Microbial Genetics Chapter 7

1
Microbial GeneticsChapter 7
2
Counting Bacteria from todays paper
Levels over 100,000/ml are considered a
significant health hazard by the WHO
3
Counting Bacteria
4
The Structure and Replication of Genomes

• Genome the entire genetic complement of an
  organism

5
The Structure of Nucleic Acids
Figure 7.1d
6
The Structure of Prokaryotic Genomes

• Contained in two structures
• Chromosomes
• Plasmids

7
Prokaryotic Chromosomes

• Main portion of DNA, along with associated
  proteins and RNA, are packaged in 1-2 distinct
  chromosomes
• Prokaryotic cells have a single copy of each
  chromosome (haploid)
• Typical chromosome circular molecule of DNA in
  nucleoid

8
Plasmids

• Small molecules of DNA that replicate
  independently
• Carry information required for their own
  replication, and often for one or more cellular
  traits
• Not essential for normal bacterial metabolism,
  growth, or reproduction
• Can confer survival advantages

9
Plasmids

• Many types of plasmids
• Fertility factors
• Resistance factors
• Bacteriocin factors
• Virulence plasmids
• Cryptic plasmids

10
The Structure of Eukaryotic Genomes

• Contained in two structures
• Nuclear DNA
• Extranuclear DNA

11
Eukaryotic Chromosomes

• Typically have more than one chromosome per cell
• Chromosomes are linear and sequestered within
  membrane-bound nucleus
• Eukaryotic cells often have two copies of each
  chromosome (diploid)

12
Extranuclear DNA of Eukaryotes

• DNA molecules of mitochondria and chloroplasts
  are circular and resemble chromosomes of
  prokaryotes
• Only codes for about 5 of RNA and proteins
• Nuclear DNA codes for 95 of RNA and proteins
• Some fungi and protozoa carry plasmids

13
DNA Replication

• An anabolic polymerization process that requires
  monomers and energy
• Triphosphate deoxyribonucleotides serve both
  functions
• Key to replication is complementary structure of
  the two strands
• Replication is semiconservative new strands
  composed of one original strand and one daughter
  strand

Animation DNA Replication (play part 1)
PLAY
14
Initial Processes in DNA Replication
Figure 7.5a
15
Initial Processes in DNA Replication

• DNA polymerase binds to each strand and adds
  nucleotides to hydroxyl group at 3' end of
  nucleic acid
• Replicates DNA only 5' to 3'
• Because strands are antiparallel, new strands
  synthesized differently
• Leading strand synthesized continuously
• Lagging strand synthesized discontinuously

16
Synthesis of the Leading Strand
Figure 7.5b
17
Synthesis of the Lagging Strand
Animation DNA Replication (play parts 2-4)
PLAY
Figure 7.5c
18
Replication of Eukaryotic DNA

• Similar to bacterial replication
• Some differences
• Use four DNA polymerases
• Thousands of replication origins

19
Gene Function

• Genotype set of genes in the genome
• Phenotype physical features and functional
  traits of organism

20
Transfer of Genetic Information

• Transcription information in DNA is copied as
  RNA nucleotide sequences
• Translation polypeptides synthesized from RNA
  nucleotide sequences
• Central dogma of genetics
• DNA transcribed to RNA
• RNA translated to form polypeptides

21
Events in Transcription

• Four types of RNA transcribed from DNA
• RNA primers
• mRNA
• rRNA
• tRNA
• Occurs in nucleoid of prokaryotes
• Three steps
• Initiation
• Elongation
• Termination

22
Initiation of Transcription
Animation Transcription
PLAY
Figure 7.8a
23
Elongation of the RNA Transcript
Figure 7.8b
24
Concurrent RNA Transcription
Figure 7.9
25
RNA Polymerase Versus DNA Polymerase

• RNA polymerase does not require helicase
• RNA polymerase slower than DNA polymerase
• Uracil incorporated instead of thymine
• RNA polymerase proofreading function is less
  efficient than DNA polymerase (more errors)

26
Termination of Transcript
Self termination
Rho-dependent termination
27
Transcription in Eukaryotes

• RNA transcription occurs in the nucleus
• Transcription also occurs in mitochondria and
  chloroplasts
• Three types of RNA polymerases
• Numerous transcription factors
• mRNA processed before translation
• Capping
• Polyadenylation
• Splicing

28
Genetic Code
PLAY
Animation Translation (play part 2 genetic code)
Figure 7.11
29
Prokaryotic mRNA
Figure 7.12
30
Eukaryotic mRNA
Figure 7.10
31
tRNA
Figure 7.13a-b
32
Ribosomes and rRNA
Figure 7.14c
33
Stages of Translation

• Three stages
• Initiation
• Elongation
• Termination
• All stages require additional protein factors
• Initiation and elongation require energy (GTP)

Animation Translation
PLAY
34
Initiation
Figure 7.15
35
Elongation
Figure 7.16.1
36
Elongation
Figure 7.16.2
37
Elongation
Figure 7.16.3
38
Elongation
Figure 7.16.4
39
Elongation
Figure 7.16.5
40
Elongation
Figure 7.16.6
41
Polyribosome
Figure 7.17a
42
Termination

• Release factors somehow recognize stop codons and
  modify ribosome to activate ribozymes which sever
  polypeptide from final tRNA
• Ribosome dissociates into subunits
• Polypeptides released at termination may function
  alone or together

43
Regulation of Genetic Expression

• 75 of genes are expressed at all times
• Other genes are regulated so they are only
  transcribed and translated when cell needs them
• Allows cell to conserve energy
• Regulation of protein synthesis
• Typically halt transcription
• Can stop translation directly

44
The Operon
Figure 7.18
45
Operons

• Inducible operons must be activated by inducers
• Lactose Operon
• Repressible operons transcribed continually
  until deactivated by repressors
• Tryptophan Operon

Animation Operons
PLAY
46
The Lactose Operon
Figure 7.19a
47
The Lactose Operon
Figure 7.19b
48
The Tryptophan Operon
Figure 7.20a
49
The Tryptophan Operon
Figure 7.20b
50
Mutations of Genes

• Mutation change in the nucleotide base sequence
  of a genome rare
• Almost always deleterious
• Rarely lead to a protein having a novel property
  that improves ability of organism and its
  descendents to survive and reproduce

Animation Mutations and DNA Repair
PLAY
51
Mutations of Genes

• Types
• Point mutations (most common) one base pair is
  affected
• Insertions, deletions, and substitutions
• Frameshift mutations nucleotide triplets after
  the mutation displaced
• Insertions and deletions

52
Effects of Mutation
Figure 7.21a-c
53
Effects of Mutation
Figure 7.21d-e
54
An English Example

• THEBOYANDTHEDOGRANFAR
• THE BOY AND THE DOG RAN FAR
• Missense
• THE BOY AND THE HOG RAN FAR
• Nonsense
• THE BOY AND .

55
English Example-Frameshift

• THEBOYANDTHEDOGRANFAR
• THE BOY AND THE DOG RAN FAR
• DEletion
• THE BOY ADT HED OGR ANF AR
• Insertion
• THE EBO YAN DTH EDO GRA NFA R

56
Mutagens

• Radiation
• Ionizing radiation induces breaks in
  chromosomes
• Nonionizing radiation induces pyrimidine dimers
• Chemical mutagens
• Nucleotide analogs disrupt DNA and RNA
  replication and cause point mutations
• Nucleotide-altering chemicals result in
  base-pair substitution mutations and missense
  mutations
• Frameshift mutagens result in nonsense mutations

57
DNA Repair
Figure 7.25a-b
58
DNA Repair
Figure 7.25c-d
59
Genetic Recombination and Transfer

• Exchange of nucleotide sequences often mediated
  by DNA segments composed of homologous sequences
• Recombinants cells with DNA molecules that
  contain new nucleotide sequences
• Vertical gene transfer organisms replicate
  their genomes and provide copies to descendants

60
Genetic Recombination and Transfer

• Horizontal gene transfer donor contributes part
  of genome to recipient three types
• Transformation
• Transduction
• Bacterial Conjugation

Animation Horizontal Gene Transfer
PLAY
61
Griffiths Experiments
Figure 7.30
62
Transformation

• Transforming agent was DNA one of conclusive
  pieces of proof that DNA is genetic material
• Cells that take up DNA are competent results
  from alterations in cell wall and cytoplasmic
  membrane that allow DNA to enter cell

63
Transduction
Figure 7.31
64
Transduction

• Generalized transduction transducing phage
  carries random DNA segment from donor to
  recipient
• Specialized transduction only certain donor DNA
  sequences are transferred

65
Bacterial Conjugation
Animation Bacterial Conjugation
PLAY
Figure 7.32a
66
Bacterial Conjugation
Figure 7.32b
67
Bacterial Conjugation
Figure 7.33
68
Transposons and Transposition

• Transposons segments of DNA that move from one
  location to another in the same or different
  molecule
• Result is a kind of frameshift insertion
  (transpositions)
• Transposons all contain palindromic sequences at
  each end

69
Transposons and Transposition

• Simplest transposons are insertion sequences
  which have no more than two inverted repeats and
  gene for transposase
• Complex transposons contain one or more genes not
  connected with transposition (e.g. antibiotic
  resistance)

Animation Transposons
PLAY