Unit 3: How Do Cells Maintain Life?
Molecular biology of the cell
Unit 3 focuses on the molecules and processes that keep cells functioning. Know the structure and roles of DNA, RNA and proteins, and how the genetic code links them. You must describe transcription in the nucleus, RNA processing and translation at the ribosome, including the roles of mRNA, tRNA and rRNA, codons and anticodons, and the concept of a gene as a sequence that produces a functional product. Understand the structure of DNA (double helix, complementary base pairing, antiparallel strands, hydrogen bonds) and how the sequence determines protein structure and therefore function.
Regulation and energy
Explain how gene expression is regulated, and how cells obtain and use energy through cellular respiration, including glycolysis, the Krebs cycle and the electron transport chain, with ATP yields as approximate. Understand photosynthesis as light-dependent and light-independent stages. Relate membrane transport, enzymes and biochemical pathways to cellular function.
Traps examiners set
- Confusing transcription (DNA to RNA, nucleus) with translation (mRNA to protein, ribosome).
- Mixing up template and coding strands, or codon and anticodon.
- Saying tRNA carries amino acids to the DNA, or that proteins are made in the nucleus.
- Forgetting that the genetic code is degenerate and universal, and that mutations may be silent.
- Assuming anaerobic respiration produces as much ATP as aerobic respiration; it does not.
Worked example and rule of thumb
A DNA template triplet TAC corresponds to mRNA codon AUG, which codes for methionine and acts as a start codon. Rule of thumb: transcribe from DNA to mRNA using complementary bases (A pairs with U in RNA), then read mRNA codons three bases at a time to find amino acids.
Experimental design
Evaluate experiments on cellular processes by identifying variables, controls, and whether conclusions are supported by the evidence presented.