14 BIOMOLECULES

A: QUICK REVISION OF THE CHAPTER

Section
(2) Grcek word for sugar is Sakkharon. Hence
14,1 NTRODUCTION carbohydrates are also called saccharides
Carbohydrates (3) e.g.(i) glucose and fructose (CH,0) are Swect
laste and are callcd sugars. in
Biomolecules Proteins (ii) Sucrose (C,,,,O,) is commonly used suvar
Nucleic acid
obtained from sugarcane or suyar beer.
14.2 CARBOHYDRATEs (iii), Lactose is the sugar present in milk.
or Starch.
(1) Carbohydrates are polyhydroxyl aldehydes (iv)

ketones or compounds which give rise such units

to
on hydrolysis.

Classiñcation of carbohydrates:
Carbohydrates (Saccharides)

Monosaccharides Oligosaccharides Polysaccharides

Do not hydrolyse into smaller units) (Yield two to ten monosaccharides number of
(Yield large
Examples: glucose, fructose, ribose units on hydrolysis) monosaccharide units on hydrolysis)
Example:starch, glycogen, cellulose

Disaccharides Trisaccharides Tetrasaccharides

(Yield two monosaccharide units (Yield three monosaccharide units (Yield four monosaccharide units
on hydrolysis) on hydrolysis) on hydrolysis)
e.g. Sucrose: (One glucose unit + e.g. Raffinose (one unit each of
:
e.g. Stachyose: (oneglucose unit + one

(C2H22 O1pne fructose unit) glucose,fructose and galactose) fructose unit + twogalactose units)
Maltose: (Two glucose units) (C24 Hy2 D4)
Lactose: (one glucose unit +
one galactose unit

(I) MONOSACCHARIDE
n
These aldose and ketose names are further modified
(1) Nomenclature of monosaccharides in accordance with the total number of carbon
As per IUPAC system of nomenclature, general atoms in the monosaccharides.
name for monosaccharides is glycose (old name of For e.g.:
glucose) (a) Glucose (C,H,,0) is an aldose with six carbons
Monosaccharide
and is thereby an aldohexose.
(b) Fructose (CH,,0) is an ketose with six carbons
and is thereby a ketohexose.
Aldose Ketose (2) GLUCOSE
(Monosaccharides (Monosaccharides
with one aldehydic with one ketonic Glucose occur in nature in free as well as in
carbonyl group) carbonyl group) combined state.
PREPARATION OF GLUCOSE:
From Sucrose From Starch
Reaction: Reaction:
+ H,0 H' (CH,0,)nt nH,0
H'
nC,H,,0,
393 K, 2-3 atm
(Sucrose) (Glucose) (Fructose) (Starch) (Glucose)
Sucrose is hydrolysed by warming with dilute Commercially glucose is obtained by hydrolysis o
hydrochloric acid or sulfuric acid for about 2 hrs. starch by boiling it with dilute sulfuric acid at 393K
under to 3 atm. pressure.
2

This hydrolysis converts sucrose into mixture of glucose

and fructose.
Glucose is separated from fructose by adding ethanol
during cooling. Glucose being almost insoluble in
alcohol crystallizes out first. The solution is filtered to
obtain crystals of glucose.
(138)
piomolecules
STRUCTURE AND 139
PROPERTIES OF GLUCOSE
Glucose has an aldohexose structure. Glucose structure is: CHO
Glucose nolecule contains one aldehydic,
that is (CHOH),
formyl group and the remaining five carbons carry
one hydroxyl group (-OH) each
CH,OH
The six carbons in glucose form one straight Molecular formula:
chain (CH,0)
CHEMICAL PROPERTIES OF GLUCOSE:
Reactions of Glucose
n-hexane on prolonged heating with Properties of Glucose
Glucose gives HI.
(1) This reactions shows that the six
CHO carbon atoms in
glucose molecules form straight chain.
(CHOH), CH,-(CH,),-CH,
CH,OH
(Glucose) (n-Hexane)
cucose forms oxime by
reaction with hydroxylaminel (2) These reaction
and gives cyanohydrins on reaction with hydrogen conirms the presence of a carbonyl
cyanide. group in glucose.
CH = N – OH
NH,OH (CHOH),
CHO CH,OH
(Osime)
(CHOH), G)uoŽine
CH,OH
CN
(Glucose)
CH - 0H
HCN I
(CHOH),

CH,OH
&lycose (Cyanohydrin)
(3)Glucose gets oxidized to a six carbon monocarboxylic(3)
acid called gluconic acid on reaction
This reaction show that, the carbonyl group in glucose
with bromine is in the form of aldehyde.
water which is a mild oxidizing agent.
CHO COOH
(CHOH), (0)
(CHOH),
Br,, water
CH,OH CH,OH
(Glucose) (Gluconic acid)
|(4) Glucose reacts with acetic anhydride to formn glucose(4) This
reaction confirms five hydroxyl groups. As glucose
pentaacetate. is a stable compound, it was further inferred
CHO that five
CHO hydroxyl groups are bonded to five different carbon
atoms in glucose molecule.
ICHOH),

CH,OH
Acetic
anhydride
S(CH,CO),0
(CH - 0
-- +
CH,), 5CH,COOH
(Acetic acid)

(Glucose) CH, - 0 -C - CH,

(Glucose pentaacetate)
(5) Glucose and pluconic acid both on oxidation with|(5) This indicates presence of a one primary alcohols
dilute nitric acid give the same dicarboxylic acid called -CH,OH)in glucose
Ccharic acid, COOH COOH
(CHOH), (0) (0)
(CHOH), (CHOH),
HNO, KNO,
CH,OH
COOH CH,OH
(Glucose)
(Saccharic acid) (Gluconic acid)
140 Ringstructure of glucose
GLUCOSE:
OPTICAL ISOMERISM IN contains Sone chemical properties of glucose
could hes
shows that it
Structural fornula of glucose cxplaincd| on the basis of this ntructure
atoms. Ring structure of glucosc fulfils he cxplanation
four chiral carbon are possible for each of
Two distinct configurations
G
properies of it.
glucose.
the four chiral carbon of of Glucose is found 1o have two cyclic structures
A structural formula containing n' number of nnd VII) which are (VI
in equilibrium with each other
maximum 2n' numbers
chiral carbon can have isomers. An aldohexose
through the open chain structure (1) in auen
stereostructures or oplical solution.
16) optical
thereforc, can cXist asonesixteen (2*=
isomers and glucose s of them. 'ÇHO
Emil Fischer, a German Nobel laureate (1902)
chiral OH
determined the coniguration of the four basis
C-5) in glucose. On the HO3- ||
carbons (C-2, C-3, C-4, measurement of optical
4

of chemical evidence and HOH

activity of various chemicals. "CIH,O11
H

'CHO 'ÇOOH 'COOH "CHOH

"CL,OH
OH
H2 OH u-D-(+) Glucose $-D-(+) Glucose
HO-HOH HO4-| HO 3 H
VI VII
E The ring structure of glucoSe is forned by reaction
H- HOH H

betwcen the formyl (-CHO) group and the alcoholic

OH OH
H5- H5 OH (-OH) group at C-5. Thus, the ring structure is a
"CH,OH "CH,OH 'COOH hemiacetal structure.
(Glucose) (Gluconic acid) (Saccharic acid) The two hemiacctal structure (VI & VIl) differ only
III in the configuration of C-1, the additional chiral
Glucose is an optically active compound and has its centre resulting from ring closure.
specifc rotation, a 120 equal to +52.70 C-1 carbon is called the anomeric carbon and the
two ring structures are called u- and B-anomers of
Glucose is also called dextrose because of its glucose.
dextrorotation. membered ring as the ring of the cyclic
It is a six
The designation (+) glucose or d-glucose imply the structure of glucose contain five carbons and one
dextrorotatory nature of glucose. oxygen.
D-glucose is another designation of glucose that Glucose is also called glucopyranose, as has
indicates the configuration of glucose rather than pyranose structure in analogy with the six
the sign of its optical rotation. membered heterocyclic compound Pyran.
D/L configuration system:
The prefix D- or L- in the name of a compound
indicates relative configuration of a stereoisomer. (Pyran)
Glyceraldehyde has one chiral carbon (C-2) and CH,OH 6
CH,0H
exist as two enantiomers. B-side 5
CHO H H
OH
CHO H
4
H OH QH H
QH
HO -
HO
OH HO
3 2
CH,OH CH,OH H
a-side 3 2
OH
H OH
D-(+)- Glyceraldehyde (a- D
-(+) - Glucopyranose) (B- D - (+) - Glucopyranose)
L- Glyceraldehyde
In the Haworth formula
IV
Conventionally (+)-glyceraldehyde is represented
V the pyranose ring is
considered to be in a perpendicular plane with
by the fischer projection formula having OH respect to the plane of paper. The carbons and
group attached to C-2. On right side (IV) and Oxygen in the ring are in
this the places as they appear
configuration is denoted by symbol D'. Similarly in the figure.
configuration of -) glyceraldehyde (V) denoted by The lower side of the ring is called
symbol L: upper side is the B-side. a-side and the
A monosaccharide is assigned D/L configuration on The -anomer has its amomeric hydroxyl
the basis of the configuration of the lowest chiral (-OH) group (at C-1) on
carbon in its Fischer Projection Formula. B-anomer has its anomeric the a-side whereas the
hydroxyl (-OH) group (at
'CHO C-1) on the B-side.
2 The groups which appear on right side in
ÇHO H OH projection formula appear on a-side in the the Fischer
3H Haworth
-OH H– formula and vice versa.
-OH Reducing nature
u Hemniacetal groupsof glucose:
H

CH,OH! HOH! a potential aldehydeof group.

glucopyranose structure 1s
CH,OH It imparts reducing
properties of glucose.
D-(+)-Glyceraldehyde D-(+)- Glucose Thus glucose give positive Tollens test
Fehling test. and positive
Bomolecules
141
Representation of Fructose struct ure:
Fructose (C,H,0,) is a laevorotatory ketohexose. Fructose in
Cnuctose is also called laevulose due tn
|20 its
laevorotation JD -92,40 Free state Combine state
Deing an a-hydroxy keto compound
fructose is a It exists as mixture of It is found in the form
reducing sugar. Íructopyranose (major) of fructofuranose ring
and fructofuranose structure.
The name furanose is given by analogwith furan, a
five membered heterocyclic compound.
'CH,OH

2C=0 HOH,
HO H
OH HO 2 -CH,0H
HO HO
HOH H 4
H
4- OH
H-OH
"CH,O1
H
H

H,OH °CH,OH
Open chain structure of fructose
a-D-(-)-Fructofuranose p-D-(-)-Fructofuranose

B-side B-side
6
HO - H,C CIl, - O1
5 2
H
HO, H

H, H, –
OH
Furan
3 3
OH (-side OH H 4-side
u-D-(-)-Fructofuranose B-D-()-Fructofuranose
Representations of fructose structure
The figure shows representation of open
structure if fructose and ring structures of
chain In the formation of the glycosidic linkage at least
one of the two monosaccharide units must use its
1-and B-anomers of fructofuranose. Ring structure anomeric hydroxyl group.
of fructose is a hemiketal.
Three most common diasaccharides are sucrose,
(II) DISACCHARIDES maltose and lactose.
Disaccharides give rise to two units of same/ (a) Sucrose
different monosaccharides on hydrolysis with dilute
acids or specific enzymes. (1) Sucrose (Cj2H,0,) its dextrorotatory (+66.59).
On hydrolysis with dilute acid or an enzyme called
Two monosaccharide units are linked together by invertase sucrose gives equimolar mixture of D-(+)
an ether oxide linkage (-0-) which is termed as glucose and D-(-) fructose.
glycosidic linkage in carbohydrate chemistry. H

Glycosidic linkage is formed by removal of a C,,H0,n + H,0 or invertase

water molecule by reaction of two hydroxyl (Sucrose)
(-OH) groups from two monosaccharide units.
C,H0, C,H,,0,
D-(*)-glucose D-(-)-fructose
CH,OH 'Cn,o11
- H,Ç
H

5 lHO
H

2
H
H H0 -
OH CH, OH
HO HO
4
3 3
OH OH OH
B-D-fructose unit
a-D-glucose unit ) a-D-glucose unit
6
– B
HO H,C
H
HO
CH, - OH u, p-1, 2-glycosidic linkage
3
OH H
a-side
B-D-fructose unit
Haworth formula of sucrose
 P'apers Solution
142 Hence maltose is a reducing sugar
sucrose contains glycosidic linkage
(2) Structure of ß- fructose. (c) Lactose:
berween C-1 of a-glucose and C-2 of (1) Lactose a
laevorotation of fructose (-92.49) (C,,H,,0,) is disaccharide
(3) Invert sugar: As the milk.
present
is larger than the dextrorotation
of glucose (+ 52.7°),
net laevorotation. Hence (2) It is formed from two monosaccharnde units,
the hydrolysis product has inversion of D-galactose and D-glucose. namely
hydrolvsis of sucrose is also called sugar.
sucrose and the product is called invert HOCH, HOÇH,
Non-reducing sugar: As the potenial aldehyde are
and
(4) HO H
units OH
ketone groups of both the monosaccharidessucrose is H H
involved in formation of the glycosidic bond
4 H
OH OH
a non-reducing sugar. 2 H
|3 3 2 H
(b) Malt ose two units of
H OH H OH
(1) (C,,H,,0, ,) is a disaccharide made of B-D-galactose B-D-glucose
D-glucose.
6 CH,oH B-1, 4-glycosidic linkage
CH,OH (3)
5
The glycosidic linkage is forrmed between C-1 of B.n
5 H H H
galactose and C-4 of glucose. Therefore the linkage
H
H H in lactose is called ß-1,4-glycosidic linkage.
OH OH H
(4) Reducing Sugar: The hemiacetal group at
HO 2 OH C-l of the
|3 3 glucose unit is not involved in glycosidic linkage but
H OH H OH is free. Hence lactose is a reducing sugar.
a-D-glucose glucose [I1) POLYSACCHARIDES
a-1,4-glycosidic bond Polysaccharides are formed by linking large number
(2) The glycosidic bond in maltose is formed between of monosaccharide units by glycosidic linkage.
C-1 of one glucose ring and C-4 of the other. The (a) Starch (b) Cellulose (c) Glycogen
e.g.
glucose ring which uses its hydroxy group at C-1 (a) Starch
is a-glucopyranose. Hence linkage is called a-1, (1) Starch is a storage carbohydrate of plants and
4-glycosidic linkage. important nutrient for humans and other animals.
(3) Maltose gives glucose on hydrolysis with dilute
(2) Starch is a polymer of a-D glucose.
acids or the enzyme maltase. (3) Starch has two components namely Amylose (15-20%)
(4) Reducing sugar: The hemiacetal group at C-1 of the
and amylopectin (80-85%)
second ring is not involved in glycosidic linkage.
Starch

Amylose Amylopectin
It is water soluble. water insoluble.
It is
I forms blue coloured complex with iodine. n It forms blue-violet coloured complex with
It contains 200-1000aglucose units linked jodine.
by a-1,4-glycosidic linkage giving rise to It is a branched chain polysaccharide
unbranched chain of variable length and formed by a-1, 4-glycosidic linkages
between a-glucose units whereas branches
are formed by a-1, 6-glycosidic 1linkage.

6 6
CH,OH CH,OH CH,0H 6
CH,OH
H H H
5
H H H H
H H
OH H H
OH H OH H OH H
3 2 3
3 2 2
H 3
OH H OH OH H OH

a-l, 4-glycosidic linkages

Amylose
Bomolecules 143

CH,OH CH,OH
H

H
H
OH OH
3

H OH H OH ) -1, 6-glycosidic linkage (branch)

Cu,o 6
"Cl, CH,OI
H H H

H H
OH H OH
3 2
H OH OH OH

u-1, 4-glycosidic linkage

Amylopectin
(b) Cellulose
1) Cellulose is the main constituent of cell wall of plant and bacterial cel. It is also main constituent
of wood and cotton.
(2) Cellulose is a straight chain polysaccharides of B-glucose units linked by B-1, 4-glycocidic bonds.
6 6
HOCH, HOÇH, HOÇH,
H H
H
H
OH H OH OH H

H H H
3 2 3 3 |2
H OH H OH H OH

B-1, 4-glycosidic link

Cellulose
(3) Chemical hydrolysis of cellulose requires use of Proteins are polyamides which are hugh motecutar
concentrated strong acids at high temperature and weight polymers of the monomer units called
pressure. This shows that the B-1, 4-glycosidic bond 0-amino acids.
is very strong and difficult to hydrolyse. (1) -AMINO ACIDS
(4) Humans do not have enzymes which can hydrolyse E ProteinHydrolysis Mixture of a-amino acids.
this linkage and hence cellulose cannot be digested a-amino acids are carboxylic acids having an amino
human beings but it serves as the fibrous content
by (-NH) group bonded to a-carbon, that is the carbon
of food useful for bowel novement. next to the carboxyl (-CoOH) group.
(c) Glycogen R- CH - COOH COOH ÇH, - COOH
(1) Glycogen constitutes storage carbohydrate of animals glycine
and is present in liver, muscles and brain. It is also NH, H,N H NH,
found in yeast and fungi. a-amino acid (a-carbon achiral)
(u-carbon chiral) R
(2) Glycogen has its structure similar to that of
(L-a-amino acid)
amylopectin but it is more highly branched. The symbol in the structure of a-amino acids
R
14.3 PROTEINS represents side chain and may contain additional
The name protein is derived from the Greelkword functional groups,
"Proteios' which means Primary' or of prime a-Amino acids have trivial names and are generally
importance'. represented by three letter symbols or sometimes by
Nutritional sources of protein = Milk, Pulses, nutes, one letter symbol.
fish, meat, etc.
Amino Acids

Acidic Amino Acids Basic Amino Acids Neutral Amino Acids

If 'R'contains a carboxyl (COOH) If R'contains amino group The other amino acids having
group the amino acid is (1°, 2°, 3°) it is called neutral or no functional group in
acidic amino acid basic amino group R' are called Neutral amino acids
 Slution
A Essential Amino acids 0-amino acids results in formation of tetrapeplides,
pentapeptides or hexapeptides respectively.
The a-amino acids which cannot be synthesised in (e) Polypeptides: When the number of
human body and have to be obtained through diet. linked by peptide bonds is more than
b 0-amino acids
These are called essential amino acid. products are called polypeptides. 10, the
Zwitter ion: wvater soluble () Amnino acid residue: The -CHR-
(a) a-amino acids are high melting,
units
or carboxylic peptide bonds are referred as amino acid linked
crystalline solids, unlike simple amines a lo) Protein are polypeptides having more residue.
are due to peculiar than
acids. These properties amino acid residue linked by peptide bonds.hundred
structure called zwitter jon structure of a-amino
(h) The two ends of a polypeptide chain of protein are
acids
acidic not identical.
(b) An -amino acid molecule contains both
group as wvell as basic amino ( C-terminal: The end having free carboxyl group jis
carboxyl (-COOH) group to
NH,) group. Proton transfer froma acidic called C-terminal.
basic group of amino acid froms salt
which is a N-termninal: The other end having free
amino group
dipolar jon called zwitter ion is called N-terminal.
H
*
In the dipeptide glycylalanine, glycine residue is
proton trasfer C
-Q N-terminal and alanine residue is C-terminal.
C-0-H
R (3) TYPES OF PROTEINS:
R
Carboxyl group Zwitter jon Depending upon the molecular shape proteins are
can donate proton classified into two types
(c) Amino acid can exist in different forms depending
upon the pH of the aqueous solution in which it is
dissolved. Consider for example, zwitter ion and the Globular Protein CFibrous Protein
other form of alanine. (a) Molecules of globular (a) Molecules of fibrous
H H H
proteins have spherical proteins have
Hgt COOH coo H,N -
coo shape. elongated, rod like
CH, CH, CH, shape.
(b) This shapes result (b) This shape is the
(B) (A) (C)
overall +l charge Zwitter ion of alanine Overall -1charge from coiling around of result of holding the
pH s2 (No net charge) pH 2 10
the polypeptide chain polypeptide chains of
pH 6 of protein. protein parallel to each
Three forms of alanine other.
(c) Globular protein (c) Hydrogen bond and
(2) PEPTIDE BOND AND PROTEIN
(a) Protein Hydrolysis have intra molecular disulphide bonds one
mixture of amino acid hydrogen bonding responsible for this
Two or More Combine through and have weak shape.
Amino acid peptide linkage Protein
intermolecular forces
as compared to fibrous
Proteins are formed by connecting a-aminoacids. protein.
(b) Peptide bond or peptide linkage is an amide formed (d) Globular protein are (d) Fibrous protein are
between two or more amino acid between -COOH ususally water soluble. insoluble in water.
and NH, group by elimination of a water molecule. (e) e.g. Insuline, egg (e) e.g. Keratin (present
(c) Example: Linking of a molecule of glycine with that albumin serum in hair, nail, woo),
of alanine.
albumin legumelin myosine (protein of
One way of doing this is to combine carboxyl group of (protein in pulses) muscles.
gycine with a-amino group of alanine. This results (4) STRUCTURE OF PROTEINS:
in elimination of a water molecule and formation
of dipeptide called glycylalanine in which the two Proteins are responsible for a variety of function in
amino acid units are linked by peptide linkage. organism.
H,N - CH, - COOH + H,N - CH - COOH
n
Proteins of hair, muscles, skin give shape to the
structure while enzymes are protein which catalyze
CH, physiological reaction.
(glycine) These diverse functions of protein can be
(alanine)
understood by studying the four level structures
H,N - CH, + CO - NH + CH - of protein namely primary, secondary tertiary and
COOH quanternary structure of protein.
(peptide bond) (a) Primary structure of protein:
CH,
(glycylalanine) (1) Primary structure of protein is the sequence
of constituent o-amino acid residues linked by
Peptide bond peptide bonds.
(d) Combination of a third
molecule of an (2) Any change in the sequence of amino acid
acid with a dipeptide would -amino
results in formation
of a tripeptide. Similarly linking residue results in a different proteins.
of four, five or six (3) Primary structure of proteins is represented by
Romciezules 145
sriting the three letter symbols of amino acid (4) As per the convention, the N-terminal amino
residues as per their sequence in the concerned acid residue as written at the left cnd and the
protein. The synbol are separated by dashes. C-terminal amino acid residue at the right end.
O=U

-----NH - CH
-C- NH - CH-C - NH – CH -C- NIH - CH - C
+N- terminal R' R" R"" R' C -
terminal >
(a) Representation by structural formula
Ala - Gly - Ser - Tyr - Gly - Gly - Lys
<-N - terminal C -
terminal
(b) Representation with amino acid symbols
Enzyme catalysis
DENATURATION OF PROTEINS:
(5) Action of an enzyme on a substrate is explained by
ial Denaruration is the process by which the lock and key mechanisn.
molecular shape of protein changes without In this the enzyme has active site on its surface.
breaking the amide/peptide bonds that form the
substrate molecule can attach to this active site
A

primay structure.
(b) High temperature, acids base and even agitation
only if it has the right size and shape.
can disrupt for a specific shape of protein. This Once in the active site, the substrate is held in the
is denatration of protein. correct orientation to react and forms the products
(c) Denaturation results in disturbing the secondary, of reaction.
tertiary or quaternary structure of protein. This Then the product leave the active site and the
causes change in properties of protein and the enzyme is then again ready to act as catalyst.
biological activity is often lost. The rate of the reaction is very high as the formation
(d) e.g. Boiling of egg coagulates egg white and of enzyme substrate complex has very low activation
conversion of milk into curd. energY.
ENZYME Some enzymes are so efficient that one enzyme
(6)
(a) Many chemical reaction takes place in our body molecule can catalyse the reaction of 10000
are brought about at the physiological pH of 7.4 substrate molecules in one second.
and the body temperature of 37°C with help of In many industrial proccsses specific reactions
biological catalysts called enzymes. are carried out by use of enzyme extracted from
(b) e.g. Insulin - an enzyme secreted by pancreas, organisms and also by used new cnzymes made
using genetic engineering
controls blood sugar levels.
Amylase - an enzyme
present in saliva, hydrolyzes Industrial Application of enzyme are:
starch. Conversion of glucose to sweet-tasting fructose,
(c) Chemical enzymes are proteins. Every living cell using glucose isomerase.
at
contains least 1000 different enzymnes.
n
Manufacture of new antibiotics, using pencillin G
(d) Most enzymes catalyse only one reaction or acylase.
one group of similar reactions. Thus enzymes Manufacture of laundry detergents, using proteases.
catalysis is highly specific. Manufacture of esters used in cosmetics, using
e.g. an enzyme that catalyses hydrolysis of amide genetically engineered enzyme.
will not work on or
ester acetal. 14.4 NUCLEIC ACID
(e) Mechanism of enzyme catalysis
One of the most remnarkable properties of living cell
substrate is their ability to produce their replicas through
thousands of generations. This becomes possible
active site
because certain type of information is passed
from one generation to the next. Such
-

unchange
enzyme information is called genetic information.
The particles present in the nucleus of the cel
which is responsible for transmission of inherent
characters are called chromosomes which are made
enzymne
up of proteins combined with biomolecules known
substrate .
as Nucleic acid.
complex There are two types of nucleic acid

product RNA DNA

(Ribonucleic acid) (Deoxyribonucleic acid)
RNAare found mainly DNA are found in the
enzymne
in the fluid of living nucleus of living cell
cells (cytoplasm)
 In this unit. we are going to understand the HO – CH, OH HO - CH,/ OH
structural aspects af nucdeic acids.
(1) NUCLEOTIDES: H/H
are nucleotides. H
The rrpeating units of nucdeic acids
Nuclec acds are unbranched poymers of repeating H

monomers called nucleoudes. i.e. Nucleic acids OH OH OH

have a ponucleoide structure).
DNA molecules contain million nucleotides.
5
RNA molecules contain few thousand
nucleotides. - OH HO- CH, OH
HO CH,
MONOMERS:
(2) THE NUCLEOTIDE
Consist of three components:
(i) A monosacchande
3
(ii) A nitrogen containing base OH OH OH No OH
(üi) A phosphate group at C-2
(i) A nonosaccharide: 2-Deozy-D-ribose
Nucleotides of both RNA and DNA contain ffve D-Ribose
(present in RNA) (present in DNA)
membered ring monosaccharide (furanose) often
called simply sugar component. Sugar Components of nucleic acids

RNA DNA
In RNA sugar components In DNA sugar components
of nucleotide unit is of nucleotide unit is
D-ribOse 2- deoxy -D-ibose
(ii) A nitrogen containing base
There are two different types of bases

Purines Pyrimidines
They have two fused rings They have single ring
example: Aldenine (A) example: Cytosine (C)
Guanine (G) Thymine (T)
Uracil (U)
NH,
H,C
N3 NH NH

1
H
H
Pyrimidine Cytosine Uracil Thymine
(Parent compound) (C)

NH,

NH

N
NH,
H 3 H H

Purine Adenine Guanine

(A) (G)
(Parent compound)
Bases in nucleic acids
(iii) A phosphate group:
Adenine (A), guanine (G) and cytosine (C) are found
These are responsible for the linkage in nucleic acid
in both RNA and DNA molecule while Uracil (U)
polymers. Phosphate group is
occurs only in RNA while thymine (T) occurs only
in DNA
= -0-P-0–
RNAcontains D-ribose A, G, C, U.
=
DNAcontains D-2-deoxyribose, A, G, C, T
Biomolecules 147
3) NUCLEOSIDES AND NUCLEOTIDES Nucleoside of RNA= base +
ribose sugar
(a) The Nucleoside is the molecule in which one of the
Nucleoside of DNA = base +
2-deoxyribose
nitrogen bases (purine/pglymidine) is bonded with
sugar molecule i.e.. A Base-sugar unit is called
nucleoside.

NH,
NH,
HO – CH, 0. OH
5'
N
HO – CH,, 0 N

OH OH
H 3 2
OH OH
(D-Ribose) Cytosine (a-ribonucleoside)
NH,
HO – NH,
CH, O.
OH

5'
HO – CH, O 9

OH 4

OH
(D-2-deoxyribose) (Adenine) (a-deoxyribonucleoside)
Formation of nucleoside
A
nucleoside is formed joining the anomeric
by (b) The Nucleotide is the molecule in which
carbon of the furanose with nitrogen of a base. phosphate group is attached to the nucleoside.
While numbering the atoms in a nucleoside, primes i.e. A
base-sugar-phosphate unit is called
() are used for furanose numbering to distinguish
Nucleotide.
them from the atoms of the base. With pyrimidine Nucleotides are joined together through phosphate
bases, thè nitrogen' atom at the 1 position bonds ester linkage.
with the 1' carbon'of the sugar. With purine bases,
the nitrogen atom at the 9 position bonds with 1

carbon of the sugar.

NH, NH,

N
O=

Ö-P-0-CH,, ( Ö-P-0-CH,

OH OH OH
(AMP) (dCMP)

Structure of nucleotides
Nucleotides are formed by adding a phosphate (4) Structure of Nucleic acids:
group to the 5-OH of a nucleoside. Thus, Nucleic acids, both DNA and RNA, are
nucleotides are monophosphates of nucleosides. polymers of nucleotides, formed by joining
Abridged names of some nucleotides are AMP, the 3'-OH group of one nucleotide with
dAMP, UMP, dTMP and so on. Here, the first capital
5-phosphate of another nucleotide.
letter is derived from the corresponding base. MP
stands for monophosphate. Small letter d' in the
beginning indicates deoxyribose in the nucleotide.
 NH,

NH,
NH, 5'
N 5' cnd Ö-p-0-CHo.
5
NH,
Ö-P-0-CH,
Õ-p-0-cH,0.

O-p-0-H,
OH Phosphodiester
linkage
OH
(dAMP) 3'
(dCMP)
OH 3' end
a
Formation of dinucleottde
One end of polynucleotide having free phosphate a The polynucleotide structure of nucleic acids can he
group of 5-position is called 5'-end, The other end represented schematically asin Fig. (a and b).
is 3 end and has free OH-group at 3 position.

Sugar-phosphate Phosphate Phosphate Sugar Phosphate Sugar

backbone Sugar
5 3'
(a)
base base base

p
(b)
-p$P -p-$ P

B B B
Ng. Polynucleotide structure of nucleic acids : Schematic representations (a) and (b)
Primary structure of nucleic acids is the sequence of gives rise to a ladderlike structure of DNA double
the nucleotides in it. This in turn is determined by helix.
the identify of the bases in the nucleotides. Adenine always forms two hydrogen bonds
Different nucleic acids have distinct primary with thymine, and guanine forms three hydrogen
- -
structure. bonds with cytosine. Thus A T and C G are
It is the sequence of bases in DNA which carries the complementary base pairs and the two strands of
genetic information of the organism. the double helix are complementary to each other.
The polynucleotide chain of nucleic acids are named It may be noted that RNA exists as single stranded
by the sequence of the bases, beginning at the 5 structure.
end using the one letter symbols of the bases.
For example: Axis of helix
EA WIIC
The name CATG means there are four nucleotide in
C

the segrment containing the bases cytosine, adenine,

thymine and guanine, in the indicated order form TAuA
the 5 end.
(5) DNA DOUBLE HELIX: Hydrogen bond
James Watson and Francis Crick put forth in 1953
a double helix model for DNA structure, which
was later verified by electron microscopy. Salient AIT Sugar phosphate
features of the Watson and Crick model of DNA are: backbone
DNA consists of two polynucleotide strands that
wind into a right-handed double helix.
The two strands run in opposite directions; one
from the 5' end to the 3' end, while the other from
the 3' end to the 5' end. A
SAIY
The sugar- phosphate backbone lies on the
G
outside of the helix and the bases lie on the inside, Base C
perpendicular to the axis of the helix.
The double helix is stabilized by hydrogen DIICA
bonding
between the bases of the two DNA strands.
This Fig. DNAdouble helix
 *SchreProtein
Iimauy Ghuce 1 poein îs the Any
equance Consituet o
aut bonds, chage i cequene af da
K auwnoridue hoked by potein reprefentd by rtttinq
ud reaJut in,rrrfferert
Fnictue te
pYoten
egyane in tte
tbree letter ywebel aino add in teis
o

yubols are repaated by dashes.

(ovietaly N-
teil aino aid reidue Qiteno
end.
let end c-teruina ain0 aid edue at ight
-
--NH-cH-c-NH-CH-2-NH-CH-NH-cH- cTeninaj
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bo Seconday etychuee
Sein gosi in
bon o
localied eons
thTee.
The dimenfdno! arangemet
Ok poin: chejo is caled
as
secondauy structue + poin
o oe amide iotage
Hydtqeo bondiq betwn N-H roton
tuctune
&Czo oyqenonothen qives econdau proteim
Tos types sec, Ghu ctuien cO wmonly pund in
&
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Th a- helix med when, palypephde 3.6

Chein tit ioto a dighd harhdk H c

rmp
oS cloctoie prol. eatues vge. -Hydrogen
Ench tu helix has 96 aiino adh Boniy
sydrDGen bondd to NH gouth
uino ad
Hydoqen bond are llel to axis
2

2R 4nups
ex
tended o tan tu heli
os
D
P-þleated 6heet En b. bleated heet, two Wore polypephde
(Called trends) up ide-by ide.
ine
hojns
ien io He plcn S
Heat
Te Czo l NH bond
HyOgen bonioq ocun ben N+H 2C=o heighlaousna chain
R
4nup oiented aLie below Heet g. Sptder draeustk
H

H
 C
Testiany trucue roes
The mengonal ehope adoped by enhre poypeptde
three
de

chan g potin is Called teataiy thuctye, 4

îs tte 5
reet
a
plinq o Chcn Io puhua iannen that Gtabiieegd itelt
The o2ces trat etabil2es teatiaiy Ghuctuc incdes hy
attrucoo, London duperson nee, dirulfid
bonding, electrottz
boncs &
covoleot bond
d. auatenay suctunes paotem
g.

When to o moye polypepide chains ith olded

tia
trutues cowe toqethe in one poten COmplex,+e reul hng
hape is caled quaten uny ttructue
 1.. equane ot Cowplenentuy ftrords o purfo n DNA olec
s'-AcG TA C-3
oiqin ropl sA Hydrgen
Cowplemeoty Gtrnd A T G-s' boneiy
ka onding belween Comp lernentany base pair
* Hydegen

-HNh
G thoee drogen bne

A Two dgen
Suga bond.