In this last session of the characteristics of cells and viruses, we will focus on viruses. We will find out the answer to the question, what are they and what are they not? Here is a brief outline of the study of viruses. We'll start with the characteristics of viruses in general, and how viruses are not considered living and thus not considered cells. Then we will look at the viral structure and viral shape in general. Next, we will look at the Baltimore classification of viruses.

Finally, we will look at a few examples of viruses according to the Baltimore classification. So, what are the main characteristics of viruses? viruses are small infectious nonliving agents. They're also considered obligate intracellular parasites that have either DNA or RNA. viruses can infect bacteria, fungi, protozoa, all the plants and animals. They do not have organelles, cytoplasm or cell nucleus.

They lacked the machinery needed for generating energy and they like large molecules. Therefore, viruses use the whole cell machinery to replicate. In terms of structure viruses have a viral genome, capsule and envelope. The viral genome contains either DNA or RNA but not both. The capsid is made of protein several smaller subunits, known as capsule mirrors makeup the capsule nucleo capsid is the name given for the nucleic acid enclosed in the capsule. Finally, the envelope is the covering on the capsule that helps the virus center horse cells.

The envelope is present in only some viruses and not others. In terms of viral shape viruses may be of helical icosahedral complex shapes. Example of a helical shape is the tobacco tobacco mosaic virus. Example of an icosahedral shape is the adenovirus responsible for the common cold example of a complex viruses the bacteriophage deforms. Which infects bacteria. With this background knowledge of viruses we cannot appreciate the Baltimore classification of viruses into groups named after David Villa Baltimore, a Nobel Prize winning biologist.

These groups are designated by Roman numerals. The Baltimore classification places viruses in one of seven classes depending on four factors, the type of nucleic acid, the strangeness, the sense and the mode of replication. The nucleic acid is either RNA or DNA but never both. For example, DNA viruses are found in classes One, two Seven and RNA viruses are found in classes two, three, that is I'm sorry for Hi, and six described it as has to do with single stranded RNA double stranded viruses. Examples of single stranded viruses are found in classes due for five, and six, whereas double stranded viruses are in classes one, three, and seven. How many viruses can further be classified according to the sentence or polarity of their RNA In the negative sense and positive sense or ambi sense RNA viruses.

Positive sense RNA viruses are similar to mRNA and thus they can immediately be translated by the host cell into mRNA. negative sense RNA is complimentary to mRNA and must be converted to a positive sense RNA by an RNA polymerase before translation. And antisense RNAs are similar to negative sense RNAs, but they can also translate the positive strand. RNA is an example of a single stranded positive sense RNA are found in class. Four and negative send signals stranded RNA viruses are found in class five. Now the replication has to do with the location of replication requiring the requirement of host cell DNA polymerase enzyme or not, and the use of a reverse transcriptase.

For example, classes one and two DNA viruses replicate within this nucleus, whereas classes four and five RNA viruses replicate within the cytoplasm. RNA viruses in general do not require host cell polymerases whereas do DNA viruses do. And finally, retroviruses, such as AIDS virus found in class seven requires a reverse transcriptase while other viruses like the hepatitis B virus found in class. Six require a single stranded RNA intermediate Net serves as a template for reverse transcription and DNA replication. Shown here is the same classification with some examples of viruses in each class just to name a few. The herpes simplex virus is a double stranded DNA virus.

The influenza virus is a negative strand, negative sense RNA virus. The HIV is a reverse RNA virus while the hepatitis B is a reverse DNA virus. So in summary, viruses are known living agents, as they do not possess many of the characteristics of life found themselves. Structurally, viruses have a viral genome capsule An envelope. In most cases, all the sub viruses lack an envelope. The bulk of your classification of viruses into groups is based on four factors, nucleic acid, strangeness, sense and replication.

This concludes the section about the characteristics of cells and viruses.