Explain the nature of the genetic code.
- We read the genetic code as a series of three bases, called a codon which codes for one amino acid.
- Each codon is non overlapping.
- It is a degenerate code - some A.A are coded for by more than one codon.
Explain the process of transcription.
- Starts when RNA polymerase attaches to the DNA double helix.
- The hydrogen bonds break.
- One of the DNA strands acts as the antisense strand (template).
- RNA polymerase joins free RNA nucleotides alongside template by complementary base pairing.
- mRNA then detaches from the DNA and leaves nucleus via nuclear pore.
- DNA winds back up.
Explain how one gene can give rise to more than one protein through post-transcriptional changes to mRNA.
- Not all DNA codes for A.A.
- Introns do not code for A.A.
- Exons do code for A.A.
- Introns and exons both copied on mRNA.
- Introns are removed by the enzyme spliceosome.
- These spliceosome enzymes sometimes also change the mRNA code slightly, so it is possible to have many subtly different proteins produced from just one gene.
- All this happens in the nucleus.
Explain the process of translation.
- mRNA attaches to ribosome and tRNA carry A.A. towards the ribosome.
- A tRNA with a complementary anti codon to the first codon on mRNA attaches itself by complementary base pairing.
- A second tRNA molecule attaches to the next codon.
- The 2 A.A attached to tRNA join by a condensation reaction to form a peptide bond. First tRNA moves away, leaving A.A.
- This process continues producing a polypeptide chain until a stop codon is reached.
- The polypeptide chain moves away from ribosome and travels to golgi.
Describe how DNA profiling is used for identification and determining genetic relationships between organisms.
- Used to determine identity and relationships between species.
- People have different repeating short sequences of introns (satellites). These show up differently in electrophoresis.
- DNA profiling can be used on animals and plants to prevent interbreeding.
- Can determine how closely related organisms are.
- Used at crime scenes to find culprit.
Describe how DNA can be amplified using PCR (polymerase chain reaction).
- You need: DNA sample, Free DNA nucleotides, Primers, Taq polymerase.
- DNA mixture heated to 95°C to break hydrogen bonds between two strands.
- Then cooled to 50-65°C so that primers can bind to strands.
- Heated to 72°C so that taq polymerase can work.
- Taq polymerase lines up free nucleotides by complementary base pairing.
- Two new copies of fragment DNA formed.
- Cycle is repeated, heated to 95°C again. (Taq does not denature at high temp.)
Describe how gel electrophoresis can be used to separate DNA fragments of different length.
Distinguish between the structure of bacteria and viruses.
- Ribosomes: 70S (smaller)
- DNA: circular, not in chromosome
- Plasmid: Contain non essential genes, can be exchanged between cells.
- Mesosome: area where respiration/photosynthesis takes place.
- Cell wall: made of peptidoglycan.
- Capsule: Thick polysaccharide layer. Used for sticking cells together, as a food reserve, as protection against desiccation (drying out) and chemicals and protection against phagocytosis.
- Lipid bilayer: Allows viral entry into host cell by endocytosis.
- Capsid: Protein coat which protects nucleic acid core (RNA or DNA).
- Extremely small.
- Can only live in host cell.
- Reverse transcriptase (HIV): an enzyme that makes a cDNA copy of the viral RNA, which is then inserted into the host’s DNA.
Describe the major routes pathogens may take when entering the body and explain the role of barriers.
- Major barriers: Skin, epithelial lining of respiratory system, stomach acid (HCl)
- Skin is made in two layers:
- a) Outer Dermis – dead, compacted cells filled with indigestible & insoluble keratin protein
- b) Inner Epidermis – site of rapid mitosis & “living” part of skin. Contains all blood vessels, glands etc.
- Barrier adaptations include:
- Sebum secreted from sebaceous glands creates acidic pH on skin (therefore harsh environment for pathogenic bacteria).
- It also contains lysozyme enzyme.
- Presence of normal flora (also called commensual organisms), which are well adapted to life on the dermis and out-compete pathogenic bacteria.
Describe the sequence of symptoms of Tuberculosis.
- TB is caused by mycobacterium tuberculosis.
- Spread by droplet infection, spreads quickly in overcrowded populations.
- If person is in good health, immune system seals off TB in tubercles.
- TB can remain dormant and become reactivated when immune system is weakened.
- When infected, bacteria destroy lung tissue, blood vessels are broken down, fluid collects.
- Causes patient to cough blood.
- Pathogen travels in bloodstream so spreads to the rest of the body.
- Loss of appetite, weight and sweating can be seen.
- If left untreated when it spreads it can cause organ failure and kill.
Describe the sequence of symptoms of HIV.
- Spread by direct contact (bodily fluids).
- HIV has specific proteins which attach to T helper cell receptors. They invade T cells and use reverse transcriptase to make copies of the virus.
- The acute phase: HIV virus rapidly infects Helper T cells.The virus population increases quickly & the population of Helper T cells falls rapidly.
- This phase ends when the immune system begins to respond to the HIV. Killer T cells begin to recognise infected Helper T cells and kill them, which slows the replication of the virus (viral population plateaus).
- Also, B cells begin to make HIV-specific antibody. The presence of this antibody in the blood can be easily tested for, which is where the term “HIV positive” comes from.
- The chronic phase: This can last for many years.
- The virus continues to replicate, but the Killer T cells keep the numbers in check.
- New Helper T cells are made continually, but their population stays low as they are continually infected by HIV and then destroyed by Killer T Cells.
- This fine balance point is affected by the person’s overall health, diet and opportunistic infections.
- The disease phase: As the numbers of virus increase and the numbers of Helper T cell fall the immune system becomes weaker and weaker.
- Eventually a second pathogen will infect the person (an opportunistic infection) which cannot be fought.
- The person will die quickly from the secondary infection.
- This is the AIDS disease state.
Describe the non-specific responses of the body to infection.
- Inflammation is the initial, rapid, localised response of tissues to damage.
- It is triggered by alarm chemicals.
- Histamine: Increases permeability of blood vessels, more WBC move into infected tissue.
- Prostaglandins: Causes vasodilation, increasing blood flow to infected area = redness.
- Lysozyme: a protease enzyme that breaks down bacterial cell walls.Found inside lysosome organelles in phagocytes, also secreted by skin, epithelial cells, lachrymal glands in eyes.
- Interferons:a hormone made by all types of WBC. Interferon has lots of functions, but the main one is to block RNA synthesis, which stops viral replication and can play a role suppressing tumour growth.
- Phagocytosis: macrophages or neurophils.
- At the end of phagocytosis, the macrophage is an antigen presenting cell (APC).
Explain the key players in the specific immune response.
- B lymphocytes: Secrete antibodies. Originate in bone marrow, mature in bone marrow.
- T lymphocytes: Assist B lymphocytes, originate in bone marrow, mature in thymus gland.
- Antibodies: Made of 4 polypeptide chains, held together by disulphide bridges.
- Highly specific due to variable region.
- Has 2 antigen binding sites.
- Constant region is the same in each antibody, can bind to phagocytes.
- MHC: (major histocompatibility complex) Are a set of surface receptors. Are unique to each cell.
Outline the process when phagocytes activate T lymphocytes.
- Antigens on APC bind to MHC on T lymph.
- This activates T lymph.
- T lymph differentiates into T helper, T memory and T killer.
- T helper activate B lymph.
- T killer release toxic chemicals which kill pathogen.
- T memory remain in blood to provide long term immunity.
Outline the process when T helper cells activate B lymphocytes.
- Antigens on pathogen bind to antibodies on B lymph.
- B lymph takes up antigens and expresses them on cell surface.
- T helper activates B lymph by binding to antigens.
- B lymph differentiates into B memory, or plasma cells
- B memory remain in blood to provide long term immunity.
- Plasma cells secrete antibodies on a large scale.
Describe how antibodies attack pathogens.
- Antibodies are specific to particular antigens.
- When they bind to antigen = antigen-antibody complex.
- Agglutination: Large antibodies can attach to several antigens, causing pathogens to group together.
- Makes it easier for phagocytes to engulf them.
- Neutralisation: Antibodies bind to antigen, preventing it from entering host cell.