Microorganisms and Viruses - AP Biology

Viruses are unique organisms in that they have a relatively simple cell structure. They have an outer protein coat, which sets them apart from other organisms. Peptidoglycan is found in bacteria, lipid bilayer is found in eukaryotic cells, and chitin is found in fungi. Membrane-bound orangelles are found in more complex cells, such as eukaryotic cells, and are not unique to viruses.

A lytic virus does not require incorporation into the host cell's DNA. Instead, a lytic virus must attach to it's host cell and enter. Once the virus enters, it can take over the hosts' cellular proteins to replication. After replication, the copies will burst from the host cell and spread throughout the extracellular environment.

Bacteria and fungi both have DNA, a plasma membrane, and a cell wall. The bacterial cell wall is made of peptidoglycan, whereas the fungal cell wall is made of chitin. Bacteria do not have nuclei and are only unicellular, while some fungi are multicellular, they are eukaryotes, and thus all have nuclei. Furthermore, eukaryotes have membrane-bound organelles. Since bacteria are prokaryotes, they lack membrane-bound organelles, but they still have ribosomes for protein synthesis (ribosomes are not membrane-bound).

Phytoplankton are the collection photosynthetic organisms (eukaryotic and prokaryotic) that are the primary producers that form the basis of most marine food chains. Zooplankton, fish, and even large whales feed on phytoplankton. Since phytoplankton are photoautotrophs, they need sunlight to perform photosynthesis, thus they are not found in the dark depths of the ocean.

Diatoms are unicellular phytoplankton that are incased in silica cell walls. These silica shells fall to the bottom of the ocean. Dinoflagellates are the bioluminescent phytoplankton with two flagella and are responsible for red tides. Diatoms have clear silica (glass) cell walls to allow sunlight into them, where they contain different pigments that absorb sunlight at various wavelengths. Note that although diatoms may form colonies, they are unicellular.

For the most part, protists are very valuable to humans. They form the base of a large portion of aquatic food chains, and without them many ecosystems would collapse. However, some protist species have caused—and continue to cause—profound problems to humans. Of the given answers, the only one not caused by a species of protist is AIDS. AIDS is caused by a retrovirus, which means that it is an RNA virus that uses reverse transcription to replicate itself in a host cell.

Though all of these answers are true of at least some protists, the most generally applicable statement is that protists generally prefer to live in environments that contain plenty of water. Species within the kingdom Protista are believed to have given rise to the other kingdoms of Eukarya (these other kingdoms being Fungi, Animals and Land Plants), so the incredible diversity found within Protista is not particularly surprising.

The only disease out of these that is caused by a prion is mad cow disease, and is therefore the correct answer. HIV is caused by a virus, syphilis and tuberculosis by bacteria, and lupus is an autoimmune disorder.

Reverse transcriptase is the epitome of viral evolution, as showcased by the HIV virus. Reverse transcriptase enables the HIV virus to create a DNA template from an RNA segment already encapsulated in the virus, but using the host cell's own DNA replication machinery.

When you think of viruses, think of them as parasites: they prey on host cells and take advantage of their resources. Viruses require the reproductive machinery of host cells in order to reproduce and use the energy in the cells to drive the synthesis processes; however, it is possible for viruses to remain virulent outside of a host for long periods of time. When not infecting a host, viruses are refrred to as virions. Essentially, viruses can survive outside of a host, but cannot replicate.

At no point in the reproductive cycle of a virus does it contain ribosomes. Instead, it uses the ribosomes found in the host cell to synthesize the proteins needed for its reproduction.

A virus may contain any of these four types of genetic material. The type of nucleic acid, and whether it is single- or double-stranded, determines how the virus integrates with the host organism. Different classes of viruses have different mechanisms for infection.

Note that retroviruses, which contain single-stranded RNA that is later converted to DNA, are often classified as a fifth type of virus.

There are a multitude of reasons as to why viruses are not classified as living organisms. In addition to the reasons given, viruses require host reproductive organelles and proteins in order to multiply. They also are not separated from the external world by a membrane or cell wall.

Human immunodeficiency virus (HIV), which is the causative agent of AIDS, primarily attacks the white blood cells. More specifically, it infects helper T-cells, macrophages, and dendritic cells. By damaging the cells of the immune system the virus creates an "immunodeficiency," weakening the body's ability to fight infection.

Virus particles, otherwise known as virions, always contain proteins. Whether they contain DNA or RNA depends solely upon the type of virion; different types of viruses will carry different types of nucleic acids. The capsids of virions contain proteins that surround the nucleic acids, whether it be DNA or RNA.

Viral genomes are incredibly diverse. Different types of viruses use different genetic materials. Viruses are capable of using double-stranded DNA, double-stranded RNA, single-stranded DNA, or single-stranded RNA.

The question states that HIV is a RNA virus; therefore, the virus must possess an enzyme that transforms the RNA genome into DNA. This enzyme is called reverse transcriptase. Since it transforms RNA to DNA it is also called an RNA-dependent DNA polymerase.

DNA-dependent RNA polymerase (required for transcription) is not present in HIV because it is not a DNA virus.

Finally, HIV does contain DNA-dependent DNA polymerase. This enzyme is required for DNA replication, which is an important process in HIV replication.

The difference between gram-positive and gram-negative bacteria is the thickness of the peptidoglycan protein layer. Gram-positive bacteria have a thick peptidoglycan layer that retains the gram stain well, making it purple under a microscope. Gram-negative bacteria have a thinner peptidoglycan layer, and do not retain the stain as well as gram positive bacteria. This makes it pink when viewed under a microscope.

Prokaryotes create their own proteins, which means that ribosomes are present in the cell. Viruses, however, rely on the ribosomes of the host cell in order to create proteins. This means that ribosomes are not present in viruses, but are present in prokaryotes.

DNA and RNA will both be found in prokaryotes. Some viruses will contain DNA, while others will contain RNA. Attachment proteins allow viruses to bind to cell membranes, but also can allow one cell to bind to another.

Recall that bacterial ribosomes contain a 30S and a 50S subunit. The ribosome itself is a 70S ribosome, as the combination of the two subunits results in a smaller mass than the two separate units. Eukaryotic ribosomes contain a 40S and 60S subunit and form an 80S ribosome.