7 key practice questions to ace your A Level Biology exam

Hey, Year 13s! Want to ace your A-Level Biology exams? Then, you need to master proteins! This post breaks down 7 challenging exam-style questions on proteins, enzymes, and cell division. Work through these, and you'll be well on your way to exam success. Let's get started!

1. Describe how calcium ions cause the myofibril to start contracting - 3 Marks

Muscle contraction revolves around myofibrils, which are the functional units of muscle fibres. Here’s how calcium ions fit into the process:

1. Binding of Calcium Ions to Troponin:
   When a nerve impulse reaches the muscle, calcium ions (Ca²⁺) are released from the sarcoplasmic reticulum. These ions then bind to a protein called troponin, which is located on the thin actin filaments. Troponin is connected to tropomyosin, a filament that wraps around actin.

2. Troponin Shape Change and Movement of Tropomyosin:
   Once calcium ions bind to troponin, the troponin changes shape. This shape change pulls on the tropomyosin, causing it to move aside. Normally, tropomyosin blocks the myosin-binding sites on actin, so when it shifts, these binding sites become exposed.

3. Myosin Binds to Actin:
   With the binding sites on actin exposed, the myosin heads (which are part of the thick myosin filaments) can now bind to actin. This interaction is key to muscle contraction. The myosin heads pull on the actin, shortening the sarcomere, and this is what causes the muscle to contract.

2. Describe how a single base substitution mutation could lead to the formation of a non-functional enzyme - 4 Marks

In Biology, it’s important to understand how changes at the DNA level can affect proteins, particularly enzymes. Here’s how a single base substitution mutation can result in a non-functioning enzyme.

1. Single Base Substitution and Amino Acid Change
   A single-base substitution mutation means that one base (A, T, C, or G) in the DNA sequence is replaced by another. This mutation can cause a change in the triplet code that corresponds to a different amino acid during protein synthesis.

2. Effect on Primary Structure
   Proteins are made up of a specific sequence of amino acids, which is called the primary structure. If a mutation causes a different amino acid to be incorporated into the chain, it changes this sequence. Remember, the order of amino acids determines how the protein will fold and what function it will have.

3. Alteration of Bonds and Tertiary Structure
   The sequence of amino acids dictates how hydrogen bonds, ionic bonds, and disulfide bonds form. These bonds are crucial because they help fold the protein into its tertiary structure, which is the protein's 3D shape. A change in the amino acid sequence means these bonds form in different places, altering the final shape of the enzyme.

4. Impact on Active Site and Functionality
   Enzymes function through their active sites, where substrates bind to form enzyme-substrate complexes. If the enzyme's tertiary structure is altered due to the mutation, the shape of the active site changes. This prevents the enzyme from binding to its substrate because the active site is no longer complementary to it. As a result, enzyme-substrate complexes cannot form, rendering the enzyme non-functional.

3. Key differences between meiosis and mitosis in cell division - 3 Marks

In A-level Biology, you may be asked to distinguish between the two types of cell division: mitosis and meiosis. Here are three main differences:

1. Number of Divisions

   Meiosis involves two rounds of division, while mitosis only goes through one division.

   • As a result, meiosis produces four daughter cells, whereas mitosis results in just two daughter cells.
     This distinction is important because the extra division in meiosis creates more variation in the cells produced.

2. Genetic Differences

   Meiosis involves two rounds of division, while mitosis only goes through one division.

   • As a result, meiosis produces four daughter cells, whereas mitosis results in just two daughter cells.
     This distinction is important because the extra division in meiosis creates more variation in the cells produced.

3. Chromosome Behaviour

   During meiosis, homologous pairs of chromosomes come together, forming pairs called bivalents, and are then separated.

   • This does not happen in mitosis, where homologous pairs do not associate and are not separated. In mitosis, the chromosomes line up individually rather than in pairs.

Bonus Point: Crossing Over

In meiosis, crossing over can occur, which means portions of chromatids are exchanged between homologous chromosomes, introducing even more genetic diversity. This process does not happen in mitosis.

4. Describe the role of ATP in translation - 2 Marks

In the process of translation, ATP plays a crucial role in protein synthesis. Here are the two key points you need to remember:

1. Energy Release
   ATP releases energy when the bond between the second and third phosphate groups is hydrolysed. This energy is essential for the various reactions involved in translation, such as forming peptide bonds.

2. Amino Acid Attachment
   ATP is required to attach amino acids to transfer RNA (tRNA). Each tRNA molecule carries a specific amino acid to the ribosome, and energy from ATP is needed for this attachment at the amino acid binding site.

This energy is also used when joining amino acids together to form peptide bonds, resulting in a polypeptide chain.

5. How is the process of mRNA production in eukaryotic vs. prokaryotic cells different? - 2 Marks

The production of mRNA varies significantly between eukaryotic and prokaryotic cells due to the presence of introns in eukaryotic DNA. Here are the key differences:

1. Initial mRNA Form

• In eukaryotic cells, the initial mRNA produced is called pre-mRNA or primary mRNA, which includes introns (non-coding regions).

• In prokaryotic cells, mRNA is produced directly from DNA without any introns, so it is not referred to as pre-mRNA.

2. Introns and Splicing

• In eukaryotes, introns must be removed from pre-mRNA through a process called splicing. This results in the formation of mature mRNA.

• In prokaryotes, since there are no introns in the DNA, splicing does not occur, and the mRNA is immediately ready for translation.

Summary:

• Eukaryotic mRNA production involves the formation of pre-mRNA, which undergoes splicing to remove introns, resulting in mature mRNA.

• Prokaryotic mRNA is produced directly from DNA, containing no introns and requiring no splicing.

6. Explain how the use of antibiotics has led to antibiotic-resistant bacteria - 4 Marks

This question revolves around the concept of natural selection and how antibiotics create a selection pressure that influences bacterial populations. Here’s a breakdown of the key points:

1. Selection Pressure

• The use of antibiotics acts as a selection pressure on bacterial populations. This means that antibiotics exert a force that affects which bacteria survive and reproduce.

2. Survival of Resistant Bacteria

• Bacteria that possess antibiotic resistance have a survival advantage in the presence of antibiotics. This means that resistant bacteria are more likely to survive when exposed to these drugs, while non-resistant bacteria are killed off.

3. Reproduction of Resistant Bacteria

• The surviving resistant bacteria reproduce, passing on the allele for resistance to their offspring. This ensures that the trait for resistance continues in subsequent generations.

4. Increase in Allele Frequency

• Over time, the frequency of the allele that confers antibiotic resistance will increase in the bacterial population. As the resistant bacteria thrive, the proportion of resistant strains becomes higher, leading to a population predominantly composed of antibiotic-resistant bacteria.

Important Note:

• Mutations and Antibiotics: It's crucial to clarify that antibiotics do not cause mutations. Mutations occur randomly, and those that confer resistance can give certain bacteria an advantage when antibiotics are used. This distinction is important to avoid misunderstandings about the nature of bacterial resistance.

7. Gas exchange in insects: Understanding spiracles and tracheae - 5 Marks

1. Name the structure through which gases will enter and leave an insect. (1 mark)
The structures through which gases enter and leave an insect are called spiracles. These are small openings located along the abdomen of the insect.

2. Name the tubes that carry the gases directly to the cells of the insect.  (1 mark)
The tubes that carry gases directly to the body cells of the insect are called tracheae. The tracheae branch into smaller tubes known as tracheoles, which facilitate the delivery of gases to individual cells.

3. Explain the movement of oxygen into body cells when an insect is at rest.  (3 mark)
When the insect is at rest, it utilises oxygen for aerobic respiration in the body cells. This consumption lowers the concentration of oxygen in the cells, creating a diffusion gradient. As a result, oxygen diffuses from areas of higher concentration (the tracheoles) to areas of lower concentration (the body cells). This movement occurs through simple diffusion, where oxygen naturally moves down the concentration gradient into the cells.

Conclusion

By understanding these 7 exam questions, you’ll not only be preparing for your A-Level Biology exams but also gaining insights into essential biological concepts. Keep practising, stay curious, and don’t hesitate to ask for help when needed. Together, we can tackle any challenge that comes our way!