Genetics ofprokaryotic 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.Mycoplasma genitalium 0,58Mb

Haemophilus influenzae 1,83Mb

Agrobacterium tumefaciens

2,84Mb

Escherichia coli 4,64Mb

Salmonella typhi 4,81Mb

Mycobacterium tuberculosis

4,41Mb

Bacillus megaterium 30MbGenome‘s sizes of some bacteria

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

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 e.g. in parasitic

protists of phylum Apicomplexa (Pasmodium,Toxoplasmosma……)

Strachan and Read: Human genetics, 2. edition

Mitochondria

Plastids

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 conjugationCells 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

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 chromosomes without „ori“ = origin of

replication.

Bacterial conjugation

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

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.

Transduction

General transduction

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.

Endosymbiotické organely

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 moleculeinducer = 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

• 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)

Lac operon - positive regulation

Thank you for your attention

Genetika, D P. SnustadM.J. Simmons, 5. edition, 2009

http://www.youtube.com/watch?v=7sZ5Nz8_cfc