Molecular biology: General theory
Author: Dr Darshana Morar-Leather
Licensed under a Creative Commons Attribution license.
of a series of nucleotides, held together in a long linear chain
Introduction
This
by a phosphodiester linkage from the 3’ hydroxyl group of
information is carried and released by RNA, and
one sugar to the 5’ phosphate group of the next (a DNA
All
organisms
store
information
as
DNA.
transformed into proteins, large organic molecules
which perform cellular functions. The “central dogma”
of molecular biology states that the flow of genetic
information is “DNA to RNA to PROTEIN”. This
strand or polynucleotide therefore comprises a sugarphosphate backbone with attached bases). Since the
phosphodiester bonds form between the third and fifth carbon
atoms of adjacent sugars, a strand of DNA has a direction or
orientation.
simplistic model is valuable in understanding the flow
of information in biological systems. The core features
Most DNA is double-stranded: two chains that constitute a
of this model are described below:
double helix. The two chains have opposite orientations and
are connected via hydrogen bridges between opposing base
groups. Each type of base on one strand forms a bond with
just one type of base on the other strand. Adenine (purine)
bonds only to thymine (pyrimidine) by two hydrogen bonds,
and guanine (purine) bonds only to cytosine (pyrimidine) by
three hydrogen bonds. This arrangement of two nucleotides
binding together across the double helix is called a base pair.
The
sequences
of
the
two
DNA
chains
are
thus
complementary.
DNA Replication
DNA replication is the process of duplicating the DNA
sequence in the parent strand to produce an exact replica
(daughter strand). Replication is semi-conservative: each one
of the two parental strands serves as a template for the new
In vivo transcription of DNA to RNA and the translation of
strand synthesis; therefore, duplicated double helices contain
RNA to protein
one parental strand and one daughter strand. DNA
polymerases are the enzymes responsible for DNA synthesis.
Deoxyribonucleic acid (DNA) contains the genetic
These enzymes use a single-stranded DNA template to
instructions needed to construct components of all
catalyse the polymerization of a complementary DNA strand.
organisms.
DNA is made up of nucleotides. A nucleotide consists of a
nitrogenous base (either a purine or a pyrimidine), a fivecarbon sugar (deoxyribose), and a phosphate group. The
four nitrogenous bases found in DNA are adenine (A),
cytosine (C), guanine (G) and thymine (T). DNA is composed
In a cell, DNA replication must happen before cell division
can occur. DNA synthesis begins at specific locations in the
genome, called "origins", where the two strands of DNA are
separated. RNA primers attach to single stranded DNA and
the enzyme DNA polymerase extends the primers to form
new strands of DNA, adding nucleotides matched to the
copy of the DNA informational strand. Unlike DNA replication,
template strand. The unwinding of DNA and synthesis of new
mRNA transcription can involve multiple RNA polymerases
strands forms a replication fork. In addition to DNA
on a single DNA template and multiple rounds of transcription
polymerase, a number of other proteins are associated with
resulting in amplification of a particular mRNA, i.e. many
the fork and assist in the initiation and continuation of DNA
mRNA molecules can be produced from a single copy of a
synthesis.
gene. This step also involves a proof-reading mechanism that
can replace incorrectly incorporated bases.
Ribonucleic acid (RNA) is made from DNA and has
numerous functions.
The stretch of DNA that is transcribed into an RNA molecule
•
RNA functions as an information carrier (mRNA)
is called a transcription unit. A transcription unit that is
•
RNA is involved in protein synthesis (rRNA and
tRNA).
translated into protein contains sequences that direct and
regulate protein synthesis in addition to coding the sequence
that is translated into protein.
RNA is a single stranded molecule but is similar to DNA as it
is also made up of a sugar (ribose, instead of deoxyribose), a
RNA is also involved in protein synthesis (rRNA
phosphate and a base. The bases come in four chemical
and tRNA) (translation)
forms; adenine (A), cytosine (C), guanine (G) and uracil (U)
Translation is the process of converting the mRNA
instead of thymine (T), which is found in DNA. There are
sequence into an amino acid sequence. It occurs in the
three primary types of RNA: messenger RNA, ribosomal RNA
cytoplasm of the cell where the ribosomes are located.
and transfer RNA.
Ribosomes are made of a small and a large subunit
RNA functions as an information carrier (mRNA)
sequence is used by the ribosome as a template to
(transcription)
Transcription is the synthesis of RNA under the direction of
DNA.
Messenger
which surround the mRNA. In translation, an mRNA
RNA
(mRNA)
is
synthesized
guide the synthesis of a chain of amino acids.
by
transcription or copying of DNA, a process similar to DNA
replication. The DNA sequence is copied by RNA polymerase
to produce a complementary RNA strand, called messenger
RNA (mRNA), because it carries a genetic message (or
instructions) from the DNA to the protein-synthesizing
machinery of the cell.
Genetic code
DNA transfers information to mRNA in the form of a code
defined by the sequence of nucleotide bases. Since DNA and
RNA are constructed from four types of nucleotides, there are
64 possible triplet sequences or codons (4x4x4); many more
than the 20 needed to specify the common amino acids
present in nature. Three of the possible codons specify the
As in DNA replication, RNA is synthesized in the 5' to 3'
direction (from the point of view of the growing RNA
transcript). Only one of the two DNA strands is transcribed
into mRNA (remember that RNA is a single-stranded
molecule), unlike DNA replication, where both strands are
copied. The DNA strand that is transcribed is called the
template strand (also known as the antisense strand), while
its complement is called the informational strand (also called
termination of the polypeptide chain. They are called "stop
codons". That leaves 61 codons to specify only 20 different
amino acids. Therefore, most of the amino acids are
represented by more than one codon. The genetic code is
therefore said to be degenerate. The vast majority of genes
are encoded with exactly the same code, known as the
genetic code. In fact there are many variant codes; so it
should be noted the genetic code is not universal.
the coding or sense strand).
Since the template strand and the informational strand are
During protein synthesis, a ribosome moves along an mRNA
complementary, and since the template strand and the
molecule from the 5' end to the 3' end and "reads" its
mRNA molecule are also complementary, it follows that the
sequence three nucleotides at a time (codon). Each amino
messenger RNA molecule produced during transcription is a
acid is specified by the mRNA's codon, which pairs with a
sequence of three complementary nucleotides (anticodon)
carried by a particular transfer RNA (tRNA) molecule. The
other end of the tRNA has the amino acid attached to the 3'OH group via an ester linkage. A tRNA molecule with an
attached amino acid is said to be "charged".
When a small subunit of a ribosome charged with a tRNA +
methionine (initiator tRNA) encounters an mRNA, it attaches
and starts to scan for a start signal or start codon (AUG).
When it finds the start sequence AUG, the codon for
methionine, the large subunit joins the small one to form a
complete ribosome and protein synthesis is initiated. A new
charged tRNA (tRNA + amino acid) enters the ribosome, at
the next codon downstream of the AUG codon. If its
anticodon matches the mRNA codon it binds and the
ribosome can link the two amino acids together (Note: if a
tRNA with the wrong anticodon (and therefore the wrong
amino acid) enters the ribosome, it cannot bind with the
mRNA and is rejected). The ribosome then moves one triplet
forward and a new charged tRNA can enter the ribosome and
the procedure is repeated. When the ribosome reaches one
of three stop codons, UAG, UAA or UGA, there are no
corresponding tRNAs to that sequence. Instead termination
proteins bind to the ribosome and stimulate the release of the
polypeptide chain (the protein), and the ribosome dissociates
from the mRNA.