Genetics of
prokaryotic organisms
113. seminar
Genome
Bacteria do not have true nuclei enclosed by membranes,
either compartments by internal membrane systems.
DNA is concentrated in
nucleoid region, prokaryotic chromosome,
which is the double stranded circle molecule.
Genes of prokaryotic chromosome
encode essential functions for cell.
Genome‘s sizes of some bacteria
Mycoplasma genitalium
0,58Mb
Haemophilus influenzae
1,83Mb
Agrobacterium
tumefaciens
Escherichia coli
Salmonella typhi
Mycobacterium
tuberculosis
Bacillus megaterium
2,84Mb
4,64Mb
4,81Mb
4,41Mb
30Mb
There are also small circular molecules of DNA –
plasmids, which encodes resistance to antibiotics or
metabolism of unusual nutrients.
Replicate independently of the main chromosome.
Fertility F-plasmid, Resistance R-plasmid, Col plasmid, Virulence plasmid
Plasmid sizes vary from 1 to over 1,000 kbp
Michael Marshall, DNA-grabbing bacteria hint at early phase of evolution, New Scientist Magazine issue 2936, 26 September 2013.
Genetic variability of prokaryotes
Prokaryotes could developed in time, adapt to new environments,
but they lack sexual cycle.
Recombination of prokaryotic genetic information is managed
by mechanisms, parasexual processes :
Transformation – genes are taken up from surrounding environment
Conjugation – genes are transferred directly from one to another
through conjugative bridge
Transduction – genes are transferred between prokaryotes
by viruses
Endosymbiosis, Somatic fusion
Endosymbiosis
Strachan and Read: Human genetics, 2. edition
Mitochondria
Plastids
proofs: circle DNA without histons and introns
Transcription begins by formylmethionin
prokaryote type of ribosomes
Some endosymb. organelles are inhibited by
antibiotics
Secondary symbiosis – between two eukaryotic organisms. Found
protists of phylum Apicomplexa (Pasmodium,Toxoplasmosma……)
e.g. in parasitic
Bacterial transformation
free intake of DNA into bacterial cells.
Only competent bacteria with relevant proteins are capable
of transformation (not Escherichia coli)
1. Experiment was done by Griffith (1928). It was proved
that the carrier of genetic information is DNA.
Avery, McLeod, McCarthy (1944) proved the same effect
with isolated DNA.
Griffith‘s experiment in 1928
Homologous parts
exchange, it is called
homologous
recombination.
Bacterial conjugation
Cells with F-plasmid are F+, without F-plasmid are F- .
Bacterial genes are transferred directly from one (F+) to
another cell (F-).
F plasmid encodes genetic information
for conjugation and the origin
of cytoplasmatic conjugative
bridge and F-pilli.
Hfr strain „High frequency of recombination“
is strain with the F plasmid integrated
into genomic DNA (episom)
Conjugation F+ into F- bacteria
Bacterial conjugation
It begins with single-strand break of donor‘s DNA.
Single-strand enters the conjugation bridge, the donor cell
synthesizes a new strand at the same time.
The second strand is also synthesized in the acceptor cell.
Then there is recombination between donor and acceptor
parts of the chromosome and excision and elimination of
incomplete
replication.
chromosomes
without
„ori“
=
origin
of
Conjugation Hfr into F- bacteria
Transduction
Bacterial genes are transferred between prokaryotes by
viruses. Bacteriophages transmit bacterial genes
spontaneously. It comes to pass for all bacteria.
Special transduction =
incorrect cut out of the genome of bacteriophage (prophage)
from the bacterial genome during the transition from
lysogenic to lytic cycle of bacteriophage
Special transduction
Transduction
General = into small number of phage particles are
packaged molecules of bacterial DNA instead of phage‘s
DNA molecules
application in history - mapping of DNA:
How often the genes are transmitted together in the general
transduction, more about that are localized closer.
General transduction
Endosymbiotické organely
Mitochondria – an origin in aerobic bacteria (Rikettsie)
Plastids – an origin in cyanobacteria
Some genes were transferred during evolution from
mitochondrial genome to eukaryotic genome.
Eukaryotic genomes have genes of prokaryotic type.
Plenty of proteins with function in mitochondria are encoded
in nucleus.
Operon model
is a functional unit common in bacteria and
phages.
Activation and inhibition of transcription
are regulated in response of conditions in
environment.
Prokaryotic genetic information is not divided into introns
and exons.
Operon
• is coordinately regulated clusters of genes,
which are transcribed into one mRNA
(polycistron transcript)
• are genes for particular metabolic pathway
and are regulated by common promoter and
are ordered on DNA following each other
Escherichia coli
Lac operon, Trp operon – model systems =
metabolic pathways of
• utilization of lactose
gen lacZ, lacY, lacA,
catabolic
pathway with negative and positive regulation
• enzymes for TRP synthesis, anabolic pathway
with negative regulation
Each operon consists of
•
•
•
•
promoter (for RNA polymerase = RNAP)
operator (for repressor)
several structural genes
terminator
repressor = allosteric protein encoded by
regulatory gene
co-repressor = product molecule
inducer = substrate molecule
Tryptophan operon
Lac operon - negative regulation
• regulatory gene produces repressor, which binds
operator and causes that RNAP is not able to
initialize transcription
• in the presence of lactose repressor is released
from operator. The repressor is changed by
inducer = lactose
RNA polymerase starts the transcription. In 2-3 minutes the amount of enzymes is
increased 1000x
Lac operon
- negative
regulation
Lac operon - positive regulation
• In the presence of glucose, E. coli preferentially uses
glucose for decomposing.
• If the level of glucosis is low, the level of cAMP is
increased.
• CAP „Catabolite activator protein“ in the
presence of cAMP attaches promotor and
activates the transcription.
• CAP is allosteric regulatory protein
Lac operon - positive regulation
Summary:
Lac operon is active only in
time, when the activator
CAP+cAMP is attached on
promotor (no glucose) and
when is not present represor
on operator (lactose present)
http://www.youtube.com/watch?v=7sZ5Nz8_cfc
Thank you for your attention
Genetika, D P. Snustad
M.J. Simmons, 5. edition, 2009