Bacteria Flashcards
Structure of bacteria
- small and unicellular/prokaryotic
- lacks true nucleus and membrane-bound organelles
- smaller 70s ribosomes
- peptidyoglycan cell wall
- chromsome …
- made out of circular, double-stranded DNA
- lacks introns
- has genes which are grouped into operons where multiple genes come under control of 1 promoter
- for packing, it is wound around histone-like proteins to compact DNA into looped domains -> supercoiling for further compaction - contains plasmid(s)
Types of peptidyoglycan cell wall (gram staining test)
- gram-positive:
(a) thicker peptidyoglycan cell wall
(b) which helps to track crystal violet dye from being washed away, thus staining the cell wall violet - gram-negative:
(a) thinker peptidyoglycan cell wall
(b) causing crystal violet dye to be easily washed away, thus cell wall is not stained violet
Plasmids
- small, circular, double-stranded extrachromosomal DNA
- contain beneficial genes which confer beneficial traits such as
(a) antibiotic resistance,
(b) toxin synthesis and
(c) enzyme production - capable of independent replication as it has its own oriR
- contain beneficial genes which confer beneficial traits such as
Differences between plasmids and chromosomes
- size
- plasmids: small
- chromosomes: larger in size - genes and their use
- plasmids:
(a) contain beneficial genes
(b) which confer beneficial traits such as antibiotic resistance, toxin synthesis and enzyme production
- chromosomes:
(a) contain genes essential for survival
(b) such as genes essential for production of enzymes for metabolism
Description of peptidyoglycan cell wall
(a) polysaccharide strands consisting of alternating monosaccharide subunits, NAM and NAG,
(b) joined via peptide cross-links between adjacent NAM residues
Transformation in prokaryotes (form of genetic exchange)
- naked DNA fragment is released when donor bacterial cell lyses
- competent recipient bacterial cell takes up DNA fragments via competence factor
- homologous recombination occurs
- to allow for incorporation of donor cell’s DNA into recipient cell’s bacterial chromosome
=> increase in genetic variation
as this results in diff combos of specific genes in prokaryotes
Transduction in prokaryotes (form of genetic exchange)
- bacteriophage injects its DNA into bacterial host cell
2a. EITHER (generalised)
host bacterial chromosome is degraded, and a small random fragment of the host DNA is accidentally incorporated/packaged into phage capsid
2b. OR (specialised)
prophage is imprecisely excised upon induction and takes with it a small portion of bacterial DNA adjacent to prophage insertion site
- resultant phage infects another bacterial cell, and homologous recombination occurs
- to allow for incorporation of donor cell’s DNA into recipient cell’s bacterial chromosome
=> increase in genetic variation
as this results in diff combos of specific genes in prokaryotes
Conjugation in prokaryotes (form of genetic exchange)
- F+ donor cell uses a sex pilus to attach to a recipient F- cell and establish a temporary cytoplasmic mating bridge
- The sugar-phosphate backbone of 1 strand of the F plasmid is nicked by endonuclease
- Nicked DNA strand moves to F- cell through cytoplasmic mating bridge
- Each parental strand becomes a template, in both donor and recipient cells
- For the synthesis of a complementary daughter DNA strand, catalysed by DNA pol
=> increase in genetic variation
as conjugation allows F- to become F+ cell,
gaining the F plasmid which
contains beneficial genes which e.g. confers antibiotic resistance
Fate of transferred DNA
- (for transformation and transduction) recombined with bacteria chromosome
- (for a transformation and transduction) replicates with bacterial chromosome /
(for conjugation) replicates by itself - (for all 3) expression of the genes in DNA
Binary fission (reproduction in bacteria)
definition:
2 equal-sized genetically identical daughter cells are produced from a single parent cell
process:
- bacterial chromosome attaches to cell membrane
- semi-conservative DNA rep occurs
- where the 2 DNA strands separate at oriR, forming a replication bubble
- and replication bubble grows bidirectionally away from single oriR until bacterial chromosome is replicated fully
- the 2 bacterial chromosomes attach to cell membrane at 2 different sites of attachment
- cell elongates and 2 chromosomes move apart
- septum develops
- as cell membrane invaginates
- cell is eventually split into 2 genetically identical daughter cells by cytokinesis
How binary fission is critical for abundance of prokaryotes
- simpler process, and thus can occur at faster rate
-> ref to ability to double in number every 20 mins or so - small size of genome, thus DNA can be replicated faster to produce daughter cells
- faster rate of evolving gives more opportunity for mutation to occur to give rise to beneficial alleles
- since prokaryotes are unicellular, each daughter cell is a new organism
How horizontal gene transfer give rise to abundance of prokaryotes
- cause advantageous/beneficial genes to be spread throughout the population
-> ref to examples of beneficial genes,
such as genes which gives it ability to survive in extreme habitats (e.g. Taq) - which allow prokaryotes to inhabit and exploit a wide range of envts
regulation of gene expression in prokaryotes
- lack of chromatin level of control
-> much less complex packing and folding of DNA - transcriptional control present,
where repressor molecules bind to operator - no control over individual genes
-> related structural genes are grouped together into operons (under the control of same promoter) - lack of post-transcriptional modification and translational control
-> transcription and translation occurs simultaneously
how cell theory applies to prokaryotes
- cell theory states that a cell is the smallest unit of life
- and all bacteria are unicellular living organisms which can
take in raw materials
/ extract useful energy from raw materials and synthesise its own molecules
/ grow in an organised manner
/ respond and adapt to the external envt - cell theory states that all cells come from pre-existing cells
- and bacteria reproduces by binary fission
description of lac operon (transcriptional control)
Usually,
1. lac repressor is in active conformation
2. and binds to operator sequence as it has a complementary conformation
3. thus RNA pol is prevented from binding to promoter and transcription cannot take place
4. thus operon is switched off
5. and hydrolysis of lactose cannot occur as B-galactosidase is not produced
However, when lactose enters the cell,
1. it forms allolactase, which functions as an inducer
2. and binds to lac repressor
3. changing it to its inactive conformation, which cannot bind to operator
4. thus RNA pol can bind to promoter, and transcription can take place
5. operon is switched on
6. and hydrolysis of lactose by B-galactosidase occurs
