or pre-replication), S (DNA Synthesis), G2 (Gap 2 or pre-mitosis), M (Mitosis), and the fifth one is G0 (Gap 0 or nondividing cells). In G1, cells grow, metabolize, make proteins, RNA, and biomolecules. In G0, there are nondividing, mostly adult cells. In synthesis, chromosome or DNA replication occurs. In Gap 2, proteins, RNA, and organelles are made, and the cell gets ready to divide. In mitosis, the division of the nucleus occurs. The cell cycle is controlled by cyclins and kinases. Cyclins bind to kinases, which give the “go ahead” signal at checkpoints of the cell cycle. More specifically, cyclins are proteins that regulate movement through the cell cycle.

6. Describe how the cell cycle is regulated. ( 3pts)

The cell cycle is regulated by cyclins, which are proteins regulating movement through the cell cycle, and kinases, which they bind to. Kinases are the ones to give the “go ahead” signal at checkpoints. Each cyclin activates different kinases, which activate some enzymes directly and signal the cell synthesize other proteins needed to progress to the next phase of the cell cycle. There is also checkpoint control, at G1 and G2 and M; they monitor the condition of DNA, chromosomes, and the mitotic spindle; proteins detect mistakes and any damage, and cell arrest or stoppage of the cell cycle occurs until the mistakes are fixed. At the G1 checkpoint, G1 cyclins accumulate when the cell gets its signals to prepare for cell division, triggering DNA replication. It also determines if the cell should divide, delay division, or enter G0. At the G2 checkpoint, G2 cells accumulate mitotic cyclin. It also triggers the start of M (metaphase), ensuring that DNA replication was a success. The M checkpoint is at the end of metaphase, and triggers exit from mitosis and cytokinesis. The daughter cells produced can then begin G1.

7. A. Summarize the event of DNA replication. ( 2 pts)

There are three steps in the process of DNA replication, which are initiation, elongation, and termination. First, the helicase (enzyme) unwinds the helix, and strand stabilizing proteins keep the DNA unwound. DNA polymerase only moves in the 3’ to 5’ direction, and synthesizes continuously on the leading strand. The primase makes RNA primer so DNA polymerase can attach and discontinuously synthesize a copy of DNA, but in short pieces called Okazaki fragments, still in the 3’ to 5’ direction. Last, the RNA primer is replaced by DNA polymerase, and ligase joins the Okazaki fragments forming a long strand of DNA.

             B. Discuss the importance of correcting DNA replication errors. ( 1 pt)

Correcting DNA replication errors is very important. Otherwise, errors and mutations in the DNA sequence can lead to birth defects, cancer, or other serious diseases. The defense against these errors is that DNA polymerase proofreads its own work and removes and replaces those errors.

8. Explain how the gene mutation in a cell can result in uncontrolled cell division called cancer.  Also, research and explain how the exposure of cells to certain chemicals and radiation increases mutation and thus increases the chance of cancer. ( 1 pt) 

Gene mutation in a cell can result in uncontrolled cell division called cancer. For example, a mutation in proto-oncogenes produces oncogenes, which are cancer genes causing cells to leave G0 and divide even when there is no signal to do so, leading to uncontrolled cell division, resulting in tumors and cancer. Genes also include the information needed to make proteins - with mutations, proteins can malfunction or not be produced at all, which inhibits normal controls of the cell cycle, and cell cycle arrest. The exposure of cells to certain chemicals and radiation increases mutation rates and the chance of cancer. This is because radiation can cause atoms to disassemble and DNA to become damaged in cells, which leads to mutated or dysfunctional cells and proteins, uncontrolled cell division, and cancer. Carcinogens cause cancer by changing a cell’s DNA, causing cells to divide at a faster than normal rate, therefore increasing the chances DNA mistakes can occur as well. For example, UV light from the sun can damage skin cells and increase the risk of skin cancer.