by Talib F. Abbas
 Virtually everyone knows that the genes, located in
the nuclei of all cells of the body, control heredity from
parents to children, but most people do not realize
that these same genes also control day-today function
of all the body’s cells.
 Each gene, which is a nucleic acid called
deoxyribonucleic acid (DNA), automatically controls
the formation of another nucleic acid, ribonucleic acid
(RNA); this RNA then spreads throughout the cell to
control the formation of a specific protein.
 Nucleosides contain a sugar linked to a nitrogen-
containing base. The physiologically important bases,
purines and pyrimidines, have ring structures These
structures are bound to ribose or 2-deoxyribose to
complete the nucleoside. When inorganic phosphate
is added to the nucleoside, a nucleotide is formed.
Nucleosides and nucleotides form the backbone for
RNA and DNA, as well as a variety of coenzymes and
regulatory molecules (eg, NAD+, NADP+, and ATP) of
physiological importance.
 DNA is made up of two extremely long nucleotide
chains containing the bases adenine (A), guanine (G),
thymine (T), and cytosine (C) . The chains are bound
together by hydrogen bonding between the bases, with
adenine bonding to thymine and guanine to cytosine.
This stable association forms a double-helical
structure. The double helical structure of DNA is
compacted in the cell by association with histones,
and further compacted into chromosomes. A diploid
human cell contains 46 chromosomes. A fundamental
unit of DNA, or a gene, can be defined as the sequence
of DNA nucleotides that contain the information for
the production of an ordered amino acid sequence for
a single polypeptide chain.
DNA
Chromosome
DNA segments
(exons) separated
by segments that
are not translated
(introns). Near the
transcription start
site of the gene is a
promoter, which is
the site at which
RNA polymerase
 The importance of DNA lies in its ability to control the
formation of proteins in the cell. It does this by means
of the so-called genetic code. That is, when the two
strands of a DNA molecule are split apart, this exposes
the purine and pyrimidine bases projecting to the side
of each DNA strand. The genetic code consists of
successive “triplets” of bases—that is, each three
successive bases is a code word.
 The top strand of DNA, reading from left to right, has
the genetic code GGC, AGA, CTT, the triplets being
separated from one another by the arrows.

Because the DNA is located in the nucleus of the cell,
yet most of the functions of the cell are carried out in
the cytoplasm, there must be some means for the DNA
genes of the nucleus to control the chemical reactions
of the cytoplasm. This is achieved through the
intermediary of another type of nucleic acid, RNA, the
formation of which is controlled by the DNA of the
nucleus. Thus, the code is transferred to the RNA; this
process is called transcription. The RNA, in turn,
diffuses from the nucleus through nuclear pores into
the cytoplasmic compartment, where it controls
protein synthesis , the process of translation.
 However, the degree of activation of respective
genes must be controlled as well; otherwise, some
parts of the cell might overgrow or some chemical
reactions might overact until they kill the cell.
Each cell has powerful internal feedback control
mechanisms that keep the various functional
operations of the cell in step with one another.
 Synthesis of a cellular biochemical product
usually requires a series of reactions, and each
of these reactions is catalyzed by a special
protein enzyme. Formation of all the enzymes
needed for the synthetic process often is
controlled by a sequence of genes located one
after the other on the same chromosomal DNA
strand. This area of the DNA strand is called an
operon, and the genes responsible for forming
the respective enzymes are called structural
genes.
 Replication of the chromosome is accomplished by the
two strands splitting apart, and each strand acting as a
template for a new DNA strand. Each parental strand
thus acts as a template for a new strand running in the
opposite direction to create a new chromosome. This
replication is said to be semi-conservative.
 The backbone of each strand consists of PhophoRibose part of the nucleic acid being joined to each
other by phosphodiester bonds. The phosphate
molecule, attached to the 5' carbon ribose atom
attaches to the hydroxy molecule at the 3' ribose
carbon molecule.
 Unwinding of parental strands, The formation of
the fork is under the control of two enzyme groups,
the topoisomerases and the helicases.
 Lengthening the Transcribed DNA strand, δ-DNA
polymerase can only add to an existing chain, it
cannot initiate transcription. Initiation is
accomplished using a strand of RNA known as RNA
primer. This is attached to the parental strand at the
initiation point. Two enzymes are involved primase and α-DNA polymerase.

 The lagging strand transcription carried out in the
usual manner, away from the replication fork, and
continues until it meets the next fragment. Thus the
lagging strand is made in fragments - called Okazaki
Fragments after scientist who first described them.
Okazaki framents are formed by adding a primer
strand of RNA catalyzed by the
enzymes primase and α-DNA polymerase.
 Removing the RNA Primer, When another Okazaki
fragment is reached, the RNA primer is removed by δDNA polymerase assisted by the enzyme flap
endonuclease.
 As the replication approaches the end of the
chromosome, a problem occurs in the lagging strand.
DNA polymerase can only add nucleic acids to the 3'
end of the strand, so when the last Okazaki fragment
is formed in the lagging, there is no way that than the
RNA primer can be reconstituted in the normal
manner. When the RNA primer breaks down we are
left with an 'overhang' at the 3' end of the parental
chain and shortening of the transcribed chain.
 Telomeres are the ends of the eucaryotic chromosome.
In humans they consist of numerous (in young
humans this can number several thousand bases)
repeats of the nucleic acid sequence TTAGGG and its
complements. This is the first protection against
degradation of the chromosome by overhang. The
DNA being shed is merely the non-functional
telomere, the TTAGGG sequence. However sooner or
later the telomeres will be used up, and the
chromosome will start shedding important genes.
When this happens, the cell will usually die.
 When cells are no longer needed or become a threat to
the organism, they undergo a suicidal programmed cell
death, or apoptosis.
 Necrotic cells may spill their contents, causing
inflammation and injury to neighboring cells.
Apoptosis, however, is an orderly cell death that results
in disassembly and phagocytosis of the cell before any
leakage of its contents occurs, and neighboring cells
usually remain healthy.
 Apoptosis is initiated by activation of a family of
proteases called caspases.