Global challenges Revision Notes

Understanding Distribution and Abundance

Distribution describes where organisms are found in a habitat, for example, whether they are spread evenly or clustered in certain areas. Abundance quantifies how many organisms are present in a specific location or area. Field investigations aim to measure both of these aspects to understand ecological patterns. For example, knowing the distribution of a plant species might reveal its preferred environmental conditions, while its abundance might indicate its success in that environment. These measurements are vital for monitoring environmental changes and assessing biodiversity.

Sampling Techniques: Quadrats and Transects

To investigate the distribution and abundance of organisms, especially plants and slow-moving animals, sampling techniques are used. A common tool is a quadrat, which is a square frame of a known area (e.g., 0.25 m² or 1 m²). To ensure unbiased data, quadrats should be placed randomly. This involves using a random number generator to select coordinates within the study area. For studying how distribution and abundance change across an environmental gradient (e.g., from dry to wet soil), a transect line is used. Quadrats are placed at regular intervals along this line, and data is recorded for each quadrat. This systematic sampling allows for the observation of patterns related to the environmental change.

Estimating Population Size and Density

Once data is collected from quadrats, it can be used to estimate the total population size and density of organisms in the larger habitat. To calculate abundance, the number of individuals of a species within each quadrat is counted. The density can be found by dividing the count by the quadrat's area. To estimate the total population, the average number of organisms per quadrat is calculated, then scaled up to the total area of the habitat. For example, if the average count in a 1 m² quadrat is 10 plants, and the habitat is 100 m², the estimated population is 10 x 100 = 1000 plants. It's important to collect a sufficient number of samples to ensure the estimate is reliable.

Methodology and Improving Reliability

A typical method involves: 1. Defining the study area. 2. Using random number generation to place quadrats to minimise bias (or placing them systematically along a transect). 3. Counting the number of chosen organisms within each quadrat. 4. Repeating this process many times (e.g., 10-20 samples) across the habitat. To improve the reliability of results, a large number of samples should be taken. Anomalous results should be re-checked or excluded if clearly erroneous. Calculating a mean from multiple samples helps to smooth out variations. Safety precautions are essential; identify potential hazards (e.g., stinging plants, slippery surfaces, deep water) and wear appropriate clothing and footwear. Always work in groups and inform someone of your location and expected return time.

What are Monoclonal Antibodies? (Higher)

Monoclonal antibodies are identical antibodies that are produced from a single clone of B-lymphocytes. This means all the antibodies produced by these cells are specific to one particular antigen. Their specificity makes them valuable tools in medicine and research, especially for targeting specific cells, such as those that are cancerous or infected.

Stimulating Antibody Production in a Host Animal (Higher)

The first step in producing monoclonal antibodies involves injecting a specific antigen into a host animal, typically a mouse. This antigen could be a protein from a pathogen or a marker found on cancer cells. The mouse's immune system responds to this injection by producing B-lymphocytes, which are white blood cells that generate antibodies specific to that antigen. These B-lymphocytes are then harvested from the animal, usually from its spleen, as this organ is rich in immune cells.

Creating Hybridoma Cells (Higher)

B-lymphocytes from the mouse are unable to divide indefinitely in a laboratory culture. To overcome this limitation, they are fused with myeloma cells. Myeloma cells are a type of cancer cell that multiplies rapidly and continuously. The fusion of a B-lymphocyte and a myeloma cell produces a hybrid cell called a hybridoma. These hybridoma cells inherit the ability to produce specific antibodies from the B-lymphocyte and the capacity for endless division from the myeloma cell. This combination is crucial for large-scale production.

Selection, Culturing, and Applications (Higher)

After fusion, the mixed cell populations are grown in conditions that select for hybridoma cells, eliminating unfused B-lymphocytes and myeloma cells. The successful hybridoma cells are then screened to identify and isolate those that are producing the desired monoclonal antibody with high specificity. Once identified, these chosen hybridoma cells are cultured in large quantities to produce vast amounts of identical monoclonal antibodies. These antibodies are then purified and can be used for various applications, including diagnosing diseases, locating specific substances in the body (e.g., in pregnancy tests), and treating diseases like cancer by delivering drugs directly to tumour cells.

1. Isolation of the Desired Gene

The first critical step in genetic engineering is to pinpoint and obtain the specific gene that codes for the desired characteristic. This gene is typically found within the donor organism's chromosomes. Specialist enzymes called restriction enzymes are used for this. These enzymes recognise and cut specific sequences of DNA, often leaving 'sticky ends' – short, single-stranded overhangs that are complementary to each other. These sticky ends are crucial for the next stage of the process because they allow the desired gene to bind effectively with the vector DNA.

2. Preparation of the Vector and Insertion of the Gene (Higher)

Once the desired gene is isolated, a 'vector' is needed to carry it into the host cell. Plasmids, which are small, circular DNA molecules found in bacteria, are commonly used as vectors. A plasmid is cut open using the *same* restriction enzyme that was used to isolate the desired gene. This ensures that the sticky ends of the plasmid are complementary to the sticky ends of the desired gene. The isolated gene is then mixed with the opened plasmid, and an enzyme called DNA ligase is used. DNA ligase forms strong phosphodiester bonds, permanently joining the desired gene into the plasmid to create 'recombinant DNA'.

3. Introduction into the Host Cell and Selection (Higher)

The recombinant DNA (plasmid containing the desired gene) is then introduced into a suitable host cell, typically a bacterium, in a process called transformation. Not all host cells will successfully take up the recombinant plasmid. Therefore, a selection mechanism is necessary to identify which host cells have been transformed. This often involves using 'marker genes' present on the plasmid, such as genes for antibiotic resistance. Host cells are grown on a medium containing the antibiotic; only those that have taken up the plasmid and consequently gained antibiotic resistance will survive and multiply. This allows scientists to select and culture only the genetically modified organisms.

4. Growth and Expression (Higher)

The selected host cells, now containing the recombinant DNA, are cultured under optimal conditions. As these cells multiply, they will replicate the recombinant DNA along with their own genetic material. If the aim is to produce a specific protein (e.g., insulin), the host cells will also express the newly introduced gene, synthesising the desired protein. This allows for the large-scale production of the desired gene product or for the modification of the host organism to exhibit the new trait. The entire process requires careful control and monitoring at each stage to ensure successful genetic modification.

Stage 1: Drug Discovery and Pre-clinical Testing

The initial stage involves identifying a disease target, followed by screening for potential chemical compounds (lead compounds) that could interact with this target. These compounds are extensively tested in laboratories using cells and tissues (in vitro) and then in animals (in vivo). Pre-clinical testing aims to determine the compound's toxicity, dosage, and initial effectiveness. This step is crucial for weeding out compounds that are unsafe or ineffective before they are given to humans. Researchers look for adverse effects and how the compound is metabolised and excreted by the body. Many potential drugs fail at this early stage due to lack of efficacy or unacceptable toxicity.

Stage 2: Clinical Trials – Testing on Humans

After successful pre-clinical testing, potential medicines proceed to clinical trials, which occur in three main phases. Phase 1 involves a small number of healthy volunteers (e.g., 20-100) to primarily assess safety, determine a safe dosage range, and study how the drug behaves in the human body. Phase 2 involves a larger group of patients with the specific disease (e.g., 100-300) to evaluate its effectiveness and further monitor for side effects. Phase 3 involves an even larger group of patients (e.g., hundreds to thousands) and compares the new medicine to existing treatments or a placebo, confirming effectiveness, monitoring side effects, and collecting data on its overall benefit-risk profile. These trials are often double-blind, meaning neither the patients nor the researchers know who is receiving the new medicine or the placebo, to reduce bias.

Stage 3: Regulatory Approval and Post-Market Monitoring

If a medicine successfully completes all three phases of clinical trials, demonstrating both efficacy and safety, the manufacturer can apply for regulatory approval from authorities like the Medicines and Healthcare products Regulatory Agency (MHRA) in the UK. These agencies review all the collected data to decide whether to license the medicine for public use. Even after approval, the medicine enters Phase 4 trials, also known as post-market surveillance. This involves ongoing monitoring of the medicine's effects in the general population over many years to detect any rare or long-term side effects that might not have appeared in the earlier, smaller trials. This continuous monitoring ensures the medicine remains safe and effective for patients.