Foundations in Microbiology PowerPoint to accompany Foundations in Microbiology Fifth Edition Talaro Chapter 9 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Microbial Genetics Chapter 9

Genetics – the study of heredity transmission of biological traits from parent to offspring expression & variation of those traits structure & function of genetic material how this material changes

Levels of genetic study

Location and forms of the genome Fig. 9.2

Levels of structure & function of the genome genome – sum total of genetic material of an organism (chromosomes + mitochondria/chloroplasts and/or plasmids) genome of cells – DNA genome of viruses – DNA or RNA chromosome – length of DNA containing genes gene-fundamental unit of heredity responsible for a given trait site on the chromosome that provides information for a certain cell function segment of DNA that contains the necessary code to make a protein or RNA molecule

Genomes vary in size smallest virus – 4-5 genes E. coli – single chromosome containing 4,288 genes; 1 mm; 1,000X longer than cell Human cell – 46 chromosomes containing 31,000 genes; 6 feet; 180,000X longer than cell

Disrupted E. Coli cell, showing all of its DNA, uncoiled

each nucleotide consists of 3 parts Nucleic acids are made of nucleotides similar to how proteins are made of amino acids each nucleotide consists of 3 parts a 5 carbon sugar (deoxyribose or ribose) a phosphate group a nitrogenous base (adenine, thymine, cytosine, guanine, and uracil)

DNA structure 2 strands twisted into a helix sugar -phosphate backbone nitrogenous bases form steps in ladder constancy of base pairing A binds to T with 2 hydrogen bonds G binds to C with 3 hydrogen bonds antiparallel strands 3’to 5’ and 5’to 3’ each strand provides a template for the exact copying of a new strand order of bases constitutes the DNA code

Fig. 2.24

Fig. 2.23

Fig. 2.25

DNA Packaging

Significance of DNA structure Maintenance of code during reproduction. Constancy of base pairing guarantees that the code will be retained. Providing variety. Order of bases responsible for unique qualities of each organism.

DNA replication is semiconservative because each chromosome ends up with one new strand of DNA and one old strand.

Semi-conservative replication of DNA

DNA replication Begins at an origin of replication Helicase unwinds and unzips the DNA double helix An RNA primer is synthesized DNA polymerase III adds nucleotides in a 5’ to 3’ direction Leading strand – synthesized continuously in 5’ to 3’ direction Lagging strand – synthesized 5’ to 3’ in short segments; overall direction is 3’ to 5’

Table 9.1

Bacterial replicon

Bacterial replicon

Bacterial replicon Fig. 9.6cde

Fig. 9.7

Flow of genetic information

What are the products that genes encode? How are genes expressed? RNAs and proteins How are genes expressed? transcription and translation

Gene expression Transcription – DNA is used to synthesize RNA RNA polymerase is the enzyme responsible Translation –making a protein using the information provided by messenger RNA occurs on ribosomes

Genotype - genes encoding all the potential characteristics of an individual Phenotype -actual expressed genes of an individual (its collection of proteins)

DNA-protein relationship Each triplet of nucleotides (codon) specifies a particular amino acid. A protein’s primary structure determines its shape & function. Proteins determine phenotype. Living things are what their proteins make them. DNA is mainly a blueprint that tells the cell which kinds of proteins to make and how to make them.

DNA-protein relationship

3 types of RNA messenger RNA (mRNA) transfer RNA (tRNA) ribosomal RNA (rRNA)

Table 9.2

DNA Transcription RNA polymerase RNA Translation ribosomes PROTEINS

Transcription RNA polymerase binds to promoter region upstream of the gene RNA polymerase adds nucleotides complementary to the template strand of a segment of DNA in the 5’ to 3’ direction Uracil is placed as adenine’s complement At termination, RNA polymerase recognizes signals and releases the transcript 100-1,200 bases long

Transcription Fig. 9.12ab

Transcription Fig. 9.12cd

Translation Ribosomes assemble on the 5’ end of a mRNA transcript Ribosome scans the mRNA until it reaches the start codon, usually AUG A tRNA molecule with the complementary anticodon and methionine amino acid enters the P site of the ribosome & binds to the mRNA

Translation

The genetic code: codons of mRNA that specify a given amino acid

Interpreting the DNA code

Translation elongation A second tRNA with the complementary anticodon fills the A site A peptide bond is formed The first tRNA is released and the ribosome slides down to the next codon. Another tRNA fills the A site & a peptide bond is formed. This process continues until a stop codon is encountered.

The events in protein synthesis

The events in protein synthesis cont.

Translation termination Termination codons – UAA, UAG, and UGA – are codons for which there is no corresponding tRNA. When this codon is reached, the ribosome falls off and the last tRNA is removed from the polypeptide.

Polyribosomal complex in bacteria Polyribosomal complex at 30,000X A) The mRNA encounters ribosomal parts immediately as it leaves the DNA B) Ribosomal factories assemble along the mRNA, each translating into a protein

Eucaryotic transcription & translation differs from procaryotic Do not occur simultaneously. Transcription occurs in the nucleus and translation occurs in the cytoplasm. Eucaryotic start codon is AUG, but it does not use formyl-methionine. Eucaryotic mRNA encodes a single protein, unlike bacterial mRNA which encodes many. Eucaryotic DNA contains introns – intervening sequences of noncoding DNA- which have to be spliced out of the final mRNA transcript.

Split gene of eucaryotes

Replication strategies in Animal viruses

Table 9.3

Multiplication of dsDNA viruses Pg. 272

Multiplication of positive-strand, single-stranded RNA viruses (+ssRNA) Pg. 273

Regulation of protein synthesis & metabolism

Operons a coordinated set of genes, all of which are regulated as a single unit. 2 types inducible – operon is turned ON by substrate: catabolic operons- enzymes needed to metabolize a nutrient are produced when needed repressible – genes in a series are turned OFF by the product synthesized; anabolic operon –enzymes used to synthesize an amino acid stop being produced when they are not needed

Lactose operon: inducible operon Made of 3 segments: Regulator- gene that codes for repressor Control locus- composed of promoter and operator Structural locus- made of 3 genes each coding for an enzyme needed to catabolize lactose – b-galactosidase – hydolyzes lactose permease - brings lactose across cell membrane b-galactosidase transacetylase – uncertain function

Lac operon Normally off Lactose turns the operon on In the absence of lactose the repressor binds with the operator locus and blocks transcription of downstream structural genes Lactose turns the operon on Binding of lactose to the repressor protein changes its shape and causes it to fall off the operator. RNA polymerase can bind to the promoter. Structural genes are transcribed.

Lactose operon in bacteria

Arginine operon: repressible Normally on and will be turned off when nutrient is no longer needed. When excess arginine is present, it binds to the repressor and changes it. Then the repressor binds to the operator and blocks arginine synthesis.

Repressible operon

Antibiotics that affect gene expression Rifamycin – binds to RNA polymerase Actinomycin D - binds to DNA & halts mRNA chain elongation Erythromycin & spectinomycin – interfere with attachment of mRNA to ribosomes Chloramphenicol, linomycin & tetracycline-bind to ribosome and block elongation Streptomycin – inhibits peptide initiation & elongation

Mutations:Changes in the Genetic Code

Mutations – changes in the DNA Point mutation – addition, deletion or substitution of a few bases Missense mutation – causes change in a single amino acid Nonsense mutation – changes a normal codon into a stop codon Silent mutation – alters a base but does not change the amino acid

Table 9.5

Excision repair

Ames Test

DNA Recombination Events: Tranmission of Genetic Material in Bacteria

Types of intermicrobial exchange conjugation requires the attachment of two related species & formation of a bridge that can transport DNA transformation transfer of naked DNA transduction DNA transfer mediated by bacterial virus

conjugation

transformation Griffith’s classic experiment in transformation

Generalized transduction: genetic transfer by means of a virus carrier

Specialized transduction: transfer of a specific material by a virus carrier

Table 9.6

Transposons –DNA segments that shift from one part of the genome to another