Mechanisms

Myofibrils contain repeating units called sacromeres Sacromeres contain actin and myosin filaments Actin filaments are thin and myosin filaments are thick Arriving action potential causes release of Ca2+ from the sacroplasmic reticulum Ca2+ binds to troponin Causing troponin and tropomyosin to move on actin exposing the binding site on actin ATP binds to myosin heads, breaking cross bridges ATP –> ADP+P Energy released from ATP hydrolysis causes myosin heads to change shape Myosin heads form cross bridges to exposed actin binding sites Myosin heads move actin (releasing ADP+P) Myosin filaments move actin filaments towards the center of sacromere Sliding of filaments shortens the sarcomere

Action potential activates voltage-gated sodium-channels; sodium ions (+) rush in to axon; Potential increases; If it increases beyond threshold, more sodium channels open; Axon depolarises, stimulating adjacent sections; Potassium channels open, potassium (+) rushes out; Potential is reduced (repolarisation); Refractory period ensures one-way conduction of action potential; sodium-potassium pump returns axon section to resting potential.

nerve impulse (AP) travels to end of presynaptic neuron; triggers influx of calcium ions causes synaptic vesicles to fuse with membrane; release neurotransmitter molecules into synaptic cleft; (neurotransmitter) crosses / diffuses across channel; (neurotransmitter) binds to receptors on next / postsynaptic neuron; causes ion channels to open on post-synaptic neuron; e.g. Na• diffuse into postsynaptic neuron by depolarizing; neurotransmitter degraded; Calcium ions pumped back into the synaptic cleft by active transport; example: acetylcholine

• Active transport sodium-potassium pump resets resting potential in the axon following nerve impulse • re-uptake of neurotransmitters to the pre-synaptic neuron following synaptic transmission • removal of Ca from pre-synaptic neuron following synaptic transmission • Simple diffusion • diffusion Of NT across synaptic cleft • diffusion of K’ ions out of axon in resting potential • Facilitated diffusion • opening of voltage-gated Na and K’ channels in action potential • opening Of voltage-gated Cal • channels at pre-synaptic terminal • Na• channels activated at post-synaptic terminal to propagate AP • Vesicle transport • influx of calcium activates vesicles containing neurotransmitters to fuse with post-synaptic membrane • exocytosis of NT from pre-synaptic neuron to synaptic cleft

Non-polar lipids enter the cell via simple diffusion as it can pass through the hydrophobic core of lipid. Hydrophobic molecules such as vitamins enter cell via facilitated diffusion, using channel proteins. Glucose and amino acids enter through active transport of sodium ions Larger molecules such as antibodies enter via endocytosis, where the membrane is folded inwards to form a vesicle

DNA replication occurs during (S phase of ) interphase, in preparation for cell division Helicase unwinds the double helix separating the strands of DNA It breaks the hydrogen bonds between the two strands Single stranded binding proteins keep the separated strands apart so that nucleotides can bind DNA gyrase moves in advance of helicase and relieves strain and prevents the DNA supercoiling again. each strand of parent DNA is used as template for the synthesis of the new strands synthesis always occurs in 5´ → 3´ direction on each new strand Therefore synthesis is continuous on leading strand (in the same direction as helicase) and dis-continuous on lagging strand (away from from helicase) This leads to the formation of Okazaki fragments on the lagging strand To synthesise a new strand first an RNA primer is synthesized on the parent DNA using RNA primase Next DNA polymerase III adds the nucleotides (to the 3´ end) added according to the complementary base pairing rules; adenine pairs with thymine and cytosine pairs with guanine; (names needed, letters alone not accepted) Nucleotides added are in the form of as deoxynucleoside triphosphate. Two phosphate groups are released from each nucleotide and the energy is used to join the nucleotides in to a growing DNA chain. DNA polymerase I then removes the RNA primers and replaces them with DNA DNA ligase next joins Okazaki fragments on the lagging strand Because each new DNA molecule contains both a parent and newly synthesised strand DNA replication is said to be semi-conservative.

DNA Gyrase Stabilises the DNA strand so that Helicase work DNA Helicase Unwinds DNA and breaks H-Bonds between Bases Single Strand Binding Proteins Attach to the DNA strand to stop it from recoiling back into a helix DNA Polymerase III Add Nucleotides in a 5’-3’ direction RNA Primase Leaves small sections of RNA (primer) on the lagging strand RNA Primers Initiation sites for DNA Polymerase III on the lagging strand Okazaki Fragments Short sections of DNA on the Lagging Strand DNA Polymerase I Removes RNA Primers, replacing them with DNA DNA Ligase Joins Okazaki fragments together

RNA polymerase binds to the promoter region This initiates transcription RNA polymerase uncoils the DNA Only one strand is used, the template strand Free nucleoside triphosphates bond to their complementary bases on the template strand Adenine binds to uracil instead of thymine As the nucleoside triphosphates bind they become nucleotides and release energy by losing two phosphate groups Transcription occurs 5’ to 3’ direction RNA polymerase forms covalent bonds between the nucleotides and keeps moving along the DNA until it reaches the terminator The terminator signals the RNA polymerase to stop transcription RNA polymerase is released and mRNA separates from the DNA The DNA rewinds

INITIATION mRNA binds to the small subunit of the ribosome. The small subunit of the ribosome moves along the mRNA molecule in a 5’ - 3’ direction until it reaches a start codon (AUG) A molecule of tRNA (with it’s amino acid, Met attached) complementary to the start codon (UAC) binds to the P site of the ribosome The large subunit of the ribosome binds to the tRNA and small subunit ELONGATION A second tRNA (with amino acid attached) complementary to the second codon on the mRNA then binds to the A site of the ribosome The amino acid carried by the tRNA in the P site is transferred to the amino acid in the A site as a consequence of the ribosome catalyzing a new peptide bond (condensation reaction). The ribosome moves one codon along the mRNA (in a 5’ – 3’ direction) The tRNA in the P site is moved to the E site and then released The tRNA in the A site is moved into P site Another tRNA binds, complementary to the next codon on the mRNA, binds to the A site. Steps 6, 7, and 8 are repeated until a stop codon is reached. TERMINATION When a stop codon is reached translation is stopped: a release factor attaches to the A site the polypeptide chain is released the ribosome complex dissembles ready for reuse translating another mRNA molecule

Production of antibodies gives specific immunity. Phagocytes ingest foreign pathogens and present its antigens on its surface. This activates T-lymphocytes. T-lymphocytes activates B-lymphocytes, which produces plasma cells containing antibodies. Antibodies are complementary to the antigens of pathogens. Some of the B-lymphocytes develop into memory cells for long-term immunity.