NUCLEOTIDES, NUCLEIC ACID
STRUCTURE AND FUNCTION
Prof. Dr. Yıldız DİNÇER
Cerrahpaşa Medical Faculty Department of Biochemistry
• Nucleotides contain;
• Base + sugar + phosphoryl group
• Purines and pyrimidines are heterocyclic
compounds including N atoms
• Nucleosides contain;
• Base + sugar (ribose or deoxyribose)
• Sugar, D-ribose or 2’-deoxy D-ribose, is linked
to base via a covalent β-N-glycosidic bond to
N-9 of a purine or to N-1 of a pyrimidine
• Nucleotides are termed ribonucleotides or
deoxyribonucleotides based on whether the
sugar is ribose or 2’-deoxyribose
• DNA nucleotides
• RNA nucleotides
Nucleotides are phosphorolated nucleosides
• Mononucleotides are nucleotides singly
phosphorolated on hydroxyl group of the
sugar
• AMP → Adenosine monophosphate
• Adenine + ribose +phosphate
• Additional phosphates linked by acid
anhydride bonds to the existing phosphate of
a mononucleotide form nucleoside di- and
triphosphates
• ADP → Adenosine diphosphate
• Adenine + ribose +phosphate +phosphate
• ATP → Adenosine triphosphate
• Adenine + ribose +phosphate +phosphate +
phosphate
Functions of nucleotides
•
•
•
•
•
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*Nucleic acid biosynthesis
*Energy production and transduction
*Protein biosynthesis
*Regulatory cascades
*İntra- and intercellular signal transduction
*Biosynthesis some biomolecules
Some properties of nucleotides
• 1.Mononucleotides have a negative charge at
physiological pH
• 2.Nucleotides absorb UV light
• 3. Many coenzymes are nucleotide derivatives
• 4. Synthetic nucleotide analogs are used in
chemotherapy
• 5. Nucleoside triphosphates have high group
transfer potential
• 6.Some nucleotides are involved in signal
transduction
• 1.Mononucleotides have a negative charge at
physiological pH. The pKs of the primary and
secondary phosphoryl groups are about 1.0
and 6.2, respectively
• Nucleosides and or free purine or pyrimidine
bases are uncharged at physiological pH
• 2.Nucleotides absorb UV light. The
conjugated double bonds of the heterocyclic
bases of purines and pyrimidines ensure that
nucleosides, nucleotides and polynucleotides
absorb UV
• Their spectra are pH- dependent but all
common nucleotides absorb light at a
wawelength close to 260 nm at pH 7,0
• Nucleotide and nucleic acid concentrations
thus often are expressed in termes of
‘absorbance at 260 nm’
3. Many coenzymes are nucleotide derivatives
• AMP is present in many coenzymes
• 4. Synthetic nucleotide analogs are used in
chemotherapy
• Chemically synthesized analogs of purines and
pyrimidines, their nucleosides and their
nucleotides find numerious applications in
clinical medicine
• Administration of an analog in which either
the heterocyclic ring or the sugar moiety has
been altered induces toxic effect when the
analog is incorporated into specific cellular
constituents
• Their effects reflect one of two processes:
• a)Inhibition by the drug of specific enzymes
essential for nucleic acid synthesis
• b)Incorporation of metabolites of the drug
into nucleic acids, thus they effect the base
pairing required for accurate replication
Examples
• 5-fluoro uracil → thymine analog
• 5-iodo deoxyuridine → thyminide analog
• 6-mercaptopurine → purine analaog
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•
•
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6-thioguanine → purine analaog
8-azaguanine → purine analaog
5 or 6-azauridine → pirimidine analaog
6-azacytidine → pirimidine analaog
• 4-hydroxypyrazolopyrimidine
• (allopurinol) → purine analaog
• Allopurinol inhibits de novo
• purine biosynthesis and
• xanthine oxidase activity.
• It is used for treatment of
• hyperuricemia and gout
• The nucleoside, cytarabine (arabinosyl
cytosine), in which arabinose replaces ribose,
is used in chemotherapy and in treatment of
viral infections
• Azathioprine is catabolized to 6mercaptopurine and is used during organ
transplantation to suppress immunological
rejection
• 5. Nucleoside triphosphates have high group
transfer potential because of acid anhydride
bonds
• High group transfer potential of nucleoside
triphosphates allows them to participate as
group transfer reagents in various reactions.
• In these reactions, cleavage of an acid
anhydride bond is coupled with a highly
endergonic reaction such as protein synthesis
or nucleic acid synthesis.
• ADP and ATP are substrates and products,respectively, for
oxidative phosphorylation
• ATP serves as the major biologic transducer of free energy
• It is the most abundant free nucleotide in mammalian
cells (1 mM)
• ATP donates some of its chemical energy by hydrolysis of
the terminal phosphoanhydride bond
• The hydrolytic cleavage of the terminal
phosphoanhydride bond in ATP separates one
of the three negatively charged phosphates
and thus relieves some of the electrostatic
repulsion in ATP
• Released Pi is stabilized by the formation of
several resonance forms not possible in ATP
• ADP2- is the other product of hydrolysis and it
immediately ionizes, releasing H+ into a
medium of very low [H+]
• For hydrolysis reactions with large, negative
standard free energy changes, the products
are more stable than the reactants
Transphosphorylations between
nucleotides
• Although we have focused on ATP as the cell’s
energy currency and donor of phosphoryl
groups, all other nucleoside triphosphates
(GTP, UTP, CTP) and all the deoxynucleoside
triphosphates (dATP, dGTP, dTTP and dCTP)
are energetically equivalent to ATP
• The free energy changes associated with
hydrolysis of their phosphoanhydride linkages
are very nearly identical with those for ATP
• In preparation for their various biological
roles, these other nucleotides are generated
as the nucleoside triphosphate (NTP) forms by
phosphoryl group transfer to the
corresponding nucleoside diphosphates
(NDPs) and monophosphates (NMPs)
• ATP is the primary high-energy phosphate
compound produced by catabolism in the
processes of glycolysis, oxidative
phosphorylation. Several enzymes carry
phosphoryl groups from ATP to the other
nucleotides
• Nucleoside diphosphate kinases, found in all
cells, catalyzes the reaction
•
• ATP+NDP(or dNDP)
Mg2+
→ ADP+NTP(or dNDP)
• Although this reaction is fully reversible the
relatively high ATP/ADP ratio in cells normally
drives the reaction to the right, with the net
formation of NTPs and dNTPs
• Phosphoryl group transfers from ATP result in an
accumulation of ADP. For example, when muscle is
concracting vigorously ADP accumulates and
interferes with ATP-dependent contraction.
• During periods of intense demand for ATP, the cell
lowers the ADP concentration, and at the same time
acquires ATP, by the action of adenylate kinase:
•
•
•
2ADP
Mg2+
→ ATP + AMP
←
• This reaction is fully reversible, so after the intense
demand for ATP ends, the enzyme can recycle AMP
by converting it to ADP which can then be
phosphorylated to ATP in mitochondria
• A similar enzyme guanylate kinase, converts GMP to
GDP at the expense of ATP. By-pathways such as
these, energy conserved in the catabolic production
of ATP is used to supply the cell with all required
NTPs and dNTPs
6.Some nucleotides are involved in signal
transduction
• cAMP and cGMP
cAMP (adenosine 3’,5’-monophosphate)
• The cyclic phosphodiester cAMP is formed from
ATP by the reaction catalyzed by adenylyl cyclase
• cAMP is a second messenger in signal
transduction
• Adenylyl cyclase activity is regulated by complex
interactions, many of which involve hormon
receptors
• As a second messenger, cAMP participates
numerous regulatory functions by activating
cAMP dependent protein kinases
• cAMP is broken down by cAMP
phosphodiesterase
• Intracellular cAMP level is maintained by the
interaction of adenylyl cyclase and
phosphodiesterase
cGMP (guanosine 3’,5’-monophosphate)
• cGMP is a second messenger in signal
transduction that can act antagonistically to
cAMP
• cGMP is formed from GMP by guanylyl cyclase
• Both adenylyl and guanylyl cyclases are
regulated by effectors that include hormones
• A phosphodiesterase hydrolyzes cGMP to
GMP
• An increase in the level of the cGMP as
response to the nitric oxide serves as the main
second messenger during events that
characterize the relaxation of smooth muscle
Other functions of free nucleotides
• 1. ADENOSINE DERIVATIVE NUCLEOTIDES
• Adenosine 3’-phosphate 5’-phosphosulfate (active
sulfate)
• This is the sulfate donor for the formation of
the sulfated proteoglycans or urinary
metabolites of drugs excreted as sulfate
conjugates
S-adenosylmethionine (SAM)
• This is a form of active methionine
• SAM serves as a methyl group donor for
methylation reactions
• SAM forms propylamine which is required in
polyamine synthesis
2. GUANOSINE DERIVATIVE NUCLEOTIDES
• Guanosine nucleotides participate in the
conversion of succinyl-CoA to succinate, a
reaction that is coupled to the substrate level
phosphorylation of GDP to GTP
• GTP is required for
• * activation of adenylyl cyclase (as a
allosteric regulator)
• **protein synthesis (as a energy source)
3. HYPOXANTHINE DERIVATIVE NUCLEOTIDES
• Hypoxanthine ribonucleotide is inosine
monophosphate (IMP)
• It is precursor for purine nucleotides
• IMP is formed by deamination of AMP and
amination of IMP re-forms AMP
• Nucleotide inosine (hypoxanthine + ribose) is
an intermediate in the purine solvage cycle
4. URACYL DERIVATIVE NUCLEOTIDES
• UDP-sugar derivatives participate in sugar
epimerization
• UDP-glucose is the glucosyl donor for biosynthesis
of glycogen and glucosyldisaccharides
• Other UDP-sugars act as sugar donors for
biosynthesis of oligosaccharides of glycoproteins
and proteoglycans
• UDP-glucuronic acid is the glucuronyl group donor
for conjugation reactions
5. CYTOSINE DERIVATIVE NUCLEOTIDES
• CTP is required for the biosynthesis of some
phosphoglycerides
• CTP is required for the biosynthesis of
sphingomyelin and other substituted
sphingosines
Polynucleotides
• Mononucleotides are covalently linked through
phosphate-group ‘bridges’
• Specifically, the 5’-OH group of one nucleotide
unit is joined to the 3’-OH group of the next
nucleotide by a phosphodiester linkage resulting
to release of one molecule water
• This results in a dinucleotide
• The term oligonucleotide is used for polymers
containing 50 or fewer nucleotides
• Polynucleotides are directional macromolecules
• Since the phosphodiester bond links 3’- and 5’carbons of adjacent monomers, each end of a
polimers is distinct. They are reffered as ‘3’-end’
or ‘5’-end’ of polynucleotides
• In the most representations displaying only the
base sequences 5’ end is shown on the left and 3’
end is shown on the right:
•
5’ GTATTGC 3’
•
• Nucleic acids are the polimers containing
many nucleotides linked by phosphodiester
linkage
• Hydrolysis of polynucleotides has a large
energy barrier and this reaction is too slow in
the absence of phosphodiesterases
• Phosphodiester linkes are hydrolyzed by
phosphodiesterases
Nucleic acid structure and function
• Genetic information is coded by DNA
• Genes control the synthesis of various types of
RNA which are involved in protein synthesis
• Nucleic acids (DNA and RNA) are the polimers
containing nucleotides as monomers.
• Covalent backbones of nucleic acids consist of
alternating phosphate and pentose residues,
and the characteristic bases may be regarded as
side groups joint to the backbone at regular
intervals
• Nucleic acids contain five major heterocyclic
base:
• Adenine (A), guanine (G), cytosine (C), tymine
(T) and uracil (U)
• DNA includes A,G,C,T
• RNA includes A,G,C,U
• Nucleic acids also contain unusual bases
• Unusual bases are the additional purines and
pyrimidines which are included by nucleic
acids at a considerably smaller quantities
• 5-methyl cytosine → DNA
• N6-methyl adenine → DNA
• N6,N6-dimethyl adenine → RNA
• 5-OH methyl cytosine → DNA
• Pseudourasil → RNA
• 7-methylguanine → RNA
• N2-methylguanine → DNA
• They serve important functions in
oligonucleotide recognition in DNA and RNA.
• The presence of specific methylated
nucleotide bases allows the DNAses arising
from infection to distinguish between native
and foreign oligonucleotides
• Methylated cytosine in the promoter region is
responsible for gene expression.
• Unusual bases play role in regulation of halflife of RNAs
Hypoxanthine and xanthine
• They are intermediates in the catabolism of
purines.
• They can be produced by spontaneous
deamination of adenine and guanine,
respectively
• Hypoxanthine is a mutagenic lesion, it pairs
with C unless repaired
• Methylated purines of the plants:
• Theophylline → Dimethylxanthine → Tea
• Caffeine → Trimethylxanthine → Coffee
• Backbones of DNA and RNA are hydrophilic
because of the;
• 1. hydrogen bonds between OH groups of the
sugars and surrounding water
• 2.Phosphate groups which are completely
ionized and negatively charged at pH 7
• These negative charges are generally
neutralized by ionic interactions with positive
charges on proteins, metal ions and
polyamines
• Monomeric units of DNA helded in the
polimeric form constitutes a single strand
• The informational content of DNA resides in
the sequence in which these monomers are
ordered
• The demonstration that DNA contained the
genetic information was first made in 1944 in
a series of experiments
• In early 1950s Watson, Crick and Wilkins
determined a model of double-stranded DNA
• This model was determined based on x-ray
diffraction data from the DNA molecule and the
observations of Chargaff
• Chargaff rule: The concentration of
deoxyadenine nucleotides equals that of
thymidine nucleotides (A=T), while the
concentration of deoxyguanosine nucleotides
equals that of deoxycytidine nucleotides (G=C)
• The two strands of this double-stranded helix are
held in register by hydrogen bonds between the
purine and pyrimidine bases of the respective
linear molecules
• The pairings between the purine and pyrimidine
nucleotides on the opposite strands are very
specific and are present between A and T; C and G
• This common form of DNA is said to be righthanded because as one looks down the
double helix the base residues form a spiral in
a clockwise direction
• The predominant tautomers of the four bases
allow A to pair only with T, and G only with C
• The two strands of the double-helical
molecule are antiparallel; one strand runs in
the 3’-5’ direction while the other in the 5’-3’
direction
• In the double-stranded DNA molecules,
genetic information resides in the sequence of
nucleotides on one strand, template strand
• This is the strand of DNA that is copied during
nucleic acid synthesis (noncoding strand)
• The opposite strand is considered the coding
strand because it matches the RNA transcript
that encodes the protein
• The two strands wind around a central axis in
the form of a double helix
• Double-stranded DNA exists in at least six
forms (A-E and Z)
• The B form is usually found under physiologic
conditions
• A single turn of B-form DNA about the axis of
the molecule contains ten base pairs
• The distance spanned by one turn of B-form
DNA is 3.4 nm, the helical diameter of the
double helix is 2 nm
• Three hydrogen bonds hold the deoxyguanosine
nucleotide to the deoxycytidine nucleotide,
whereas A and T are helded by two hydrogen
bonds
• G-C bonds are much more resistant to
denaturation than A-T bonds
The denaturation of DNA (melting)
• Double stranded DNA can be separated into two
component strands in solution by increasing the
temperature or decreasing the salt concentration
• As a result of denaturation, optical absorbance of
bases increases
• Double stranded DNA molecule exhibits properties
of a rigid rod and in solution is a viscous material
because of the stacking of the bases and hydrogen
bonding but it loses its viscosity upon denaturation
• DNA rich in G-C pairs melts at a higher
temperature than that rich in A-T pairs
• Separated strands of DNA will renature when
physiologic temperature and salt conditions
are achieved
• The rate of reassociation depends upon the
concentration of the complementary strands
• There are grooves in the DNA molecule
• A major groove and a minor groove wind
along the molecule parallel to the
phosphodiester backbones.
• In these grooves, regulatory proteins can interact
specifically with exposed atoms of the
nucleotides
• Therefore these proteins recognize and bind to
specific nucleotide sequences without disturbing
the base pairing
• Regulatory proteins can control the expression of
specific genes via such interactions
• DNA exists in relaxed and supercoiled forms
• In bacteria and many DNA containing animal
viruses the ends of the DNA molecules are
joined to create a closed circle with no
terminal
• This structure does not destroy polarity but
eliminates all free 3’ and 5’ hydroxyl and
phosphoryl groups
• Closed circles may exist in relaxed or
supercoiled form
• Supercoils are introduced when a closed circle
is twisted around its own axis or when a linear
piece of dublex DNA is twisted
• This is an energy-requiring process and puts
the DNA under stress
• Negative supercoils are formed when the
molecule is twisted in the opposide direction
of clockwise turns of the right-handed double
helix found in B-DNA. Such DNA is said to be
underwound
• The transition to another form is required
energy
• One such transition occurs during the strand
separation just before the replication
• Enzymes that catalyze topologic changes of
DNA are called as topoisomerases
• Topoisomerases can relax or insert supercoils
using ATP
• The best characterized is bacterial gyrase
which induced negative supercoilig in DNA
• DNA provides a template for replication and
transcription
• The genetic information stored in DNA serves for two
purposes:
• a)It provides the information for the synthesis of all
protein molecules of the organism
• b)It provides the information inherited by daughter
cells
• Both these functions require that the DNA molecule
serves as a template for the transcription of the
information into RNA and for the replication of
information into daughter DNA molecules
• Replication of the DNA molecule occurs in a
semiconservative manner. Thus, when each
strand of the double-stranded parent DNA
molecule separated during replication, each
serves as a template on which a new
complementary strand is synthesized
• Each daughter cell contains DNA molecules
with information identical to that which the
parent possessed;
• Each daughter cell the DNA molecule of the
parent cell has been only semiconserved
Chemical structure of RNA
• The chemical nature of RNA differs from that of DNA
• RNA is also formed by purines and pyrimidines linked by
3’-5’ phosphodiester bonds
• Although sharing many features with DNA , RNA
possesses several specific differences:
• 1. In RNA, the sugar moiety to which the phosphates and
purine and pyrimidines are attached is ribose instead of
deoxyribose of DNA
• 2. The pyrimidine components of RNA is differ from those
of DNA. RNA contains the A, G,C but does not contain T
(with a rare exception), instead of T, U is present in RNA
• 3.RNA exist as a single strand. However, given
the proper complementary base sequence
with opposite polarity, single strand RNA is
capable of folding back on itself like a hairpin
thus acquiring double-stranded structure
• 4. Since the RNA molecule is a single strand
complementary to only one of the two strands of
a gene, its guanine content does not necessarily
eaqual its cytosine content; and its adenine
content does not necessarily eaqual its uracil
content
• 5. RNA can be hydrolyzed by alkali to 2’, 3’cyclic
diesters of mononucleotides but those molecules
can not be formed from alkali-treated DNA
because of the absence of a 2’- OH group
• The alkali lability of RNA is useful both
diagnostically and analytically
• Information within the RNA is contained in its
sequence of purine and pyrimidine nucleotides.
• The sequence is complementary to the template
strand of the gene that was transcribed
• Because of this complementarity, a RNA
molecule can bind specifically via the basepairing rules to its template DNA strand; but it
does not bind to coding strand
• The nucleotide sequence of the RNA molecule is
the same as that of the coding strand of the
gene, except for U replacing T
RNA types and their funtions
• Cytoplasmic RNA molecules that serve as a templates
for protein synthesis are designated as messnger RNAs
(mRNA). mRNA molecules transfer genetic information
from DNA to protein-synthesizing machinary
• Many other cytoplasmic RNA molecules have structural
roles. They contribute to the formation of ribosomes
(ribosomal RNA, rRNA) or serve as adapter molecules
(transfer RNA, tRNA) for the translation of RNA
information into specific sequences of polymerized
amino acids
• Some RNA molecules have intrinsic catalytic
activity. The activity of these ribozymes often
involved in the cleavage of a nucleic acid
• In human cells there are small nuclear RNA
(snRNA) species. These are not directly
involved in protein synthesis but that may
have roles in RNA processing
• These relatively small molecules vary in size
from 90 to 300 nucleotides
• The genetic material for some animal and
plant viruses is RNA rather than DNA
• Many animal RNA viruses are, retroviruses,
transcribed by an RNA-dependent DNA
polymerase , the so-called reverse
transcriptase, to produce a double- stranded
DNA copy of their RNA genome
• In many cases, the resulting double- stranded
DNA transcript is integrated into the host
genome and subsequently serves as a
template for gene expression and from which
new viral RNA genomes can be transcribed
• A newly synthesized RNA molecule is called as
primary transcript. Primary transcript is
processed depending on the type of RNA
• In all prokaryotic and eukaryotic organisms,
three main classes of RNA molecules exist.
Each differs from the others by size, function
and general stability
1. Messenger RNA (mRNA)
• This is the most heterogenous class in size and
stability
• All members of the class function as
messengers conveying the information in a
gene to the protein-synthesizing machinery
• Each serves as a template for a specific
sequence of amino acids that is polimerized to
form a specific protein
• mRNAs have some unique chemical characteristics:
• The 5’ terminal of mRNA is capped by a 7methylguanosine triphosphate
• The cap is involved in the recognition of mRNA by
translating machinery, and it probably helps stabilize the
mRNA by preventing the attack of 5’-exonucleases
• The protein synthesis begins translating the mRNA into
proteins at the 5’or capped terminal
• The other end of most mRNAs has attached a
polymer of adenylate residues 20-250
nucleotides in lenght so called the poly A tail
• The funtion of the poly A tail seems that it
maintains the stability of the mRNA by
preventing the attack of 3’-exonucleases
2. Transfer RNA (tRNA)
• tRNA molecules vary in lenght from 74 to 95
nucleotides
• tRNA molecules serve as a adapters for the
translation of the information in the sequence of
nucleotides of mRNA into specific amino acids
• There are at least 20 species of tRNA molecules in
every cell. At least one, often several
corresponding to each of the 20 amino acid are
required for protein synthesis
• Although each specific tRNA differs from the
others in its sequence of nucleotides, the
tRNA molecules as a class have many features
in common
• The primary structure of all tRNA molecules
allows extensive folding and intrastrand
complementarity to generate a secondary
structure
• This structure appears like a cloverleaf
• All tRNA molecules contain four main arms
• The acceptor arm consists of a base-paired
stem that terminates in the sequence CCA (5’
to 3’). It is through an ester bond to the 3’
hydroxyl group of the adenosyl moiety that
the carboxyl groups of amino acids are
attached
• The other arms have base-paired stems and
unpaired loops
• The anticodon arm at the end of a base-paired
stem recognizes the triplet nucleotide or codon of
the template mRNA. It has a nucleotide sequence
complementary to the codon and is responsible
for the specifity of the tRNA
• The D arm is named for the presence of the
base dihydrouridine, and the TΨC arm for the
sequence T, pseudouridine, and C
• The extra arm is the most variable feature of tRNA.
It accounts for the differences in lenght of the
tRNAs; and it provides a basis for classification.
• Class 1 tRNAs have an extra arm that is 3-5 bp long
• Class 2 tRNAs have an extra arm that is 13-21 bp
long
• The secondary structure of tRNA molecules is
maintained by the base pairing in these arms
and this is a consistent feature :
• The TΨC and anticodon arms have 5 bp
• The D arm has 3-4 bp
• The acceptor arm has 7 bp
3. Ribosomal RNA (rRNA)
• A ribosome is a cytoplasmic nucleoprotein
structure that acts as the machinery for the
synthesis of proteins from the mRNA templates
• On the ribosomes, the mRNA and tRNA
molecules interact to translate into a specific
protein molecule information transcribed from
the gene
• During the active protein synthesis, many
ribosomes are associated with an mRNA
molecule in an assembly called the polysome
• The mammalian ribosome contains two major
nucleoprotein subunits, a larger one with a
molecular weight of 2.8x106 (sedimentation
velocity is 60S*) and a smaller subunit with a
molecular weight of 1.4x106 (40S)
• The 60S subunit contains a 5S ribosomal RNA,
a 5.8S rRNA and a28S rRNA; there are also
probably more than 50 specific polypeptides
• *Svedberg unit (sedimentation coefficient)
• The 40S subunit is smaller and contains 18S
rRNA and approximately 30 polypeptide chains
• All of the rRNA molecules, except the 5S rRNA
are processed from a single 45S precursor RNA
molecule in the nucleus
• 5S rRNA has its own precursor that is
independently transcribed
• The highly methylated rRNA molecules are
packaged in the nucleus with the specific
ribosomal proteins; but in the cytoplasm,
ribosomes remain quite stable and capable of
many translaton cycles
• Ribosomal RNA molecules are necessary for
ribosomal assembly and seem to play key
roles in the binding of mRNA to ribosomes
and its translation
Small stable RNA (snRNA)
• A large number of discrete, highly conserved
and small stable RNA species are found in the
mammalian cells
• The majority of these molecules exist as
ribonucleoproteins and are distributed in the
nucleus, cytoplasm or in both
• They are involved in mRNA processing and
gene regulation
• Of the several snRNAs, U1,U2,U4,U5 and U6
are involved in intron removaland the
processing of primary transcript into mRNA.
The U4 and U6 snRNAs are required for poly
(A) processing
Nucleases
• Nucleic acids are digested by nucleases
• Nucleases exhibit specifity to deoxyribonucleic
acids are referred to as deoxyribonucleases
• Those which specifically hydrolyze ribonucleic
acids are ribonucleases
• Enzymes capable of cleaving internal
phosphodiester bonds are referred to as
endonucleases
• Some of the endonucleases are capable of
hydrolyzing both strands of a double-stranded
molecule, whereas others can only clevage
single strands of nucleic acids
• Endonuclease classes that recognize specific
sequences in DNA are present. They are
referred as restriction endonucleases and are
widely used in molecular genetic
investigations
• Some nucleases are capable of hydrolyzing a
nucleotide only when it is present at a
terminal of a nucleic acid. These enzymes are
referred to as exonucleases
• Exonucleases can act in one direction only;
3’→5’ or 5’→3’