Question 1

How does the lac operon work in E. coli?

Accepted Answer

The lac operon is a set of genes in E. coli that code for enzymes needed to metabolise lactose. It is regulated by a repressor protein that binds to the operator region, blocking transcription. When lactose is present, it is converted to allolactose, which binds to the repressor and changes its shape, causing it to detach. However, if glucose is also present, the operon remains off due to catabolite repression: low cAMP levels mean the CAP protein cannot bind to activate transcription. So, the operon is fully induced only when lactose is present and glucose is absent.

Question 2

What is the difference between siRNA and miRNA?

Accepted Answer

Both siRNA (small interfering RNA) and miRNA (microRNA) are small non-coding RNAs that regulate gene expression post-transcriptionally. siRNA is typically derived from exogenous double-stranded RNA (e.g., from viruses) and leads to cleavage of complementary mRNA, resulting in gene silencing. miRNA is usually encoded by the cell's own genome and binds to partially complementary sequences in target mRNAs, often inhibiting translation or causing mRNA degradation. Both use the RISC complex, but siRNAs are more specific and cause complete degradation, while miRNAs fine-tune expression.

Question 3

How do cells communicate with each other?

Accepted Answer

Cells communicate via signalling molecules such as hormones, neurotransmitters, or growth factors. These molecules bind to specific receptors on the target cell's surface (or inside for steroid hormones). This binding triggers a signal transduction pathway, often involving second messengers like cAMP or calcium ions, which amplify the signal. The signal eventually leads to a cellular response, such as changes in gene expression, enzyme activity, or cell division. Examples include adrenaline binding to β-adrenergic receptors, activating G-proteins and adenylyl cyclase, increasing cAMP, and activating protein kinase A.

Question 4

What is apoptosis and why is it important?

Accepted Answer

Apoptosis is programmed cell death, a controlled process that eliminates unwanted or damaged cells without causing inflammation. It is crucial for development (e.g., removing webbing between fingers), maintaining tissue homeostasis, and eliminating cells with DNA damage. Apoptosis is triggered by internal signals (e.g., DNA damage) or external signals (e.g., death ligands binding to receptors). Key players include caspases, which are proteases that dismantle the cell, and the Bcl-2 family, which regulates mitochondrial outer membrane permeabilisation. Dysregulation of apoptosis can lead to cancer (too little) or neurodegenerative diseases (too much).

Question 5

How do transcription factors control gene expression?

Accepted Answer

Transcription factors are proteins that bind to specific DNA sequences (e.g., enhancers, promoters) to regulate transcription. Activators bind to enhancers and recruit RNA polymerase and co-activators, increasing gene expression. Repressors bind to silencers and block transcription by preventing RNA polymerase binding or recruiting co-repressors. In eukaryotes, transcription factors often work in complexes and can be activated by signalling pathways (e.g., phosphorylation). For example, the transcription factor p53 is activated by DNA damage and promotes expression of genes involved in cell cycle arrest or apoptosis.

Question 6

What is alternative splicing and why is it important?

Accepted Answer

Alternative splicing is a process where a single pre-mRNA can be spliced in different ways to produce multiple mature mRNA variants, each encoding a different protein isoform. This allows a single gene to generate a diverse range of proteins with different functions. For example, the DSCAM gene in fruit flies can produce over 38,000 isoforms through alternative splicing. This mechanism increases proteomic diversity without requiring additional genes and is important for processes like cell differentiation, immune response, and neuronal wiring. Errors in splicing can lead to diseases such as spinal muscular atrophy.

Cellular control

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

Before You Start

Likely Command Words

Ready to test yourself?

Related Topics in OCR A-Level Biology

Analysis

Biodiversity

Biological membranes

Biological molecules

How to Revise Cellular control — OCR A-Level Biology

Examiner Tips for Cellular control

Common Mistakes in Cellular control

Key Marking Points