Download Organic Chemistry Chapters 2 and 3

Organic Chemistry
Chapters 2 and 3
• Organisms are composed of matter
– Anything that takes up space and has
mass
– An element is a substance that cannot be
broken down to other substances by
chemical reactions.
– A compound is a substance consisting of
two or more elements combined in a fixed
ratio
• Ex. NaCl
• Life requires 25 of the 92 natural elements.
– Carbon, oxygen, hydrogen, and nitrogen make
up 96% of living matter
– Phosphorus, sulfur, calcium, potassium, and a
few other elements make up the rest of the 4%.
• Trace elements are those required by an
organism in only minute quantities.
– An atom is the smallest unit of matter that
still retains the properties of an element
• Subatomic particles
– Neutrons- neutral, nucleus
– Protons- positive, nucleus
– Electrons- negative, electron cloud
• Measure the mass of atoms and subatomic particles
in daltons
– The dalton is the same as the atomic mass unit, or amu)
• Protons and neutrons have masses close to 1 dalton
• Atomic number- number of protons in an element
– Written as a subscript to the left of the symbol for the
element. (2He)
• Mass number- sum of protons and neutrons in the
nucleus of an atom.
– Written as a superscript to the left of an element’s symbol
4 He
2
• Atomic weight- total weight of an atom (including
electrons)
• Isotopes
– Different forms of atom
– Different number of neutrons
– Nature- elements occur as a mixture of its
isotopes
– Behave identically in chemical reactions
– Radioactive isotopes- the nucleus decays
spontaneously, giving off particles and energy.
• When the decay leads to a change in the number of
protons, it transforms the atom to an atom of a different
element.
– Used to date fossils
– Acts as tracers to follow atoms through metabolism
– Diagnostic tools
• Energy levels of electrons
– Electrons are directly involved in the chemical
reactions between atoms
– An atom’s electrons vary in the amount of energy
they possess
• Electrons of an atom have potential energy because of
their position in relation to the nucleus.
– The more distant the electrons are from the nucleus the
greater their potential energy.
– Different states of potential energy that electrons have
are called energy levels, or electron shells.
• Electron configuration and chemical
properties
• First shell can hold no more than 2 electrons
• Second shell holds a maximum of 8 electrons
– Chemical behavior of an atom depends mostly
on the number of electrons in its outermost shell
called valence electrons
• Electron orbitals
– The three dimensional space where an
electron is found 90% of the time is called
an orbital.
• Each electron shell consists of a number of
orbitals of specific shapes.
• No more than 2 electrons can occupy the
same orbital.
– S shaped orbitals are spherical and hold 2
electrons
– P orbitals are dumbbell shaped
Chemical bonding
• Covalent bonds
– Sharing of a pair of valence electrons by two
atoms
• Molecule- two or more atoms held together by covalent
bonds
• Structural formula H-H
• Molecular formula H2
• Double covalent bond- sharing two pairs of valence
electrons
– Nonpolar and polar covalent bonds
• Electronegativity- attraction of an atom for the
electrons of a covalent bond
– The more electronegative an atom, the more
strongly it pulls shared electrons toward itself.
– Nonpolar covalent bond- electrons are shared
equally
– Polar covalent bond- one atoms is bonded to a
more electronegative atom, electrons are not
shared equally
» Ex. water
• Ionic bond
– Two atoms are so unequal in their attraction for valence
electrons that the more electronegative atom strips an
electron completely away from its partner.
– a charged atom (or molecule) is called an ion
• When the charge is positive, the ion is called at cation
• Anion- negatively charged ion
– Ionic bond- attraction between cation and anions
– Ionic compounds (salts)- compounds formed by ionic
bonds
• Found in nature as crystals of various sizes and shapes
– Ion also applies to entire molecules that are electrically
charged.
– Environment affects the strength of ionic bonds.
• Weak chemical bonds play important roles in the
chemistry of life
– Hydrogen bonds
• Forms when a hydrogen atom covalently bonded to one
electronegative atom is also attracted to another
electronegative atom.
– Usually oxygen or nitrogen atoms
– Van der Waals interactions
• Enables all atoms and molecules to stick to one another
• Occur only when atoms and molecules are very close
together.
• A molecule’s biological function is
related to its shape
– Molecule has a characteristic size and
shape
• Shapes are determined by the positions of the
atoms’ orbitals.
– Determines how most biological
molecules recognize and respond to one
another.
• Chemical reactions make and break
chemical bonds
• Reactants- starting materials
• products
• Coefficients indicate the number of molecules
involved
• All atoms of the reactants must be accounted
for in the products
• Some reactions go to completion, but most
reactions are reversible.
– Factors affecting the rate of reaction
• Concentration of reactants
• Concentration of products
• Chemical equilibrium- point at which the
reactions offset one another exactly
– Does not mean that the reactants and products are
equal in concentration
Water
• Polar
– Oxygen is more electronegative than hydrogen, electrons
spend more time closer to oxygen
– Shaped like a wide V
– Properties of water arise from attractions between polar
molecules
• Cohesion
– Water molecules stick to each other as a result of
hydrogen bonding.
– Contributes to the transport of water against
gravity in plants.
– Adhesion- clinging of one substance to another
• Adhesion to the walls of the vessels helps counter the
downward pull of gravity
– Surface tension- measure of how difficult it is to
stretch or break the surface of a liquid.
• Water has a greater surface tension that most liquids.
• Stabilizes Temperature
– Water stabilizes air temperatures by
absorbing heat from air that is warmer
and releasing the stored heat to air that is
cooler.
– Heat and temperature
• Anything that moves has kinetic energy
– Faster a molecule moves, the greater its kinetic energy
• Heat is a measure of the total quantity of kinetic energy
due to molecular motion in a body of matter
• Temperature measures the intensity of heat due to the
average kinetic energy of the molecules.
• Whenever two objects of different temperature are
brought together, heat passes from the warmer to the
cooler body until the two are the same temperature.
• Celsius Scale
• Calorie- amount of heat energy it takes to raise the
temperature of 1g of water by 1 degree C
• Kilocalorie
• Joule- one joule equals 0.239 cal( cal=4.184J)
– Specific heat
• The amount of heat that must be absorbed or lost for 1g
of that substance to change its temperature by 1°C
– 1 calorie per gram per degree C (1cal/g/°C)
• Relatively high specific heat
– Evaporative cooling
• Evaporation (vaporization) - transformation from a liquid
to a gas
– Some evaporation occurs at any temperature
• Heat of vaporization- quantity of heat a liquid must
absorb for 1 g of it to be converted from the liquid to the
gaseous state.
– Water has a high heat of vaporization
– Helps moderate Earth’s climate
• Evaporative cooling
– As a liquid evaporates, the surface of the liquid remains
behind cools down
– Hottest molecules are the most likely to leave as a gas
• Expansion upon freezing
– Less dense as a solid than as a liquid.
– Due to hydrogen bonding
– Important factor in the fitness of the
environment
• Solvent
– solution
• Liquid that is a completely homogeneous mixture of two
or more substances
• Solvent- dissolving agent; solute- substance that is
dissolved
• Aqueous solution- water is the solvent
– Not a universal solvent but a very versatile
solvent due to its polarity
– Hydration shell- sphere of water molecules
around each dissolved ion.
– Can dissolve ionic compounds and polar
molecules that are water soluble (sugars)
– Hydrophobic and hydrophilic substances
• Hydrophilic- has an affinity for water
– Used even if doesn’t dissolve because molecules
are too large
• Hydrophobic-substances that do not have an
affinity for water
– Non-ionic and nonpolar
– Repeal water
– Solute concentration
• Most of the chemical reactions that occur in organisms
involve solutes dissolved in water
• Calculate the concentrations of solutes dissolved in
aqueous solutions
– A mole is equal in number to the molecular weight of a
substance
– Molecular weight is the sum of the weights of all the
atoms in a molecule.
– Advantage of measuring in moles is that a mole of one
substance has exactly the same number of molecules as
a mole of another substance.
– The number of molecules in a mole (Avogadro’s number)
is 6.02 x 1023
– Molarity- the number of moles of solute per liter of
solution
Dissociation of Water
• Hydrogen ion- single proton with a charge of +1
• Hydroxide ion- water molecule that lost a proton
(OH-) and has a charge of -1
• Proton binds to the other water molecule, making a
hydronium ion (H3O+)
• Easier to think of it as the dissociation of water into
a hydrogen ion and a hydroxide ion
• Hydrogen and hydroxide ions are very reactive
– Changes in their concentrations can drastically affect a
cell’s proteins and other complex molecules.
– Pure water : concentrations are equal
• pH
– Acids and bases
• Acids increase the hydrogen ion concentration
of a solution (ex. HCl)
• Bases reduce hydrogen ion concentration
– Some bases reduce directly by accepting hydrogen
ions (Ammonia, NH3)
– Other bases reduce the hydrogen ion concentration
by dissolving to form hydroxide ions, which then
combine with hydrogen ions to form water (NaOH)
• HCl is a strong acid and NaOH is a strong
base because they dissociate completely.
– pH scale
• In any aqueous solution, the product of the H+
and OH- concentrations is constant at 10-14.
• The pH scale, which ranges from 0 to 14,
compresses the range of H+ and OHconcentrations by employing logarithms
– The pH of a solution is defined as the negative
logarithm (base 10) of the hydrogen ion
concentration.
– pH declines as H+ concentration increases.
– Acids are below 7, bases are above 7
– Most biological fluids are within the pH 6 to pH 8
range.
– Buffers
• Internal pH of most living things is close to 7
• Buffers are substances that minimize changes in the
concentrations of H+ and OH- in a solution.
– Work by accepting hydrogen ions from the solution when
they are in excess and donating hydrogen ions to the
solution when they have been depleted.
• Acid precipitation
– Uncontaminated rain has a pH of about 5.6 allowing the
formation of carbonic acid from carbon dioxide and water
– Acid precipitation refers to rain, snow, or fog that is more
acidic than 5.6
– Caused primarily by the presence in the atmosphere of
sulfur oxides and nitrogen oxides
– Effect of acids in lakes and streams is most pronounced in
the spring, as snow begins to melt.
– Direct effects on forests and other terrestrial life is
controversial
Organic Compounds
Carbon
• Compounds containing carbon are said to be
organic
– Organic chemistry- branch of chemistry that specializes in
the study of carbon compounds
• Versatile building blocks
– carbon usually completes its valence
shell by sharing elections with other atoms
in four covalent bonds.
• Tetravalence is one facet of carbon’s
versatility that makes large, complex
molecules possible.
• Variation in carbon skeletons contributes to diversity
– Carbon chains form the skeletons of most organic
molecules
• Can vary in length and may be straight, branched, or
arranged in closed rings
– Hydrocarbons are organic molecules consisting only of
carbon and hydrogen
• Hydrophobic (bonds between C and H are nonpolar)
• Store a relatively large amount of energy
– Isomers
• Same molecular formula but different structures and
different properties
• Structural isomers
– Differ in the covalent arrangements of their atoms
– Location of double bonds
• Geometric isomers
– Same covalent partnership, but differ in their spatial
arrangements
• Enantiomers
– Mirror images of each other
– Usually one isomer is biologically active and the other is
inactive.
– Important in pharmaceutical industry
Functional Groups
• Functional groups
– Components of organic molecules that are most commonly
involved in chemical reactions
– Each functional group behaves consistently from one
organic molecule to another, and the number and
arrangement of the groups help give each molecule its
unique properties.
– Six functional groups most important in the chemistry of life
are:
•
•
•
•
•
•
Hydroxyl
Carbonyl
Carboxyl
Amino
Sulfhydryl
Phosphate
– All are hydrophilic, increasing solubility in water
• Hydroxyl group
– A hydrogen atom is bonded to an oxygen
atom, which in turn is bonded to the
carbon skeleton of the organic molecule
– Compounds containing hydroxyls are
called alcohols.
• Names end in –ol (ex. Ethanol)
• Abbreviated as –OH or –HO
– is polar
• Carbonyl Group
– Consists of a carbon atom joined to an
oxygen atom by a double bond
– Aldehyde- carbonyl on end of compound
• proponal
– Ketone- carbonyl anywhere else in
compound
• Acetone
– Acetone and proponal are isomers
• Carboxyl
– Oxygen atom is double-bonded to a carbon atom
that is also bonded to a hydroxyl group (-COOH)
– Called carboxylic acids or organic acids
• Ex. Formic acid, acetic acid
– Source of hydrogen ions
• Amino Group
– Consists of a nitrogen atom bonded to 2
hydrogen atoms and to the carbon
skeleton (-NH2)
– Called amines
• Ex. Glycine
– Also has a carboxyl group
– Most organic compounds have 2 or more different
functional groups
– Compounds having both amino and carboxyl
groups are called amino acids
• Sulfhydryl Group
– sulfur atom bonding to an atom of
hydrogen (-SH)
– Called thiols
• Phosphate group
– Have a phosphate ion covalently attached
by one of its oxygen atoms to the carbon
skeleton.
– One of functions is to transfer energy
between organic molecules
Polymer Principles
• Most macromolecules are polymers
– A polymer is along molecule consisting of many similar or
identical building blocks linked by covalent bonds.
• Repeating units that serve as building blocks of a polymer are
small molecules called monomers.
– Condensation reaction- monomers are connected by a
reaction in which two molecules are covalently bonded to
each other through loss of a water molecule
• Specifically a dehydration reaction because the molecule lost
is water
• One molecule provides a hydroxyl group (-OH) while the
other provides a hydrogen (-H)
• Cell must expend energy
• Process occurs only with the help of enzymes
– Polymers are disassembled to monomers
by hydrolysis
• Reverse of dehydration
• Break with water
• Polymer variety
– Each cell has thousands of different kinds
of macromolecules
– Molecules are constructed from only 40 to
50 common monomers and some others
that occur rarely.
• Key is arrangement
Carbohydrates
• Include both sugars and their polymers
– Simplest- monosaccharide (single sugar)
– Disaccharides- double sugars
– Polysaccharides- many sugars
• Monosaccharides
– Have molecular formulas that are some multiple of CH2O.
• Glucose is the most common
– Has a carbonyl group and multiple hydroxyl groups
– Classifying sugars
• Depending on location of the carbonyl group, a sugar is either
an aldose (aldehyde sugar) or a ketose (ketone sugar)
– Glucose is an aldose, fructose (structural isomer of glucose) is a
ketose
• Most names for sugars in end –ose
• Size of carbon skeleton
– Ranges from three to seven carbons long
» Glucose, fructose, and other sugars that have six carbons
are called hexoses. Trioses (3 carbon sugars) and
pentoses (five carbon sugars) are also common.
• Spatial arrangement of their parts around asymmetric carbons
– Ex. Glucose and galactose (differ only in placement of parts)
– Most sugars form rings
– Monosaccharides are major nutrients for
cells
– Functions
• major fuel for cellular work
• Carbon skeletons serve as raw materials for
the synthesis of other types of small organic
molecules such as amino acids and fatty acids
• Disaccharides
– Two monosaccharides joined by a
glycosidic linkage
• Glycosidic linkage is a covalent bond formed
between two monosaccharides by a
dehydration reaction.
• Polysaccharides
– Are macromolecules, polymers with a few
hundred to a few thousand monosaccharides
joined by glycosidic linkages.
– Some serve as storage material, hydrolyzed as
needed to provide sugar for cells
– Others serve as building material for structures
that protect the cell or the whole organism
– Architecture and function of a polysaccharide are
determined by its sugar monomers and my the
positions of its glycosidic linkages.
– Storage polysaccharides
• Starch is a storage polysaccharide of plants
– Consists mainly of glucose monomers
– Most of monomers are joined by 1-4 linkages
(number 1 carbon to number 4 carbon)
– Angle of these bonds makes the polymer helical.
» Simplest form of starch, amylose, is
unbranched
» Amylopectin, a more complex form of starch, is
a branched polymer with 1-6 linkages at the
branch points.
– Plants store starch as granules within cellular structures called
plastids, including chloroplasts
– Starch represents stored energy
– Most animals have enzymes that can hydrolyze plant starch
• Animals store a polysaccharde called glycogen
– Humans and other vertebrates store glycogen mainly in liver and
muscle cells
– Hydrolysis of glycogen in these cells releases glucose when the
demand for sugar increases.
– Stored fuel cannot sustain an animal for long
– Glycogen bank is depleted in about a day unless it is replenished
by consumption of food
– Structural polysaccharides
• Organisms build strong materials from structural
polysaccharides
• Cellulose is a major component of the though walls that
enclose plant cells
– Two different ring structures for glucose
» Alpha (α) and Beta (β)
– In cellulose, all of the glucose molecules are in the β
configuration
– , a cellulose molecule is straight (never branched), and its
hydroxyl groups are free to hydrogen bond with the hydroxyls of
other cellulose molecules lying parallel to it.
– In plant cell walls, parallel cellulose molecules held together in
this way are grouped into units called microfibrils.
– Enzymes that digest starch by hydrolyzing α linkages are
unable to hydrolyze the β linkages of cellulose.
• Chitin
– Another important structural polysaccharide used by
arthropods to build exoskeletons
Lipids
• Does not include polymers
• Grouped together because they share
one important trait: little or no affinity
for water
• Hydrophobic behavior is based on
molecular structure
• Consist mostly of hydrocarbons
• Fats store large amounts of energy
– Constructed from two kinds of smaller molecules: glycerol
and fatty acids
• Glycerol is an alcohol with three carbons, each containing a
hydroxyl group
• A fatty acid has a long carbon skeleton, usually 16 or 18
carbon atoms in length with a carboxyl group at one end.
– The nonpolar C-H bonds in the hydrocarbon chains of fatty acids
make the molecule hydrophobic.
• Making a fat, three fatty acids each join to a glycerol by an
ester linkage
– Ester linkage- a bond between a hydroxyl group and a carboxyl
group
– Called a triacylglycerol or triglyceride
• Fatty acids vary in length and the number and locations
of double bonds
– Saturated fats
» No double bonds between the carbon atoms
composing the chain, then as many hydrogen atoms
as possible are bonded to the carbon skeleton
» Most animal fats are saturated
» Solid at room temperature
– Unsaturated fats
» Has one or more double bonds, formed by the
removal of hydrogen atoms from the carbon skeleton
» Fats of plants and fishes
» Usually liquid at room temperature and often referred
to as oils
» Kinks where double bonds are located prevent the
molecules from packing closely enough together to
solidify
• Major function of fats is energy storage
– A gram of fat stores more than twice as much
energy as a gram of a polysaccharide, such as
starch.
– Humans and other mammals stock their long term
food reserves in adipose cells which swell and
shrink as fat is deposited and withdrawn from
storage.
» Adipose tissue also cushions such vital organs
as the kidneys and a layer of fat beneath the
skin insulates the body
• Phospholipids
– Similar to fats, but only have two fatty acid tails rather than three
– Phospholipids show ambivalent behavior toward water
• Their tails, which consist of hydrocarbons, are hydrophobic and are
excluded from water
• The phosphate group and its attachments form a hydrophilic head
that has an affinity for water.
• micelle, a phospholipid droplet with the phosphate heads on the
outside, in contact with water; hydrocarbon tails restricted to the water
free interior of the micelle
– At the surface of a cell, phospholipids are arranged in a bilayer
• Hydrophilic heads are on the outside, hydrophobic tails point toward
the interior
• Forms a boundary between the cell and its external environment
• Major components of cell membranes
• Steroids
– Lipids characterized by a carbon skeleton
consisting of four fused rings
– Different steroids vary in the functional groups
attached to this ensemble of rings.
– Cholesterol
• Common component of animal membranes and is also
the precursor from which other steroids are
synthesized.
– Many hormones, including vertebrate sex
hormones, are steroids produced from
cholesterol
Proteins
• Used for structural support, storage, transport of
other substances, signaling from one part of the
organism to another, movement, and defense
against foreign substances.
• As enzymes, proteins regulate metabolism by
selectively accelerating chemical reactions in the
cell
• All proteins are polymers constructed from the same
set of 20 amino acids.
– Polymers of amino acids are called polypeptides
– A protein consists of one or more polypeptides folded and
coiled into specific conformations
• Polypeptides
– Amino acids are organic molecules possessing
both carboxyl and amino groups.
• α carbon attached to:
– Amino group, carboxyl group, hydrogen atom, and a
variable group symbolized by R
• The R group, also called the side chain, differs with the
amino acid
– Physical and chemical properties of the side chain
determines the unique characteristics of a particular
amino acid.
» Nonpolar, hydrophobic
» Polar, hydrophilic
» Electrically charged (either acidic or basic)
– Amino acids are joined by catalyzing a
dehydration reaction
• Resulting bond is a peptide bond.
• Repeated to form a polypeptide
• At one end of the polypeptide is a free amino
group, and at the opposite end is a free
carboxyl group.
– Chain has an amino end (N-terminus) and a
carboxyl end (C-terminus)
• Protein function
– A functional protein is not just a
polypeptide chain, but one or more
polypeptides precisely twisted, folded, and
coiled into a molecule of unique shape.
• Amino acid sequence determines the 3-D
shape (conformation)
• A protein’s specific conformation determines
how it works
• Function of a protein depends on its ability to
recognize and bind to another molecule
– Four levels of Protein Structure
• Primary Structure
–
–
–
–
Unique sequence of amino acids
Like the order of letters in a very long word
Determined by inherited genetic information
Even slight changes can affect a protein’s conformation
and ability to function
• Secondary structure
– Segments of the polypeptide chain repeatedly coiled or
folded
– Result of hydrogen bonds at regular intervals along the
polypeptide backbone
» α helix- delicate coil held together by hydrogen
bonding between every 4th amino acid
» β pleated sheet-two or more regions of the
polypeptide chain lie parallel to each other.
Hydrogen bonds between the parts of the backbone
in the parallel regions hold the structure together
• Tertiary structure
– Consists of irregular contortions from interactions
between side chains (R groups) of the various
amino acids.
» Hydrophobic interaction
» Disulfide bridges- form where 2 cysteine
monomers are brought close together by the
folding of the protein.
» Hydrogen bond
» Ionic bond
• Quaternary Structure
– The overall protein structure that results from the
aggregation of two or more polypeptide chains.
– Protein conformation
• Depends on the physical and chemical conditions of the
protein’s environment
– If the pH, salt concentration, temperature, or other aspects of the
environment are altered, the protein may unravel and lose its
native conformation, called denaturation
» is biologically inactive
» Most proteins become denatured when transferred from an
aqueous environment to an organic solvent (ether or
chloroform)
» Chemicals that disrupt the hydrogen bonds, ionic bonds,
and disulfide bridges that maintain a protein’s shape.
» Excessive heat
» When a protein in a test-tube solution has been denatured
by heat or chemicals, it will often return to its functional
shape when the denaturing agent is removed.
• Chaperonins (chaperone proteins)
– Protein molecules that assist the proper folding of other
proteins
– Do not specify the correct final structure of a polypeptide
– Keep the new polypeptide segregated from “bad
influences” in the cytoplasmic environment while it fold
spontaneously.
– Determining the structure of a protein
• X-ray crystallography
– Main method used to determine 3-D shape
– Depends on the diffraction (deflection) of an x-ray beam
by the individual atoms in a crystal of protein
Nucleic Acids
• The amino acid sequence of a
polypeptide is programmed by a unit of
inheritance known as a gene
– Genes consist of DNA which is a polymer
belonging to the class of compounds
known as nucleic acids
• Nucleic Acids store and transmit hereditary
information
– Two types of nucleic acids
• DNA
• RNA
– DNA
• Is the genetic material that organisms inherit from their
parents
• Consists of hundreds or thousands of genes
• Encoded within the structure is the information that
programs all the cell’s activities.
– RNA
• DNA directs the synthesis of messenger RNA (mRNA).
• The mRNA molecule then interacts with the cell’s
protein-synthesizing machinery to direct the production
of a polypeptide
• DNARNAprotein
• Eukaryotic cell
– Ribosomes are located in the cytoplasm
– DNA- nucleus
– mRNA conveys the genetic instructions for building
proteins from the nucleus to the cytoplasm
• Prokaryotic cells
– Lack nuclei
– Use RNA to send a message from the DNA to the
ribosomes and other equipment of the cell that transalt
the coded information into amino acid sequences.
• Nucleic Acid Structure
– Are polymers of nucleotides
• Nitrogenous base
• Pentose sugar (5 Carbon)
• Phosphate group
– Types of nitrogenous bases
• Pyrimidine
–
–
–
–
Has a six membered ring of carbon and nitrogen atoms
Cytosine
Thymine
Uracil
• Purines
– Larger, with the six membered ring fused to a five membered
ring
– Adenine
– Guanine
• Adenine , Guanine, and Cytosine are found in both types of
Nucleic acid
– Thymine is only found in DNA
– Uracil- RNA
– Pentose sugar
• Ribose in RNA
• Deoxyribose in DNA
– Lacks an oxygen atom on its number 2 carbon
– Nucleoside monophosphate- or nucleotide
– Polynucleotide- nucleotides are joined by covalent bonds
called phosphodiester linkages between the phosphate of
nucleotide and the sugar of the next
• Results in a backbone with a repeating pattern of sugarphosphate units
– The sequence of bases along a DNA (or mRNA) polymer is
unique for each gene.
• Number of possible base sequences is limitless
• Linear order of bases in a gene specifies the amino acid
sequence (Primary structure) of a protein
• Inheritance is based on replication of the DNA
double helix
– RNA molecules consist of a single polynucleotide chain
– DNA has 2 polynucleotides that spiral around an imaginary
axis to form a double helix
•
•
•
•
1953 Watson and Crick
Sugar-phosphate backbones are the outside of the helix
Nitrogenous bases are paired in the interior
Polynucleotides are held together by hydrogen bonds
between the paired bases and by van der Waals interactions
between the stacked bases.
– A goes with T
– G goes with C
– Both strands are complementary to each other
• Genes (DNA) and their products (proteins)
document the hereditary background of an
organism.
– DNA sequences determine amino acid
sequences of proteins
– Siblings have greater similarity in their DNA and
proteins than do unrelated individuals of the
same species
– Two species that appear to be closely related
based on fossil and anatomical evidence should
also share a greater proportion of their DNA and
protein sequences than do more distantly related
species.