Download 8. DNA,RNA Membranes, Cytoskeleton

NUCLEIC ACIDS
[REVISE HIGHER NOTES]
• DNA and RNA are information carrying molecules
• DNA: info storage & transmission
• RNA: protein synthesis
• Based on a SUGAR PHOSPHATE BACKBONE
• Base coding A-T and G-C enables a variety and
diversity of proteins
Nucleotide
NUCLEOTIDES
• Monomer of nucleic acid
• Consists of 3 main parts :
PENTOSE sugar (deoxyribose/ribose)
a PHOSPHATE group (PO42-)
a nitrogenous base PURINE or a PYRIMIDINE
• a
•
•
PURINE
PYRIMIDINE
double or fused ring structure
single ring structure
ADENINE, GUANINE
CYTOSINE, THYMINE &
URACIL (only found in RNA)
PURINE
PYRIMIDINE
double or fused ring structure
single ring structure
PHOSPHODIESTER BOND
• Chains of nucleotides (polynucleotides)
formed by DEHYDRATION SYNTHESIS
reaction between the phosphate group of one
nucleotide and the hydroxyl group on the
sugar of another nucleotide
• This bonding gives polynucleotides a defined
polarity reflecting the component nucleotides
Phosphodiester bond
DNA
• a double stranded helix
• two polynucleotide chains that run
in opposite directions (anti-parallel)
• one purine pairing with its
complementary pyrimidine base
• the helix is the only shape that
accommodates the purinepyrimidine base pair and maintains
stable hydrogen bonds
RNA
• 3 types of RNA which are SINGLE stranded but can fold
to give 3D shapes or conformations:
• mRNA – information transcribed from a DNA molecule
and transports it to a ribosome
• tRNA - collects amino acids and transports them to a
ribosome to be fitted according to the messenger RNA
code
• rRNA (ribosomal RNA) - provides a major structural
support component of the ribosome
POLYMERASE ENZYMES
DNA REPLICATION: enables a complete copy of
the genome to be passed on to each daughter
cell during mitosis
TRANSCRIPTION OF DNA into RNA:
mechanism by which genes are expressed
DNA polymerase
DNA LIGASE
This enzyme forms phosphodiester bonds which are used
to join DNA molecules or fragments together to produce
recombinant DNA (recDNA)
Polymerases, ligases and restriction endonucleases
(cut DNA) are important components of a genetic
engineer’s ‘toolkit’.
They are used to manipulate DNA
CELL MEMBRANES
The cell membrane/plasma membrane represents the
barrier that separates the cell’s contents from the
surrounding environment and controls what moves in and
out
In eukaryotic cells, membranes are also used to generate
compartments within the cell, each with a specialised
function e.g. golgi apparatus, endoplasmic reticulum,
lysosomes etc.
MEMBRANE FUNCTIONS
Provides selectively permeable barriers
Compartmentalisation
Localises reactions in the cell
Transport of solutes (active transport)
Signal transduction – receptor proteins on the membrane
surface recognise and respond to different stimulating
molecules, enabling specific responses to be generated
Cell to cell recognition – the external surface of the
membrane represents the cell’s biochemical “personality”
THE PLASMA MEMBRANE
TYPES OF MEMBRANE PROTEINS
• Proteins approx. 50% of the mass of the plasma
membrane and can be classified into different groups
depending on their arrangement in the membrane
and/or their function
•
•
•
•
INTRINSIC - proteins may be embedded ce
TRANSMEMBRANE – proteins run through completely
EXTRINSIC – proteins may be on surface
Glycoproteins – proteins may have carbohydrates
attached
FUNCTIONS OF MEMBRANE PROTEINS
The main functions of these membrane
proteins are as follows:
Transport
Cell recognition
Receptor sites
Enzymes
Intercellular Junctions
TRANSPORT PROTEINS
Transport non-diffusable
Channel proteins – provide a ‘pore’ across the membrane
through which molecules (usually small and charged) can
diffuse
Carrier proteins – these are more specific with binding
sites for only one solute
Both these proteins permit passive transport (with a
concentration gradient this is called facilitated diffusion)
To transport molecules against the concentration
gradient, special types of the carrier proteins are needed.
These harness energy to drive the transport process
during active transport e.g. sodium-potassium pump
CELL RECOGNITION PROTEINS
• usually glycoproteins
• the carbohydrate chain of the glycoprotein
projects out of the cell
• the immune system can recognise it’s own
cells and organs e.g. ABO blood group
antigens:
• A = glycoprotein antigen A
• O = no glycoprotein antigens
RECEPTOR PROTEINS
• These have a specific conformation (shape) that
allows binding of a particular molecule (the ligand)
• The binding of the ligand will then trigger a response
in the cell
ENZYMES
• A protein that catalyses a specific reaction
• Some receptor proteins have enzymatic activity;
the cytoplasmic portion of the protein catalyses
a reaction in response to binding a ligand
INTERCELLULAR JUNCTIONS
PLASMODESMATA
although each plant cell is encased cell wall, fine
strands of cytoplasm, called plasmodesmata,
extend through pores in the cell wall connecting
the cytoplasm of each cell with that of its
neighbours allowing direct exchange of materials
• In ANIMALS, there are 3 types…
• Spot desmosome – dense protein deposits that hold
adjacent cells together like rivets. Mechanical strength is
provided by the intracellular filaments passing from one
desmosome to another
• Tight junction – adjacent membrane proteins are bonded
together preventing movement of materials in the space
between the cells e.g. between epithelial cells lining the
small intestine
• Gap junction – doughnut shaped proteins from each cell
joined together to form tiny channels allowing the
passage of small molecules such as ions, amino acids and
sugars
THE CYTOSKELETON
•
The eukaryotic cell is a 3D
structure. It has a cytoskeleton
anchored to proteins in the plasma
membrane
•
These proteins both maintain shape
and allow movement
•
The cytoskeleton is a dynamic
structure, as the microfilaments and
microtubules can depolymerise and
repolymerise very easily
MICROFILAMENTS
INTERMEDIATE
FILAMENTS
MICROTUBULES
THE CYTOSKELETON
•
The cytoskeleton is made up of 3 components, in order of increasing
diameter. They are …
1) Actin filaments/microfilaments
2) Intermediate filaments
3) Microtubules
MICROFILAMENTS
• These are composed of actin (protein)
• 2 strands of protein molecules twisted together
about 7nm in diameter
• These are present throughout the cell but are
most highly concentrated just inside the plasma
membrane
• They are important in all cell movement and
contraction
Actin fibres in a cell stained
with a fluorescent strain
specific for actin
INTERMEDIATE FILAMENTS
• tough fibrous protein strands twisted
together about 10nm in diameter
• very stable structures provide mechanical
strength to animal cells which lack the
strong cell walls of plants
The nucleus in epithelial cells is
held within the cell by a basket
like network of intermediate
filaments made of keratins which
have been stained here using a
fluorescent stain
MICROTUBULES
•
•
•
•
•
hollow tubes (like straws)
tubulin protein (a globular protein)
cylindrical arrangement
a relatively rigid structure
Microtubules only form around a centrosome
(organising centre)
• The centrosome provides a “place” from which the
microtubules form.
• important in cell division as part of the spindle fibre
network
• can move components within the cell
Microtubules growing in
vitro from an isolated
centrosome
FUNCTIONS
All of these
components give
mechanical support
and shape to the
cell
Primary importance of the cytoskeleton is in cell motility.
The cytoskeleton extends throughout the cytoplasm
determines the internal movement of cell organelles,
cell locomotion and muscle fibre contraction
Sodium-potassium pump
Cells maintain sodium and potassium against
concentration gradient.
Transmembrane protein and ATP involved.
3 Sodium pumped out and 2 potassium pumped in.
ATP changes shape of protein with phosphate.
(ATPase)
Nerve impulse transmission and ion balance.
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