Divisions of Life and Viruses - GRE Subject Test: Biology

The phylum chordata has a few key characteristics. They are deuterostomes, meaning that the anus arises from the blastopore. They have a coelom that arises from the mesoderm during development, and at some point they have a tail, pharyngeal slits, and a notochord.

Vertebrata is a subphylum of chordata, so not all chordates are vertebrates (though all vertebrates are chordates).

Centrioles are generally only found in animal cells, but have been found in a variety of eukaryotes. There are a small subset of plants that do contain centrioles. Some plant cells have flagella and peroxisomes.

Only bacterial have peptidoglycan cell walls. The other traits are typical of at least some known archaea. The s-layer involves crystalline arrays of proteinaceous subunits, which are common to some archaea and bacteria.

The main reaction that happens during a gram stain is the detection of the presence of peptidoglycan. Most bacterial cell walls contain peptidoglycan, but in varying amounts. If the cell contains large amounts of peptidoglycan, it will retain the crystal violet dye and display a purple coloration (this is gram-positive). If the cell does not contain a large quantity of peptidoglycan, it will display the counterstain and appear pink (this is gram-negative).

The gram-stain does not dictate whether or not a species belongs to the domain archaea. Archaea cell walls do not contain peptidoglycan and will not react to the stain. Note that archaea and bacteria are different domains altogether, though they share some similar characteristics.

There are three principal forms of bacteria. Spherical bacteria are termed cocci. Rod-shaped bacteria are called bacilli. Spiral bacteria are rigid (spirilla), flexible (spirochetes), or curved (vibrios).

Streptococcus, the causative agent of strep throat, is a genus of gram-postitive (purple staining) cocci (spheres), in which the cells tend to form chains. Bacteria are described by a combination of their response to gram stain and their appearance. For example, "gram-positive streptococcus" indicate chains of spheres that stain purple.

The peptidoglycan layer of bacteria retains the Gram stain. In general, Gram-negative bacteria are more virulent than Gram-positive bacteria. This is due in part to an outer layer of lipopolysaccharides present on Gram-negative bacteria that invokes immune responses (particularly via a component called lipid-A), and are somewhat more difficult to target with antibiotics.

All of these answers are true, except the statement that protists are definitionally unicellular. Protists are an incredibly diverse group of organisms that are difficult to classify because of the vast differences that exist between protist species; it is easier to classify protists based on what they are not, rather than what they are. Multicellular protists include species of seaweeds called kelp. Though kelp resemble plants, they are actually multicellular groups of protists that lack true stems, roots or leaves (though kelp often have structures resembling these plant structures).

Endosymbiosis is an intriguing theory that seeks to explain the presence of complex structures such as mitochondria and chloroplasts inside of plant and animal cells. Mitochondria's similarities to free-living proteobacteria, and chloroplasts' similarities to free-living cyanobacteria suggest that at one point these structures were organisms that became engulfed by larger cells. According to the theory of endosymbiosis, mitochondria and chloroplasts lived inside their hosts in a symbiotic relationship, eventually evolving to lose their capacity to live independently and becoming organelles that support their host cell.

Bryophytes are nonvascular plants, such as mosses. Gametophytes are species that have haploid cells during their mature lives, while sporophytes are predominantly diploid during their adult phases.

Bryophytes have the gametophyte generation as dominant, with the sporophytes relying on the parental gametophyte. Starting with the evolution of seedless vascular plants, the gametophytes become reduced and are no longer the dominant life cycle. By the formation of angiosperms (seed plants), the gametophytes have become dependent on the parental sporophyte.

Plants have a multicellular haploid stage called the gametophyte. Gametophytes () produce gametes () through mitosis, which combine to produce a zygote (). The zygote grows into a multicellular, diploid sporophyte (), which produces spores () through meiosis. Those spores give rise to multicellular gametophytes.

All plants can be classified as either bryophytes or tracheophytes. Plants that contain transport vessels (xylem and phloem) are tracheophytes, while those without transport vessels are bryophytes. All plants contain cell walls and chloroplasts, but only a tracheophyte would contain xylem. Plasmids are structures that are almost exclusively found in bacteria or protozoans.

HIV is a retrovirus, meaning that it contains RNA as its genetic material, and thus reverse transcriptase to code for DNA from the RNA template. It also has a protein coat, and must use proteases in order to degrade the host cell membrane to inject its RNA. It does not, however, have ribosomes. It will eventually use the host ribosomes to translate its own genetic material.

The question addresses the Baltimore classification system for viral genetic information. Viruses contain some form of DNA or RNA, but never both.

Retroviruses carry RNA and when they infect a host cell, they use their own reverse transciptase to made DNA from that RNA; therefore, within the virus, there are only the molecules that make up RNA. Uracil is used in RNA place of thymine, which can be found in DNA. Cytosine, guanine, and adenine can be found in both DNA and RNA.

Neisseria synthesizes proteins as a free living prokaryote. Viruses are generally dependent on host cellular machinery, and synthesize their proteins upon hijacking of a host ribosome.

All viruses contain a protein coat (or capsule) that protects the genetic material (which can be either DNA or RNA). In some instances (such as herpes simplex), the virus can also contain an lipid envelope that serves as a membrane.

A lysogenic infection involves the virus entering the cell and incorporating its nucleic acid into the host genome. RNA viruses are capable of this type of infection, but must first have their RNA be converted to DNA using the enzyme reverse transcriptase. The DNA genome remains dormant in the cell as it undergoes cellular divisions, increasing the number of cells that contain the viral DNA. This stage is known as the latent period.

Eventually, the viral genome will be stimulated and enter into the lytic viral cycle. During this phase, the virus hijacks the cell ribosomes and produces large quantities of the viral genome and proteins. Eventually the virus causes the cell to lyse, releasing more virions into the host.

The correct answer is provirus. A retrovirus uses its own reverse transcriptase to synthesize DNA from its RNA. This DNA is then incorporated into the host genome via the integrase enzyme. The viral DNA in the host genome is referred to as a provirus; the host cell transcribes and translates the provirus to produce new copies of the virus, along with its own genome.

The correct answer is pseudopeptidoglycan. This is molecularly different than peptidoglycan, which comprises the bacterial cell wall. However, both pseudopeptidoglycan and peptidoglycan cell walls are similar in function, shape and structure. Cellulose comprises the cell walls of plants, chitin comprises the cell walls of fungi and glycogen is the animal form of stored glucose, and is found in the liver and skeletal muscle.