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Microbial Genetics > Genetic Transfer in Prokaryotes
Genetic Transfer in Prokaryotes
• Generalized Recombination and RecA
• Bacterial Transformation
• Bacterial Transduction
• Prokaryotic Reproduction
• Complementation
• Gene Transfer in Archaea
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Microbial Genetics > Genetic Transfer in Prokaryotes
Generalized Recombination and RecA
• Homologous recombination can vary among different organisms and cell types,
but most forms involve the same basic steps.
• Homologous recombination is a major DNA repair process in bacteria. It is also
important for producing genetic diversity in bacterial populations.
• Homologous recombination has been most studied and is best understood for
Escherichia coli.
Steps in the pre-synapsis phase of homologous
recombination in bacteria
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Microbial Genetics > Genetic Transfer in Prokaryotes
Bacterial Transformation
• Transformation results in the genetic alteration of the recipient cell.
• Exogenous DNA is taken up into the recipient cell from its surroundings through
the cell membrane(s).
• Transformation occurs naturally in some species of bacteria, but it can also be
effected by artificial means in other cells.
Transformation
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Microbial Genetics > Genetic Transfer in Prokaryotes
Bacterial Transduction
• Transduction does not require physical contact between the cell donating the
DNA and the cell receiving the DNA (which occurs in conjugation), and it is
DNAase resistant.
• Transduction happens through either the lytic cycle or the lysogenic cycle.
• Transduction is especially important because it explains one mechanism by which
antibiotic drugs become ineffective due to the transfer of antibiotic-resistance
genes between bacteria.
Transduction
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Microbial Genetics > Genetic Transfer in Prokaryotes
Prokaryotic Reproduction
• Binary fission is a type of reproduction in which the chromosome is replicated and
the resultant prokaryote is an exact copy of the parental prokaryate, thus leaving
no opportunity for genetic diversity.
• Transformation is a type of prokaryotic reproduction in which a prokaryote can
take up DNA found within the environment that has originated from other
prokaryotes.
• Transduction is a type of prokaryotic reproduction in which a prokaryote is
infected by a virus which injects short pieces of chromosomal DNA from one
bacterium to another.
Modes of prokaryote reproduction
• Conjugation is a type of prokaryotic reproduction in which DNA is transferred
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between prokaryotes by means of a pilus.
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Microbial Genetics > Genetic Transfer in Prokaryotes
Complementation
• A complementation test answers the question: "Does a wild-type copy of gene X
rescue the function of the mutant allele that is believed to define gene X? ".
• Complementation arises because loss of function in genes responsible for
different steps in the same metabolic pathway can give rise to the same
phenotype.
• When strains are bred together, offspring inherit wildtype versions of each gene
from either parent.
Complementation Test
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Microbial Genetics > Genetic Transfer in Prokaryotes
Gene Transfer in Archaea
• Archaea while being very different from eukaryotes and bacteria, there are many
commonalities at the the genetic level between them.
• Horizontal gene transfer can explain the similarities between the genes found in
the three domains of life and indeed there is evidence that horizontal gene
transfer occurs with Archaea species.
• Archaea can be infected by double-stranded DNA viruses, which can account for
gene transfers, as well like bacteria, Archaea may conjugate.
Archaeal viral infection
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Appendix
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Microbial Genetics
Key terms
• archaea a taxonomic domain of single-celled organisms lacking nuclei that are fundamentally from bacteria.
• binary fission the process whereby a cell divides asexually to produce two daughter cells
• Complementation In genetics, complementation refers to a relationship between two different strains of an organism which both
have homozygous recessive mutations that produce the same phenotype (for example, a change in wing structure in flies) but
which do not reside on the same (homologous) gene.
• conjugation the temporary fusion of organisms, especially as part of sexual reproduction
• eukaryotic Having complex cells in which the genetic material is organized into membrane-bound nuclei.
• exogenous Produced or originating outside of an organism.
• expression Gene expression is the process by which information from a gene is used in the synthesis of a functional gene
product.
• genetic Relating to genetics or genes.
• homologous Showing a degree of correspondence or similarity.
• homozygous of an organism in which both copies of a given gene have the same allele
• lysogenic cycle A form of viral reproduction involving the fusion of the nucleic acid of a bacteriophage with that of a host,
followed by proliferation of the resulting prophage.
• lytic cycle The normal process of viral reproduction involving penetration of the cell membrane, nucleic acid synthesis, and lysis
of the host cell.
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Microbial Genetics
• mutation Any heritable change of the base-pair sequence of genetic material.
• pilus a hairlike appendage found on the cell surface of many bacteria
• recombination The formation of genetic combinations in offspring that are not present in the parents
• transcription Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA
are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from
DNA to RNA by the action of the correct enzymes.
• transduction horizontal gene transfer mechanism in prokaryotes where genes are transferred using a virus
• transduction Transduction is the process by which DNA is transferred from one bacterium to another by a virus.
• transformation the alteration of a bacterial cell caused by the transfer of DNA from another, especially if pathogenic
• transformation In molecular biology transformation is genetic alteration of a cell resulting from the direct uptake, incorporation
and expression of exogenous genetic material (exogenous DNA) from its surroundings and taken up through the cell
membrane(s).
• translation Translation is the communication of the meaning of a source-language text by means of an equivalent targetlanguage text.
• translocase An enzyme that assists in moving another molecule, usually across a membrane.
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Microbial Genetics
Steps in the pre-synapsis phase of homologous recombination in bacteria
Beginning of the RecBCD pathway. This model is based on reactions of DNA and RecBCD with Mg2+ ions in excess over ATP. Step 1: RecBCD binds
to a DNA double strand break. Step 2: RecBCD initiates unwinding of the DNA duplex through ATP-dependent helicase activity. Step 3: RecBCD
continues its unwinding and moves down the DNA duplex, cleaving the 3' strand much more frequently than the 5' strand. Step 4: RecBCD encounters a
Chi sequence and stops digesting the 3' strand; cleavage of the 5' strand is significantly increased. Step 5: RecBCD loads RecA onto the 3' strand. Step
6: RecBCD unbinds from the DNA duplex, leaving a RecA nucleoprotein filament on the 3' tail.
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Microbial Genetics
Modes of prokaryote reproduction
Besides binary fission, there are three other mechanisms by which prokaryotes can exchange DNA. In (a) transformation, the cell takes up prokaryotic
DNA directly from the environment. The DNA may remain separate as plasmid DNA or be incorporated into the host genome. In (b) transduction, a
bacteriophage injects DNA into the cell that contains a small fragment of DNA from a different prokaryote. In (c) conjugation, DNA is transferred from one
cell to another via a mating bridge that connects the two cells after the pilus draws the two bacteria close enough to form the bridge.
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Microbial Genetics
Complementation Test
Example of a complementation test. Two strains of flies are white eyed because of two different autosomal recessive mutations which interrupt different
steps in a single pigment-producing metabolic pathway. Flies from Strain 1 have complementary mutations to flies from Strain 2 because when they are
crossed the offspring are able to complete the full metabolic pathway and thus have red eyes.
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Microbial Genetics
Archaeal viral infection
Cell of Sulfolobus infected by virus STSV1 observed under microscopy. Two spindle-shaped viruses were being released from the host cell. The strain of
Sulfolobus and STSV1 (Sulfolobus tengchongensis Spindle-shaped Virus 1) were isolated by Xiaoyu Xiang and his colleagues in an acidic hot spring in
Yunnan Province, China. At present, STSV1 is the largest archaeal virus to have been isolated and studied. Its genome sequence has been sequenced.
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Microbial Genetics
Transformation
Illustration of bacterial transformation. DNA from dead cells gets cut into fragments and exits the cell. The free-floating DNA can then be picked up by
competent cells. The exogenous DNA is incorporated into the host cell's chromosome via recombination.
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Microbial Genetics
Transduction
Transduction is the process by which DNA is transferred from one bacterium to another by a virus. It also refers to the process whereby foreign DNA is
introduced into another cell via a viral vector.
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Wikipedia. "Transduction (genetics)en." Public domain http://en.wikipedia.org/wiki/File:Transduction_(genetics)en.svg View on Boundless.com
Microbial Genetics
What is the role of the Chi site in homologous recombination
mechanisms?
A) It marks the location of breaks in double-strand DNA.
B) It triggers the loading of RecA proteins onto DNA.
C) It forms a D-loop.
D) It causes the replication fork to collapse.
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Microbial Genetics
What is the role of the Chi site in homologous recombination
mechanisms?
A) It marks the location of breaks in double-strand DNA.
B) It triggers the loading of RecA proteins onto DNA.
C) It forms a D-loop.
D) It causes the replication fork to collapse.
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Microbial Genetics
All of the following are associated with natural transformation
EXCEPT:
A) DNA translocase
B) calcium chloride
C) pilin
D) secretins
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Microbial Genetics
All of the following are associated with natural transformation
EXCEPT:
A) DNA translocase
B) calcium chloride
C) pilin
D) secretins
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Microbial Genetics
Which of the following is a true statement concerning
transduction?
A) Transduction is not susceptible to DNases.
B) Transduction transfers genetic material between cells.
C) Transduction does not require physical contact between cells.
D) All of these answers
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Microbial Genetics
Which of the following is a true statement concerning
transduction?
A) Transduction is not susceptible to DNases.
B) Transduction transfers genetic material between cells.
C) Transduction does not require physical contact between cells.
D) All of these answers
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Microbial Genetics
A combination of bacterial and viral genes are transferred during:
A) generalized transduction
B) both specialized and generalized transduction
C) specialized transduction
D) neither specialized nor generalized transduction
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Microbial Genetics
A combination of bacterial and viral genes are transferred during:
A) generalized transduction
B) both specialized and generalized transduction
C) specialized transduction
D) neither specialized nor generalized transduction
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Microbial Genetics
Which of the following modes of reproduction is correctly paired
with its description?
A) Transformation: DNA is transferred between prokaryotes by means of
a pilus
B) Transduction: DNA is transferred to a prokaryote by a virus or
bacteriophage
C) Binary Fission: a prokaryote fuses to another prokaryote to take up
DNA present in the environment
D) Conjugation: a prokaryote replicates its DNA and produces an exact
copy of itself
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Microbial Genetics
Which of the following modes of reproduction is correctly paired
with its description?
A) Transformation: DNA is transferred between prokaryotes by means of
a pilus
B) Transduction: DNA is transferred to a prokaryote by a virus or
bacteriophage
C) Binary Fission: a prokaryote fuses to another prokaryote to take up
DNA present in the environment
D) Conjugation: a prokaryote replicates its DNA and produces an exact
copy of itself
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Microbial Genetics
Two strains of an organism that share the same mutant
phenotype are crossed, and they have offspring that exhibit the
wild-type phenotype. What is the simplest explanation for what is
occurring here?
A) The mutation in one strain is suppressing the mutation in the other
strain.
B) The two strains have mutations in the same gene.
C) One of the strains has a dominant wild-type copy of the gene
responsible for the phenotype.
D) The two strains have mutations in different genes.
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Microbial Genetics
Two strains of an organism that share the same mutant
phenotype are crossed, and they have offspring that exhibit the
wild-type phenotype. What is the simplest explanation for what is
occurring here?
A) The mutation in one strain is suppressing the mutation in the other
strain.
B) The two strains have mutations in the same gene.
C) One of the strains has a dominant wild-type copy of the gene
responsible for the phenotype.
D) The two strains have mutations in different genes.
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Microbial Genetics
Why is horizontal gene transfer thought to have occurred between
the Archaea and other domains of life?
A) Archaea share many genes and proteins with other organisms.
B) Archaea can be infected by viruses.
C) Archaea efficiently transmit their genes from parental generations to
offspring.
D) Most Archaean genes lack introns.
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Microbial Genetics
Why is horizontal gene transfer thought to have occurred between
the Archaea and other domains of life?
A) Archaea share many genes and proteins with other organisms.
B) Archaea can be infected by viruses.
C) Archaea efficiently transmit their genes from parental generations to
offspring.
D) Most Archaean genes lack introns.
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Microbial Genetics
Attribution
• Wikipedia. "Complementation (genetics)." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Complementation_(genetics)
• Wiktionary. "homozygous." CC BY-SA 3.0 http://en.wiktionary.org/wiki/homozygous
• Boundless Learning. "Boundless." CC BY-SA 3.0 http://www.boundless.com//microbiology/definition/complementation
• Wiktionary. "mutation." CC BY-SA 3.0 http://en.wiktionary.org/wiki/mutation
• Wikipedia. "Homologous recombination." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Homologous_recombination#In_bacteria
• Wiktionary. "genetic." CC BY-SA 3.0 http://en.wiktionary.org/wiki/genetic
• Wiktionary. "recombination." CC BY-SA 3.0 http://en.wiktionary.org/wiki/recombination
• Wiktionary. "homologous." CC BY-SA 3.0 http://en.wiktionary.org/wiki/homologous
• Wikipedia. "Horizontal gene transfer." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Horizontal_gene_transfer
• Wikipedia. "Archaea." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Archaea
• Wikipedia. "Archaea." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Archaea#Genetics
• Wiktionary. "translation." CC BY-SA 3.0 http://en.wiktionary.org/wiki/translation
• Wiktionary. "transcription." CC BY-SA 3.0 http://en.wiktionary.org/wiki/transcription
• Wikipedia. "archaea." CC BY-SA 3.0 http://en.wikipedia.org/wiki/archaea
• Wikipedia. "Transformation (genetics)." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Transformation_(genetics)
• Wiktionary. "translocase." CC BY-SA 3.0 http://en.wiktionary.org/wiki/translocase
• Wiktionary. "eukaryotic." CC BY-SA 3.0 http://en.wiktionary.org/wiki/eukaryotic
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Microbial Genetics
• Wiktionary. "expression." CC BY-SA 3.0 http://en.wiktionary.org/wiki/expression
• Wiktionary. "exogenous." CC BY-SA 3.0 http://en.wiktionary.org/wiki/exogenous
• Wiktionary. "transformation." CC BY-SA 3.0 http://en.wiktionary.org/wiki/transformation
• Wikipedia. "Transduction (genetics)." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Transduction_(genetics)
• Wiktionary. "transduction." CC BY-SA 3.0 http://en.wiktionary.org/wiki/transduction
• Wiktionary. "lysogenic cycle." CC BY-SA 3.0 http://en.wiktionary.org/wiki/lysogenic+cycle
• Wiktionary. "lytic cycle." CC BY-SA 3.0 http://en.wiktionary.org/wiki/lytic+cycle
• Connexions. "Structure of Prokaryotes." CC BY 3.0 http://cnx.org/content/m44605/latest/?collection=col11448/latest
• Wiktionary. "binary fission." CC BY-SA 3.0 http://en.wiktionary.org/wiki/binary+fission
• Wiktionary. "pilus." CC BY-SA 3.0 http://en.wiktionary.org/wiki/pilus
• Wiktionary. "conjugation." CC BY-SA 3.0 http://en.wiktionary.org/wiki/conjugation
• Wiktionary. "transduction." CC BY-SA 3.0 http://en.wiktionary.org/wiki/transduction
• Wiktionary. "transformation." CC BY-SA 3.0 http://en.wiktionary.org/wiki/transformation
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