BACTERIAL GENETICS - MODULE Flashcards by ARRIANE CYREL CAMACHO

DNA is a macromolecule composed of repeating units called

nucleotides,

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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)]

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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

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The DNA molecule consists of two strands of nucleotides resembling a

ladder with a sugar-phosphate backbone

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Each rung of the ladder consists of a of pair bases, which combine in specific pairs

(A with T, and C with G)

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According to base pairing rules, hydrogen bonds bind the nitrogenous bases of the two separate polynucleotide strands to
make a

double-stranded DNA

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the sequence on one strand is [?] to that on the other.

complementary

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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

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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

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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

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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

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is the inheritable genetic makeup of an organism that codes for a particular trait.

Genotype

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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

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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

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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

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Bacterial cell has how many chromosome/s

one (1) chromosome

consists of double stranded DNA
arranged in a circular form

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Bacterial chromosome is found in the region of the cytoplasm called

nucleoid

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carry most bacterial genes

Chromosomes

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During cell division, duplication of chromosome occurs so that each daughter cell receives an

identical set

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are circular, double-stranded DNA. They are small ranging from 1.5 to 400 kbp; extra-chromosomal; can replicate
autonomously in bacterial cells

PLASMIDS

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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

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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

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a repressed virus — carried passively without replication and causing lysis of the infected bacterium

temperate phage

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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

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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

1. Attachment and infection 2. Replication of the phage DNA 3. Assembly and package 4. Lysis of the host bacterium

The virulent phage DNA is injected into the bacterium

1. Attachment and infection

This directs synthesis of the phage components and the in disintegration of the bacterial chromosome into small pieces

2. 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.

3. Assembly and package

This releases the phages and transducing particles.

4. 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

1. Attachment and infection 2. Lysogeny and lysogenic conversion 3. Replication 4. Most cells continue to divide 5. Spontaneous induction 6. Reactivation of phage DNA, assembly and package 7. Lysis of the host bacterium

The temperate phage DNA is injected into the bacterium, and represses its own replication (R, repressor molecules).

1. 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.

2. 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.

3. Replication

showing no evidence of viral infection

4. 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.

5. 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.

6. Reactivation of phage DNA, assembly and package

The induced cell lyses and infectious temperate phages are released.

7. 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