Download Chapter 19 Biochemistry - American Public University System

Introductory Chemistry
Fourth Edition
Nivaldo J. Tro
Chapter 19
Biochemistry
American Public University System
© 2012 Pearson Education, Inc.
19.1 The Human
Genome Project
• The similarities
between parents and
their children are
caused by genes,
inheritable
blueprints for making
organisms.
• The structure at the
bottom of this image
is DNA, the molecular
basis of genetic
information.
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19.1 The Human Genome Project
• A 15-year project to map the human genome, which
contains all of the genetic material of a human being.
• The Human Genome Project was possible because of
decades of research in biochemistry, the study of the
chemical substances and processes that occur in
plants, animals, and microorganisms.
• The mapping of the human genome revealed that
humans have 20,000–25,000 genes.
• Understanding single nucleotide polymorphisms,
differences from one individual to another, can help
identify individuals who are susceptible to certain
diseases.
• An understanding of gene function can lead to smart
drug design.
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19.1 The Human Genome Project
• Human genes can provide the blueprint for certain
types of drugs.
• Interferon, a drug taken by people with multiple
sclerosis, is a complex compound normally found
in humans.
• The blueprint for making interferon is in the human
genome.
• Scientists have been able to take this blueprint out
of human cells and put it into bacteria, which then
synthesize the needed drug.
• The drug is harvested from bacteria, purified, and
given to patients.
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19.2 The Cell and Its Main Chemical
Components
• The cell is the smallest structural unit of living
organisms that has the properties traditionally
associated with life.
• A cell can be an independent living organism or a
building block of a more complex organism.
• Some cells contain a nucleus, the part of the cell that
contains genetic material.
• The perimeter of the cell is bound by a cell
membrane that holds the contents of the cell together.
• The region between the nucleus and the cell
membrane is called the cytoplasm.
• The cytoplasm contains a number of specialized
structures that carry out much of the cell’s work.
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19.2 The Cell and Its Main Chemical
Components
• The cell is the
smallest
structural unit
of living
organisms.
• The primary
genetic
material is
stored in the
nucleus.
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19.2 The Cell and Its Main Chemical
Components
The main chemical components of the cell
can be divided into four classes:
• carbohydrates
• lipids
• proteins
• nucleic acids
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19.3 Carbohydrates: Sugar, Starch, and
Fiber
• Carbohydrates are the primary molecules
responsible for short-term energy storage
in living organisms.
• Carbohydrates form the main structural
components of plants.
• Carbohydrates often have the general
formula (CH2O)n.
• Structurally, carbohydrates are aldehydes
or ketones containing multiple −OH
groups.
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Glucose (C6H12O6), Fructose (C6H12O6), and
Galactose (C6H12O6)
• Glucose is an aldehyde
(it contains the −CHO
group) with −OH groups
on most of the carbon
atoms.
• The many −OH groups
make glucose soluble in
water and blood, which
is important in glucose’s
role as the primary fuel
of cells.
• Glucose is easily
transported in the
bloodstream and is
soluble within the
aqueous interior of a
cell.
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Glucose is an example of a monosaccharide, a
carbohydrate that cannot be broken down into simpler
carbohydrates. Monosaccharides such as glucose
rearrange in aqueous solution to form ring structures.
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Glucose, fructose, and galactose are examples of
hexoses, six-carbon sugars. The most common
monosaccharides in living organisms are pentoses
and hexoses. Fructose and galactose form rings that
are isomers of glucose.
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Glucose (C6H12O6), Fructose (C6H12O6), and
Galactose (C6H12O6)
• Fructose, also known as fruit sugar, is a
hexose found in many fruits and
vegetables and is a major component of
honey.
• Galactose, also known as brain sugar, is a
hexose usually found combined with other
monosaccharides in substances such as
lactose.
• Galactose is also present within the brain
and nervous system of most animals.
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Monosaccharides Combine to Form
Disaccharides
• Two monosaccharides can react, eliminating
water to form a carbon–oxygen–carbon bond
called a glycosidic linkage that connects
the two rings. The resulting compound is a
disaccharide, a carbohydrate that can be
decomposed into two simpler carbohydrates.
• The link between individual monosaccharides
is broken during digestion, allowing the
individual monosaccharides to pass through
the intestinal wall and enter the bloodstream.
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Glucose and fructose condense to form sucrose.
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Monosaccharides can link together to form
polysaccharides, long, chainlike molecules
composed of many monosaccharide units.
Polysaccharides are a type of polymer—chemical
compounds composed of repeating structural units in a
long chain.
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19.3 Carbohydrates: Sugar, Starch, and Fiber
• Monosaccharides and disaccharides are
simple sugars or simple carbohydrates.
• Polysaccharides are complex
carbohydrates.
• Some common polysacchharides include
starch and cellulose, both of which are
composed of repeating glucose units.
• A third kind of polysaccharide is glycogen.
Glycogen has a structure similar to starch,
but the chain is highly branched. In animals,
excess glucose in the blood is stored as
glycogen until it is needed.
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Difference between Starch and Cellulose
• The difference between starch and cellulose is
the link between the glucose units.
• In starch, the oxygen atom joining neighboring
glucose units points down (as conventionally
drawn) relative to the planes of the rings, a
configuration called an alpha linkage.
• In cellulose, the oxygen atoms are roughly
parallel with the planes of the rings but pointing
slightly up (as conventionally drawn), a
configuration called a beta linkage.
• This difference in linkage causes the
differences in the properties of starch and
cellulose.
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Difference between Starch and Cellulose
• Starch is common in potatoes and grains. It is a soft,
pliable substance that we can easily chew and
swallow.
• During digestion, the links between individual glucose
units are broken, allowing glucose molecules to pass
through the intestinal wall and into the bloodstream.
• Cellulose—also known as fiber—is a stiffer and more
rigid substance. Cellulose is the main structural
component of plants.
• The bonding in cellulose makes it indigestible by
humans.
• When we eat cellulose, it passes right through the
intestine undigested.
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19.4 Lipids
• Lipids are chemical components of the
cell that are insoluble in water but soluble
in nonpolar solvents.
• Lipids include fatty acids, fats, oils,
phospholipids, glycolipids, and steroids.
• Insolubility in water makes lipids an ideal
structural component of cell membranes.
• Lipids are used for long-term energy
storage and for insulation.
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Lipids: Fatty Acids
• One class of lipids is the fatty acids,
carboxylic acids with long hydrocarbon tails.
The general structure for a fatty acid is:
where R represents a hydrocarbon chain
containing 3 to 19 carbon atoms.
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Fatty acids may be saturated or unsaturated.
• Myristic acid is an example of a saturated fatty
acid; its formula is CH3(CH2)12COOH. Its carbon
chain has no double bonds.
• Myristic acid occurs in butterfat and in coconut oil.
• Other fatty acids—called monounsaturated or
polyunsaturated fatty acids—have one or more
double bonds in their carbon chains.
• Oleic acid is an example of a monounsaturated
fatty acid; its formula is
CH3(CH2)7CH:CH(CH2)7COOH.
• Oleic acid occurs in olive oil, peanut oil, and
human fat.
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Fatty acids may be saturated or unsaturated.
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Fatty acids differ only in their R group. The long hydrocarbon
tails of fatty acids make them insoluble in water.
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Lipids: Fats and Oils
• Fats and oils are
triglycerides,
triesters composed of
glycerol linked to
three fatty acids, as
shown in the block
diagram.
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Triglycerides form by the reaction of glycerol with
three fatty acids. The bonds that join the glycerol
to the fatty acids are called ester linkages.
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Tristearin—the main component of beef fat—is
formed from the reaction of glycerol and three
stearic acid molecules.
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Lipids: Fats and Oils
• If the fatty acids in a triglyceride are
saturated, the triglyceride is called a
saturated fat and tends to be solid at room
temperature.
• Lard and many animal fats are examples of
saturated fat.
• If the fatty acids in a triglyceride are
unsaturated, the triglyceride is called an
unsaturated fat, or an oil, and tends to be
liquid at room temperature.
• Canola oil, olive oil, and most other vegetable
oils are examples of unsaturated fats.
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Other Lipids: Phospholipids The basic structure is the
same as triglycerides, except that one of the fatty acid
groups is replaced with a phosphate group.
• Unlike a fatty
acid, which is
nonpolar, a
phosphate group
is polar and often
has another
polar group
attached to it.
• The phospholipid
molecule has a
polar section
and a nonpolar
section.
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In the structure of a phosphatidylcholine, a
phospholipid found in the cell membranes,
the polar part of the molecule is hydrophilic
(has a strong affinity for water), while the
nonpolar part is hydrophobic (avoids water).
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Other Lipids: Phospholipids and Glycolipids
• Glycolipids have similar structures and properties to
phospholipids.
• The nonpolar section of a glycolipid is composed of a
fatty acid chain and a hydrocarbon chain.
• The polar section is a sugar molecule such as
glucose.
• Phospholipids and glycolipids are often schematically
represented as a circle with two long tails.
• The structure of phospholipids and glycolipids is ideal
for constructing cell membranes; the polar parts
interact with the aqueous environments of the cell and
the nonpolar parts interact with each other.
• Lipid bilayer membranes encapsulate cells and many
cellular structures.
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The circle represents the polar
hydrophilic part of the
molecule, and the tails
represent the nonpolar
hydrophobic parts.
Cell membranes are
composed of lipid bilayers, in
which phospholipids or
glycolipids form a double layer.
In this bilayer, the polar heads
of the molecules point outward
and the nonpolar tails point
inward.
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Other Lipids:
Steroids are
lipids that
contain the
four-ring
structure
shown here.
Some common
steroids
include
cholesterol,
testosterone,
and estrogen.
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Other Lipids: Steroids
• Cholesterol serves many important
functions in the body.
• Like phospholipids and glycolipids,
cholesterol is part of cell membranes.
• Cholesterol serves as a precursor for the
body to synthesize other steroids such as
testosterone, a principal male hormone,
and estrogen, a principal female hormone.
• Hormones are chemical messengers that
regulate many body processes, such as
growth and metabolism. They are secreted
by specialized tissues and transported in
the blood.
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Chemistry and Health
Dietary Fats
• Most of the fats and oils in our diet are triglycerides.
• During digestion, triglycerides are broken down into fatty acids,
glycerol, monoglycerides, and diglycerides.
• These products pass through the intestinal wall and then
reassemble into triglycerides before they are absorbed into the
blood. This process is slower than the digestion of other food
types, and eating fats and oils gives a lasting feeling of fullness.
• The Food and Drug Administration (FDA) recommends that fats
and oils compose less than 30% of total caloric intake. The FDA
also recommends that no more than one-third of those fats
(10% of total caloric intake) should be saturated fats.
• A diet high in saturated fats increases the risk of artery
blockages that can lead to stroke and heart attack.
Monounsaturated fats may help protect against these threats.
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19.5 Proteins
• From a biochemical perspective, proteins have a
broad definition.
• Within living organisms, proteins do much of the work
of maintaining life.
• Most of the chemical reactions that occur in living
organisms are catalyzed or enabled by proteins.
• Proteins that act as catalysts are called enzymes.
Without enzymes, life would be impossible.
• Proteins are the structural components of muscle,
skin, and cartilage.
• Proteins transport oxygen in the blood, act as
antibodies to fight disease, and function as hormones
to regulate metabolic processes.
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What are proteins?
• Proteins are polymers of amino acids.
• Amino acids are molecules containing an amine group, a
carboxylic acid group, and an R group (also called a side
chain). The general structure of an amino acid is:
• In a protein, an R group does not necessarily mean a pure
alkyl group.
• Amino acids differ from each other only in their R groups.
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What are proteins?
• The R groups, or side chains, of different
amino acids can be very different chemically.
• Alanine has a nonpolar side chain (—CH3)
while serine has a polar one (—CH2OH).
• Aspartic acid has an acidic side chain
(—CH2COOH), while lysine has a basic one
((—CH2)4NH2).
• When amino acids are strung together to
make a protein, these differences determine
the structure and properties of the protein.
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20
Common
Amino
Acids
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• Amino acids link together because the amine end of
one amino acid reacts with the carboxylic acid end
of another amino acid.
• The resulting bond is a peptide bond, and the
resulting molecule is called a dipeptide. Short
chains of amino acids are called polypeptides.
• Functional proteins contain hundreds or even
thousands of amino acids joined by peptide bonds.
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19.6Protein Structure
• In proteins, amino acids interact with one another, causing the
protein chain to twist and fold in a very specific way.
• The exact shape that a protein takes depends on the types of
amino acids and their sequence in the protein chain.
• Different amino acids and different sequences result in
different shapes.
• Insulin is a protein that recognizes muscle cells because their
surfaces contain insulin receptors, molecules that fit a specific
portion of the insulin protein. If insulin were a different shape,
it would not latch onto insulin receptors on muscle cells and
therefore would not do its job.
• The shape, or conformation, of proteins is crucial to their
function.
• There are four levels of protein structure analysis: primary
structure, secondary structure, tertiary structure, and
quaternary structure.
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Protein structure
(a) Primary structure
is the amino acid
sequence.
(b) Secondary
structure refers to
small-scale
repeating patterns
such as the helix or
the pleated sheet.
(c) Tertiary structure
refers to the largescale bends and
folds of the protein.
(d) Quaternary
structure is the
arrangement of
individual
polypeptide chains.
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Proteins: Primary Structure
• The primary protein structure is the sequence of amino
acids in its chain. Primary structure is maintained by the
covalent peptide bonds between individual amino acids.
• For example, one section of the insulin protein has the
sequence
Gly-Ile-Val-Glu-Gln-Cys-Cys-Ala-Ser-Val-Cys.
• Each three-letter abbreviation represents an amino acid.
• The first amino acid sequences for proteins were determined
in the 1950s.
• Today, the amino acid sequences for thousands of proteins
are known.
• Changes in the amino acid sequence of a protein, even minor
ones, can have devastating effects on the function of a
protein.
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Proteins: Primary Structure Hemoglobin is composed of
four protein chains, each containing 146 amino acid units. The
substitution of glutamic acid for valine in just one position on two
of these chains results in sickle-cell anemia, in which red blood
cells take on a sickle shape that ultimately leads to damage of
major organs.
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Secondary Protein Structure: The Alpha-Helix
The structure is maintained by hydrogen-bonding interactions between NH
and CO groups along the peptide backbone of the coiled protein strand.
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Secondary Protein
Structure:
The beta-pleated
sheet is maintained
by interactions
between the
peptide backbones
of neighboring
protein strands.
In this structure,
the chain is
extended (as
opposed to coiled)
and forms a zigzag
pattern like an
accordion pleat.
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Secondary Protein Structure
• Some proteins—such as keratin, which
composes hair—have the α-helix pattern
throughout their entire chain.
• Some proteins—such as silk—have the βpleated sheet structure throughout their entire
chain.
• Since its protein chains in the β-pleated sheet
are fully extended, silk is inelastic.
• Many proteins have some sections that are
β-pleated sheet, other sections that are αhelix, and still other sections that have less
regular patterns called random coils.
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Everyday Chemistry
Why Hair Gets Longer When It Is Wet
• Hair is composed of a protein called keratin. The
secondary structure of keratin is α-helix throughout,
meaning that the protein has a wound-up helical
structure. This structure is maintained by hydrogen
bonding.
• Individual hair fibers are composed of several strands
of keratin coiled around each other.
• When hair is dry, the keratin protein is tightly coiled,
resulting in the normal length of dry hair.
• When hair becomes wet, water molecules interfere
with the hydrogen bonding that maintains the α-helix
structure.
• The result is the relaxation of the α-helix structure and
the lengthening of the hair fiber. Completely wet hair is
10
to 12% longer than dry hair.
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Tertiary Protein Structure
TERTIARY STRUCTURE consists of the large-scale
bends and folds due to interactions between the R
groups of amino acids that are separated by large
distances in the linear sequence of the protein
chain.
These interactions include:
• hydrogen bonds
• disulfide linkages (covalent bonds between sulfur
atoms on different R groups)
• hydrophobic interactions (attractions between
large nonpolar groups)
• salt bridges (acid–base interactions between
acidic and basic groups)
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Tertiary Protein Structure
• Proteins with structural functions—such as keratin,
which composes hair, or collagen, which composes
tendons and much of the skin—tend to have tertiary
structures in which coiled amino acid chains align
parallel to one another, forming long, water-insoluble
fibers.
• These kinds of proteins are called fibrous proteins.
• Proteins with nonstructural functions—such as
hemoglobin, which carries oxygen, or lysozyme, which
fights infections—tend to have tertiary structures in
which amino acid chains fold in on themselves,
forming water-soluble globules that can travel through
the bloodstream.
• These kinds of proteins are called globular proteins.
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Quaternary Protein Structure
• Many proteins are composed of more than
one amino acid chain.
• Recall that hemoglobin is composed of
four amino acid chains—each chain is
called a subunit.
• The quaternary protein structure
describes how these subunits fit together.
• The same kinds of interactions between
amino acids maintain quaternary structure
and tertiary structure.
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Interactions that create tertiary and quaternary structure include hydrogen
bonds, disulfide linkages, hydrophobic interactions, and salt bridges.
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To summarize protein structure:
• Primary structure is simply the amino acid sequence. It
is maintained by the peptide bonds that hold amino
acids together.
• Secondary structure refers to the small-scale repeating
patterns often found in proteins. These are maintained
by interactions between the peptide backbones of
amino acids that are close together in the chain
sequence or adjacent to each other on neighboring
chains.
• Tertiary structure refers to the large-scale twists and
folds within the protein. These are maintained by
interactions between the R groups of amino acids that
are separated by long distances in the chain sequence.
• Quaternary structure refers to the arrangement of
chains (or subunits) in proteins. It is maintained by
interactions between amino acids on the individual
chains.
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19.7 Nucleic Acids: Molecular Blueprints
• What ensures that proteins have the correct amino
acid sequence? The answer lies in nucleic acids.
• Nucleic acids contain a chemical code that
specifies the correct amino acid sequences for
proteins.
• Nucleic acids can be divided into two types:
deoxyribonucleic acid, or DNA, which exists
primarily in the nucleus of the cell; and ribonucleic
acid, or RNA, which is found throughout the entire
interior of the cell.
• Like proteins, nucleic acids are polymers.
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The individual units composing nucleic acids are
nucleotides. Each nucleotide has three parts: a
phosphate, a sugar, and a base. In DNA, the sugar is
deoxyribose, while in RNA the sugar is ribose.
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Components of DNA
DNA is a polymer of
nucleotides.
Each nucleotide has
three parts: a sugar
group, a phosphate
group, and a base.
Nucleotides are joined
by phosphate
linkages.
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Every nucleotide in DNA has the
same phosphate and sugar, but
can have one of four different
bases.
In DNA, the four bases are
adenine (A), cytosine (C),
guanine (G), and thymine (T).
In RNA, the sugar is different,
and the base uracil (U) replaces
thymine.
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19.7 Nucleic Acids: Molecular Blueprints
• The order of bases in a nucleic acid chain specifies the
order of amino acids in a protein.
• Since there are only four bases and about 20 different
amino acids to be specified, a single base cannot code
for a single amino acid.
• It takes a sequence of three bases—called a codon—to
code for one amino acid.
• The genetic code—the understanding of which amino
acid is coded for by which specific codon—was
discovered in 1961.
• It is nearly universal– the same codons specify the same
amino acids in nearly all organisms.
• In DNA the sequence AGT codes for the amino acid
serine and the sequence TGA codes for the amino acid
threonine.
• In a rat, a bacterium, or a human, the code is the same.
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19.7 Nucleic Acids: Molecular Blueprints
Codons A sequence of three nucleotides with
their associated bases is called a codon.
Each codon codes for one amino acid.
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19.7 Nucleic Acids: Molecular Blueprints
• A gene is a sequence of codons within a DNA
molecule that codes for a single protein.
• Because proteins vary in size from 50 to
thousands of amino acids, genes vary in length
from 50 to thousands of codons.
• Each codon is like a three-letter word that
specifies one amino acid.
• String the correct number of codons together in
the correct sequence, and you have a gene, the
instructions for the amino acid sequence in a
protein.
• Genes are contained in structures called
chromosomes—46 in humans—within the nuclei
of cells.
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Organization of
the genetic
material:
Chromosomes
Genes
Codons
Nucleotides
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19.8 DNA Structure
• The ability of DNA to copy
itself is related to its
structure.
• DNA is stored in the
nucleus as a doublestranded helix.
• The bases on each DNA
strand are directed
toward the interior of the
helix, where they
hydrogen-bond to bases
on the other strand.
• The hydrogen bonding
between bases is not
random.
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19.8 DNA Structure
• Each base is
complementary—
capable of precise
pairing—with only
one other base.
• Adenine (A)
hydrogen-bonds only
with thymine (T), and
cytosine (C)
hydrogen-bonds only
with guanine (G).
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DNA REPLICATION
When a cell is about to divide, the
DNA within its nucleus unwinds
and the hydrogen bonds joining
the complementary bases
break, forming two single
parent strands.
With the help of enzymes, a
daughter strand
complementary to each parent
strand—with the correct
complementary bases in the
correct order—is formed.
The hydrogen bonds between the
strands then re-form, resulting
in two complete copies of the
original DNA, one for each
daughter cell.
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19.7 Nucleic Acids: Molecular
Blueprints Protein Synthesis
• Humans and animals must synthesize the
proteins they need to survive from the
dietary proteins that they eat.
• Dietary protein is split into its constituent
amino acids during digestion.
• These amino acids are reconstructed into
the correct proteins—those needed by the
particular organism—in the organism’s
cells.
• Nucleic acids direct the process.
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19.7 Nucleic Acids: Molecular
Blueprints Protein Synthesis
• When a cell needs to make a particular protein, the gene—the
section of the DNA that codes for that specific protein—
unravels.
• The segment of DNA corresponding to the gene acts as a
template for the synthesis of a complementary copy of that
gene in the form of another kind of nucleic acid, messenger
RNA (or mRNA).
• The mRNA moves out of the cell’s nucleus to a cell structure
within the cytoplasm called a ribosome.
• At the ribosome, protein synthesis occurs.
• The mRNA chain that codes for the protein moves through the
ribosome.
• As the ribosome “reads” each codon, the corresponding
amino acid is brought into place and a peptide bond forms
with the previous amino acid.
• As the mRNA moves through the ribosome, the protein (or
polypeptide) is formed.© 2012 Pearson Education, Inc.
• Protein Synthesis The mRNA strand that
codes for a protein moves through the ribosome.
• At each codon, the correct amino acid is brought
into place and bonds with the previous amino
acid.
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19.7 Nucleic Acids: Molecular
Blueprints Protein Synthesis
To summarize:
• DNA contains the code for the sequence of amino acids in proteins.
• A codon—three nucleotides with their bases—codes for one amino
acid.
• DNA strands are composed of four bases, each of which is
complementary—capable of precise pairing—with only one other
base.
• A gene—a sequence of codons—codes for one protein.
• Chromosomes are molecules of DNA found in the nuclei of cells.
Humans have 46 chromosomes.
• When a cell divides, each daughter cell receives a complete copy of
the DNA—all 46 chromosomes in humans—within the parent cell’s
nucleus.
• When a cell synthesizes a protein, the base sequence of the gene
that codes for that protein is transferred to mRNA. The mRNA then
moves out to a ribosome, where the amino acids are linked in the
correct sequence to synthesize the protein.
• The general sequence is DNA  RNA  protein.
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Chemistry and Health
Drugs for Diabetes
• Diabetes is a disease in which a person’s body does not
make enough insulin, the substance that promotes the
absorption of sugar from the blood. Consequently, diabetics
have high blood sugar levels, which can—over time—lead to
a number of complications, including kidney failure, heart
attacks, strokes, blindness, and nerve damage.
• One treatment for diabetes is the injection of insulin, which
can help manage blood sugar levels and reduce the risk of
these complications.
• Insulin is a human protein and cannot be easily synthesized in
the laboratory.
• For many years, the primary source was animals, particularly
pigs and cattle. Although animal insulin worked to lower blood
sugar levels, some patients could not tolerate it.
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Chemistry and Health
Drugs for Diabetes
• Today, diabetics inject human insulin.
• Scientists were able to remove the gene for insulin
from a sample of healthy human cells.
• They inserted that gene into bacteria, which
incorporated the gene into their genome.
• When the bacteria reproduced, they passed on exact
copies of the gene to their offspring. The result was a
colony of bacteria that all contained the human insulin
gene.
• The chemical machinery within the bacteria expressed
the gene—meaning the bacteria synthesized the
human insulin that the gene codes for.
• Today insulin made in this way is harvested from the
cell cultures and bottled for distribution to diabetics.
© 2012 Pearson Education, Inc.
Chapter 19 in Review
• The Cell: The main chemical components of the cell can be divided
into four categories: carbohydrates, lipids, proteins, nucleic acids.
• Carbohydrates are aldehydes or ketones containing multiple —OH
groups. Monosaccharides include glucose and fructose.
Disaccharides, such as sucrose and lactose, are two
monosaccharides linked together by glycoside linkages.
Polysaccharides include starch and cellulose. Polysaccharides are
also called complex carbohydrates.
• Lipids are chemical components of the cell that are insoluble in
water but soluble in nonpolar solvents. Important lipids include fatty
acids, triglycerides, phospholipids, glycolipids, and steroids.
• Proteins are polymers of amino acids. Amino acids are molecules
composed of an amine group on one end and a carboxylic acid on
the other. Between these two groups is a central carbon atom that
has an R group attached. Amino acids link together by means of
peptide bonds. Functional proteins are composed of hundreds or
thousands of amino acids.
© 2012 Pearson Education, Inc.
Chapter 19 in Review
Protein Structure:
• Primary protein structure is the linear amino acid sequence in
the protein chain. It is maintained by the peptide bonds.
• Secondary structure refers to the small-scale repeating
patterns found in proteins. These are maintained by
interactions between the peptide backbones of amino acids
that are close together in the chain sequence or on
neighboring chains.
• Tertiary structure refers to the large-scale twists and folds
within the protein. These are maintained by interactions
between R groups of amino acids that are separated by long
distances in the chain sequence.
• Quaternary structure refers to the arrangement of chains in
proteins. Quaternary structure is maintained by interactions
between amino acids on the individual chains.
© 2012 Pearson Education, Inc.
Chapter 19 in Review
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Nucleic Acids, DNA Replication, and Protein Synthesis: Nucleic acids,
including DNA and RNA, are polymers of nucleotides.
In DNA, each nucleotide contains one of four bases: adenine (A), cytosine
(C), thymine (T), and guanine (G). The order of these bases contains a
code that specifies the amino acid sequence in proteins.
A codon, a sequence of three bases, codes for an amino acid.
A gene, a sequence of hundreds to thousands of codons, codes for a
protein. Genes are contained in cellular structures called chromosomes.
Complete copies of DNA are transferred from parent cells to daughter cells
via DNA replication.
In this process, the two complementary strands of DNA within a cell unravel
and two new strands that complement the original strands are synthesized.
In this way, two complete copies of the DNA are made, one for each
daughter cell.
When a cell synthesizes a protein, the base sequence of the gene that
codes for that protein is transferred to mRNA. The mRNA then moves out to
a ribosome, where the amino acids are linked in the correct sequence to
synthesize the protein.
The general sequence is: DNA  RNA  protein.
© 2012 Pearson Education, Inc.