StemUp: OCR A A level Biology 6.1.1 Cellular control

What are the three main types of gene mutations? (3)

- Insertion: One or more bases are added

- Deletion: One or more bases are removed

- Substitution: One or more bases are swapped for another

How can gene mutations occur, and what can increase the mutation rate? (2)

- Gene mutations can occur spontaneously during DNA replication

- Mutagenic agents such as UV radiation, ionising radiation, certain chemicals, and viruses can increase the mutation rate

What is a neutral (silent) mutation, and how does it affect protein production? (3)

- Silent mutations occur when a change in the DNA base sequence doesn't alter the amino acid sequence

- Due to the degenerate nature of the genetic code

- The function of the protein is not affected

How do frameshift mutations affect protein production? (3)

- Frameshift mutations occur when a base is added or removed

- Causing a shift in the reading frame of the genetic code

- This alters the order of amino acids and may result in a non-functional protein

What are the effects of harmful mutations and give an example? (4)

- Result in non-functional proteins or prevent protein synthesis

- Negatively affecting the phenotype

- Cystic fibrosis is caused by a deletion in the CFTR gene

- Leading to a misfolded protein that affects multiple systems in the body

What is a beneficial mutation, and how does it affect an organism? (3)

- A mutation that results in a new and useful characteristic in the phenotype

- It has an advantageous effect, increasing the organism's chance of survival

- Example: A mutation in bacteria that allows an enzyme to break down a wider range of antibiotics, increasing antibiotic resistance

How can a mutation prevent transcription? (2)

- If a mutation occurs at the start of a gene, it can prevent RNA polymerase from binding to the DNA

- This prevents transcription, and the protein coded for by the gene won't be synthesised

How can a mutation result in a non-functional enzyme? (8)

- Change in base sequence

- Change in triplet code

- Change in amino acid sequence

- Change in primary structure

- Alteration of hydrogen, ionic, or disulfide bonds

- Change in tertiary structure

- Active site changes shape, and the substrate is no longer complementary

- The enzyme can no longer form an enzyme-substrate complex

Why don't all mutations cause a change in the protein? (3)

- A mutation may involve substitution of one base in the mRNA

- The new codon may be different, but due to the degenerate nature of the genetic code, it may still code for the same amino acid

- The primary sequence remains unchanged, so the final polypeptide is unaffected

Why is a deletion mutation more likely to result in a non-functional protein than a substitution mutation? (3)

- Deletion causes a frame shift, affecting all base triplets downstream of the deletion

- This changes most of the amino acids after the deletion, significantly altering the polypeptide structure

- Substitution only affects one base and one codon, so the effect on the polypeptide is less severe

Why are errors in DNA replication more dangerous than errors in transcription? (2)

- Errors in DNA replication are inherited and will have a permanent effect on the organism and its daughter cells

- Errors in transcription are temporary and only affect specific cells, making them less damaging and not inherited

What is the role of transcription factors in gene expression? (3)

- Proteins that bind to DNA to activate or deactivate genes

- They control gene expression by increasing or decreasing the rate of transcription

- Activators increase the rate of transcription, while repressors decrease it

What is an operon, and how does it regulate gene expression in prokaryotes? (3)

- An operon is a section of DNA containing structural genes, control elements (promoter, operator), and a regulatory gene

- The promoter is where RNA polymerase binds, and the operator is where transcription factors bind to control transcription

- The regulatory gene codes for an activator or repressor to regulate transcription

What happens to the lac operon in E.coli in the absence of lactose? (4)

1. The regulatory gene (lacI) produces the lac repressor

2. The repressor binds to the operator site

3. This blocks transcription by preventing RNA polymerase from binding to the promoter

4. The genes lacZ, lacY, and lacA are not transcribed

What happens to the lac operon in E.coli in the presence of lactose? (4)

1. Lactose binds to the lac repressor

2. This changes the shape of the repressor, preventing it from binding to the operator site

3. RNA polymerase is free to bind to the promoter and transcribe the structural genes

4. The genes lacZ, lacY, and lacA are transcribed, producing enzymes to digest lactose

What happens to pre-mRNA during post-transcriptional control? (3)

- Introns (non-coding sequences) are removed from pre-mRNA through splicing

- Exons (coding sequences) are joined together to form mature mRNA

- This mature mRNA leaves the nucleus and is used in translation to form proteins

How are proteins activated during post-translational control? (4)

- Some proteins need to be activated after synthesis

- Molecules like hormones and sugars trigger the production of cyclic AMP (cAMP) inside cells

- cAMP alters the protein's 3D structure, making it functional (e.g., changes an enzyme's active site)

- Example: PKA is activated when cAMP binds, releasing its active subunits

What is the role of Hox genes in the development of body plans? (3)

- Hox (homeobox) genes control the development of an organism's body plan

- They contain homeobox sequences, which code for a protein region called the homeodomain

- The homeodomain binds to specific sites on DNA, acting as a transcription factor to regulate the expression of developmental genes

Why have homeobox sequences remained highly conserved across species? (3)

- Homeobox sequences are vital for body plan development

- Mutations in these genes could drastically alter the body plan and affect many other genes

- Such mutations are likely to be lethal, preventing them from being passed on to future generations

What is apoptosis? (3)

- Programmed cell death

- An ordered and controlled process where a cell breaks down through a series of biochemical events leading to its death

- It is distinct from necrosis, which results from damage and uncontrolled cell death

What are the key steps involved in apoptosis? (3)

1. Enzymes inside the cell break down components like proteins and DNA

2. The cell shrinks and breaks into fragments

3. The fragments are engulfed and digested by phagocytes

How do mitosis and apoptosis control body development? (3)

- Mitosis produces genetically identical daughter cells, creating the bulk of body parts

- Apoptosis refines body structures by removing unwanted cells, e.g., separation of fingers and toes in humans

- Both processes are regulated by genes to ensure proper body form development.

What stimuli regulate mitosis and apoptosis during body development? (4)

- Internal stimuli

- E.g., DNA damage can trigger apoptosis or pause mitosis for repairs

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- External stimuli

- E.g., stress or nutrient availability can stop mitosis, while pathogens may trigger apoptosis