23. Replication and Recombination Flashcards
1
Q
what are transposable elements?
A
- “Jumping Genes”
- chromosomes of bacteria, viruses, eukaryotic cells contain piences of DNA that move around genome
– process called transposition
– important role in generation and transfer of new gene combinations - DNA segments carry genes required for this process
– consequently, move baout chromosome are transposable elements (transposons) - transposition does not require extensive homology between transposon and destination site
2
Q
what is the behaviour of transposable elements?
A
- similar to lysogenic phages
– but originate in one position of chromosome, then move to different location in same chromosome - transposon differ from phages
– lacking virus cycle and plasmids
– not able to reproduce autonomously
– exist apart from chromosome
3
Q
what are the types of transposons?
A
- simple transposons
- complex transposons
4
Q
what are simple transposons considered as?
A
- contain palindromic sequences at each end
– nucleotide sequences read same backwards and forwards - bound by inverted repeat (IR) sequences on each side
5
Q
what are complex transposons considered as?
A
- include, not only, transposase gene
– in insertion sequence - other genes as well
– bound by insertion sequences - R-factors and cell marker genes
– often contained in complex transposons
6
Q
what are insertion sequences (IS elements)?
A
- short sequence of DNA (750-1600 bp)
– containing only genes for enzymes required for transposition - bound at both ends by identical sequences of nucleotides
– in reverse orientation
– Inverted Repeats (15-25 bases long, vary between IS elements) - terminal inverted repeats are inverted complements of each other
– complement of ACGCTA (inverted repeat on right side of TE) is TGCGAT (reverse order of terminal inverted repeat on left side) - one of roles of terminal inverted repeats are to be recognised by transposase
– between inverted repeats is gene coding for enzyme transposase, required for transposition - flanking direct repeats
– no actually transposable element, rather play role in insertion of TE - after TE excises, repeats left as ‘footprints’
– can alter expression of gene in which they have been left, even after related TE has moved to another location on genome
7
Q
what is the mechanism of transposition?
A
- Tn5 transposase
- first step
– individual molecules of transposase bind to specific sits at ends of transposon DNA - next step
– looping of transposon DNA forms synaptic complex, bringing two ends of transposable element close together - once synaptic complex formed
– Tn5 transposase cuts transposon DNA away from flanking donor DNA - after cleavage
– Tn5 transposase/DNA complex moves freely until encounters and binds to target DNA - process called
– strand transfer
– transposase catalyses insertion of transposon DNA into traget DNA, completing transposition process
8
Q
Review Paper**
A
–
9
Q
What is the replication of the bacterial genome?
A
- duplication of chromosome and separation of copies
- continues elongation of cell and movement of copies
- division into two daughter cells
10
Q
what is the bacterial genome?
A
- one circular DNA genome
– single origin of replication
– bi-directional DNA replication - bacteria may contain plasmids
– smaller circular DNA molecules
– autonomously replicated - bacteria divide by binary fission
– asexual reproduction - progeny are genetically identical
– clonal replication
11
Q
what is the process of bacterial genome replication?
A
- enzymes unwind parental double helix
- proteins stabilise unwound parental DNA
- leading strand is synthesised continuously by DNA polymerase
- lagging strand is synthesised discontinuously
– RNA polymerase synthesises short RNA primer which is extended by DNA polymerase - DNA polymerase digests RNA primer and replaces with DNA
- DNA ligase joins discontinuous fragments of lagging strand
12
Q
what is bacterial DNA mutation?
A
- stability of nycleic acid sequence important for life
- sequence chages occur
– resulting in altered phenotypes - changes in base sequence of DNA
– inheritable - can be:
– harmful / lethal
– helpful
– silent
13
Q
what are the causes of mutations?
A
- spontaneous mutations
– in cells, result of errors in replication
– develop in absence of any added agent - physical and chemical agents
– induced mutations as result of exposure to mutagen
– UV radiation
14
Q
what type of mutations occur?
A
- silent
- missense
- nonsense
- frameshift
- large deletions/insertions
15
Q
what is a silent mutation?
A
- mutation occurs but no resultant phenotypic change
– due to codon degeneracy - mutation not detected
– except at level of DNA / mRNA sequence
16
Q
what is a missense mutation?
A
- single base substitution in DNA
– changes a codon for one amino acid into codon for another - EG.
– GGC specifies Glycine; changes to AGC specifying Serin - expression / effect of mutation can vary
– expressed at protein level, resulting effect on protein function may range
— from complete loss of activity -> no change
17
Q
what are nonsense mutations?
A
- causes early termination of translation
– results in truncated polypeptide - results in conversion of sense codon to nonsense / stop codon
- phenotypic effect largely dependent on relative location of mutation
18
Q
what are frameshift mutations?
A
- arise from insertion/deletion of 1 or 2 base pairs within coding region of a gene
- usually very deleterious and yield mutant phenotypes
– result from synthesis of nonfunctional proteins - often produces stop codon and shorter peptide
19
Q
how do we calculate rate of incidence of mutations?
A
- if doubling time = 20 min
– 2^3 cells/hr from single cell - if spontaneous mutation rate = 1*10^-7 /gene
– (12 hr)(day)(10^10)(10^-7)=10^3 mutations/gene/day - if bacteria = 4000 genes
– (410^3 genes) = 410^9 mutations/day
20
Q
A
