BACTERIAL GENETICS - MODULE Flashcards
DNA is a macromolecule composed of repeating units called
nucleotides,
Each nucleotide consists of a
pentose sugar (deoxyribose)
phosphate group
nitrogen-containing nucleobase [adenine (A), and guanine (G) or a pyrimidine base — cytosine (C), and thymine (T)]
The nucleotides are joined to one another in a chain by covalent bonds between the sugar of one nucleotide and the phosphateof the next, resulting in an alternating [?]
sugar-phosphate backbone
The DNA molecule consists of two strands of nucleotides resembling a
ladder with a sugar-phosphate backbone
Each rung of the ladder consists of a of pair bases, which combine in specific pairs
(A with T, and C with G)
According to base pairing rules, hydrogen bonds bind the nitrogenous bases of the two separate polynucleotide strands to
make a
double-stranded DNA
the sequence on one strand is [?] to that on the other.
complementary
The two strands of nucleotides are coiled around each other to form a [?], a structure like a spiral ladder forming a double-stranded DNA that is helical
double helix
are small sections of the DNA molecule that codes for production of proteins; These are the fundamental units of heredity which is transferred from a parent to offspring and is held to determine some characteristic of the offspring
Genes
is the sum total of genes present in cell or organism; Each contains all of the information needed to build and maintain that organism
Genome
The genome size is expressed in [?]. For example, in humans, 3 billion base pairs (3 gbp) is are contained in all cells that have a nucleus.
base pairs
is the inheritable genetic makeup of an organism that codes for a particular trait.
Genotype
refers to actual, expressed or observable trait; governed by the genotype; may be affected by environmental factors thus, its
expression differs in different situations
Phenotype
Among the bacteria, [4 answers] are some of the environmental influences that may dictate whether the genetic trait may be expressed or not. For example, Serratia marcescens form red colonies at 24 oC, and white colonies at 37 oC.
temperature, ph, age, humidity
Different types of DNA may be found in a bacterial cell. The genes are carried on:
bacterial chromosome, (2) plasmid, (3) phage, or (4) transposon
Bacterial cell has how many chromosome/s
one (1) chromosome
- consists of double stranded DNA
- arranged in a circular form
Bacterial chromosome is found in the region of the cytoplasm called
nucleoid
carry most bacterial genes
Chromosomes
During cell division, duplication of chromosome occurs so that each daughter cell receives an
identical set
are circular, double-stranded DNA. They are small ranging from 1.5 to 400 kbp; extra-chromosomal; can replicate
autonomously in bacterial cells
PLASMIDS
They carry genes that code for specialized functions, e.g, fertility genes that direct conjugation, many of the genes for antibiotic resistance, and most bacterial exotoxins.
PLASMIDS
process where table pieces of phage (prophage) DNA may be inserted into the bacterial chromosome and replicates with the bacterial DNA following infection with a temperate phage
lysogeny
a repressed virus — carried passively without replication and causing lysis of the infected bacterium
temperate phage
Besides the repressor protein, this prophage DNA may also direct synthesis of
another protein. Most notable are gene products that make bacteria more
pathogenic. This enhanced virulence is called
lysogenic conversion
are large, mobile genetic elements, with DNA sequences of several kbp , that can move themselves or a copy from one molecule of DNA to another, so are referred j to as “jumping genes”
Transposons
carry genes for specialized functions but do not contain genetic information for replication
transposons
is the unilateral transfer of genetic information from a donor cell to a recipient cell
Genetic exchange
is not an essential step in the life cycle of bacteria. But, it is beneficial and may bring together combination of genes that enables the recombinant bacteria to carry out a valuable new function
genetic exchange
is the gene transfer from one bacterial cell to another involving direct cell-to-cell contact
Conjugation
Conjugation is controlled by [?] that carries the genes that code for sex pili formation which bring the two cells in physical contact
F factor (fertility plasmid)
have F factor, therefore form sex pili
Donor (male) cells
lack F factors and are called F- cells
Recipient (female) cells
have F factor in its free state (not incorporated into the bacterial chromosome)
F+ cells
have F factor that is integrated into the bacterial chromosome
Hfr cells
denotes high frequency of recombination, referring to the fact that Hfr cells donate copies of genes on the bacterial chromosome
“Hfr”
Types of conjugal crosses:
F+ x F- conjugation
Hfr x F- conjugation
It is mediated by the sex pilus that forms the conjugative tube through which the F factor passes
Attachment between F+ and F- cells
Only a single strand of the DNA duplex is transferred. The area that is lost is reduplicated so the donor always stays the same genotype. The transfer of the plasmid is fairly quick so that it is transferred in its entirety. No bacterial genes are transferred
Transfer of the F factor from the F+ cell to the F- cell
The donor cell remains F+ cell. The recipient cell is converted to F+ cell
Only the F factor has been transferred and it remains a free element in the cytoplasm. The F factor will replicated independently of the chromosome, thus, there is no genetic recombination
Separation of both cells
Other plasmids are transferred by similar mechanisms with F+ x F- conjugation
R (resistance) plasmids
Col plasmids (bacteriocinogenic plasmids)
virulence plasmids
It is facilitated by the sex pilus
Attachment of the Hfr cell to an F- cell
The first half of F factor will be transferred first and then the bacterial genes in the linear order. Most of the integrated F factor enters the recipient cell last
Only a single strand of the DNA duplex is
transferred. The area that is lost is reduplicated so that the donor always stays the same genotype. It takes approximately 90 minutes for a complete transfer to occur. Because the conjugative bridge is so fine, mating is normally interrupted, so the entire genome of the donor cell is not transferred
Transfer of the F factor from the Hfr cell to the F- cell
The donor cell remains Hfr. The recipient gets some donor chromosomal genes
that can recombine with its own chromosome, but does not become Hfr since the complete F factor is not transferred. It becomes a recombinant F- cell
Separation of cells
Destruction of cell does not necessarily destroy its
genetic material
When bacteria [?], they often release their DNA into the surrounding medium.
lyse
When this DNA is introduced into another viable cell, it retains its ability to
direct the
synthesis of specific proteins
The uptake of free (or naked) extracellular DNA in the environment by a competent cell and subsequent integration into its chromosome by a competent is called
transformation
The [?] cell often acquires new characteristics as a result.
recipient
Competency is dependent on several factors:
- Stage of growth — logarithmic phase
- Alterations in the cell membrane that makes it permeable to the DNA molecule
- Synthesis of receptor sites on the bacterial cell surface
Only a few bacterial species are capable of
natural transformation
Naturally competent transformable bacteria, of medical importance, are found in several genera and include
Haemophilus influenzae
Neisseria gonorrhoeae
Neisseria meningitidis
Streptococcus pneumoniae
Specific DNA sequences are required for uptake of the DNA, thus restricting genetic
exchange to a [?] or closely related species.
single species
is induced in the laboratory by a variety of techniques. The procedure involves
extraction of the DNA, and treatment of cells with high salt (e.g., CaCl2 ) and temperature shock to render the cell membrane more permeable to DNA and make them readily take up DNA. This is fundamental in genetic engineering.
Artificial (or forced) transformation
is the transfer of DNA between bacteria by means of a phage (bacteriophage)
Transduction
Phages come in two major types:
Virulent phage
Temperate phage
infects bacterial cells, replicates, and and kills the host bacteria, usually by lysing — lytic cycle
Virulent phage
often have repressor genes to prevent phage replication; integrates their DNA into the bacterial chromosome where it stably stays as a prophage without lysing the host bacteria. — lysogeny
Temperate phage
In the absence of [?] they also may replicate lytically.
functional repressor protein,
two (2) types of transduction
generalized and specialized
is mediated virulent phages
Generalized transduction
steps in Generalized transduction
- Attachment and infection
- Replication of the phage DNA
- Assembly and package
- Lysis of the host bacterium
The virulent phage DNA is injected into the bacterium
- Attachment and infection
This directs synthesis of the phage components and the in disintegration of the bacterial chromosome into small pieces
- Replication of the phage DNA
Phage DNA and protein coat (capsid) are packaged into phages . Occasionally, bacterial DNA are packaged in a phage capsid by mistake — these are called transducing particles.
- Assembly and package
This releases the phages and transducing particles.
- Lysis of the host bacterium
When the transducing particle infects a new bacterium, it injects the DNA from the [?], rather than viral nucleic acid.
donor bacterium
The recipient bacterium receives the donated DNA and incorporates it into its own chromosome, producing a [?] with a new genotype.
recombinant cell
Because the virus is defective (biologically inactive as a virus), it is unable to initiate lytic cycle. The [?] survives and can use the new genetic material
transduced cell
Generalized transduction is so-called because any bacterial DNA fragment can be [?] within the phage and can be [?] from one cell to another.
packaged; transferred
is mediated by temperate phages
Specialized transduction
steps in Specialized transduction
- Attachment and infection
- Lysogeny and lysogenic conversion
- Replication
- Most cells continue to divide
- Spontaneous induction
- Reactivation of phage DNA, assembly and package
- Lysis of the host bacterium
The temperate phage DNA is injected into the bacterium, and represses its own replication (R, repressor molecules).
- Attachment and infection
Lysogeny - - - t h e p h a g e D N A integrates into the chromosome of the host bacterium. The integrated phage DNA is now called prophage. Ly s o g e n i c c o n v e r s i o n — t h e bacterium (lysogenic cell) acquires new traits.
- Lysogeny and lysogenic conversion
Prophage is replicated with the host chromosome. All daughter cells are lysogenic cells qnd have the potential for producing temperate phages.
- Replication
showing no evidence of viral infection
- Most cells continue to divide
In a small percentage of lysogenic cells, prophage DNA excises from the bacterial chromosome. This occurs spontaneously but is enhanced by irradiation with UV light or exposure to agents that inter fere with DNA replication.
- Spontaneous induction
Following excision, phage DNA replicates and progeny temperate phages assemble and become packaged within a capsid.When prophage separates from the bacterial chromosome, it carries with it a segment of bacterial DNA in the same capsid, becoming a specialized transducing particle.
- Reactivation of phage DNA, assembly and package
The induced cell lyses and infectious temperate phages are released.
- Lysis of the host bacterium
Because prophages are inserted only at special site on the bacterial chromosome, only bacterial DNA [?] to this site can be transferred in specialized transduction
adjacent
The medical significance of lysogenic conversion is illustrated by its role in the [?] of certain bacteria
pathogenesis
Phage-coded pathogenic
factors:
• [1] of Salmonella
• [2] of Clostridium botulinum (causing botulism)
• [3] of Streptococcus pyogenes (causing scarlet fever)
• [4] of Corynebacterium diphtheriae
[1] O antigen
[2] Botulinum toxin
[3] Erythrogenic toxins
[4] Diphtheria toxin
is a permanent alteration in the base sequence of the gene (DNA) resulting in phenotypic change; genetic variation in bacteria may result from this
Mutation
Mechanisms of mutation:
Insertion
Deletion
Substitution
takes place when a nitrogenous base (nucleobase) is added to the nucleotide sequence
Insertion
is the removal of a nucleobase from the nucleotide chain
Deletion
arise when there is mispairing between complementary bases. It occurs when one or more nucleobases in a nucleotide sequence is replaced with another base
Substitution
Mutations can be [?] by a mutagen in the environment.
spontaneous, or induced
is a random, undirected alteration of the DNA base sequence that arise as a consequence of mistakes in DNA replication. This type of mutation occurs naturally in the environment.
Spontaneous mutation
are caused by exposure to external influences or agents that result in the DNA to breakdown. Such external influences include chemicals, radiation, viruses, diet and lifestyle
Induced mutation
agents that induce mutations causing the DNA to break down
mutagens
converts the nucleobase adenine (A) to a form that no longer pairs with thymine (T) but cytosine (C). Thus, when DNA containing such modified adenines replicates, one daughter DNA molecule will have a base-pair sequence different from that of the parent DNA.
Nitrous acid (HNO2)
ionize atoms and molecules with the formation of highly reactive ions and free radicals. Some of these ions can combine with bases in DNA, resulting in errors in DNA replication and repair that produce mutations
X rays and gamma rays
is a non-ionizing component of ordinary sunlight. Its most important effect on DNA is the formation of harmful covalent bonds between certain bases. Adjacent thymines in a DNA strand can cross-link to form thymine dimers. Such dimers, unless repaired, may cause serious damage or death to the cell because it cannot properly transcribe or replicate such DNA
Ultraviolet (UV) light
