Download Nucleic acids store and transmit hereditary information

Nucleic acids store and transmit hereditary
information
• There are two types of nucleic acids: ribonucleic acid
(RNA) and deoxyribonucleic acid (DNA).
• DNA provides direction for its own replication.
• DNA also directs RNA synthesis and, through RNA,
controls protein synthesis.
• Organisms inherit DNA from their parents.
– Each DNA molecule is very long and usually consists
of hundreds to thousands of genes.
– When a cell reproduces itself by dividing, its DNA is
copied and passed to the next generation of cells.
A nucleic acid strand is a polymer of
nucleotides
• Nucleic acids are polymers of monomers called
nucleotides.
• Each nucleotide consists of three parts: a nitrogen
base, a pentose sugar, and a phosphate group.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• The nitrogen bases, rings of carbon and
nitrogen, come in two types: purines and
pyrimidines.
– Pyrimidines have a single six-membered ring.
– The three different pyrimidines, cytosine (C),
thymine (T), and uracil (U) differ in atoms attached
to the ring.
– Purine have a six-membered ring joined to a fivemembered ring.
– The two purines are adenine (A) and guanine (G).
• The pentose joined to the nitrogen base is ribose
in nucleotides of RNA and deoxyribose in
DNA.
– The only difference between the sugars is the lack of
an oxygen atom on carbon two in deoxyribose.
– The combination of a pentose and nucleic acid is a
nucleoside.
• The addition of a phosphate group creates a
nucleoside monophosphate or nucleotide.
• Polynucleotides are synthesized by connecting the sugars
of one nucleotide to the phosphate of the next with a
phosphodiester link.
• This creates a repeating backbone of sugar-phosphate
units with the nitrogen bases as appendages.
Example of A-T base pair as found
within DNA double helix
Example of G-C base pair as found
within DNA double helix
Inheritance is based on replication of the
DNA double helix
• An RNA molecule is single polynucleotide chain.
• DNA molecules have two polynucleotide strands
that spiral around an imaginary axis to form a
double helix.
– The double helix was first proposed as the structure
of DNA in 1953 by James Watson and Francis Crick.
• The sugar-phosphate backbones of the two
polynucleotides are on the outside of the helix.
• Pairs of nitrogenous
bases, one from each
strand, connect the
polynucleotide chains
with hydrogen bonds.
• Most DNA molecules
have thousands to
millions of base pairs.
Fig. 5.30
• Because of their shapes, only some bases are
compatible with each other.
– Adenine (A) always pairs with thymine (T) and
guanine (G) with cytosine (C).
• With these base-pairing rules, if we know the
sequence of bases on one strand, we know the
sequence on the opposite strand.
• The two strands are complementary.
plasma membrane
• The plasma membrane separates the living cell
from its nonliving surroundings.
• This thin barrier, 8 nm thick, controls traffic into
and out of the cell.
• Like other membranes, the plasma membrane is
selectively permeable, allowing some substances
to cross more easily than others.
• The main macromolecules in membranes are lipids
and proteins, but include some carbohydrates.
• The most abundant lipids are phospholipids.
• Phospholipids and most other membrane
constituents are amphipathic molecules.
– Amphipathic molecules have both hydrophobic regions
and hydrophilic regions.
• The phospholipids and proteins in membranes create
a unique physical environment, described by the
fluid mosaic model.
– A membrane is a fluid structure with proteins embedded
or attached to a double layer of phospholipids.
• Attempts to build artificial membranes provided
insight into the structure of real membranes.
– In 1917, Irving Langmuir discovered that
phosphilipids dissolved in benzene would form a
film on water when the benzene evaporated.
• The hydrophilic heads were immersed in water.
Fig. 8.1a
• In 1925, E. Gorter and F. Grendel reasoned that
cell membranes must be a phospholipid bilayer,
two molecules thick.
• The molecules in the bilayer are arranged such
that the hydrophobic fatty acid tails are
sheltered from water while the
hydrophilic phosphate
groups interact
with water.
Fig. 8.1b
• Actual membranes adhere more strongly to
water than do artificial membranes composed
only of phospholipids.
• One suggestion was that proteins on the surface
increased adhesion.
• In 1935, H. Davson and
J. Danielli proposed a
sandwich model in
which the phospholipid
bilayer lies between two
layers of globular
proteins.
Fig. 8.2a
• In 1972, S.J. Singer and G. Nicolson presented
a revised model that proposed that the
membrane proteins are dispersed and
individually inserted into the phospholipid
bilayer.
– In this fluid mosaic
model, the hydrophilic
regions of proteins
and phospholipids are
in maximum contact
with water and the
hydrophobic regions
are in a nonaqueous
environment.
Fig. 8.2b
• A specialized
preparation technique,
freeze-fracture, splits a
membrane along the
middle of the
phospholid bilayer
prior to electron
microscopy.
• This shows protein
particles interspersed
with a smooth matrix,
supporting the fluid
mosaic model.
Fig. 8.3
Membranes are fluid
• Membrane molecules are held in place by relatively
weak hydrophobic interactions.
• Most of the lipids and some proteins can drift
laterally in the plane of the membrane, but rarely
flip-flop from one layer to the other.
Fig. 8.4a
• The lateral movements of phospholipids are rapid,
about 2 microns per second.
• Many larger membrane proteins move more slowly
but do drift.
– Some proteins move in very directed manner, perhaps
guided/driven by the motor proteins attached to the
cytoskeleton.
– Other proteins never move, anchored by the cytoskeleton.
Fig. 8.5
• Membrane fluidity is influenced by temperature
and by its constituents.
• As temperatures cool, membranes switch from a
fluid state to a solid state as the phospholipids are
more closely packed.
• Membranes rich in unsaturated fatty acids are more
fluid that those
dominated by saturated
fatty acids because the
kinks in the unsaturated
fatty acid tails prevent
tight packing.
Fig. 8.4b
• The steroid cholesterol is wedged between
phospholipid molecules in the plasma membrane
of animals cells.
• At warm temperatures, it restrains the movement
of phospholipids and reduces fluidity.
• At cool temperatures, it maintains fluidity by
preventing tight packing.
Fig. 8.4c
Membranes are mosaics of structure and
function
• A membrane is a collage of different proteins
embedded in the fluid matrix of the lipid bilayer.
Fig. 8.6
• Proteins determine most of the membrane’s
specific functions.
• The plasma membrane and the membranes of
the various organelles each have unique
collections of proteins.
• There are two populations of membrane
proteins.
– Peripheral proteins are not embedded in the lipid
bilayer at all.
– Instead, they are loosely bounded to the surface of
the protein, often connected to the other population
of membrane proteins.
– Integral proteins penetrate the hydrophobic core of
the lipid bilayer, often completely spanning the
membrane (a transmembrane protein).
• Where they contact the core, they have hydrophobic
regions with nonpolar amino acids, often coiled into alpha
helices.
• Where they are in
contact with the
aqueous environment,
they have hydrophilic
regions of amino acids.
Fig. 8.7