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MCB100 Introductory Microbiology March 8 2019 Chapter 7 Microbial Genetics

This chapter covers the regulation of the E. coli Lac Operon, protein synthesis by ribosomes, and the properties of nucleic acids and the genetic code.

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MCB100 Introductory Microbiology March 8 2019 Chapter 7 Microbial Genetics

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  1. MCB100Introductory Microbiology March 8 2019Chapter 7 Microbial Genetics Regulation of the E. coli Lac Operon

  2. Exam 2 will be on Wednesday, March 27, 2019. Place: If your last name starts with A – Pe: please take exam in room 112 Gregory If your last name starts with Ph – Z: please take exam in room 213 Gregory Covers chapters: 5, 6, 5, 7, 5, 9 &10 and 5 Review Session: 7:00 – 8:20 pm, Monday, March 25 Place: Room 124 Burrill Hall

  3. Protein Synthesis by Ribosomes - Translation The ribosome is a complex enzyme that is made of rRNA molecules and 50 different proteins. The ribosome reads the information in mRNA and with the use of tRNA molecules assembles an amino acid chain following the instructions in the mRNA chain. See figures 7.15 and 7.16 on page 207 of your textbook. Translation is the process of protein synthesis by ribosomes. The ribosome moves along the mRNA reading the codons and adding one amino acid to the growing peptide chain for each codon. The anticodon is a sequence of three nucleotides in the tRNA molecule that is complementary to a codon. The matching of a compatible anticodon to a codon assures that the proper amino acid will be inserted into the protein. See figure 7.17, 7.18 and 7.19 on pages 208 - 210 of your text book.

  4. Nucleic Acids Which statement about nucleic acids and nucleic acid functions is incorrect? A. DNA acts as a template for the synthesis of all 3 major types of RNA and is also the genetic material in all cells. B. A ribosome is basically a complicated enzyme that synthesizes new molecules of protein. Specific RNA molecules form a critical part of the structure of ribosomes. C. Messenger RNA binds to amino acids and helps to align them within the ribosome during protein synthesis. D. A codon is a sequence of three nucleotides in a molecule of mRNA while an anticodon is a sequence of three nucleotides in a molecule of tRNA. E. The site where replication, or synthesis of DNA, begins is called the “origin”.

  5. Nucleic Acids Which statement about nucleic acids and nucleic acid functions is incorrect? A. DNA acts as a template for the synthesis of all 3 major types of RNA and is also the genetic material in all cells. B. A ribosome is basically a complicated enzyme that synthesizes new molecules of protein. Specific RNA molecules form a critical part of the structure of ribosomes. C. Messenger RNA binds to amino acids and helps to align them within the ribosome during protein synthesis. D. A codon is a sequence of three nucleotides in a molecule of mRNA while an anticodon is a sequence of three nucleotides in a molecule of tRNA. E. The site where replication, or synthesis of DNA, begins is called the “origin”.

  6. - the genetic code - mutations - genetic recombination The Genetic Code The genetic code is the key for translating information which is contained within a nucleic acid sequence into information that is in the form of a protein sequence. The genetic code consists of 64 codons, each codon is a sequence of three nucleotides and each codon specifies the insertion of one particular amino acid into a growing protein chain. The start codon AUG tells the ribosome where to begin translation. Stop codons UAA, UAG and UGA tell the ribosome to stop elongating a protein chain because the job is done. The Triplet Codon: Why are there 64 codons for 20 amino acids? There are 4 letters in the nucleotide language: A, G, C and U (or T). If a codon was two letters long, there would be 16 possible codons: AA, AG, AC, AU, GA, GG, GC, GU, CA,CG,CC,CU, UA, UG,UC, UU. A third nucleotide gives 43 possible sequences, 4 x 4 x 4 = 64. This is sufficient variety to specify 20 different common amino acids.

  7. The Universal Genetic Code In Bacteria, mitochondria and chloroplasts, translation starts with N-formylmethionine. In Eukaryotes & Archaea, translation starts with methionine. Internal AUG codons encode methionine in Bacteria and Eukaryotes. In some instances UGA encodes incorporation of the unusual amino acid, selenocysteine. In some prokaryotes UAG encodes incorporation of pyrrolysine.

  8. Fundamental Properties of the Genetic Code i) The genetic code is universal. With a few exceptions, the genetic code is the same in all living organisms on Earth. ii) The genetic code is called a triplet code. It takes 3 base pairs of DNA to contain UCA encodes Serine the information needed to specify the use of one amino acid in a protein. AUC encodes Isoleucine A codon is 3 bases of mRNA. iii) There are 64 codons in the genetic code. 61 are called sense codons because they direct the incorporation of specific amino acids into a growing protein chain, the other 3 are called nonsense codons because they cause the termination of translation. iv) The genetic code is unambiguous. There is only one amino acid that a particular codon will specify. CUU encodes Leucine ACG = Threonine ACG = Threonine ACG = Threonine

  9. Fundamental Properties of the Genetic Code v) The genetic code is said to be degenerate. Most amino acids can be specified for by more than one codon. For example: 6 codons: UUA UUG CUU CUC CUA and CUG, all specify leucine 2 codons specify aspartic acid: GAU and GAC 2 codons specify phenylalanine: UUU and UUC 4 codons specify valine: GUU, GUC, GUA, GUG there are two amino acids that are specified by only one codon each; methionine is always specified by an AUG codon tryptophan is always specified by UGG iii) The genetic code is not punctuated. There are no commas, periods, semicolons etc.

  10. The Genetic Code and Mutations Which statement is CORRECT? A. If a codon that reads UGG (tryptophan) changes to UAG (stop) it is called a silent mutation. B. If a sequence within a gene that reads GUU UGC (valine, cysteine) changes to read GUU UUG C (valine, leucine) it is called a frameshift insertion mutation. C. If a codon that reads ACA (threonine) changes to ACG (threonine) it is called a nonsense mutation. D. If a codon that reads ACA (threonine) changes to GCA (alanine) it is called a missense mutation. E. All mutations are fatal.

  11. The Genetic Code and Mutations Which statement is CORRECT? A. If a codon that reads UGG (tryptophan) changes to UAG (stop) it is called a silent mutation. B. If a sequence within a gene that reads GUUUGC (valine, cysteine) changes to read GUUUUG C (valine, leucine) it is called a frameshift insertion mutation. C. If a codon that reads ACA (threonine) changes to ACG (threonine) it is called a nonsense mutation. D. If a codon that reads ACA (threonine) changes to GCA (alanine) it is called a missense mutation. E. All mutations are fatal.

  12. Fundamental Properties of the Genetic Code Instructions for things like: where should RNA polymerase begin transcribing the DNA, what AUG in the mRNA is the real translation start site, where does the RNA polymerase stop transcription, and other necessary signals, are sequences of nucleotides that are often found near structural genes. Example: Transcription starts at a promoter. Another Example: Translation starts at the AUG near the Ribosome Binding Site.

  13. The genome of a single stranded RNA virus must be copied by an RNA-dependent RNA polymerase.Retrovirus RNA is copied by reverse transcriptase to make a DNA copy of the RNA genetic material. Viral variations of the central dogmaViruses with RNA genomes+ strand RNA viruses (example: Picornaviruses, includes: polio and rhinoviruses) - strand RNA viruses (example: Orthomyxoviruses, such as influenza) Positive-sense viral RNA is identical to viral mRNA and can be translated directly by the host cell’s ribosomes. Negative-sense viral RNA is complementary to mRNA and must be copied by an RNA-dependent RNA polymerase to make a positive-sense mRNA before translation.dsRNA viruses (example: Reoviruses, includes rotovirus)retroviruses (example: HIV)Viruses with ssDNA genomes (example: Parvovirus)

  14. MUTATIONS - A mutation is a stable change in the DNA of an organism that can be passed on to the offspring of that organism. - Mutations are permanent and rare. - A mutation produces a genotypic change. - Many mutations affect the phenotype of the organism, but not all of them. Mutations result from errors in DNA base pairing during replication. The enzyme that makes the new DNA strand, DNA polymerase, is very faithful but it’s not perfect. There are 3 main characteristics of mutations, they are: 1) rare, 2) permanent and 3) inherited.

  15. There are 3 types of mutation: 1) substitution - one nucleotide is replaced by another 2) deletion - one or more nucleotides are left out 3) insertion - one or more extra nucleotides are added Substitutions within a gene can change a codon. There are 3 types of codon change: a. missense mutationsencode a different amino acid, this can inactivate the enzyme Example: changing a GAG codon (glutamic acid) to GUG (valine). b. nonsense mutationsencode an early stop codon, this usually eliminates the enzyme Example: Changing CAA (Glutamine) to UAA (STOP). c.neutral mutations (also known as silent mutations)are minor changes in the DNA that have no effect on an enzyme Example: changing a GUU codon to GUC has no effect, both encode valine. Mutations can occur spontaneously when DNA polymerase makes an error when synthesizing a new strand of DNA. A mutagenic agent causes damage to DNA that increases the likelihood of an error being made by DNA polymerase. Common mutagens include: x-rays, UV light, and many chemicals (including: coal tar derivatives, LSD and DNA base analogs such as 5-bromouracil).

  16. Mutations Which one of the following types of mutations would probably not cause a serious problem? A. There is one additional nucleotide base pair inserted into the middle of a gene. B. A codon in the middle of a gene gets changed from UGG (tryptophan) to UAG. C. The mutation deletes a ribosome binding site. D. The mutation changes the -10 box of a promoter sequence. (assume a bacterial cell) E. A codon in the middle of a gene gets changed from UCA (serine) to UCG (serine).

  17. Mutations Which one of the following types of mutations would probably not cause a serious problem? A. There is one additional nucleotide base pair inserted into the middle of a gene. B. A codon in the middle of a gene gets changed from UGG (tryptophan) to UAG. C. The mutation deletes a ribosome binding site. D. The mutation changes the -10 box of a promoter sequence. (assume a bacterial cell)E. A codon in the middle of a gene gets changed from UCA (serine) to UCG (serine).

  18. GENETIC RECOMBINATION The vast majority of eukaryotic organisms, including most fungi, algae and protozoa, have well developed sexual reproduction that includes the formation of diploid cells by the fusion of gametes (haploid sex cells such as sperm and ovum). Bacteria do not form diploid cells, they do not form gametes or undergo meiosis. DNA exchange in bacteria is limited to the one-way transfer of DNA from a donor cell to a recipient. Sexual reproduction produces an individual with a combination of two complete haploid genomes.

  19. Three Ways Genetic Recombination Occurs in Bacteria: Transformation – the recipient cell takes up naked DNA Conjugation – one-way transfer of DNA from a donor cell to a touched recipient Transduction – transfer of DNA from a virus-infected cell via a defective phage particle

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