1.list and describe the structural components of viruses.
2. explain why viruses are obligate intracellular parasites.
3. explain how a virus identifies its host cell.
4. describe bacterial defenses against phages.
5. distinguish between the lytic and lysogenic reproductive cycles, using phage as an example.
6. describe the reproductive cycle of an hiv retrovirus.
7. what are several catagories of virus and how do they differ?
8. list some characteristics that viruses share with living organisms and explain why viruses do not fit our usual definition of life.
9. define and describe mobile genetic elements. explain why plasmids, transposons, and viruses are all considered mobile genetic elements. viruses, viroids and prions.
10. explain how viral infections may cause disease.
11. describe the three processes that lead to the emergence of new viral diseases.
12. distinguish between the horizontal and vertical routes of viral transmission in plants.
13. describe viroids and prions.

A virus is a fragment of code embedded in a legitimate program. Virus are self-replicating and are designed to infect other programs. They can wreak havoc in a system by modifying or destroying files causing system crashes and program malfunctions. On reaching the target machine a virus dropper(usually trojan horse) inserts the virus into the system.

For more details, refer this.

Various types of virus :

File Virus : This type of virus infects the system by appending itself to the end of a file. It changes the start of a program so that the control jumps to its code. After the execution of its code, the control returns back to the main program. Its execution is not even noticed. It is also called Parasitic virus because it leaves no file intact but also leaves the host functional.

Boot sector Virus : It infects the boot sector of the system, executing every time system is booted and before operating system is loaded. It infects other bootable media like floppy disks. These are also known as memory virus as they do not infect file system.

boot sector virus flowchart

Macro Virus : Unlike most virus which are written in low-level language(like C or assembly language), these are written in high-level language like Visual Basic. These viruses are triggered when a program capable of executing a macro is run. For example, macro virus can be contained in spreadsheet files.

Source code Virus : It looks for source code and modifies it to include virus and to help spread it.

Polymorphic Virus : A virus signature is a pattern that can identify a virus(a series of bytes that make up virus code). So in order to avoid detection by antivirus a polymorphic virus changes each time it is installed. The functionality of virus remains same but its signature is changed.

Encrypted Virus : In order to avoid detection by antivirus, this type of virus exists in encrypted form. It carries a decryption algorithm along with it. So the virus first decrypts and then executes.

Stealth Virus : It is a very tricky virus as it changes the code that can be used to detect it. Hence, the detection of virus becomes very difficult. For example, it can change the read system call such that whenever user asks to read a code modified by virus, the original form of code is shown rather than infected code.

Tunneling Virus : This virus attempts to bypass detection by antivirus scanner by installing itself in the interrupt handler chain. Interception programs, which remain in the background of an operating system and catch viruses, become disabled during the course of a tunneling virus. Similar viruses install themselves in device drivers.

Multipartite Virus : This type of virus is able to infect multiple parts of a system including boot sector,memory and files. This makes it difficult to detect and contain.

Armored Virus : An armored virus is coded to make it difficult for antivirus to unravel and understand. It uses a variety of techniques to do so like fooling antivirus to believe that it lies somewhere else than its real location or using compression to complicate its code.

1. nucleoid, capsid, envelope, enzymes.

A. Nucleoids represents the viral chromosomes, it is made up of single molecular which is either linear or coiling.

B. Capsid is the proteinacious covering around the virus that protects the nucleoids from damage and it consists of subunits called capsomere and campaigners.

C. Envelope is the loose membrane covering that occur in some viruses consisting of protein from lipids and carbohydrates(from host).

D. Enzymes are occasionally enzymes lysozymes is present in the region that comes in contact with host cell in bacteria pages.

2. Due to their nature of behaving like simple chemical compounds outside the living host.

3. Virus recognises it's host cell through the receptors they carry and virus can also trick the cell into taking them by bulk transport called endocytosis.

4. The development of CRISPR immune system to fight against phages.

5. Lytic involves reproduction of viruses by using host cell for virus manufacturing while lysogenic cycles involves integration of viral genome into host DNA and reproducing with it.

6. The life cycle of hic

a. Attachment and entry- this happens in two stages attachment and fusion.

B. Reverse Transcription- when hiv enters the cell it must be reverse transcribed into proviral DNA before integrating into the DNA of host.

C. Integration- after conversion into DNA, hiv integrate enzyme attaches itself to the proviral DNA strands it passes through the wall of cell nucleus.

D. Transcription and Translation- they become active when they encounter an infectious agent.

7. Bacterial viruses are classified under viruses that affect the ecoil.

-DNA phages of T series where the T phages of ecoil are larger lyric phages that contain single molecule of double stranded DNA

-Temprate pages: bacterial phage lambda

-Small DNA phage: the genome of some bacteria phage encoded 10-12 protein

-RNA phage- some e coil bacterial phage contains a genome composed of RNA instead of DNA.

B. ANIMAL viruses are classified by genome type and MNRA synthesis path way.

-DNA viruses (classes 1 and 2) commonly used in the study of DNA replication.

-RNA viruses(classes 3 and 4) have RNA genome and all animal cells belong to this class.

8. They are made of protein.

They contain genetic codes needed for reproduction.

-viruses do not fit our normal life because viral reproduction is not possible without a host cell and it doesn't consume energy to survive that is metabolic processes within the cell.

9. The DNA sequences which can move around the genome changing their number of copies or location often affecting the nearest genes.

- plasmids and trasposoons are mobile transferable genetic elements through bacterial conjugation which is a horizontal mechanism gene transfer while viruses can move due to their adaptive replication strategy.

10. They cause infectious diseases by entering a living and normal cell, thereby using the cell to reproduce other viruses of them selves and infecting the immune system of the living cell.

11. -introduction of viral pathogens into a new host species.

-establishment of pathogens in New host.

-dissemination of pathogens among large number of individual of new host species.

12. In a nut shell, transmission of viruses occur horizontally is the transmission of viruses among plants of same generation while vertical transmission is between full grown plants and the seed from it to ensure transmission from current generation to the next generation.

13. Virions is the infectious form of viruses as it exists outside the host cell designed for transmission of nucleic acid genome amongst hosts while a prion is neither bacterial nor fungal nor viral and contains no genetic materials.

Explanation:2. They have long and non living thing characteristics in nature.

4. This is an act of copying a snipet of phages they come in contact with into a memory to help build against subsequent attacks by viruses in future.

6. The life cycle of hiv consists of 4 stages which includes the process of entering , reverse integration where the RNA enzymes reverse transcribed into DNA , integration where it passes through the walls of the cell and finally the last stage where it becomes activated.

N.B

Most of the answers required explanations above in order to avoid repetition of answers.

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