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 : a PENTOSE sugar (deoxyribose/ribose) a PHOSPHATE group (PO42-) a nitrogenous base PURINE or a PYRIMIDINE

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 purine-pyrimidine 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 1) Actin filaments/microfilaments 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 growing in vitro from an isolated centrosome 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 Primary importance of the cytoskeleton is in cell motility. 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. internet