MICROBIAL GENETICS – MOLECULAR BIOLOGY
DNA REPLICATION AND GENE EXPRESSION
The genetics of the cell encompass the replication and expression of the cell’s hereditary
information. The hereditary information of all living cells is encoded in the cell’s
deoxyribonucleic acid molecule(s) (DNA). The information within the DNA determines
the metabolic and structural nature of the organism. The double helical nature of the
DNA macromolecule is critical for its replication. The revelation of the DNA double
helix by James Watson and Francis Crick in 1953 revolutionized biology. This discovery
of the structure of DNA quickly revealed how hereditary information is transmitted from
one generation to the next.
Replication of the hereditary information of a cell involves synthesizing new DNA
molecules that have the same nucleotide sequence as the genome of the parental
organism, a process that requires great precision (Replication = DNA
DNA). The
genome of the progeny must contain the appropriate information to permit the survival
and growth of the organism. Because changes in the sequence of nucleotides can alter
the characteristics of an organism considerably, the process of DNA replication is
designed to ensure that the progeny receive an accurate copy of the genetic information
of the parent cell.
Expression of genetic information involves using information encoded within the DNA to
direct the synthesis of proteins. DNA contains regulatory genes that control gene
expression. By specifying and regulating protein synthesis, the genetic informational
macromolecules define and control the metabolic capabilities of microorganisms. The
order of nucleotides in the DNA is used to specify the order of amino acids in a protein.
The information in the DNA molecule is initially transferred to ribonucleic acid (RNA)
molecules in a process called transcription (Transcription = DNA
RNA). The
message encoded in the mRNA molecule is then translated into the sequence of amino
acids that comprise the protein (Translation = RNA
protein).
DNA (DEOXYRIBONUCLEIC ACID)
DNA is the macromolecule that stores the hereditary information of the cell. It is
composed of subunits, called nucleotides that are like the letters of the “genetic
alphabet”. The order of the nucleotides specifies the genetic information of the cell and
contains the mechanisms for controlling genetic expression. As such, DNA is sometimes
called the “master molecule”. The sequence of nucleotides within the DNA molecule
encodes all the potential properties of that cell by determining the sequence of amino
acids in a particular protein. This is like saying that the arrangement and number of
letters used to create a word define its meaning. The genetic code, based on only the “few
letters” (nucleotides) in its “alphabet”, provides the necessary chemical basis for
encoding the genetic information and thus creating the great diversity of living
organisms.
Deoxyribonucleotides
DNA macromolecules are made up of numerous subunits called deoxyribonucleotides.
These deoxyribonucleotides often are referred to as nucleotides, a genetic term that also
describes the ribonucleotides in RNA. Each deoxyribonucleotide consists of a nucleic
acid base, the sugar deoxyribose, and phosphate.
Four different nucleic acid bases occur in the nucleotides of DNA: adenine, guanine,
cytosine, and thymine. Adenine (A) and guanine (G) are purines, which are moleclules
composed of two fused rings. Cytosine (C) and thymine (T) are pyrimidines, which
have only one ring. Purines and pyrimidines are heterocyclic molecules: their rings
contain two kinds of atoms, carbon and nitrogen, instead of just carbon. The nucleic acid
bases are attached to the deoxyribose sugars to form deoxyribonucleosides, and the
deoxyribonucleosides are joined to a phosphate group on carbon 5′ of the sugar to form
the deoxyribonucleotide subunits of DNA.
GENETIC MUTATION, RECOMBINATION, AND MAPPING
Changes in the sequence of nucleotides of a cell’s DNA occur by mutation (from the
Latin word mutare, meaning to change). Various types of mutations introduce
modifications into DNA with varying degrees of frequency. Mutations produce multiple
allelic forms of the same gene and recombinational processes permit further
redistribution of genetic information. (Alleles: corresponding forms of a gene. When both
copies of the gene are identical, the cell is homozygous. When the corresponding copies
of the gene differ, the cell is heterozygous). Recombination involves exchange of DNA
segments from differing genomes. This establishes new combinations of genes.
Heritable changes in the sequence of nucleotides of cells introduce variability into the
gene pool of microbial populations. Genetic variability typically occurs within a
population or within cells of a given organism. The genes of one bacterial cell may differ
slightly, for example, from the genes of another bacterial cell within the same species.
Heterogeneity within the gene pool may give some organisms a competitive advantage
for survival. This forms the basis for evolution according to the Darwinian principle of
survival of the fittest. Diversity within the gene pool establishes the basis for the
selective evolution of microorganisms. Recombinant DNA technology also permits the
directed formation of cells with specific genes that may come from divergent sources.
Names of the most common bases in DNA and RNA and corresponding names of
nucleosides and nucleotides containing these bases.
Nucleic acid base
Adenine (DNA or RNA)
Cytosine (DNA or RNA)
Guanine (DNA or RNA)
Thymine (DNA)
Uracil (RNA)
Nucleoside
Adenosine
Cytidine
Guanosine
Thymide
Uridine
Nucleotide
Adenylate (or adenylic acid)
Cytidylate (or cytidylic acid)
Guanylate (or guanylic acid)
Thymidylate (or thymidylic acid)
Uridylate (or uridylic acid)
MUTATIONS
A mutation is a heritable change in the nucleotide sequence of a cell’s DNA. Changes in
the cell’s hereditary molecules sometimes occur as a result of mistakes made during
DNA replication. These mutations sometimes occur during DNA replication despite the
mechanisms that are designed to ensure the fidelity of the process, which includes the
proofreading activities of DNA polymerases.
Types of mutations
1. Base substitutions
A base substitution mutation occurs when one pair of nucleotide base in the DNA is
replaced by another.
2. Missense mutations
Type of a base substitution that results in the change in the amino acid inserted into
the polypeptide chain specified by the gene in which the mutation occurs.
3. Silent mutations
Mutations that do not alter the phenotype of an organism and therefore go undetected.
4. Nonsense mutation
A mutation in which a codon specifying an amino acid is altered to a nonsense codon.
5. Polor mutations
Mutations that prevent the translation of subsequent polypeptides coded for in the
same mRNA molecule.
6. Suppressor mutation
A mutation that alleviates the effects of an earlier mutation at a different locus.
7. Lethal mutations
Mutations that result in the death of a microorganism or its inability to reproduce.
8. Nutritional mutations
Mutations that alter the nutritional requirements of the progeny of a microorganism.
RECOMBINATION
Recombination occurs when there is an exchange of genetic information among
different DNA molecules that results in a reshuffling of genes. This process can
produce numerous new combinations of genetic information. Recombination of
genetic information from two different cells produces progeny that contain genetic
information derived from two potentially different genomes.
Recombinant DNA technology
Recombinant DNA technology is the intentional recombination of genes from
different sources by artificial means. This is the basis for the creation of new genetic
varieties of organisms, which is known a genetic engineering. It is the foundation of
the “biotechnological revolution.”
Genetic engineering uses enzymes to cut out target genes and to join DNA to form
recombinant DNA that may come from diverse sources; for example, bacterial and
human DNA can be combined in a recombinant DNA molecule by genetic
engineering. Bacterial production of Human Insulin comes as a result of genetic
engineering. Human insulin (humulin) is produced by recombinant strains of
Escherichia coli. One recombinant strain is genetically engineered to produce the A
protein and a second recombinant strain, the B protein. The two proteins are then
chemically combined to produce commercial humulin. The humulin produced
functions as normal human insulin.
Genetic engineering holds great promise in industry and medicine because various
proteins of economic importance or use in curing disease may be produced.
However, the potential of genetic engineering to short-circuit evolution has raised
numerous ethical, legal, and safety questions. Concerning the development of
genetically engineered organisms designed for deliberate release into the
environment, the questions of whether novel genomes will survive in the
environment, whether they will transfer to other microorganisms and spread, and
whether this dissemination could represent a serious biological hazard to human
health and the environment are being actively debated.
In spite of the stated concern, there is little question that through genetic engineering
the quality of human life can be improved. Therefore, one must weigh these ethical
questions against the benefits that can be derived through genetic engineering.
GENETIC AND PHYSICAL MAPPING
Recombinants formed from different allelic forms of multiple genes can be used to
determine the relative locations of genes on a chromosome, thus producing a genetic
map of the chromosome. Genetic mapping is based on recombinational frequency
analysis that reveals new combinations of alleles that contain mutations (genetic
differences between the alleles). The extent of recombination between genes on the
same chromosome, for example, on a bacterial chromosome, can be used to establish
a genetic map. The occurrence of recombinants that result from mating is used to
establish a map showing the order and relative locations (loci) of genes.
The map distance between two genes is given by the formula:
Map distance = number of recombinants x 100
Total number of progeny