Chap 18

1. Chap 18 The Genetics of Viruses and Bacteria

3. Structure of Virus • Approximately 20 nm in diameter • Their genome can contain DNA or RNA. • Enclosed by a protein coat  capsid • Contain no metabolic enzymes or ribosomes. • Contain a mechanism of replication dependent on host cell to produce necessary viral components. • Some viruses introduce genetic variation into the host cell

6. Viral Replication • Viruses hijack the transcription/translation systems in host cells to produce viral proteins. • RNA viruses replicate and assemble in the cytoplasm of the host cell

7. RNA viruses • RNA viruses replicate and assemble in the cytoplasm of the host cell. • Replication error prone, no proof reading ability. • High rates of mutation • Hard for immune system and technology to target.

8. DNA viruses • Most are double stranded genomes but some are single stranded. • Replication takes place in eukaryotic host cells in nucleus.

9. Retroviruses • Contain reverse transcriptase that allows the reversed transcription of the viral RNA genome into DNA. • These newly transcribed fragments are than integrated into host genome. • The viral DNA becomes replicated every time the host cell divides.

10. Reproductive cycles • Generally, a virus can only reproduce in its host cell. • Each virus can infect only a limited range of host cells. • Identification through a “lock and key” between proteins on the external surface of the virus and specific receptors on the host cell surface.

12. The Lytic cycle • Reproduction that culminates in the death of the host cell. • Infection, assembly, lysing of the host cell and repeat!!!

14. The Lysogenic Cycle • Integrates its viral DNA into host DNA. • Can become lytic or lysogenic based on “early gene” expression. • Regulatory proteins determine the cycle direction by competing for binding sites on the viral DNA embedded in host DNA. • Not permanent, can be influenced by environmental factors or stressors.

17. Bacteria and Diversity • The main bacterial genome consists of one double stranded circular DNA and a small amount of associated proteins. • In addition to the chromosome, many contain plasmids. • Rapid reproduction, mutation and genetic recombination contribute to genetic diversity of bacteria.

18. Plasmid • A small circular, self replicating piece of DNA. • Separate from the bacterial chromosome. • Contain a small number of genes that are not required for the survival and reproduction of the bacterium under normal conditions.

20. Binary Fission • An asexual process of bacterial reproduction, yet prone to mutations. • Single origin of replication and continuous replication in both directions. • Mutation rates individually are rare, but significant as a whole when reproductive rates are high and short generations.

21. Mechanisms of recombination • The imperfect nature of DNA replication and repair increase variation in organisms. • 4 mechanisms are used by bacteria to alter their genome (horizontal transmission). • Transformation is the alteration by the uptake of naked, foreign DNA. • Transduction occurs when phages carry bacterial genes from one host to another. • Conjugation is the direct transfer of genetic material between bacterial cells

22. Transformation is the alteration by the uptake of naked, foreign DNA. • Transduction occurs when phages carry bacterial genes from one host to another. • Conjugation is the direct transfer of genetic material between bacterial cells. • Transposition is when a DNA sequence that can change its position within the genome, sometimes creating or reversing mutations and altering the cell's genomic size.

25. Bacteria regulation of gene expression • The Central Dogma of molecular genetics was stated by Francis Crick that information flow moves from DNA to proteins. • To regulate this product it is most logical to interfere at the beginning  mRNA • Regulatory proteins act by modifying the ability for a RNA poly to bind to the promoter

26. Evolution promotes prokaryotic cells to grow and divide rapidly, to exploit transient resources, allowing organisms to respond quickly to changes in the external environment. • Primary control is to adjust what enzymes are present in response to the quantity and type of available nutrients. • Regulation must be reversible allowing cell to adjust to its environnment.

27. Regulatory Proteins • Regulatory proteins act by modifying the ability of RNA poly to bind to the promoter. • These proteins bind to specific sequences that either block or stimulate transcription by facilitating the binding of RNA polymerase to the promoter. • Effectors • Operators • Activators • Repressors

28. Operons • An operon is a cluster of genes that code for a related function. • Acts as a single transcription unit that encodes multiple enzymes necessary for a biochemical pathway. Therefore they are regulated together. Induced operons – enzymes fro a certain pathway are produced in response to a substrate. Repressible operons –occurs when bacteria capable of making enzymes do not produce them.

29. Negative Control • Repressor proteins are allosteric in nature and bind to the operator regulatory sites. • They are dependent on effector molecules which bind to repressors altering their conformational shape. • This shape change will either enhance or abolish the binding of the repressor to the DNA.

30. Positive Control • Allosteric activators bind to DNA and stimulate initiation of transcription. • Effectors are required also.