Bio things that you got wrong

Why are 2 genetically different primers used during PCR?

DNA is double-stranded and anti-parallel, which means that two genetically different primers are needed to anneal to the two 3' ends of the two strands.

When the amount of glucose increases in a yeast cell, it would be expected that the amount of CO2 that is produced?

Increases. Glucose is an input of cellular respiration, whereas CO2 is an output; therefore, as more glucose becomes available, more CO2 will be produced.

What is the vector in insulin production?

In this process, the vector carries the DNA from one cell to another; in this case, it is the recombinant plasmid.

Explain how CRISPR-Cas9 technology could edit genes to change the rate of photosynthesis in plants.

• CRISPR-Cas9 can be used to edit the gene that codes for the enzyme Rubisco.

• The Rubisco protein can be edited to bind more to CO2 (or less to O2), which could increase the rate of photosynthesis. 

OR

• Genes could be edited to increase the amount of chlorophyll.

• More chlorophyll would increase the amount of sunlight captured and hence more water split to provide more H for glucose production.

Explain the differences in the adaptations that affect the process of photosynthesis between C4 and C3 plants. (3 marks)

• Sugarcane plants split the location of the Calvin cycle into two locations, whereas, in C3 plants, there is no separation.

• In sugarcane, the Calvin cycle begins in mesophyll cells and then finishes in bundle sheath cells, whereas, in C3 plants, the Calvin cycle only occurs in the mesophyll cells.

• Sugarcane plants have this adaptation to reduce the amount of photorespiration and to increase the amount of photosynthesis that occurs, which is not a concern for C3 plants.

possible application of CRISPRCas9 technology on plants.

• To increase crop yield.

• To increase the nutritional value of crops.

• To improve the appearance of fruit or vegetables to appeal to consumers.

• To change the ripening period.

• To increase the shelf life of fruit or vegetables.

• To improve drought or pest resistance.

some physical and chemical barriers and why

Physical barriers

• An intact skin barrier; to prevent the entry of pathogens after contact with a contaminated surface.

• Mucus; to trap any virus that enters through respiratory droplets.

• Cilia; to sweep the virus away from the airways (to the digestive tract).

Chemical barriers

• Acidic sweat; to destroy the virus on the skin.

• Stomach acid; to destroy the ingested/swallowed virus.

• Lysosomal enzymes in the saliva; to break down the virus (if it entered through the mouth).

importance of herd immunity (3 marks)

• Herd immunity is when an overwhelming/significant majority of a population is vaccinated,

• which leads reduced spread of measles, as there are fewer hosts carrying the virus.

• It also contributes to the protection of those that cannot be vaccinated, such as babies, the elderly or the immunocompromised.

. Identify one limitation of relative dating

• This technique only gives the age of a fossil relative to another fossil; it does not give the actual age of a fossil.

• Sometimes, rock strata shift positions, which makes it difficult to date the age of fossils accurately.

List two structural trends in the evolution of hominins from the Australopithecus genus to the Homo genus.

• An increased cranial capacity/brain size.

• A reduced arm-to-leg ratio.

• A more-central foramen magnum.

• Shorter and less-curved fingers and toes.

• A less prominent brow ridge/flatter face.

• A more-parabolic/less-rectangular jaw.

Provide reasons which explain how there can be disagreement between scientists when classifying different human species using the same fossil record. (4 marks)

• There are many gaps in the fossil record.

• As new discoveries of fossils are made, this can alter previous views or ideas that were held by scientists.

• Some fossils are incomplete or only very small fragments are found.

• There can be different interpretations of the limited evidence collected so far in the fossil record.

mitochondrial DNA (4 marks)

• Maternal inheritance – mtDNA is inherited from the mother only, providing a much more direct genetic lineage

• No recombination – As mtDNA is passed from the mother, no recombination occurs, This ensures that mtDNA sequences remain relatively stable across generations, making it easier to track mutations over time

• Higher mutation rate – Mitochondria produce reactive oxygen species, which cause sequences to mutate at a higher rate

• High copy number – As every cell has mitochondria, large amounts of mtDNA can be gathered for sequencing

• No proof-reading enzymes to correct mutations as they occur so mutations accumulate faster.

explain how the scientists would have designed the sgRNA molecule and outline the function of sgRNA in editing the gene

Scientists identify a gene that acts as a template and create a complementary sgRNA strand that binds with Cas9. sgRNA then guides Cas9 to the target gene.

Compare the process of CAM plants and C4 plants (3 marks)

• In CAM plants, initial carbon fixation is separated from the rest of the Calvin cycle over time. In C4 plants, the initial carbon fixation is separated from the rest of the Calvin cycle over space, rather than time.

• In CAM plants, during the night, the plants open their stomata to take in CO2 and use it to produce glucose during the day when their stomata are closed. C4 plants have their stomata open during the day.

• In C4 plants, initial carbon fixation occurs in a mesophyll cell and the rest occurs in bundle-sheath cells. In CAM plants, only mesophyll cells are involved.

describe what occurs during attenuation when levels of tryptophan are high (3 marks)

The attenuator sequence contains codons for two tryptophan amino acids. The ribosome arrives at this sequence, and translates these codons, adding tryptophan to the peptide chain.

This causes the mRNA molecule to fold and form a terminator hairpin loop.

This folding causes the mRNA molecule to separate from the DNA template strand and causes RNA polymerase to detach. Protein synthesis then ceases.

identify the impact on gene flow for an isolated populatin and explain how an isolated population could benefit from this process

• Since the population was initially isolated, gene flow could not occur and the population would have had a comparatively low genetic diversity.

• The benefit of introducing organisms with desirable traits is an increase in genetic diversity of the population or an increase in the biological fitness of the population as a result of the introduction of alleles for favourable traits.

what is the benefit of having 2 doses instead of 1 dose of a vaccine? (2 marks)

• The first dose leads to the activation of B cells and development of a small number of memory cells.

• The second dose increases the number of memory cells for a much faster response to subsequent infection and greater antibody production.

advantages of using mtDNA

·         there are many more copies of mtDNA in each cell

·         mtDNA has a predictable rate of mutation

·         mtDNA mutations accumulate or are not repaired

·         all changes in nucleotides are a result of mutations rather than recombination

·         mtDNA inherited from mother only allowing for a more direct line of relationships to be established.

Why is using CRISPR Cas9 more advantageous over transgenic organisms? (2 marks)

CRISPR-Cas9 edited organisms do not have a gene from another species inserted into the genome.

This reduces unknown harmful effects on people OR reduces risk of disease OR improves public perception / increases consumption.

Difference between endemic and epidemic

why fossils may not give scientists a more detailed understanding of the relatedness of homo sapiens (3 marks)

• There are limited fossils finds / the fossil record is incomplete, so some species may be missing from the record.

• Some samples may only be a small part of the organism (e.g. finger bone), making it difficult to compare species.

• Closely related Homo species may not show any great differences in physical structure so it is harder to determine their relatedness.

major trends in evolution from the genus Australopithecus to homo

·       increase in cranial size

·       reduction in size of brow ridges

·       reduction in jaw size

·       face becomes flatter

·       longer leg-to-arm ratio.

controlled variables for gel electrophoresis

·       same concentration of agarose gel

·       same person loading the DNA into the gel

·       same volume of DNA loaded into the same gel

·       DNA run in gel for the same amount of time

·       same dye used.

General role of coenzymes

·       assisting enzymes in catalysing reactions OR lowering activation energy

·       cycling between loaded and unloaded forms

·       carrying energy (e.g. ATP)

·       carrying protons/hydrogen (ions) and/or electrons (e.g. NADH, FADH₂, NADPH).

compare repression and attenuation (6 marks)

In repression, when tryptophan (trp) levels are high, two trp amino acids bind to the repressor, causing a conformational shape change in the repressor, enabling it to bind to the operator. This inhibits RNA polymerase from binding to the promoter to transcribe the structural genes.

In attenuation, when trp levels are high, RNA polymerase begins transcription of the leader or attenuator sequence. Within this sequence, there are two trp codons. As the ribosome does not stall here, a hairpin termination loop forms, resulting in the detachment of both RNA polymerase and the ribosome.

Both mechanisms occur to regulate trp levels, conserving energy in prokaryotic cells.

Difference between exposing a plant to green and blue light compared to white light

• White light is made up of different colours, e.g. green and blue light, or different wavelengths.

• Green light is reflected / not absorbed.

• Blue light is absorbed.

• Light is absorbed in photosynthesis which produces glucose and then starch.

outline 2 ongoing challenges for treatment strategies as a result of bacteria resistance (2 marks)

• not enough research or development into new drugs or vaccines

• more time or money required to develop new drugs or vaccines

• doctors overprescribing new drugs or patients not completing the full course of treatment

• epidemics or pandemics more likely, making herd immunity difficult to achieve.

Outline how human insulin is made using recombinant plasmids (5 marks)

• Restriction endonucleases are used to cut, e.g. the plasmid or human DNA.

•  DNA ligase joins the DNA sequences into the plasmid.

• The plasmid is inserted by heat shock into the bacteria which is transformed OR each plasmid is inserted into separate bacteria.

•  Antibiotic selection or another suitable method (e.g. insulin genes inserted next to a gene coding for beta-galactosidase protein) is used to determine success.

• Processing of the protein such as joining insulin polypeptide chains A and B occurs to create functional insulin.

Describe the purpose of inserting the genes for insulin production next to a gene for the production of the protein β-galactosidase.

The gene that codes for the production of the protein β-galactosidase acts as a marker gene

The gene for insulin production is inserted next to the gene for β-galactosidase so that the insulin gene can be expressed in bacteria as part of a fusion protein. This ensures that the insulin is produced efficiently and allows scientists to easily identify and purify the insulin protein from bacterial cells.

advantages of booster shot

• The advantage of the booster shots is that they enable a faster and stronger immune response.

• The first dose of the vaccine results in the production of B memory cells, which remain in the lymphatic system in case of re-infection. However, only a small number of these cells are produced. Each subsequent exposure to the antigen increases the number of B memory cells.

humoral response (4 marks)

• Phagocytosis: The immune response to an extracellular pathogen begins when antigen-presenting cells phagocytose the pathogen. 

• Presentation: These cells then present the antigen to T helper cells in the lymph node, which become activated and release cytokines and bind to stimulate B cells with complimentary binding sites on their antibody to the antigen.  

• Expansion: The selected B cells undergo clonal expansion and differentiate into plasma cells and B memory cells. 

• Antibodies: The plasma cells produce specific antibodies to the antigen (refer to the pathogen if given in question) and are released into the bloodstream, where they help identify and eliminate the pathogen. 

cell mediated response

• Presentation: Cell-mediated immunity begins when a pathogenic antigen is presented by an antigen-presenting cell and recognized by a complementary T helper cell

• Cytokines: Once selected, the T helper cell releases cytokines to activate a matching naive T cell with a complementary receptor to the antigen. (colonial selection) 

• Expansion: This T cell undergoes clonal expansion and differentiates into cytotoxic T cells and T memory cells. 

• Destroy: The cytotoxic T cells leave the lymph nodes then target infected cells by binding to infected cells that display the antigen on their MHC1 marker. They then release death ligands that trigger apoptosis.