ReplicationReplication

Learning Objectives:Learning Objectives:

• To be able to describe the process of To be able to describe the process of replication of the influenza and HIV replication of the influenza and HIV virusvirus

• To be able to describe the course of To be able to describe the course of infection, signs, symptoms and infection, signs, symptoms and transmission if Influenza and AIDStransmission if Influenza and AIDS

ReplicationReplication

• Though the details of virus infection and Though the details of virus infection and replication vary greatly with host type, replication vary greatly with host type, all viruses share 6 basic steps in their all viruses share 6 basic steps in their replication cycles. These are: replication cycles. These are: – AttachmentAttachment– PenetrationPenetration– UncoatingUncoating– ReplicationReplication– AssemblyAssembly– Release Release

• This is called lytic replicationThis is called lytic replication

• The virus must first attach itself to the host cell. The virus must first attach itself to the host cell. This is usually accomplished through special This is usually accomplished through special glycoprotiens on the exterior of the capsid, glycoprotiens on the exterior of the capsid, envelope or tail. envelope or tail.

• Next, penetration occurs, either of the whole Next, penetration occurs, either of the whole virus or just the contents of the capsid. If the virus or just the contents of the capsid. If the entire capsid enters, the genetic material must entire capsid enters, the genetic material must be uncoated to make it available to the cell's be uncoated to make it available to the cell's replication machinery. replication machinery.

• Replication of genetic material takes place, as Replication of genetic material takes place, as well as the production of capsid and tail well as the production of capsid and tail proteins. proteins.

• Once all of the necessary parts have been Once all of the necessary parts have been replicated, individual virus particles are replicated, individual virus particles are assembled and released. assembled and released.

• Release often takes place in a destructive Release often takes place in a destructive manner, bursting and killing the host cell. manner, bursting and killing the host cell.

Lysogenic Lysogenic

• Some viruses, when they invade a cell Some viruses, when they invade a cell do not undergo the full infective cycle.do not undergo the full infective cycle.

• They integrate themselves into the They integrate themselves into the hosts DNA.hosts DNA.

• When the cell replicates, it also When the cell replicates, it also replicates the virus.replicates the virus.

• When the virus reassembles it re-enters When the virus reassembles it re-enters the lytic phase, releasing new copies of the lytic phase, releasing new copies of the virus and rupturing the host cellthe virus and rupturing the host cell

Viral DNA Viral DNA injected into injected into host cellhost cell

Assembly of Assembly of new virusesnew viruses

Lysis of cellLysis of cell

Viral DNA Viral DNA integrated into integrated into

hosts DNAhosts DNA

Lytic cycleLytic cycleLysogenic Lysogenic

cyclecycle

• In the lytic phase virus particles infect In the lytic phase virus particles infect host cells and are replicated. host cells and are replicated.

• In the lysogenic phase viral genetic In the lysogenic phase viral genetic material that has entered the host cell material that has entered the host cell becomes incorportated in the cell and becomes incorportated in the cell and lies dormant. It is then passed on to the lies dormant. It is then passed on to the progeny of the infected cells. progeny of the infected cells.

• Eventually, the lytic phase will start Eventually, the lytic phase will start again, and cells that were never again, and cells that were never infected themselves, but carry the viral infected themselves, but carry the viral genetic material will begin to produce genetic material will begin to produce new virus particles. new virus particles.

• What are the six steps in virus replicationWhat are the six steps in virus replication• attachment, penetration, uncoating, attachment, penetration, uncoating,

replication, assembly and release. replication, assembly and release. • What is contained within the capsid of a What is contained within the capsid of a

virus? virus? • The capsid contains the genetic material, The capsid contains the genetic material,

either DNA or RNA, and sometimes either DNA or RNA, and sometimes enzymes.enzymes.

• How is the genetic material of viruses How is the genetic material of viruses replicated? replicated?

• Viruses have no replication capabilities Viruses have no replication capabilities themselves, so they must use the themselves, so they must use the replication machinery of their host cells.replication machinery of their host cells.

Human Influenza Virus (type A,B or C)Human Influenza Virus (type A,B or C)• A type of A type of myxovirusmyxovirus (any of a group of RNA

viruses including those that cause influenza and mumps)Influenza A viruses infect a wide variety of Influenza A viruses infect a wide variety of mammals, including man, horses, pigs, mammals, including man, horses, pigs, ferrets and birds ferrets and birds

• Influenza A viruses are a major human Influenza A viruses are a major human pathogen, associated with epidemics and pathogen, associated with epidemics and pandemics. pandemics.

• Influenza B viruses infect humans only Influenza B viruses infect humans only and cause disease, but generally not a and cause disease, but generally not a severe as A types severe as A types

• Influenza C viruses infect humans only, Influenza C viruses infect humans only, but are not believed to cause disease. but are not believed to cause disease.

Method of transmission Method of transmission

• Air-bourne droplets infection from the Air-bourne droplets infection from the coughs and sneezes of infected coughs and sneezes of infected individualsindividuals

• Infected people are infected from a day Infected people are infected from a day before symptoms appear until a week before symptoms appear until a week afterwardsafterwards

Organ affectedOrgan affected

• The virus invades the epithelial cells of the upper respiratory tract and reproduces inside them, killing many cells in the process.

• The dead cells increase the amount and thickness of the mucus produced which irritates the throat causing coughing.

• Secondary bacterial infection is common

Incubation timeIncubation time

• Infected people are infected from a day Infected people are infected from a day before symptoms appear until a week before symptoms appear until a week afterwardsafterwards

• Symptoms usually lasts about 7-10 days, Symptoms usually lasts about 7-10 days, with the fever and aches improving after 2-3 with the fever and aches improving after 2-3 days and the cough and sore throat lasting days and the cough and sore throat lasting longer. longer.

• However, some people have symptoms However, some people have symptoms which go on for much longer - weakness and which go on for much longer - weakness and depression sometimes lasts weeks.depression sometimes lasts weeks.

SymptomsSymptoms

• Fever (usually above 38°C) • Chills • Muscle aches and pain • Severe headache • Weakness and tiredness • Loss of appetite • Sore throat • Cough, and • Chest pain

TreatmentTreatment

Most people got better on their own with Most people got better on their own with simple relief for their symptoms. This simple relief for their symptoms. This includes: includes:

• Rest. The most important thing in flu is to Rest. The most important thing in flu is to give your body a chance to heal itself by give your body a chance to heal itself by resting. Lie down and sleep if possible. resting. Lie down and sleep if possible.

• Regular pain relief with aspirin, paracetamol Regular pain relief with aspirin, paracetamol or ibuprofen or ibuprofen

• Keep well-hydrated with plenty of warm Keep well-hydrated with plenty of warm fluids.fluids.

• Remedies for sore throat, headache and Remedies for sore throat, headache and blocked nose.blocked nose.

PreventionPrevention

• Vaccination is difficult due to genetic Vaccination is difficult due to genetic changes in the influenza viruschanges in the influenza virus

• Vaccinations based on a variety of Vaccinations based on a variety of antigens are used to protect vulnerable antigens are used to protect vulnerable groupsgroups

• Prevention would require isolation of Prevention would require isolation of flu victims which is not practical.flu victims which is not practical.

Human immunodeficiency virus (HIV)

• HIV is a retrovirus HIV is a retrovirus (ie one (ie one that contains a single strand of that contains a single strand of RNA and the enzyme reverse RNA and the enzyme reverse transcriptase)transcriptase)

• Through infected semen or vaginal Through infected semen or vaginal secretions during sexual activitysecretions during sexual activity

• Through infected blood in transfusions Through infected blood in transfusions or contaminated needlesor contaminated needles

• Across the placenta or in breast milk Across the placenta or in breast milk from infected mothersfrom infected mothers

Method of transmission Method of transmission

• HIV in the blood attaches to cells that HIV in the blood attaches to cells that carry the ‘CD4’ antigen. These include carry the ‘CD4’ antigen. These include T Lymphocytes and macrophage white T Lymphocytes and macrophage white blood cells.blood cells.

Organ affectedOrgan affected

Incubation timeIncubation time

• After entering the cell it becomes a provirus After entering the cell it becomes a provirus – that is – that is a form of the virus which is capable a form of the virus which is capable of being integrated into the host genome. of being integrated into the host genome.

• It is replicated over a period of 8 – 10 yearsIt is replicated over a period of 8 – 10 years• Eventually the virus particles are Eventually the virus particles are

reassembled and emerge into the blood, reassembled and emerge into the blood, rupturing and killing the T cells in the rupturing and killing the T cells in the process. This leaves the immune system process. This leaves the immune system compromisedcompromised

SymptomsSymptoms• During the latent period there are no During the latent period there are no

symptoms, but the individual is infectious.symptoms, but the individual is infectious.• After this period symptoms begin to appear After this period symptoms begin to appear

that include:that include:– TirednessTiredness– FeverFever– Weight lossWeight loss– Diarrhoea Diarrhoea

• This is followed by the more serious This is followed by the more serious symptoms of AIDSsymptoms of AIDS

AIDS

• As the immune system is no longer As the immune system is no longer functioning there is no defence against a functioning there is no defence against a variety of opportunistic infectionsvariety of opportunistic infections

• The most common are:The most common are:– Kaposi’s sarcomaKaposi’s sarcoma– TBTB– PneumoniaPneumonia

• All of these are usually fatalAll of these are usually fatal

Treatment Treatment

• There is, as yet, no cure or vaccination There is, as yet, no cure or vaccination for HIV or AIDSfor HIV or AIDS

• Drugs such as AZT can delay the onset Drugs such as AZT can delay the onset of symptomsof symptoms

PreventionPrevention

• Safe sexSafe sex

• Use only sterile needlesUse only sterile needles

• Screening of blood productsScreening of blood products

What is a bacteriophage?What is a bacteriophage?b)b) What do they infect?What do they infect?c)c) Why are bacteriophages important Why are bacteriophages important industrially?industrially?d)d) How do they infect a cell?How do they infect a cell?e)e) Why is the life cycle of a bacteriophage Why is the life cycle of a bacteriophage described as lytic?described as lytic?

Do you consider viruses to be alive?Do you consider viruses to be alive?Give reasons to justify your answer.Give reasons to justify your answer.

Read the information on RETROVIRUSES.Read the information on RETROVIRUSES.a) a) Give 2 features of all retrovirusesGive 2 features of all retrovirusesb) b) Name a retrovirusName a retrovirusc)c) Why are reverse transcriptase and Why are reverse transcriptase and retroviruses widely used in genetic engineering?retroviruses widely used in genetic engineering?