Structure and Function of Nucleic Acids
*Figures not available
Nucleic acids, such as deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), are important
biological molecules which allow the genetic code of an organism to be carried and synthesised.
They are polymers which are made up nucleotide monomers. A nucleotide is a molecule consisting
of a pentose sugar, a phosphate group and a nitrogenous base. DNA consists of a deoxyribose sugar,
phosphate group and the four bases adenine, cytosine, guanine and thymine. The overall structure
of the DNA molecule is a double-stranded helix shape with the sugar-phosphate complex (known as
the sugar-phosphate backbone) making up each strand, connected by the nucleotide bases which
are held together through hydrogen bonding. The bases form complimentary base pairs – adenine
and thymine, and guanine and cytosine. The bases can only bond in their complementary pairings.
Adenine and thymine are connected by 2 hydrogen bonds, while guanine and cytosine are
connected by 3 hydrogen bands. Hydrogen bonds are relatively weak on their own, but when there
are millions these bonds joined together, they are very strong. The sequence of the nucleotide bases
within the strand of DNA encodes biological information, such as instructions for making proteins.
DNA replication is a process of producing two identical DNA molecules from a single molecule. DNA
replication is one of the most basic cellular processes and is required for the growth and repair of
multicellular organisms. First the super-coiled DNA must be relaxed in a series of reactions catalysed
by the enzyme topoisomerase, and the double helix is unwound by the initiator protein. DNA
helicase then attaches and breaks apart the hydrogen bonds between base pairs, separating the two
strands. As they are separated, an enzyme called primase attaches to each strand to form a short
stretch of nucleoties called a primer, shown in red in figure 1 below.
Figure 1: Opening and Priming of DNA molecule.
Next, another enzyme, DNA polymerase, attaches to the primer and begins travelling down the DNA
strands in the 3’-5’ direction, as seen in figure 2.
Figure 2: DNA Polymerase attaches to Primer
As DNA polymerase travels down the strand, it adds new nucleotides to the exposed bases, drawing
free-floating nucleotides from the surroundings to build a new strand on top of the existing one (1).
The nucleotides that are added must contain the complimentary base to the base on the template
strand. The leading strand is synthesised continuously, while the lagging strand is synthesised in
sections, known as Okazaki Fragments, which are sealed together using DNA ligase. Figure 3 shows
the new DNA strands forming.
Figure 3: New DNA Strands Forming
Once both strands have been elongated, two replication forks will meet in the same stretch of DNA,
and the primers are removed and replaced with DNA nucleotides. Any gaps are filled in with DNA
ligase. This is known as termination and is illustrated in figure 4 below.
Figure 4: Two Forks Meet
The product of replication is 2 new DNA strands, each made up of one parental strand and one
daughter strand. The 5’ ends of the daughter strands contain a short RNA primer, shown in figure 5
below.
Figure 5: New Strands With Short RNA Primers
, The primers are removed but cannot be replaced so the chromosomes are shortened slightly during
each round of replication. Figure 6 below shows the final product of replication.
Figure 6: Final Product of Replication
The other most common nucleic acid is ribonucleic acid, or RNA. Like DNA, RNA is a polymer or
nucleotide monomers. The sugar in RNA is ribose acid, and the nitrogenous base thymine is replaced
with uracil, which is complementary to adenine. The rest of the bases and the phosphate group are
the same, but the overall structure differs to that of DNA. While DNA is a double-stranded helix
shape, RNA is a single-stranded, right-handed helix. As it is a single strand, areas which are
complimentary to itself can bond together and form complex structures. There are several types of
RNA, including messenger RNA, transfer RNA, ribosomal RNA and small interfering RNA. They are
involved in different biological processes such as transcription, translation and gene silencing. The
structure of an RNA molecule and the uracil base are shown in figure 7 below.
Figure 7: RNA Molecule
Messenger RNA (mRNA) is an RNA molecule which carries the complimentary genetic code of a gene
from the nucleus to the ribosomes which are in the cytoplasm or on the rough endoplasmic
reticulum. The molecule is processed in the nucleus after transcription. The molecule consists of a
protective cap, two untranslated regions which help the molecule to attach to the ribosome, a
coding sequence and a Poly-A tail – a series of 20-50 adenine bases which are attached by
poly(A)polymerase to enable the molecule to be efficiently translated and protected from RNases
attacks. This structure is shown in figure 8 below.
Figure 8: Messenger RNA Molecule
Transfer RNA (tRNA) is an RNA molecule containing self-complimentary sequences which result in
the stem-and-loop structure seen in figure 9. Most cells have around 40-50 tRNA molecules and
each contain around 75-95 nucleotides. These molecules are essential to translation, as they transfer
amino acids during protein synthesis, giving them their name. There is a specific tRNA molecule
which binds to each of the 20 amino acids, to be transferred to the growing peptide chain.
Figure 9: Transfer RNA Molecle
Ribosomal RNA (rRNA) is found in the ribosomes. They combine with proteins and enzymes in the
cytoplasm to form ribosomes, where protein synthesis occurs. These structures travel along the
mRNA molecule and facilitate the assembly of amino acids during translation (2). They have self-
complimentary and non-self-complimentary regions, which create stem-and-loop structures like
tRNA, as the diagram in figure 10 shows.
*Figures not available
Nucleic acids, such as deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), are important
biological molecules which allow the genetic code of an organism to be carried and synthesised.
They are polymers which are made up nucleotide monomers. A nucleotide is a molecule consisting
of a pentose sugar, a phosphate group and a nitrogenous base. DNA consists of a deoxyribose sugar,
phosphate group and the four bases adenine, cytosine, guanine and thymine. The overall structure
of the DNA molecule is a double-stranded helix shape with the sugar-phosphate complex (known as
the sugar-phosphate backbone) making up each strand, connected by the nucleotide bases which
are held together through hydrogen bonding. The bases form complimentary base pairs – adenine
and thymine, and guanine and cytosine. The bases can only bond in their complementary pairings.
Adenine and thymine are connected by 2 hydrogen bonds, while guanine and cytosine are
connected by 3 hydrogen bands. Hydrogen bonds are relatively weak on their own, but when there
are millions these bonds joined together, they are very strong. The sequence of the nucleotide bases
within the strand of DNA encodes biological information, such as instructions for making proteins.
DNA replication is a process of producing two identical DNA molecules from a single molecule. DNA
replication is one of the most basic cellular processes and is required for the growth and repair of
multicellular organisms. First the super-coiled DNA must be relaxed in a series of reactions catalysed
by the enzyme topoisomerase, and the double helix is unwound by the initiator protein. DNA
helicase then attaches and breaks apart the hydrogen bonds between base pairs, separating the two
strands. As they are separated, an enzyme called primase attaches to each strand to form a short
stretch of nucleoties called a primer, shown in red in figure 1 below.
Figure 1: Opening and Priming of DNA molecule.
Next, another enzyme, DNA polymerase, attaches to the primer and begins travelling down the DNA
strands in the 3’-5’ direction, as seen in figure 2.
Figure 2: DNA Polymerase attaches to Primer
As DNA polymerase travels down the strand, it adds new nucleotides to the exposed bases, drawing
free-floating nucleotides from the surroundings to build a new strand on top of the existing one (1).
The nucleotides that are added must contain the complimentary base to the base on the template
strand. The leading strand is synthesised continuously, while the lagging strand is synthesised in
sections, known as Okazaki Fragments, which are sealed together using DNA ligase. Figure 3 shows
the new DNA strands forming.
Figure 3: New DNA Strands Forming
Once both strands have been elongated, two replication forks will meet in the same stretch of DNA,
and the primers are removed and replaced with DNA nucleotides. Any gaps are filled in with DNA
ligase. This is known as termination and is illustrated in figure 4 below.
Figure 4: Two Forks Meet
The product of replication is 2 new DNA strands, each made up of one parental strand and one
daughter strand. The 5’ ends of the daughter strands contain a short RNA primer, shown in figure 5
below.
Figure 5: New Strands With Short RNA Primers
, The primers are removed but cannot be replaced so the chromosomes are shortened slightly during
each round of replication. Figure 6 below shows the final product of replication.
Figure 6: Final Product of Replication
The other most common nucleic acid is ribonucleic acid, or RNA. Like DNA, RNA is a polymer or
nucleotide monomers. The sugar in RNA is ribose acid, and the nitrogenous base thymine is replaced
with uracil, which is complementary to adenine. The rest of the bases and the phosphate group are
the same, but the overall structure differs to that of DNA. While DNA is a double-stranded helix
shape, RNA is a single-stranded, right-handed helix. As it is a single strand, areas which are
complimentary to itself can bond together and form complex structures. There are several types of
RNA, including messenger RNA, transfer RNA, ribosomal RNA and small interfering RNA. They are
involved in different biological processes such as transcription, translation and gene silencing. The
structure of an RNA molecule and the uracil base are shown in figure 7 below.
Figure 7: RNA Molecule
Messenger RNA (mRNA) is an RNA molecule which carries the complimentary genetic code of a gene
from the nucleus to the ribosomes which are in the cytoplasm or on the rough endoplasmic
reticulum. The molecule is processed in the nucleus after transcription. The molecule consists of a
protective cap, two untranslated regions which help the molecule to attach to the ribosome, a
coding sequence and a Poly-A tail – a series of 20-50 adenine bases which are attached by
poly(A)polymerase to enable the molecule to be efficiently translated and protected from RNases
attacks. This structure is shown in figure 8 below.
Figure 8: Messenger RNA Molecule
Transfer RNA (tRNA) is an RNA molecule containing self-complimentary sequences which result in
the stem-and-loop structure seen in figure 9. Most cells have around 40-50 tRNA molecules and
each contain around 75-95 nucleotides. These molecules are essential to translation, as they transfer
amino acids during protein synthesis, giving them their name. There is a specific tRNA molecule
which binds to each of the 20 amino acids, to be transferred to the growing peptide chain.
Figure 9: Transfer RNA Molecle
Ribosomal RNA (rRNA) is found in the ribosomes. They combine with proteins and enzymes in the
cytoplasm to form ribosomes, where protein synthesis occurs. These structures travel along the
mRNA molecule and facilitate the assembly of amino acids during translation (2). They have self-
complimentary and non-self-complimentary regions, which create stem-and-loop structures like
tRNA, as the diagram in figure 10 shows.
