The Biological Molecules:
Nucleic acids
BYBY
Introduction
• Nucleic acids are known to be found in all kinds of life forms including in
viruses.
• The nucleic acids contain the genetic information necessary for coordinating
activities in the organism.
• Nucleic acids were isolated by Fredrick Meischer (1868) from the nuclei of pus
cells.
• Nucleic acids are naturally occurring biological molecules that are capable of
being broken down to yield phosphoric acid, sugars, and a mixture of organic
bases.
• Nucleic acids are the main information-carrying molecules of the cell, and, by
directing the process of protein synthesis, they determine the inherited
characteristics of every living thing.
• Nucleic acids are polymers of nucleotides.
• They can also be called polynucleotides.
• There are two types of NAs: DNA and RNA.
• The nucleotides combine with each other to form a polynucleotide.
• Each nucleotide is made up of three components:
✓ a nitrogenous base
✓ a pentose sugar
✓ a phosphate group
Structure of a nucleotide
1. Nitrogenous bases
• The nitrogenous bases are organic molecules and are so named because they
contain carbon and nitrogen.
• They are bases because they contain an amino group that has the potential of
binding an extra hydrogen, and thus, decreasing the hydrogen ion
concentration in its environment, making it more basic.
• Two types of organic bases occur in nucleotides:
✓ Purines
✓ Pyrimidines
i.
Purines
ii. Pyrimidines
• Large
• Smaller
• Double-ring molecules
• Single-ring molecules
• Found in both DNA and RNA
• They include:
• Examples include:
✓adenine (A) and guanine (G)
✓Cytosine (C, in both DNA and RNA)
✓Thymine (T, in DNA only)
✓Uracil (U, in RNA only)
2. Pentose sugar
• The pentose sugar in DNA is deoxyribose and
in RNA it is ribose.
• The difference between the sugars is the
presence of the hydroxyl group on the second
carbon of the ribose and hydrogen on the
second carbon of the deoxyribose.
• The carbon atoms of the sugar molecule are
numbered as 1′, 2′, 3′, 4′, and 5′
3. Phosphate group
• The phosphate residue is attached to
the hydroxyl group of the 5′ carbon of
one sugar and the hydroxyl group of
the 3′ carbon of the sugar of the next
nucleotide.
Formation of polynucleotides
• When a nucleic acid polymer forms, the phosphate group of one nucleotide
binds to the hydroxyl group of another, releasing water and forming a
phosphodiester bond.
• The phosphate residue is attached to the hydroxyl group of the 5′ carbon of
one sugar and the hydroxyl group of the 3′ carbon of the sugar of the next
nucleotide, which forms a 5′3′ phosphodiester linkage.
Q. Why phosphodiester bond?
• In nucleic acids, the backbone of the
macromolecule consists of alternating
pentose sugars and phosphates (sugar—
phosphate—sugar—phosphate).
• The bases are attached to the sugars and
project from the chain.
• The nucleotides are joined by
phosphodiester linkages between the sugar
of one nucleotide and the phosphate of the
next.
Classification of Nucleic acids
• We have two types of nucleic acids based on their structure.
• These are:
✓ Deoxyribonucleic acid (DNA)
✓ Ribonucleic acid (RNA)
i.
Deoxyribonucleic acid (DNA)
• DNA molecules in organisms exist not as single chains folded into complex
shapes, like proteins, but rather as double chains.
• Two DNA polymers wind around each other like the outside and inside rails of
a circular staircase.
• Such a winding shape is called a helix, and a helix composed of two chains
winding about one another, as in DNA, is called a double helix.
• DNA’s helical staircase is a basepair, consisting of a base in one chain attracted
by hydrogen bonds to a base opposite it on the other chain.
• These hydrogen bonds hold the two chains together as a duplex.
• Because hydrogen bonds can form between the bases in a base-pair, the
double helix is stabilized as a duplex DNA molecule composed of two
antiparallel strands, one chain running 3′ to 5′ and the other 5′ to 3′.
• The base-pairing rules are rigid:
✓ adenine can pair only with thymine (in DNA) or with uracil (in RNA)
✓ cytosine can pair only with guanine in both DNA and RNA.
• These rules are also known as Chargaff’s rules.
• The bases that participate in base-pairing are said to be complementary to
each other.
• The structure of DNA. Hydrogen bond
formation (dashed line) between the
organic bases, called base-pairing,
causes the two chains of a DNA duplex
to bind to each other and form a
double helix.
• DNA plays a role in storing hereditary
information in cells.
• In order to synthesize new strand in DNA, it uses a methos known as the semiconservative method of DNA replication.
• The semi-conservative method of DNA replication states that the two original
DNA strands (i.e., the two complementary halves of the double helix) separate
during replication; each strand then serves as a template for a new DNA
strand, which means that each newly synthesized double helix is a
combination of one old (or original) and one new DNA strand.
Semi-conservative method of DNA
replication
ii. Ribonucleic acid (RNA)
• RNA is similar to DNA, but with slight major differences.
• Firstly, RNA molecules are usually single stranded molecules.
• Secondly, RNA contains ribose sugars in which the number 2 carbon is bonded
to a hydroxyl group. In DNA, this hydroxyl group is replaced by a hydrogen
atom.
• Thirdly, RNA molecules utilize uracil in place of thymine.
• RNA uses the information stored in DNA to specify protein synthesis.
• RNA can exist in many different forms.
• Three different types of RNA:
✓ Messenger RNA (mRNA)
✓ Ribosomal RNA (rRNA)
✓ Transfer RNA (tRNA)
i.
Messenger RNA (mRNA)
• mRNA is transcribed from DNA and contains the genetic blueprint to make
proteins.
• Used to transcribe the genetic code from the DNA into a form that can be read
by the ribosomes and used to make proteins.
• mRNA carries genetic information from the nucleus to the cytoplasm of a cell.
• Prokaryotic mRNA does not need to be processed and can proceed to
synthesize proteins immediately.
• In eukaryotes, a freshly transcribed RNA transcript is considered a pre-mRNA
and needs to undergo maturation to form mRNA.
• A pre-mRNA contains non-coding and coding regions known as introns and
exons, respectively.
• During pre-mRNA processing, the introns are spliced, and the exons are joined
together.
ii. Transfer RNA (tRNA)
• tRNAs are RNA molecules that are
located in the cellular cytoplasm and are
involved in the synthesis of proteins.
• The primary function of a tRNA is to
carry amino acids to the ribosomes for
protein synthesis.
iii. Ribosomal RNA (rRNA)
• rRNA forms ribosomes, which are essential in protein synthesis.
• rRNA directs the translation of mRNA into proteins.
• A ribosome contains a large and small ribosomal subunit.
Functions of Nucleic acids
• Nucleic acids are involved in the:
✓ Storage of genetic information
✓ Expression of genetic information
✓ Encodes information needed for cells to synthesize proteins
TAKE HOME MESSAGE
• Nucleic acids are a class of biological molecules needed by organisms.
• NAs are made up of nucleotides.
• Nucleotides consist of: pentose sugar, nitrogenous bases and PO4 group.
• Two types of NAs: DNA and RNA.
• DNA is double-stranded and has the following bases: A, C, G & T
• RNA is single-stranded, molecule and has the following bases: A, C, G & U
• RNA has different forms: mRNA, rRNA & tRNA.