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striction endonucleases can destroy foreign DNA that bears no "fingerprint" (modification) signifying "self." These modifications take the form of methy-lation of the DNA bases by modification enzymes.

Bacterial restriction endonucleases are invaluable tools in modern gene cloning techniques. The process is termed gene "cloning" because it involves replication of DNA that has been manipulated in vitro in a suitable host cell so as to produce identical copies of this DNA: molecular clones or gene clones.

The technique simplifies the replication of DNA, making experimental manipulations easier. On the other hand, the bacteria can also be used to synthesize gene products of the foreign genes. Such foreign proteins are called recombinant proteins. Bacterial plasmids often function in the role of vectors into which the sequences to be cloned are inserted. Fig. 3.24 illustrates the principle of gene cloning in simplified form.

Table 3.3 lists the most important terms used in the field of bacterial genetics.

Table 3.3 Glossary of Important Terminology in Bacterial Genetics








Effector molecules


F factor Gene

Genome Genotype Hfr cells

Inductor Integron

Triplet sequences of transfer RNA complementary to the codons of mRNA

See nucleoid

Genetic unit, identical to "gene"

Key relating the DNA nucleotide (n = 3) sequence to the polypeptide amino acid sequence

Sequence of three nucleotides, triplet

See effector molecules

Loss of a DNA sequence in a replicon

Small molecules that inactivate (= inductor) or activate (= corepressor) a regulator protein by means of an allosteric effect

Historical term; characterizes a replicon (e.g., F plasmid) occurring either in the cytoplasm or integrated in the bacterial chromosome

Prototype of a conjugative plasmid (fertility factor)

DNA segment containing the information used in synthesis of a polypeptide or RNA

All of the genetic information contained in a cell

The totality of genetically determined characteristics

Coli bacteria with F factor integrated into their chromosomes, therefore capable of transferring chromosomal genes at a high frequency by means of conjugation (Hfr = high frequency of recombination)

See effector molecules

Genetic structure containing the determinants for a site-specific recombination system; responsible for integration or excision of mobile gene cassettes

Table 3.3 Continued: Glossary of Important Terminology...

Inverted repeats IS

Cassette Clone

Conjugation Lysogenic bacteria

Lysogenic conversion Messenger RNA

Modification enzymes








Promoter Prophage Regulator



Legitimate or homologous recombination

Nucleotide sequences repeated in reverse order at the ends of transposable DNA

Insertion sequences; transposable DNA elements

Sequence in a gene that can be transferred to other genes by homologous recombination

Population of identical cells or DNA molecules

Transfer of hereditary material in a pairing process

Cells with a phage genome (prophage) integrated into their chromosomes

Change in cell phenotype brought about by prophage genes

Synthesized at the DNA by transcription; carries genetic information to the ribosomes

Methylases that label DNA as "self" by methylation

A permanent alteration of the genome

Nuclear region, nucleus equivalent

DNA sequence of an operon; regulator binding site

Regulatory unit comprising the promoter, operator, structural genes, and terminator

Unilateral gene transfer from a donor to a receptor

The totality of characteristics expressed in a bacterial cell

Extrachromosomal, autonomous, in most cases circular DNA molecule

Recognition and binding site for RNA polymerase

Phage genome integrated into the chromosome

Regulatory protein that controls gene transcription; repressor or activator

Functional unit of genes at different loci controlled by the same regulator

Replacement of a DNA sequence by a homologous sequence from a different genome; breakage and reunion model

Table 3.3 Continued: Glossary of Important Terminology...



Insertion of transposable DNA



Integration or excision of a DNA sequence by means of


homologous recombination in a specific DNA segment com

prising only a small number of nucleotides


Reproduction, duplication of DNA


DNA molecule that replicates autonomously


Enzymes that recognize and cleave specific DNA nucleotide




DNA duplication mechanism in which one old strand is con


served in each of the two new double strands

Conjugal (or sex)

Surface structures essential to conjugation in Gram-negative


rod bacteria

Sigma factors

Proteins that temporarily associate with prokaryotic RNA

polymerase for specific promoter binding


Circular DNA molecule additionally twisted about the helical

axis in the opposite direction


Sequence marking the end of a transcription process


Gene transfer using bacteriophages as vehicles

Transfer RNA

Specifically binds an amino acid (aminoacyl tRNA) and trans-

fers it to the ribosome


Transfer of genes from a donor in the form of "naked" DNA


RNA synthesis at DNA


Ribosomal synthesis of polypeptides


Transposition enzyme; facilitates illegitimate recombination


Translocation of a mobile DNA element within a replicon or

between different replicons


Transposable DNA; frequently contains—in addition to the

genes for transposition—determinants that change the

phenotype of a bacterial cell

Triplet code

Three nucleotides coding for one amino acid


Vehicle for foreign (passenger) DNA; usually a plasmid or

phage genome

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