1 VirusesViruses Page 328 CHAPTER 18

2 Fig. 18.1, Page Viruses are much smaller than bacteria 4- A virus is a genome حامض نووى enclosed in a protective coat غطاء واقى 3- Viruses are not cells 2- Virus is about 20nm in diameter

3 Genome ( DNA/RNA ) Protein coat () Protein coat (capsid) Virus is a genome enclosed in a protective coat Fig. 18.2a & b, Page 330

4 Viral Capsid and Envelope A protein shell that encloses the viral genome.A protein shell that encloses the viral genome. It is rode-shaped, helical, polyhedral or more complex.It is rode-shaped, helical, polyhedral or more complex. Capsomeres: Are the protein units that form capsid.Capsomeres: Are the protein units that form capsid. A- Capsid الجدار الفيروسى Page 329, 330 Capsomeres ( proteins ) Capsid Sometimes further rapped يُغَلَّف in a membranous envelope (Viral envelope الغلاف الفيروسى ), eg. Influenza virus. viral envelope Membranous envelop ( viral envelope )

5 viral envelopesSome viruses have viral envelopes, membranes cloaking their capsids. These envelopes are derived from the membrane of the host cell. Fig. 18.2c, Page 330 B- Envelop الغطاء الفيروسى

6 Fig. 18.1, Page 328 Viral Genome: (Hereditary material المادة الوراثية) Viral genomes may consist of: - double-stranded DNA (dsDNA), - single-stranded DNA (ssDNA), - double-stranded RNA (dsRNA), - single-stranded RNA (ssRNA). depending on the specific type of virus. The viral genome is usually organized as a single linear or circular molecule of nucleic acid. four genesseveral hundred The smallest viruses have only four genes, while the largest have several hundred.

7 - DNA enclosed in a protein coat ( sometimes, membranous envelop also ) - Can be crystallised يتبلور Key CharactersViruses Key Characters Viruses - Reproduced only within a living host cell - Reproduced only within a living host cell ( obligate parasitism تطفل إجبارى ). - They lack يفتقد enzymes for metabolism - Have no ribosomes for making their own proteins - Each type of virus infects a limited range of host cells (host range مدى الإصابة ) Human cold AIDS Most viruses of eukaryotes attack specific tissues. eg. Human cold viruses infect only the cells lining the upper respiratory tract, and AIDS virus binds only to certain white blood cells (Immune system).

8 Virus-Reproduction within a living host cell 1-Inters the cell and releases its genome 2- Replicates using host nucleotides and enzymes 3- Produce a new capsid units using host cell resources. 4- The new viral DNA and proteins assemble to form new viruses Page 331 (Fig. 18.3) Living Cell Viral RNA Capsid Proteins obligate parasitism تطفل إجبارى

9 A viral infection begins when the genome of the virus enters the host cell. Once inside, the viral genome commandeers its host, reprogramming the cell to copy viral nucleic acid and manufacture proteins from the viral genome. The nucleic acid molecules and capsomeres then self-assemble into viral particles and exit the cell. Fig. 18.3, Page 331

10 Viruses with an outer envelope use the envelope to enter the host cell.Viruses with an outer envelope use the envelope to enter the host cell. –Glycoproteins on the envelope bind to specific receptors on the host’s membrane. –The envelope fuses with the host’s membrane, transporting the capsid and viral genome inside. –The viral genome duplicates and directs the host’s protein synthesis machinery to synthesize capsomeres with free ribosomes and glycoproteins with bound ribosomes. –After the capsid and viral genome self- assemble, they bud from the host cell covered with an envelope derived from the host’s plasma membrane, including viral glycoproteins. These enveloped viruses do not necessarily kill the host cell.These enveloped viruses do not necessarily kill the host cell. Fig. 18.6, Page 334

11 Human Immunodeficiency Virus (HIV), the virus that causes AIDS (Acquired Immuno-Deficiency Syndrome) is a retrovirusHuman Immunodeficiency Virus (HIV), the virus that causes AIDS (Acquired Immuno-Deficiency Syndrome) is a retrovirus. The viral particle includes: 1)an envelope with glyco- proteins, 2)a capsid containing two identical RNA strands as its genome 3)Two copies of reverse transcriptase. Fig. 18.7a, Page 336

12 bacteriophages phagesViruses that infect bacteria, are called bacteriophages or phages. It has a 20-sided capsid-head that encloses their DNA and protein tail piece that attaches the phage to the host and injects the phage DNA inside. Phages reproduce by Lytic Cycle (تحـللية مميتــــة) and/or Lysogenic cycle (تحـللية دورة غير).Phages reproduce by Lytic Cycle (تحـللية مميتــــة دورة) and/or Lysogenic cycle (تحـللية دورة غير). Fig. 18.2d Page 330 Bacteria infecting Viruses

13 Phages reproductive cycles within bacteria: Bacteriophages (on E. coli) BacteriaBacteria VirusVirus 1)- lytic cycle (التحـللية) 1)- lytic cycle (التحـللية الدورة), The phage reproductive cycle results in the death of the host. lyses –In the last stage, the bacterium lyses (breaks open) and releases the phages produced within the cell to infect others. Virulent phages فيروسات قاتلة reproduce only by a lytic cycle.

14 Fig. 18.4, Page 332 Phage T4 (virulent viruses) فيروس مميت

15 The phage genome replicates without destroying the host cell. phage lambda ( ג ) lytic and lysogenic cyclesTemperate phages, like phage lambda ( ג ), may use both lytic and lysogenic cycles. Within the host, the virus’ circular DNA engages in either the lytic or lysogenic cycle. During a lytic cycle, the viral gene immediately turn the host cell into a virus-producing factory, and the cell soon lyses and releases its viral products.Temperatevirus فـيــروس غير مميت 2)- lysogenic cycle (التحـلليةغير) 2)- lysogenic cycle (التحـللية الدورة غير) Phage lambda (ג)

16 Fig. 18.5, Page 333 2)- Lysogenic cycle (التحـلليةغير) 2)- Lysogenic cycle (التحـللية الدورة غير) Unit 3 Unit 3

17 Only 1 tail fibre Temperate virus فيروس غير مميت أحيانا Phage lambda (ג) Phage T4 (virulent viruses) فيروس مميت Many tail fibre lytic cycle (التحـللية) (التحـللية الدورة) Lysogenic cycle (التحـلليةغير) (التحـللية الدورة غير) lytic cycle (التحـللية) (التحـللية الدورة) &