7a. Differentiate DNA from RNA.
As their names indicate, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are both nucleic acids.
- How are DNA and RNA chemically different?
- How are DNA and RNA functionally different?
Any nucleic acid is a polymer made up of monomers called nucleotides. Any nucleotide consists of three components:
- A pentose (five-carbon sugar)
- A nitrogenous base attached to the pentose
- A phosphate group also attached to the pentose
For DNA, the specific pentose in each of its nucleotides is deoxyribose, whereas RNA features ribose as the pentose in each of its nucleotides. There are normally four kinds of DNA nucleotides, because there are four normal nitrogenous bases used in DNA:
RNA also features four kinds of nucleotides, and the nitrogenous bases are nearly the same as for DNA, except that RNA uses uracil instead of thymine.
DNA functions for self-replication (before a cell divides into two cells) and for transcription, a process that produces RNA. There are different functional categories of RNA including mRNA, tRNA, rRNA, and others. Read this overview of DNA and RNA to refresh your knowledge of the similarities and differences between these important macromolecules.
7b. Recognize the phases of mitosis.
- What is mitosis?
- What are the phases of mitosis?
- What types of cells undergo mitosis?
Mitosis is division of a cell nucleus. Since only eukaryotic cells feature a nucleus, only eukaryotic cells undergo mitosis. Mitosis is part of eukaryotic cell division (the other part is cytokinesis, which is division of the cytoplasm). Mitosis occurs in the following phases, listed in the order in which they occur:
Review these phases by watching this lecture on mitosis.
7c. Describe the cell cycle.
The cell cycle includes all parts of the normal lifetime of a single, eukaryotic cell. It starts when a cell is created through the division of a previous cell, and it ends when that cell undergoes its own cell division to produce two new cells.
- What are the phases of the cell cycle?
- What are the major events occurring in each phase of the cell cycle?
A single cell cycle consists of four major phases of unequal length:
- The G1 phase includes most of the normal lifetime of the cell. During G1, the cell goes about using its DNA as instructions for building proteins that allow the cell to metabolize and function for its specific purpose. During this time, the DNA has not yet replicated.
- The S phase is the first step in a cell’s preparation for cell division. During the S phase, the DNA is replicated, yielding two identical copies of the DNA (one for each of two cells that will be created when this cell divides).
- The G2 phase follows the S phase. Like the G1 phase, the G2 phase features much protein synthesis and metabolism, but most of this activity is concentrated on preparing for cell division.
- The M phase includes mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm). By the end of the M phase, two separate cells have been created from the original cell, and each of these two cells enters its own cell cycle.
As you review the cell cycle by reading this section, pay particular attention to the diagram in Figure 1.
7d. Describe what occurs in each of the phases of mitosis.
Recall that mitosis (division of a eukaryotic cell's nucleus) occurs in five phases.
- Prophase: This is the first phase of mitosis. The microtubules that make up the mitotic spindle begin forming on the two centrioles, and those centrioles start to move to opposite poles.
- Prometaphase: Construction of the mitotic spindle is completed, and the nuclear envelope disintegrates, allowing microtubules of the mitotic spindle to connect to replicated chromosomes.
- Metaphase: Replicated chromosomes (each consisting of two identical sister chromatids) move along the spindle tubules until all replicated chromosomes are aligned at the metaphase plate, midway between the poles.
- Anaphase: Each pair of sister chromatids (one pair for each replicated chromosome) separate and move toward opposite poles. At this point, they are no longer called chromatids; rather each is an unreplicated chromosome.
- Telophase: The unreplicated chromosomes reach opposite poles. Each pole becomes a new nucleus, as each pole becomes enclosed by a new nuclear envelope.
In most cases, cytokinesis (division of the cytoplasm) occurs near the end of telophase, in which case the original cell is separated into two distinct cells, each with its own nucleus.
Be sure to review the particular events of each mitotic phase by viewing this lecture.
7e. Explain the purpose of mitosis.
Mitosis is a particularly important event in the eukaryotic cell cycle.
- Why is mitosis important?
- Under what conditions does a cell undergo mitosis?
Recall that mitosis is division of a eukaryotic cell’s nucleus. One of the hallmarks of mitosis is that when the original nucleus divides, two genetically identical nuclei are produced. That is, the chromosomes in one nucleus are exactly the same as the chromosomes in the other nucleus. Moreover, each nucleus is genetically identical to the nucleus in the original cell (before mitosis). Therefore, mitosis creates two nuclei from one, and each of those nuclei can serve as the nucleus of a new cell. When cytokinesis accompanies mitosis, the cytoplasm divides, forming two distinct cells. Each cell contains its own nucleus (created by mitosis). Cell division that features mitosis allows one parent cell to divide into two, genetically identical daughter cells. Creating new cells is important for several reasons:
- Mitosis allows for cell proliferation for the purpose of development of a unicellular zygote into a multicellular organism.
- Mitosis allows for cell proliferation for the purpose of growth of a multicellular organism.
- Mitosis allows for creation of new cells to replace cells that have been damaged by injury or infection in a multicellular organism.
- Mitosis is a means for a unicellular, eukaryotic organism to reproduce.
As you review mitosis and appreciate its importance, think about how mitosis (as part of the M phase) fits into the cell cycle.
Unit 7 Vocabulary
This vocabulary list includes terms that might help you with the review items above and some terms you should be familiar with to be successful in completing the final exam for the course.
Try to think of the reason why each term is included.
- Cell Cycle
- Deoxyribonucleic Acid
- G1 Phase
- G2 Phase
- M Phase
- Metaphase Plate
- Mitotic Spindle
- Nitrogenous Base
- Nuclear Envelope
- Nucleic Acid
- Ribonucleic Acid
- S Phase