L3 - Prokaryotic genomes Flashcards
what shape are bacterial chromosones always
circular
circular DNA takes up a lot of space so needs to be ‘supercoiled’
= large negatively charged ring needs to be scrunched by positively charged proteins
explain how supercoiling makes bacterial chromones/DNA harder to access
taking elastic band and twistuing it round = makes it smaller
BUT makes it harder to get into DNA than when its a large covalently closed circle
explain the division/repliaction of bacterial chromosones
replication is bi-directional and usally from 1 origin (OriC)
copies must be segrefated when they divide and replication si timed to coincide with cell division
= 1 whole chromosone is passed down to eacg daughter cell
describe some of the regions within prokayptic genomes
most genes (open reading frames code for mRNAs = proteins
ORFs can overlap meaning 2 genes share the same sequnce but in different reading frames
transcription and translation are coupled due to no compartmentalisation
non-coding DNA is involved in gene expression = promoters
what does the size of a prokayptic genome depend on
number of genes
no introns means that the bigger the genome the larger the number of coding genes
= intracellular/niche pathogens often have fewer genes as they do not need to be able to adapt to different environments
extracellular organims have more
what are ‘housekeeping genes’
Genes that code for for core functions
is there proof reading in prokaryotic DNA polymerase’s
yes
but polymerases can be tricked into making errors for increased mutation rate
explain the 2 possible effects of homolopgous recombination when there are duplicate gene segments either in same or opposite orientation
Opposite:
A B C D E C B A
the sequnces line up due to base specific homology causing a hairpin loop
Recombination event takes place
= the DNA between the duoplicate segments is inverted/flipped = gene inversion
= A B C E D C B A
Same:
A B C D E A B C
Chromosone loops so that duplicates align together (not a hairpin loop)
Recombination event takes place
= The dNA between the repeats is cut out from the chromosome = A B C D E
= also have a shorter chromosome = A B C
= gene excision or deletion event
name a few ways that prokaryotes can evolve over time
Gene duplication/divergent evolution:
= gene is copied and the new gene changes over time to gain new function
can respond to environment BUT very slow
= think of hameoglobin and myoglobin = came from ancient duplication
Gene loss:
= loss of a regulatory gene can have a dramatic change
Plasmids:
transferable between bacteria = new functions and advantages
Transposible elements:
can pass chromosnal genes onto plasmids for transposition between bacteria
Phages:
viruses infect bacteria and inset into genome
what is conjugation
proscess of transferring plasmids
happens by chance
effect of conjugation
transferred plamids may be useful and maintained or a drasin onr esources and kicked out
some plasmids refuse to be kicked out and weaken bacteria as it takes enrgy for the bacterua to repliacte it
are there just single copies of plasmid in bacteria
Not always
can have multiple copies of the same plasmid OR just 1
what are insertion sequnces
simplest kind of transposible elements
made of:
1. Transposase gene = codes for eznyme to cut it out and pastes it
2. Inverted repeats (IRs) on either side of the gene that are recognised by transposase
what are composite transposons
movement of genes due to insertion sequnces flanking a group of genes and turning them into a ‘transposon’
- An insertion sequence jumps into one spot on a chromosome.
- The same type of IS jumps into a nearby spot, but further downstream.
- Now, you have a random DNA segment flanked by two IS elements.
- Transposase sees the IS elements → treats the whole region in between as a transposon.
- That whole segment — IS + random DNA + IS — can now jump together onto a plasmid or another part of the genome.
whats a complex transposon
No flanking Insertion sequnces are present - one continous stretch of DNA including:
1. Transposase gene (gene for movement)
2. Resolvase gene (helps seperate DNA intermediates in movement)
3. accessory genes (antibotic R for example)
this transposon copies itself leaving the orginal behind
what is transduction
Bacteriophage (virus that infects bacteria) accidentally packages host bacterial DNA instead of its own
When this phage infects a new bacterium, it injects the previous host’s DNA, which can recombine with the recipient’s genome
what are inegrons
like shopping trolleys:
has a core frame like a trolley including a promoter and recombinase gene:
= recombinase is like the mechanism that lets you add or remove compartments from the trolley as you go through the store.
inserts gene cassettes from a variety of places and collects them
