A nucleic acid consists of four kinds of bases linked to a sugar-phosphate backbone.

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A nucleic acid consists of four kinds of bases
linked to a sugar-phosphate backbone.
A monomer unit
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The double-helical structure of DNA facilitates
the replication of the genetic material
(10 nucleotides)
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The double helix can be reversibly melted
Single stranded DNA absorbs light more
effectively than does double-helical
DNA-----Hypochromism.
The absorbance of a DNA solution at 260nm
increased when the double helix is melted into
single strands.
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? In eukaryotes, the processes of transcription
and translation are separated both spatially and
in time. Transcription of DNA into mRNA occurs in
the nucleus. Translation of mRNA into
polypeptides occurs on ribosomes.
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Lipids and Cell Membrane
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Fatty acid
Head group
Structure of biomembrane
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Electron micrograph of red blood cell plasma
membrane
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Electron micrograph of red blood cell plasma
membrane
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Confocal image of ERD-GFP (an ER marker)
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Common feagures of biological membrane
1. Membranes are sheet-like structures, only two
molecules thick, that form closed boundaries
between different compartments. The thickness of
most membranes is between 6 nm and 10 nm. 2.
Membrane consist mainly of lipids and proteins.
Their mass ratio ranges from 14 to 41.
Membranes also contain carbohydrates that are
linked to lipids and proteins. 3. Membrane lipids
have both hydrophilic and hydrophobic moieties.
These lipid bilayers are barriers to the flow of
polar molecules. 4. Specific proteins mediate
distinctive functions of membrane.
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Common feagures of biological membrane
5. Membranes are noncovalent assemblies. The
constituent proteins and lipid molecules are held
together by many noncovalent interactions. 6.
Membranes are asymmetric. The two faces of
biological membranes always differ from each
other. 7. Membranes are fluid structures. Lipid
and protein molecules diffuse rapidly, unless
they are anchored by specific interactions.
Membranes can be regarded as two-dementional
solutions of oriented proteins and lipids. 8.
Most cell membranes are electrically polarized.
Membrane potential plays a key role in transport,
energy conversion, and excitability.
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Fatty Acids are key conponents of lipids
Fatty acid
Head group
Structure of biomembrane
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General Features of Fatty Acid Structure The
elements of fatty acid structure are quite
simple. There are two essential features 1. A
long hydrocarbon chain ?The chain length
ranges from 4 to 30 carbons 12-24 is most
common. ? The chain is typically linear,
and usually contains an even number of carbons.
2. A carboxylic acid group
The many fatty acids which occur naturally arise
primarily through variation of chain length and
degree of saturation.
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Carbon-Carbon Double Bonds Carbon-carbon double
bonds (unsaturations) are found in naturally
occurring fatty acids. There may be one double
bond or many, up to six in important fatty acids.
Fatty acids with one double bond are the most
prevalent in the human body, comprising about
half of the total. Fatty acids with two or more
double bonds occur in lesser quantities, but are
extremely important. When double bonds occur
they are almost always cis. If there is more than
double bond, they occur at three-carbon
intervals, e.g., -CC-C-CC-. This is called the
divinylmethane pattern.
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Classification of Fatty Acids One system of
fatty acid classification is based on the number
of double bonds. ? 0 double bonds saturated
fatty acids
?1 double bond monounsaturated fatty acids
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? 2 or more double bonds polyunsaturated fatty
acids
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Phospholipids are the major class of membrane
lipids
Hydrophobic
hydrophilic
Amphipathic
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Cholesterol is a lipid based on a steriod Nucleus
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Cholesterol is a lipid based on a steriod Nucleus
4 linked hydrocarbon rings
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Cholesterol is a lipid based on a steriod Nucleus
Interact with phospholipid head group
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Membrane formation is a consequence of the
amphipathic nature of the lipids
A micelle
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leaflets
A section of a bilayer membrane
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The favored structure for most phospholipids and
glycolipids in aqueous media is a bimolecular
sheet rather than a micelle.
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Hydrophobic interactions have three significant
biological consequences 1. Lipid bilayers have
an inherent tendency to be extensive. 2. Lipid
bilayers tends to close on themselves so that
there are no edges with exposed hydrocarbon
chains, and so they form compartments. 3. Lipid
bilayer are self-sealing because a hole in a
bilayer is energetically unfavorable.
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Lipid bilayer are highly impeameable to ions
and most polar molecules.
Water is a conspicuous exception. It traverses
such membrane because of its small size, high
concentration, and lack of a complete charge.
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Proteins carry out most membrane processes
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SDS-acrylamide gel patterns of membrane proteins
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Immunofluorescent staining of a plasmamembrane
protein
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Proteins associate with the lipid bilayer in a
variety of ways
Integral membrane protein a, b, c. Peripheral
membrane protein d, e.
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Proteins interact with membranes in a variety of
ways
Protein can span the membrane with alpha helices
which are the most common structural motif in
membrane proteins.
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A channel protein can be formed from Beta strands
hydrophilic
hydrophobic
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Lipids and many membrane proteins diffuse rapidly
in the plane of the membrane
FRAP Fluorescence recovery after photobleaching
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The fluid mosaic model allows lateral movement
but not rotation through the membrane
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Membrane fluidity is controlled by fatty acid
composition and cholesterol content
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? All biological Membranes asymmetric. ? The
outer and inner surface of all known biological
membranes have different components and different
enzymatic activities.
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Proteins are targeted to specific compartments by
signal sequences
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Proteins are targeted to specific compartments by
signal sequences
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Proteins are targeted to specific compartments by
signal sequences
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Phase separation in model membranes
Micron-scale fluid/fluid phase separation in
giant unilamellar vesicles (GUVS) composed of
cholesterol, SM, DOPC, and ganglioside GM1.
Tangential confocal section of GUV imaged at
23 C. Alexa488-cholera toxin B (A488-CTB) bound
to Lo-preferring GM1 partitions complementarily
to the Ld-preferring carbocyanine lipid probe
C120 DiI in phase-separated GUVs (scale bar,
5 µm).
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Lipid rafts and domains in the plasma membrane
Giant plasma membrane vesicles (GPMVs) isolated
from RBL-2H3 mast cells spontaneously phase
separate into coexisting fluid phases. GPMVs were
generated from cells pre-labeled with
Alexa488-cholera toxin B and lissamine rhodamine
B sulfonyl-DOPE (Rh-DOPE). Equatorial confocal
section of a GPMV ( 25 µm diameter) was imaged
at 15C. A488-CTB bound to GM1 shows partitioning
complementary to Rh-DOPE and prefers the Lo phase
in GPMVs.
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Summary ? Many common features underlie the
diversity of biological membrane ? Fatty acid are
key constituents of lipid ? There are three
common types of membrane lipids phosphlipids,
glycolipids and cholesterol. ? Protein associate
with the lipid bilayer in a variety of way. ?
Lipids and many membrane proteins diffuse rapidly
in the plane of the membrane
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Selected readings De Weer, P. 2000. a century of
thinking about cell membranes. Annu. Rev.
Physiol. 62 919-926 White SH and wimley WC.
1999. Membrane protein folding and stability
Physical principles. Annu. Rev. Biophys. Biomol.
Struct. 28 319-365. Gunnar von Heijne. 2006.
Membrane-protein topology. Nature Reviews
Molecular Cell Biology 7, 909 918 John F.
Hancock. 2006. Lipid rafts contentious only from
simplistic standpoints. Nature Reviews Molecular
Cell Biology 7, 456-462