Mechanisms Flashcards
Explain how skeletal muscles contract.
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
Explain how a nerve impulse passes along a non-myelinated neuron.
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.
Explain how a nerve impulse passes along a non-myelinated neuron.
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
Outline the use of the four methods of membrane transport in nerves and synapses.
- 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
Describe the processes involved in absorbing different nutrients across the cell membrane of villus epithelium cells lining the small intestines.
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
Give a detail summary of DNA replication.
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.
Outline the enzymes and molecules involved in DNA replication.
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
Explain the process of transcription in prokaryotes, including the role of the promoter region, RNA polymerase, nucleoside triphosphates and the terminator.
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
Process of translation
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
Explain the production of antibodies.
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.
