Download Microbial Genetics - Lecture Slides | BIOS 213 and more Exams Bacteriology in PDF only on Docsity!
Microbial Genetics
Chapter 8
Terminology
- Genetics Study of what genes are, how they carry information, how information is expressed, and how they are replicated
- Gene Segment of DNA that encodes a functional product, usually a protein
- Genome All of the genetic material in a cell
- Genomics Molecular study of genomes
- Chromosome Structures contain DNA that physically carry genetic information
- Plasmid Extra-chromosomal, self-replicating, gene-containing, circular
- Transposon Small segment of DNA that can move (be transposed) from one region of DNA molecule to another
- Genotype All genes of an organism
- Phenotype Expression of the genes at a given growth condition
E. coli chromosome
E. coli genome is ~4.6 million base pairs: 1 mm long, 1,000 times length of the cell
Flow of Genetic Information
Figure 8.
- Polymer of nucleotides: Adenine, Thymine, Guanine, Cytosine
- Double helix
- Deoxyribose (sugar) - phosphate “backbone"
- Strands held together by hydrogen bonds between AT and GC
- Strands are antiparallel
DNA Structure
Figure 8.
DNA primase
RNA primer
Lagging strand is synthesized discontinuously
Parental double strand
DNA polymerase III Okazaki fragment DNA ligase
DNA polymerase I Digests RNA primer And replaces it with DNA
DNA ligase joins the discontinuous fragments
DNA polymerase III
DNA polymerase
The leading strand is synthesized continuously
SSB proteins stabilize the single strand parental DNA
Enzymes unwind and untangle the DNA helix
- Synthesis always in 5′ → 3 ′ direction
- Each nucleotide added by Dehydration Synthesis
- SSB (single strand binding) protein keeps single strands apart
- Leading strand synthesized continuously by DNA polymerase
- Lagging strand synthesized discontinuously
- RNA primer made by DNA primase
- Okazaki fragments made by DNA polymerase
- DNA Polymerase I removes RNA primer and fills gap with DNA
- DNA ligase joins completed Okazaki fragments
Overview
This is the URL for the DNA replication interactive cartoon.
http://www.wiley.com/legacy/college/boyer/0470003790/animations/replication/replication.swf
- Bacterial DNA replication is bidirectional
DNA Replication
Figure 8.
- DNA is transcribed to make
RNA (mRNA, tRNA, & rRNA)
- Transcription begins when
RNA polymerase binds to
the promoter sequence
- Transcription proceeds in
the 5′ → 3 ′ direction
- Transcription stops when it
reaches a terminator signal
Transcription
NOT ALL RNAs ENCODE PROTEINS
Transcription Process
Figure 8.10.
Translation Process
P site A site
Figure 8.10.
Translation Process
Figure 8.10.
Translation Process
Figure 8.10.
Translation Process
Translation Process
To Think About
- mRNA sequence is dictated by DNA sequence
- Sequence of tRNA base-pairing is dictated by the mRNA
sequence
- The amino acid sequence of a protein is dictated by the
order in which tRNAs with their amino acids base pair
- So…. the sequence of the protein is determined by the DNA
- Change the DNA sequence, and you are likely to change the
protein sequence
- Change the protein sequence, and the protein may be less
efficient, or even non-functional
Simultaneous Transcription and
Translation in Bacteria
This is the URL for the DNA replication interactive cartoon.
http://www.wiley.com/legacy/college/boyer/0470003790/animations/replication/replication.swf
Other Genetic Elements: Plasmids
Episomal elements
Can replicate independently of the chromosome
Conjugative plasmid (F factor)
Carries genes for sex pili and transfer of the plasmid
Dissimilation plasmids
Encode enzymes for catabolism of unusual compounds
R factors
Encode antibiotic resistance
- Segments of DNA that can move from one region of DNA to another
- Contain insertion sequences for cutting and resealing DNA (transposase)
- Complex transposons carry other genes
Transposons
Figure 8.30a, b
- Vertical gene transfer
- Horizontal gene transfer
Genetic Transfer and
Recombination
Occurs during reproduction (from one generation of cells to the next)
Occurs between cells of the same generation
Horizontal Gene Transfer in Bacteria
- Transformation: genes transferred from one
bacterium to another as “naked” DNA in solution
- Conjugation: genes transferred from one bacterium
to another through cell-to-cell contact (donor and
recipient cells)
- Transduction: genes transferred from donor cell to
recipient cell through a bacteriophage (virus)
Pili Provide a Channel for DNA transfer
(Conjugation)
Conjugation
Figure 8.27a
Conjugation – Hfr Formation)
Figure 8.27b
F factor integrated into chromosomal DNA to form Hfr cell Hfr: high frequency recombination
Conjugation of Hfr Cell
Figure 8.27c
Transduction
Figure 8.
Recombinant
1
Phage protein coat Bacterial chromosome
2
3
Bacterial DNA Phage DNA
4 Recipient cell^5
Donor bacterial DNA
Recipient bacterial DNA
Recombinant cell
A phage infects the donor bacterial cell.
Phage DNA and proteins are made, and the bacterial chromosome isbroken down into pieces.
Occasionally during phage assembly,pieces of bacterial DNA are packaged in a phage capsid. Then the donor cell lyses and releases phage particles containing bacterial DNA.
A phage carrying bacterial DNA infects a new host cell, the recipient cell.
Recombinant can occur, producing a recombinant cell with a genotype different from both thedonor and recipient cells.
T 4 Attachment to E. coli Cell Wall and Injection of
DNA
- Point mutation (base substitution): change in one base
- Missense mutation: result in change in codon and amino acid
Missense Mutation
Figure 8.17a, b
- Nonsense mutation: introduces a stop codon
Nonsense Mutation
Figure 8.17a, c
- Frameshift mutation: Insertion or deletion of one or more nucleotide pairs which change the triplet codon reading
Frameshift Mutation
Figure 8.17a, d
- Ionizing radiation (X-rays and gamma rays) causes the formation of ions that can react with nucleotides and the deoxyribose- phosphate backbone.
- Chromosomal breaks
Mutation: Radiation Mutagen
causes thymine dimers
separates thymine dimers
excision repairs mutations
Mutation: UV light mutagen
Figure 8.
Detection of Mutations – Replica Plating
Operon
Figure 8.14.
Bacterial genes are often structured as operons,
which contain the promoter, operator, and
structural genes. The operon is regulated by a
regulatory gene product which is not part of the operon
With a repressor
Without a repressor
A Model for Repression
Regulation of Gene Expression
lac operon – Repressed
Regulation of Gene Expression
Figure 8.14.
lac operon –
Inducible operon
Regulation of Gene Expression
Figure 8.14.
Tryptophan operon
Regulation of Gene Expression
Repressible
operon
Tryptophan operon
