Microbial Genetics

1. Microbial Genetics

2. What is the genetic material? • DNA • Nucleotide base pairs • A-T, C-G • Chromosomes • Bacteria: circular • Chromatin • Genetics • Genes • Genome • Genetic code

3. What is the purpose of DNA? • Recipe for making _____________ • Genotype vs phenotype

4. What’s special about bacterial DNA? • Circular • Attached to PM at various pts

5. How is DNA copied for replication? • Recall DNA structure • Replication Overview • DNA helicase unwinds double helix • DNA polymerase III copies at replication fork • Replication is 5’ to 3’ • Ligase “glues” nucleotides • Animation

6. What is semiconservative replication?

7. What are the specifics of DNA replication? • DNA strands are antiparallel • Bidirectional replication animation • Rolling circle animation • Replication always starts at new 5’ end • Leading strand • Lagging strand • Okazaki fragments (started with an RNA primer) • Error rate: 1 in 1010

8. Protein Synthesis

9. How does DNA direct protein synthesis? • Via RNA • Three types of RNA • tRNA, rRNA, mRNA • Overview: • mRNA is copy of DNA gene • Created by transcription • Protein made during translation • Ribosome “reads” triplet genetic code • tRNA delivers appropriate amino acid

10. What happens in transcription? • mRNA created • Often translation occurs while transcription happens • Base pairing • No T in RNA, instead U • Begins with RNA polymerase attaching to promoter region of coding strand • Stops when reaches terminator region • transcription process

11. What happens in translation? • Ribosome attaches to mRNA • Reads codons • Code is redundant (degenerate) • 20 aas, but 61 codons • 3 stop codons (nonsense codes) • Start codon (______) in bacteria codes for formylmethione • tRNA brings in appropriate aa • Matches to tRNA anticodon

12. How does the ribosome “read” the mRNA? • Ribosome finds start codon • 30S attaches, then 50S • First tRNA to P site • Second to A site • First tRNA transfers aa to aa on A site tRNA • Ribosome shifts • Moves 5’  3’ • New tRNA into now open A site • Process repeats • translation

13. How many ribosomes can work at once?

14. Is it the same process in eukaryotes? • Pretty much • Exception: • DNA is inside nucleus • Post-transcriptional modifications • 5’ cap • 3’ poly-A tail • Introns removed

15. Control of Genes, Mutations and DNA Recombination

16. What controls gene expression? • Majority of genes are constitutive • Protein produced at constant rate • Repression • Inhibit gene expression • Repressors: proteins that repress • Induction • Turning on transcription • Inducer: substance that induces • Lac operon model demonstrates these two processes

17. What is induction? • Induction • Turning on transcription • Inducer: substance that induces • Lac operon model demonstrates these two processes

18. What is repression? • Repression • Inhibit gene expression • Repressors: proteins that repress

19. What is the lac operon model? • Study of E. coli • Inducible system when lactose is present • Three genes for lactose consumption • next to each other on chromosome • These are structural genes • DNA nearby is control region • Includes promoter and operator • Together these are the operon • Lac operon = 3 lac genes + operon region • General regulation animation • Animation

20. Regulation of Gene Expression Figure 8.13

21. What happens if the DNA code is wrong? • Called a mutation • Causes change to mRNA sequence which can affect translation and thus ___________ • Spontaneous • Induced • Excision repair

22. Nonsense mutation Results in a nonsense codon Mutation Figure 8.16a, c

23. Frameshift mutation Insertion or deletion of one or more nucleotide pairs Mutation Figure 8.16a, d

24. What types of mutation are there? • Original: • THE BIG FLY HAD ONE RED EYE. • Point mutation: • THE BIT FLY HAD ONE RED EYE. • Frame shift mutation: • Addition: • THE BIT GFL YHA DON ERE DEY E. • Deletion of G in BIG: • THE BIF LYH ADO NER EDE YE. • Which is more dangerous? • mutations movie • Spontaneous mutations • Mutagens • Can affect pathogeneticity

25. What can be a mutagen? • Chemicals • Nitrous acid • Nucleoside analog • Similar to normal nitrogenous base • Causes mismatching of base pairs • AZT (azidothymidine) does this • Radiation • Formation of thymine dimers • Light-repair enzymes (photolyases) • animation

26. Mutagens vs Carcinogens: what the difference? • Mutagens • Carcinogens • Ames test • Identifies possible carcinogens by identifying mutagens • Looks to see how many mutate Salmonella cells revert to a nonmutant form

27. How can bacteria pass DNA? • Genetic recombination • Exchange of genes between chromosomes • Gives new combinations • Vertical gene transfer • Parent to offspring • Horizontal transfer • 3 types…

28. What is conjugation? • Horizontal gene transfer (1% of population) • Donor bacterial cell gives DNA to recipient cell • Recipient now has recombinant DNA • Conjugation • Process in E. coli • Donor is F+, recipient F- • F= fertility factor • Hfr cell (high frequency of recombination • When F factors integrate into chromosome • Conjugation-plasmid • Conjugation-chromosome

29. What is transduction? • Virus transfer DNA • general vs. specialized • animation

30. What is transformation? • Gene transferred to recipient bacterium • Griffith (1928) • Studied Streptococcus pneumoniae • Two strains: one virulent, one not • Transformation animation How could this happen???!!

31. What are transposons? • Jumping genes • Can be transferred to other cells