L23: Genes- Structure, Replication And Expression In Bacteria Flashcards
DNA
Present in nucleoid region of bacterial cells
Long term storage of genetic info
Encodes all other cellular components (RNA & proteins)
Highly compacted and organised
DNA double helix: 2 anti-parallel DNA polymers
Each DNA strand: reverse complement of other. Run 5’ to 3’ in opp directions (anti-parallel)
Complementary base pairing
Each nucleotide pair is a single bp
Bacterial chromosome features
Eukaryotes: nucleus surrounded by nuclear membranes
Bacteria: no nucleus: microscopically visible structure -> nucleoid
Bacterial chromosome: condensation
Branched loops (plectonemes) arranged in linear order and radiate from core of DNA organising factors (nucleoid associated proteins- NAPs
Circular chromatin fibre can be organised as ring or intertwined
Origin of replication: only start point of replication
Termination site: end point for replication
Chromosome organised so ori and ter sites found at poles of cell
Chromosome is physically attached to cytoplasmic membrane
NAPs effect on transcription
5 predominant NAPs which bend or bridge DNA
DNA bending NAPs change conc during growth phases
Histone-like nucleoid structuring protein (HNS): dimers of HNS packed side by side between 2 adjacent DNA strands, preferentially binds curved DNA
Factors for inversion stimulation (FIS): sequence specific DNA binding protein, bends DNA 50-90degrees, acts as transcriptional activator, promotes favourable DNA supercoiling
DNA replication
Semi conserved
Isomerase unwinds supercoil
Helicase unwinds DNA double helix exposing nitrogenous bases -> 2 replication forks
Strands are kept apart by ss DNA binding proteins (SSB)
DNA replicated 5’ to 3’
DNA pol binds ssDNA, recruits deoxyribonucleoside triphosphates and matches them to template strand by complementary base pairing and catalyses phosphodiester bond
Ozaki fragments are synthesised by RNA primase using template strand, extended from 3’ end by DNA pol III
-> 2 ds copies of DNA, each with one strand of original DNA
Termination and resolution of chromosomal replication
Because chromosome is a circle, 2 copies of chromosome are intertwined or catenated
ds break introduced into dsDNA by topoisomerase
Strands joined to create dimer at 2 ter sites
Homologous recombination occurs at ter -> Holliday junctions (strange exchange between each dsDNA pair)
Dimer resolved by FtsK-XerCD recombination machinery which cleaves Holliday junction
DNA ligase repairs phosphodiester bonds in backbones -> uninterrupted backbone
DNA replication and cell division in prokaryotes
- DNA replication is initiated at ori
- Expansion phase where bacterial cell size is increased almost 2-fold
- Resolution of 2 chromosomes which are moved to opp poles of cell by actin-like cytoskeleton in cytoplasm
- Septum of new cell wall material is synthesised to separate 2 cells
- Cells released from one another.
- > each daughter cell receives copy of chromosomal DNA
Is all DNA transcribed?
No
Intergenic regions between genes may not be transcribed
Do all genes encode mRNA?
No
Some genes encode for tRNA or rRNA
Genes organisation of bacterial chromosomes
Genes: working subunits of DNA containing particular set of instructions- encode gene products (tRNA, rRNA, mRNA/polypeptide)
Represented as arrows
Found as both strands of DNA
