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Proteins have many structures, resulting in a wide
range of functions
• Proteins account for more than 50% of
the dry mass of most cells
• The monomers (building units) of
proteins are amino acids.
• Cells use 20 amino acids to make
thousands of proteins
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Amino Acid Monomers
• Amino acids are organic molecules with carboxyl
and amino groups
a carbon
Amino
group
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Carboxyl
group
Amino acids are linked by peptide bonds formed by
dehydration reactions
Peptide
bond
(a)
Peptide
bond
Side chains
Backbone
(b)
Amino end
(N-terminus)
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Carboxyl end
(C-terminus)
Polypeptides
• Polypeptides are polymers of amino acids
• A protein consists of one or more polypeptides
Lysozyme:
an
antibacterial
enzyme
(protein)
found in
human tears.
It is made of
one
polypeptide.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Hemoglobin protein
is made of four
polypeptides
Fig. 5-22
Normal hemoglobin
Primary
structure
Sickle-cell hemoglobin
Primary
structure
Val His Leu Thr Pro Glu Glu
1
2
3
Secondary
and tertiary
structures
4
5
6
7
subunit
Secondary
and tertiary
structures
Val His Leu Thr Pro Val Glu
1
2
3
Exposed
hydrophobic
region
Quaternary
structure
Normal
hemoglobin
(top view)
Quaternary
structure
Sickle-cell
hemoglobin
Function
Molecules do
not associate
with one
another; each
carries oxygen.
Function
Molecules
interact with
one another and
crystallize into
a fiber; capacity
to carry oxygen
is greatly reduced.
10 µm
Red blood
cell shape
Normal red blood
cells are full of
individual
hemoglobin
moledules, each
carrying oxygen.
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4
5
6
7
subunit
10 µm
Red blood
cell shape
Fibers of abnormal
hemoglobin deform
red blood cell into
sickle shape.
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Four Levels of Protein Structure
• Primary structure
• Secondary structure
• Tertiary structure
• Quaternary structure
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• Primary structure, the sequence of amino
acids in a protein, is like the order of letters in a
long word
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Figure 3.7 The Four Levels of Protein
Structure (Part 1)
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Secondary Structure
• The coils and folds of secondary structure result from
hydrogen bonds between atoms of the polypeptide
backbone (NOT the amino acid side chain or R-groups).
• Typical secondary structures are a coil called an alpha
helix and a folded structure called a beta pleated sheet.
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Figure 3.7 The Four Levels of Protein
Structure (Part 2)
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Fig. 5-21d
Abdominal glands of the
spider secrete silk fibers
made of a structural protein
containing -pleated sheets.
The radiating strands, made
of dry silk fibers, maintain
the shape of the web.
The spiral strands (capture
strands) are elastic, stretching
in response to wind, rain,
and the touch of insects.
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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,
– van der Waals interactions
– Strong covalent bonds called disulfide bridges may
reinforce the protein’s conformation.
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Figure 3.7 The Four Levels of Protein
Structure (Part 3)
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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
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Concept 5.5: 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
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The Roles of Nucleic Acids
• There are two types of nucleic acids:
– Deoxyribonucleic acid (DNA)
– Ribonucleic acid (RNA)
• DNA replicates in order for the cells to divide
• DNA directs the synthesis of messenger RNA
(mRNA) and, through mRNA, controls protein
synthesis
• Protein synthesis occurs in ribosomes
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Concept 3.1 Nucleic Acids Are Informational
Macromolecules
• DNA’s information is encoded in the sequence
of bases. DNA has two functions:
– Replication
– Information is copied to RNA and used to specify
amino acid sequences in proteins.
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LE 5-25
DNA directs the
synthesis of
messenger RNA
(mRNA) and,
through mRNA,
controls protein
synthesis
DNA
Synthesis of
mRNA in the nucleus
mRNA
NUCLEUS
CYTOPLASM
mRNA
Movement of
mRNA into cytoplasm
via nuclear pore
Ribosome
Synthesis
of protein
Polypeptide
Amino
acids
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
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LE 5-26a
5 end
Nucleoside
Nitrogenous
base
Phosphate
group
Nucleotide
3 end
Polynucleotide, or
nucleic acid
Pentose
sugar
Nitrogenous bases
• 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.
Pyrimidines
Nitrogenous bases
Cytosine
C
Thymine (in DNA) Uracil (in RNA)
T
U
Purines
Adenine
A
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Guanine
G
Pentose sugar
• In DNA, the sugar is deoxyribose
• In RNA, the sugar is ribose.
Pentose sugars
Deoxyribose (in DNA)
Ribose (in RNA)
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The DNA Double Helix
• A DNA molecule has two
polynucleotides spiraling around
an imaginary axis, forming a
double helix
• The nitrogenous bases in DNA
form hydrogen bonds in a
complementary fashion: A always
pairs with T, and G always pairs
with C
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Figure 3.4 DNA
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Concept 3.1 Nucleic Acids Are Informational
Macromolecules
• The two strands are antiparallel (running in
opposite directions), and the double helix is
right-handed.
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RNA
• RNA is made of one
polynucleotide (one strand of
nucleotides)
• The nucleotide of RNA is made
of:
– A nitrogenous bases,
Adenine (A), Uracil (U),
Guanine (G), or Cytosine
(C).
– A ribose sugar.
– A phosphate group.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 3.1 Nucleic Acids Are Informational
Macromolecules
• DNA replication and transcription depend
on base pairing:
•
5′-TCAGCA-3′
•
3′-AGTCGT-5′
•
•
transcribes to RNA with
the
•
sequence 5′-UCAGCA3′.
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Differences between DNA and RNA
DNA
RNA
Composed of two strands
of polynucleotides twisted
together helically to form a
double helix
Composed of one strand
of polynucleotides.
Contains the 5-carbon
sugar Deoxyribose
Contains the 5-carbon
sugar Ribose
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Differences between DNA and RNA
DNA
Contains the nitrogenous
bases Adenine (A),
Thymine (T), Guanine (G),
and Cytosine (C)
RNA
Contains the nitrogenous
bases Adenine (A),
Uracil (U), Guanine (G),
and Cytosine (C)
Adenine pairs with Thymine
and Guanine pairs with
Cytosine.
Larger molecule.
Shorter than DNA
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