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
oxyribonucleic acid molecule
(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
lix 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 inform
ation 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 co
py 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 re
gulating 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 pr
otein (Translation = RNA protein).
DNA (DEOXYRIBONUCLEIC ACID)
DNA is the macromolecule that stores the hereditary information of the cell. It is
composed of subunits, called
that are like the letters of the “genetic
he 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
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
DNA macromolecules are made up of numerous subunits call
These deoxyribonucleotides often are referred to as
, a genetic term that also
describes the ribonucleotides in RNA. Each deoxyribonucleotide consists of a nucleic
acid base, the sugar deoxyribose, and phosphate.
different nucleic acid bases occur in the nucleotides of DNA: adenine, guanine,
cytosine, and thymine.
, which are moleclules
composed of two fused rings.
ly 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
yribonucleosides 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 mut
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. (
: corresponding forms of a gen
e. When both
copies of the gene are identical, the cell is
. When the corresponding copies
of the gene differ, the cell is
). Recombination involves exchange of DNA
segments from differing genomes. This establishes new combinations
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 bacteri
al 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 t
he 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 co
me 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)
Adenylate (or adenylic acid)
Cytosine (DNA or RNA)
Cytidylate (or cytidylic acid)
Guanine (DNA or RNA)
Guanylate (or guanylic acid)
Thymidylate (or thymidylic acid)
Uridylate (or uridylic acid)
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
that are designed to ensure the fidelity of the process, which includes the
proofreading activities of DNA polymerases.
Types of mutations
A base substitution mutation occurs when one pair of nucleotide base in the DNA is
replaced by ano
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.
Mutations that do not alter the phenotype of an organ
ism and therefore go undetected.
A mutation in which a codon specifying an amino acid is altered to a nonsense codon.
Mutations that prevent the translation of subsequent polypeptides coded for in the
same mRNA molecule.
A mutation that alleviates the effects of an earlier mutation at a different locus.
Mutations that result in the death of a microorganism or its inability to reproduce.
Mutations that alter the
nutritional requirements of the progeny of a microorganism.
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 c
ombinations 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
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
. 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 produ
ction of Human Insulin comes as a result of genetic
Human insulin (humulin)
is produced by recombinant strains of
. One recombinant strain is genetically engineered to produce the A
protein and a second recombinant strain, th
e 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 import
ance 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 hu
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
ap 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
me, 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 g
enes is given by the formula:
Map distance = number of recombinants x 100
Total number of progeny