biochemistry the structure and function of macromolecules
TRANSCRIPT
Biochemistry
The Structure and Function of Macromolecules
Carbon—Backbone of Biological Molecules
• Although cells are 70–95% water, the rest consists mostly of carbon-based compounds
• Carbon is unique in its ability to form large, complex, and diverse molecules
• Proteins, DNA, carbohydrates, and other molecules that distinguish living matter are all composed of carbon compounds
Organic chemistry-the study of carbon compounds
• Organic compounds range from simple molecules to colossal ones
• Most organic compounds contain hydrogen atoms in addition to carbon atoms
• With four valence electrons, carbon can form four covalent bonds with a variety of atoms
• Needs 4 electrons - single, double or triple bonds
• This tetravalence makes large, complex molecules possible - can form long chains or rings
Carbon Molecules• In molecules with multiple carbons, each carbon
bonded to four other atoms has a tetrahedral shape• However, when two carbon atoms are joined by a
double bond, the molecule has a flat shapeMolecularFormula
StructuralFormula
Ball-and-StickModel
Space-FillingModel
Methane
Ethane
Ethene (ethylene)
Carbon Molecules• The electron configuration of carbon gives it covalent
compatibility with many different elements• The valences of carbon and its most frequent partners
(hydrogen, oxygen, and nitrogen) are the “building code” that governs the architecture of living molecules
Hydrogen
(valence = 1)
Oxygen
(valence = 2)
Nitrogen
(valence = 3)
Carbon
(valence = 4)
Carbon Skeleton Diversity
• Carbon is a versatile atom
• Carbon can use its bonds to form an endless diversity of carbon skeletons
• Carbon chains form the skeletons of most organic molecules
LengthEthane Propane
Butane 2-methylpropane(commonly called isobutane)
Branching
Double bonds
Rings
1-Butene 2-Butene
Cyclohexane Benzene
Hydrocarbons
• Hydrocarbons are organic molecules consisting of only carbon and hydrogen
• Many organic molecules, such as fats, have hydrocarbon components
• Hydrocarbons can undergo reactions that release a large amount of energy
Isomers
• Isomers are compounds with the same molecular formula but different structures and properties:– Structural isomers have
different covalent arrangements of their atoms
– Geometric isomers have the same covalent arrangements but differ in spatial arrangements
– Enantiomers are isomers that are mirror images of each other
Structural isomers differ in covalent partners, as shown in this example of two isomers of pentane.
Geometric isomers differ in arrangement about a double bond. In these diagrams, X represents an atom or group of atoms attached to a double-bonded carbon.
cis isomer: The two Xsare on the same side.
trans isomer: The two Xsare on opposite sides.
L isomer D isomer
Enantiomers differ in spatial arrangement around an asymmetric carbon, resulting in molecules that are mirror images, like left and right hands. The two isomers are designated the L and D isomers from the Latin for left and right (levo and dextro). Enantiomers cannot be superimposed on each other.
Enantiomers• Enantiomers are important in the pharmaceutical industry• Two enantiomers of a drug may have different effects• Differing effects of enantiomers demonstrate that organisms
are sensitive to even subtle variations in molecules
L-Dopa(effective againstParkinson’s disease)
D-Dopa(biologicallyInactive)
Functional Groups• Distinctive properties of organic molecules
depend not only on the carbon skeleton but also on the molecular components attached to it
• Certain groups of atoms called functional groups are often attached to skeletons of organic molecules
• Functional groups are the parts of molecules involved in chemical reactions
• The number and arrangement of functional groups give each molecule its unique properties
Functional Groups• The six functional groups that are most important in the
chemistry of life:– Hydroxyl group– Carbonyl group– Carboxyl group– Amino group– Sulfhydryl group– Phosphate group
STRUCTURE
(may be written HO—)
NAME OF COMPOUNDS
Alcohols (their specific names
usually end in -ol)
Ethanol, the alcohol present in
alcoholic beverages
FUNCTIONAL PROPERTIES
Is polar as a result of the
electronegative oxygen atom
drawing electrons toward itself.
Attracts water molecules, helping
dissolve organic compounds such
as sugars (see Figure 5.3).
STRUCTURE
NAME OF COMPOUNDS
Ketones if the carbonyl group is
within a carbon skeleton
EXAMPLE
Acetone, the simplest ketone
A ketone and an aldehyde may
be structural isomers with
different properties, as is the case
for acetone and propanal.
Aldehydes if the carbonyl group is
at the end of the carbon skeleton
Acetone, the simplest ketone
Propanal, an aldehyde
FUNCTIONAL PROPERTIES
STRUCTURE
NAME OF COMPOUNDS
Carboxylic acids, or organic acids
EXAMPLE
Has acidic properties because it isa source of hydrogen ions.
Acetic acid, which gives vinegarits sour taste
FUNCTIONAL PROPERTIES
The covalent bond betweenoxygen and hydrogen is so polarthat hydrogen ions (H+) tend todissociate reversibly; for example,
Acetic acid Acetate ion
In cells, found in the ionic form,which is called a carboxylate group.
STRUCTURE
NAME OF COMPOUNDS
Amine
EXAMPLE
Because it also has a carboxyl
group, glycine is both an amine and
a carboxylic acid; compounds with
both groups are called amino acids.
FUNCTIONAL PROPERTIES
Acts as a base; can pick up a
proton from the surrounding
solution:
(nonionized)
Ionized, with a charge of 1+,under cellular conditions
Glycine
(ionized)
STRUCTURE
(may be written HS—)
NAME OF COMPOUNDS
Thiols
EXAMPLE
Ethanethiol
FUNCTIONAL PROPERTIES
Two sulfhydryl groups can
interact to help stabilize protein
structure (see Figure 5.20).
STRUCTURE
NAME OF COMPOUNDS
Organic phosphates
EXAMPLE
Glycerol phosphate
FUNCTIONAL PROPERTIES
Makes the molecule of which it
is a part an anion (negatively
charged ion).
Can transfer energy between
organic molecules.
Biochemistry: The Molecules of Life
• Within cells, small organic molecules are joined together to form larger molecules
• Macromolecules are large molecules composed of thousands of covalently connected atoms
Biochemistry• Living organisms are composed of both inorganic
and organic molecules• Inorganic molecules
– Relatively small, simple molecules that usually lack C (a few have one C atom).
– Examples: CO2, NH3, H2O, O2, H2
• Organic molecules– Larger, more complex, based on a backbone of C atoms
(always contain C as a major part of their structure). – Examples: C6H12O6, C2H5COOH
Macromolecules - Polymers• A polymer is a long molecule consisting of many similar
building blocks called monomers• Most macromolecules are polymers, built from monomers• An immense variety of polymers can be built from a small set
of monomers• Three of the four classes of life’s organic molecules are
polymers:– Carbohydrates– Proteins– Nucleic acids
Polymers• Monomers form larger
molecules by condensation reactions called dehydration reactions
• Polymers are disassembled to monomers by hydrolysis, a reaction that is essentially the reverse of the dehydration reaction
Short polymer Unlinked monomer
Dehydration removes a watermolecule, forming a new bond
Dehydration reaction in the synthesis of a polymer
Longer polymer
Hydrolysis adds a watermolecule, breaking a bond
Hydrolysis of a polymer
Carbohydrates• Carbohydrates serve as fuel and building material• They include sugars and the polymers of sugars• The simplest carbohydrates are monosaccharides,
or single (simple) sugars• Carbohydrate macromolecules are
polysaccharides, polymers composed of many sugar building blocks
Sugars
• Monosaccharides have molecular formulas that contain C, H, and O in an approximate ratio of 1:2:1
• Monosaccharides are used for short term energy storage, and serve as structural components of larger organic molecules
• Glucose is the most common monosaccharide
• Monosaccharides are classified by location of the carbonyl group and by number of carbons in the carbon skeleton
• 3 C = triose e.g. glyceraldehyde • 4 C = tetrose • 5 C = pentose e.g. ribose, deoxyribose • 6 C = hexose e.g. glucose, fructose, galactose • Monosaccharides in living organisms generally
have 3C, 5C, or 6C:
Monosaccharides
Triose sugars(C3H6O3)
GlyceraldehydeAld
ose
sK
eto
s es
Pentose sugars(C5H10O5)
Ribose
Hexose sugars(C5H12O6)
Glucose Galactose
Dihydroxyacetone
Ribulose
Fructose
Monosaccharides• Monosaccharides serve as a
major fuel for cells and as raw material for building molecules
• The monosaccharides glucose and fructose are isomers– They have the same chemical
formula– Their atoms are arranged
differently
• Though often drawn as a linear skeleton, in aqueous solutions they form rings Glucose Fructose
Monosaccharides
• In aqueous solutions, monosaccharides form rings
Linear andring forms
Abbreviated ringstructure
29
H H H H H
H
OH OH
OH O
OH H
OH O
CH2OH
Ribose Glyceraldehyde
Triose (3-carbon sugar)
Pentoses (5-carbon sugars)
Deoxyribose
H H
H
H
H
H
OH
OH
O C
C
C 4
5
1
3 2
4
5
1
3 2
1
3
2
CH2OH
Disaccharides• A disaccharide is formed when a dehydration reaction joins two
monosaccharides• Disaccharides are joined by the process of dehydration synthesis• This covalent bond is called a glycosidic linkage
Glucose
Maltose
Fructose Sucrose
Glucose Glucose
Dehydrationreaction in thesynthesis of maltose
Dehydrationreaction in thesynthesis of sucrose
1–4glycosidic
linkage
1–2glycosidic
linkage
Disaccharides
• Lactose = Glucose + Galactose• Maltose = Glucose + Glucose• Sucrose = Glucose + Fructose• The most common disaccharide is
sucrose, common table sugar• Sucrose is extracted from sugar cane and
the roots of sugar beets
Polysaccharides• Complex carbohydrates are called polysaccharides
• They are polymers of monosaccharides - long chains of simple sugar units
• Polysaccharides have storage and structural roles
• The structure and function of a polysaccharide are determined by its sugar monomers and the positions of glycosidic linkages
(a) Starch
(b) Glycogen
(c) Cellulose
Storage Polysaccharides - Starch
• Starch, a storage polysaccharide of plants, consists entirely of glucose monomers
• Plants store surplus starch as granules within chloroplasts and other plastids
Chloroplast Starch
1 µm
Amylose
Starch: a plant polysaccharide
Amylopectin
Storage Polysaccharides - Glycogen
• Glycogen is a storage polysaccharide in animals
• Humans and other vertebrates store glycogen mainly in liver and muscle cells
Mitochondria Glycogen granules
0.5 µm
Glycogen
Glycogen: an animal polysaccharide
Structural Polysaccharides• Cellulose is a major
component of the tough wall of plant cells
• Like starch, cellulose is a polymer of glucose, but the glycosidic linkages differ
• The difference is based on two ring forms for glucose: alpha () and beta ()– Polymers with alpha
glucose are helical
– Polymers with beta glucose are straight
a Glucose
a and b glucose ring structures
b Glucose
Starch: 1–4 linkage of a glucose monomers.
Cellulose: 1–4 linkage of b glucose monomers.
Cellulose • Enzymes that digest starch by
hydrolyzing alpha linkages can’t hydrolyze beta linkages in cellulose
• Cellulose in human food passes through the digestive tract as insoluble fiber
• Some microbes use enzymes to digest cellulose
• Many herbivores, from cows to termites, have symbiotic relationships with these microbes
Cellulosemolecules
Cellulose microfibrilsin a plant cell wall
Cell walls Microfibril
Plant cells
0.5 µm
Glucosemonomer
Chitin• Chitin, another structural polysaccharide, is found in the
exoskeleton of arthropods
• Chitin also provides structural support for the cell walls of many fungi
• Chitin can be used as surgical thread
Lipids• Lipids are the one class of large biological molecules
that do not form polymers• Utilized for energy storage, membranes, insulation,
protection• Greasy or oily substances• The unifying feature of lipids is having little or no
affinity for water - insoluble in water • Lipids are hydrophobic becausethey consist mostly
of hydrocarbons, which form nonpolar covalent bonds
Fats• The most biologically important lipids are fats,
phospholipids, and steroids• Fats are constructed from two types of smaller molecules:
glycerol and fatty acids• Glycerol is a three-carbon alcohol with a hydroxyl group
attached to each carbon• A fatty acid consists of a carboxyl group attached to a long
carbon skeleton
Dehydration reaction in the synthesis of a fat
Glycerol
Fatty acid(palmitic acid)
Fatty Acids• A fatty acid has a long hydrocarbon chain with a
carboxyl group at one end.
• Fatty acids vary in length (number of carbons) and in the number and locations of double bonds
• Saturated fatty acids have the maximum number of hydrogen atoms possible and no double bonds
• Unsaturated fatty acids have one or more double bonds, – Monounsaturated (one double bond)– Polyunsaturated (more than one double bond)
• H can be added to unsaturated fatty acids using a process called hydrogenation
• The major function of fats is energy storage
Stearate Oleate
Fats• Fats separate from water because water molecules form
hydrogen bonds with each other and exclude the fats
• In a fat, three fatty acids are joined to glycerol by an ester linkage, creating a triacylglycerol, or triglyceride
Ester linkage
Fat molecule (triacylglycerol)
Glycerides
• Glycerol + 1 fatty acid = monoglyceride Glycerol + 1 fatty acid = monoglyceride
• Glycerol + 2 fatty acids = diglyceride Glycerol + 2 fatty acids = diglyceride
• Glycerol + 3 fatty acids = triacylglycerol Glycerol + 3 fatty acids = triacylglycerol (also called a triglyceride or fat.)(also called a triglyceride or fat.)
Saturated Fats• Fats made from saturated fatty acids are called saturated
fats
• Most animal fats are saturated
• Saturated fats are solid at room temperature
• A diet rich in saturated fats may contribute to cardiovascular disease through plaque deposits
Saturated fat and fatty acid.
Stearic acid
Unsaturated Fats• Fats made from unsaturated fatty acids are called
unsaturated fats
• Plant fats and fish fats are usually unsaturated
• Plant fats and fish fats are liquid at room temperature and are called oils
Unsaturated fat and fatty acid.
Oleic acid
cis double bondcauses bending
Fat Sources
• Most animal fats contain saturated fatty acids and tend to be solid at room temperature
• Most plant fats contain unsaturated fatty acids. They tend to be liquid at room temperature, and are called oils.
Phospholipids• In phospholipids, two of the –OH groups on glycerol are joined to
fatty acids. The third –OH joins to a phosphate group which joins, in turn, to another polar group of atoms.
• The phosphate and polar groups are hydrophilic (polar head) while the hydrocarbon chains of the 2 fatty acids are hydrophobic (nonpolar tails).
Structural formula Space-filling model Phospholipid symbol
Hydrophilichead
Hydrophobictails
Fatty acids
Choline
Phosphate
Glycerol
Hyd
rop
ho
bic
tai
lsH
ydro
ph
i lic
hea
d
Micelle
Phospholipid bilayer Water
Water
Water Lipid head (hydrophilic)
Lipid tail (hydrophobic)
Phospholipids• When phospholipids are added to water, they orient so that the
nonpolar tails are shielded from contact with the polar H2O may form micelles
• Phosopholipids also may self-assemble into a bilayer, with the hydrophobic tails pointing toward the interior
Phospholipids• The structure of phospholipids results in a bilayer arrangement
found in cell membranes
• Phospholipids are the major component of all cell membranes
WATERHydrophilichead
Hydrophobictails
WATER
Steroids• Steroids are lipids characterized by a carbon
skeleton consisting of four fused rings• Cholesterol, an important steroid, is a component
in animal cell membranes• Testosterone and estrogen function as sex
hormones
Proteins• Proteins have many structures, resulting in a wide
range of functions• They account for more than 50% of the dry mass
of most cells• Protein functions
– Structural support / storage / movement - fibers – Catalysis - Enzymes– Defense against foreign substances– Immunoglobulins– Transport – globins, membrane transporters– Messengers for cellular communications - hormones
Proteins• A protein is composed of one or more polypeptides that
performs a function• A polypeptide is a polymer of amino acids joined by
peptide bonds to form a long chain• Polypeptides range in length from a few monomers to
more than a thousand• Each polypeptide has a unique linear sequence of amino
acids• A protein consists of one or more polypeptides which are
coiled and folded into a specific 3-D shape. • The shape of a protein determines its function.
Amino Acids• Amino acids are monomers of polypetides
• They composed of a carboxyl group, amino group, and an “R”Group
• Amino acids differ in their properties due to differing side chains, called R groups
• Cells use 20 amino acids to make thousands of proteins
Aminogroup
Carboxylgroup
carbon
O
O–
H
H3N+ C C
O
O–
H
CH3
H3N+ C
H
C
O
O–
CH3 CH3
CH3
C C
O
O–
H
H3N+
CH
CH3
CH2
C
H
H3N+
CH3
CH3
CH2
CH
C
H
H3N+ C
CH3
CH2
CH2
CH3N+
H
C
O
O–
CH2
CH3N+
H
C
O
O–
CH2
NH
H
C
O
O–
H3N+ C
CH2
H2C
H2N C
CH2
H
C
Nonpolar
Glycine (Gly) Alanine (Ala) Valine (Val) Leucine (Leu) Isoleucine (Ile)
Methionine (Met) Phenylalanine (Phe)
C
O
O–
Tryptophan (Trp) Proline (Pro)
H3C
Figure 5.17
S
O
O–
Amino Acids
O–
OH
CH2
C C
H
H3N+
O
O–
H3N+
OH CH3
CH
C C
HO–
O
SH
CH2
C
H
H3N+ C
O
O–
H3N+ C C
CH2
OH
H H H
H3N+
NH2
CH2
O
C
C C
O
O–
NH2 O
C
CH2
CH2
C CH3N+
O
O–
O
Polar
Electricallycharged
–O O
C
CH2
C CH3N+
H
O
O–
O– O
C
CH2
C CH3N+
H
O
O–
CH2
CH2
CH2
CH2
NH3+
CH2
C CH3N+
H
O
O–
NH2
C NH2+
CH2
CH2
CH2
C CH3N+
H
O
O–
CH2
NH+
NH
CH2
C CH3N+
H
O
O–
Serine (Ser) Threonine (Thr)Cysteine
(Cys)Tyrosine
(Tyr)Asparagine
(Asn)Glutamine
(Gln)
Acidic Basic
Aspartic acid (Asp)
Glutamic acid (Glu)
Lysine (Lys) Arginine (Arg) Histidine (His)
Amino Acids
Amino Acids and Peptide Bonds
• Two amino acids can join by condensation to form a dipeptide plus H2O.
• The bond between 2 amino acids is called a peptide bond.
Protein Conformation and Function• A functional protein consists
of one or more polypeptides twisted, folded, and coiled into a unique shape
• The sequence of amino acids determines a protein’s three-dimensional conformation
• A protein’s conformation determines its function
• Ribbon models and space-filling models can depict a protein’s conformation
A ribbon model
Groove
Groove
A space-filling model
Four Levels of Protein Structure• The primary structure of a protein is its unique sequence of amino
acids
• Secondary structure, found in most proteins, consists of coils and folds in the polypeptide chain
• Tertiary structure is determined by interactions among various side chains (R groups)
• Quaternary structure results when a protein consists of multiple polypeptide chains
Amino acidsubunits
pleated sheet
helix
Primary Structure
• Primary structure, the sequence of amino acids in a protein, is like the order of letters in a long word
• Primary structure is determined by inherited genetic information
Amino acidsubunits
Carboxyl end
Amino end
Secondary Structure• The coils and folds of secondary structure result from
hydrogen bonds between repeating constituents of the polypeptide backbone
• Typical secondary structures are a coil called an alpha helix and a folded structure called a beta pleated sheet
Amino acidsubunits
pleated sheet
helix
Tertiary Structure• Tertiary structure is
determined by interactions between R groups, rather than interactions between backbone constituents
• These interactions between R groups include hydrogen bonds, ionic bonds, hydrophobic interactions, and van der Waals interactions
• Strong covalent bonds called disulfide bridges may reinforce the protein’s conformation
Hydrophobicinteractions andvan der Waalsinteractions
Polypeptidebackbone
Disulfide bridge
Ionic bond
Hydrogenbond
Quaternary Structure• Quaternary structure results when two or more polypeptide
chains form one macromolecule• Collagen is a fibrous protein consisting of three
polypeptides coiled like a rope• Hemoglobin is a globular protein consisting of four
polypeptides: two alpha and two beta chains
Chains
ChainsHemoglobin
Iron
Heme
CollagenPolypeptide chain
Polypeptidechain
Levels of Protein Structure
68
Interactions that Contribute to a Interactions that Contribute to a Protein’s ShapeProtein’s Shape
68 68 68
69
Sickle-Cell Disease: A Simple Change in Primary Structure
• A slight change in primary structure can affect a protein’s conformation and ability to function
• Sickle-cell disease, an inherited blood disorder, results from a single amino acid substitution in the protein hemoglobin
Red bloodcell shape
Normal cells arefull of individualhemoglobinmolecules, eachcarrying oxygen.
10 µm 10 µm
Red bloodcell shape
Fibers of abnormalhemoglobin deformcell into sickleshape.
Sickle Cell Anemia
Primarystructure
Secondaryand tertiarystructures
1 2 3
Normal hemoglobin
Val His Leu
4Thr
5Pro
6Glu Glu
7Primarystructure
Secondaryand tertiarystructures
1 2 3
Sickle-cell hemoglobin
Val His Leu
4Thr
5Pro
6Val Glu
7
Quaternarystructure
Normalhemoglobin(top view)
Function Molecules donot associatewith oneanother; eachcarries oxygen.
Quaternarystructure
Sickle-cellhemoglobin
Function Molecules interact withone another tocrystallize intoa fiber; capacityto carry oxygenis greatly reduced.
Exposedhydrophobicregion subunit subunit
Conformation• In addition to primary structure, physical and chemical conditions can
affect conformation
• Alternations in pH, salt concentration, temperature, or other environmental factors can cause a protein to unravel
• This loss of a protein’s native conformation is called denaturation
• A denatured protein is biologically inactive
Denaturation
Renaturation
Denatured proteinNormal protein
Nucleic Acids
• Nucleic acids store and transmit hereditary information
• The amino acid sequence of a polypeptide is programmed by a unit of inheritance called a gene
• Genes are made of DNA, a nucleic acid
The Roles of Nucleic Acids
• There are two types of nucleic acids:– Deoxyribonucleic acid (DNA)– Ribonucleic acid (RNA)
• DNA provides directions for its own replication• DNA directs synthesis of messenger RNA
(mRNA) and, through mRNA, controls protein synthesis
• Protein synthesis occurs in ribosomes
NUCLEUS
DNA
CYTOPLASM
mRNA
mRNA
Ribosome
Aminoacids
Synthesis ofmRNA in the nucleus
Movement ofmRNA into cytoplasmvia nuclear pore
Synthesis of protein
Polypeptide
The Structure of Nucleic Acids
• Nucleic acids are polymers called polynucleotides• Each polynucleotide is made of monomers called
nucleotides• Each nucleotide consists of a nitrogenous base, a
pentose sugar, and a phosphate group• The portion of a nucleotide without the phosphate
group is called a nucleoside
5 end
3 end
Nucleoside
Nitrogenousbase
Phosphategroup
Nucleotide
Polynucleotide, ornucleic acid
Pentosesugar
Phosphate group
Sugar
Nitrogenous base
N
N
O
4’
5’
1’
3’ 2’
2 8
7 6
3 9 4
5
P CH2
O
– O
O –
OH R OH in RNA
H in DNA
O
N
NH2
N 1
Nucleotide Monomers• Nucleotide monomers are made
up of nucleosides and phosphate groups
• Nucleoside = nitrogenous base + sugar
• There are two families of nitrogenous bases: – Pyrimidines have a single six-
membered ring– Purines have a six-membered ring
fused to a five-membered ring
• In DNA, the sugar is deoxyribose• In RNA, the sugar is ribose
Nitrogenous bases
Pyrimidines
Purines
Pentose sugars
CytosineC
Thymine (in DNA)T
Uracil (in RNA)U
AdenineA
GuanineG
Deoxyribose (in DNA)
Nucleoside components
Ribose (in RNA)
Nitrogenous bases
• Purines have a double ring structure and include adenine (A) and guanine (G).
• Pyrimidines have a single ring structure and include cytosine (C), thymine (T), and uracil (U) – found only in RNA
Adenine
Guanine
C C
N N
N
C
H
N
C
C H
O
H
Cytosine (both DNA and RNA)
Thymine (DNA only)
Uracil (RNA only)
H C C
N C
H
N
C
NH2
N
N
C H O C C
N C
H
N
C H
H
O C C
N C
H
N
C
O
H H3C
H
O C C
N C
H
N
C
O
H H
H
P U R I N E S
P Y R I M I D I N E S
NH2
NH2
Nucleotide Polymers• Nucleotide polymers are linked
together, building a polynucleotide• Adjacent nucleotides are joined by
covalent bonds that form between the –OH group on the 3´ carbon of one nucleotide and the phosphate on the 5´ carbon on the next
• These links create a backbone of sugar-phosphate units with nitrogenous bases as appendages
• The sequence of bases along a DNA or mRNA polymer is unique for each gene
The DNA Double Helix• A DNA molecule has two
polynucleotides spiraling around an imaginary axis, forming a double helix
• In the DNA double helix, the two backbones run in opposite 5´ to 3´ directions from each other, an arrangement referred to as antiparallel
• One DNA molecule includes many genes
• The nitrogenous bases in DNA form hydrogen bonds in a complementary fashion: A always with T, and G always with C
Sugar-phosphatebackbone
3 end5 end
Base pair (joined byhydrogen bonding)
Old strands
Nucleotideabout to beadded to anew strand
5 end
New strands
3 end
5 end3 end
5 end
ATP• Adenosine triphosphate (ATP), is the primary energy-
transferring molecule in the cell • ATP is the “energy currency” of the cell• ATP consists of an organic molecule called adenosine
attached to a string of three phosphate groups• The energy stored in the bond that connects the third
phosphate to the rest of the molecule supplies the energy needed for most cell activities