Gene Expression and Genetics for AP Biology
Practice problems for these concepts can be found at: Molecular Genetics Review Questions for AP Biology
Let's cover some vocabulary before diving into this section:
Promoter region: a base sequence that signals the start site for gene transcription; this is where RNA polymerase binds to begin the process.
Operator: a short sequence near the promoter that assists in transcription by interacting with regulatory proteins (transcription factors).
Operon: a promoter/operator pair that services multiple genes; the lac operon is a wellknown example (Figure 11.7).
Repressor: protein that prevents the binding of RNA polymerase to the promoter site.
Enhancer: DNA region, also known as a "regulator," that is located thousands of bases away from the promoter; it influences transcription by interacting with specific transcription factors.
Inducer a molecule that binds to and inactivates a repressor (e.g., lactose for the lac operon).
The control of gene expression is vital to the proper and efficient functioning of an organism. In bacteria, operons are a major method of gene expression control. The lactose operon services a series of three genes involved in the process of lactose metabolism. This contains the genes that help the bacteria digest lactose. It makes sense for bacteria to produce these genes only if lactose is present. Otherwise, why waste the energy on unneeded enzymes? This is where operons come into play—in the absence of lactose, a repressor binds to the promoter region and prevents transcription from occurring. When lactose is present, there is a binding site on the repressor where lactose attaches, causing the repressor to let go of the promoter region. RNA polymerase is then free to bind to that site and initiate transcription of the genes. When the lactose is gone, the repressor again becomes free to bind to the promoter, halting the process.
Because gene expression in eukaryotes involves more steps, there are more places where gene control can occur. Here are a few examples of eukaryotic gene expression control:
Transcription: controlled by the presence or absence of particular transcription factors, which bind to the DNA and affect the rate of transcription.
Translation: controlled by factors that tend to prevent protein synthesis from starting. This can occur if proteins bind to mRNA and prevent the ribosomes from attaching, or if the initiation factors vital to protein synthesis are inactivated.
DNA methylation: addition of CH3 groups to the bases of DNA. Methylation renders DNA inactive. Barr bodies, discussed in Chapter 10, are highly methylated.
These are only a few of the examples of gene expression control that occur in eukaryotes. Do not get lost in the specifics.
The Genetics of Viruses
A virus is a parasitic infectious agent that is unable to survive outside of a host organism. Viruses do not contain enzymes for metabolism, and they do not contain ribosomes for protein synthesis. They are completely dependent on their host. Once a virus infects a cell, it takes over the cell's machinery and uses it to produce whatever it needs to survive and reproduce. How a virus acts after it enters a cell depends on what type of virus it is. Classification of viruses is based on many factors:
Genetic material: DNA, RNA, protein, etc.?
Capsid: type of capsid?
Viral envelope: present or absent?
Host range: what type of cells does it affect?
All viruses have a genome (DNA or RNA), and a protein coat (capsid). A capsid is a protein shell that surrounds the genetic material. Some viruses are surrounded by a structure called a viral envelope, which not only protects the virus but also helps the virus attach to the cells that it prefers to infect. The viral envelope is produced in the endoplasmic reticulum (ER) of the infected cell and contains some elements from the host cell and some from the virus. Each virus has a host range, which is the range of cells that the virus is able to infect. For example, the HIV virus infects the T cells of our body, and bacteriophages infect only bacteria.
A special type of virus that merits discussion is one called a retrovirus. This is an RNA virus that carries an enzyme called reverse transcriptase. Once in the cytoplasm of the cell, the RNA virus uses this enzyme and "reverse transcribes" its genetic information from RNA into DNA, which then enters the nucleus of the cell. In the nucleus, the newly transcribed DNA incorporates into the host DNA and is transcribed into RNA when the host cell undergoes normal transcription. The mRNA produced from this process gives rise to new retrovirus offspring, which can then leave the cell in a lytic pathway. A well-known example of a retrovirus is the HIV virus of AIDS.
Once inside the cell, a DNA virus can take one of two pathways—a lytic or a lysogenic pathway. In a lytic cycle, the cell actually produces many viral offspring, which are released from the cell—killing the host cell in the process. In a lysogenic cycle, the virus falls dormant and incorporates its DNA into the host DNA as an entity called a provirus. The viral DNA is quietly reproduced by the cell every time the cell reproduces itself, and this allows the virus to stay alive from generation to generation without killing the host cell. Viruses in the lysogenic cycle can sometimes separate out from the host DNA and enter the lytic cycle. (Like a bear awaking from hibernation.)
Viruses come in many shapes and sizes. Although many viruses are large, viroids are plant viruses that are only a few hundred nucleotides in length, showing that size is not the only factor in viral success. Another type of virus you should be familiar with is a prion—an incorrectly folded form of a brain cell protein that works its magic by converting other normal host proteins into misshapen proteins. An example of a prion disease that has been getting plenty of press coverage is "mad cow" disease. Prion diseases are degenerative diseases that tend to cause brain dysfunction—dementia, muscular control problems, and loss of balance.