A liver cell and a neuron both derived from the same embryo carry identical DNA sequences, yet they produce very different sets of proteins. What best explains this?
ASomatic mutations during development altered the DNA sequence in each cell type
BOnly a regulated subset of genes are transcribed in any given cell type, determined by the cell's developmental context
CNeurons use a different genetic code than liver cells, so the same DNA produces different amino acids
DPost-translational modifications change proteins so extensively that the underlying DNA no longer matters
Same DNA does not mean same proteins — this is the key insight of regulated gene expression. Cells control which genes are transcribed, how pre-mRNAs are spliced, which mRNAs are stabilized, and how efficiently they are translated. A liver cell and a neuron express radically different gene sets despite identical DNA — this is what allows ~200 distinct cell types to arise from one genome. Option A is wrong: somatic mutations occur but are not the primary mechanism of differentiation. Option C is wrong: all human cells use the same genetic code.
Question 2 Multiple Choice
How can roughly 20,000 human protein-coding genes generate over 100,000 distinct protein isoforms?
AEach gene is copied multiple times in different chromosomal locations, creating natural variation
BPost-translational modifications add chemical groups that create proteins with new amino acid sequences
CAlternative splicing of pre-mRNA allows a single gene to produce multiple distinct mRNA variants, each encoding a different protein isoform
DReverse transcription occasionally creates gene duplicates that diverge within the same organism
Alternative splicing is the primary mechanism. The pre-mRNA contains all exons, but the spliceosome can include or exclude different exons in the mature mRNA, producing multiple transcripts from one gene. Each splice variant encodes a different isoform. Option B is wrong: post-translational modifications add chemical groups but do not change the amino acid sequence and are not the source of the 100,000+ isoform count.
Question 3 True / False
Post-translational modifications like phosphorylation can alter a protein's activity or subcellular location without changing its amino acid sequence.
TTrue
FFalse
Answer: True
True. PTMs such as phosphorylation, glycosylation, ubiquitination, and acetylation add or remove chemical groups after the polypeptide is assembled. These changes alter the protein's charge, conformation, binding partners, stability, or targeting — functioning as molecular switches — without touching the underlying amino acid sequence. This layer of regulation allows rapid, reversible responses to signals without requiring new rounds of transcription and translation.
Question 4 True / False
The central dogma of molecular biology states that information cannot flow from RNA to DNA under any circumstances.
TTrue
FFalse
Answer: False
False. While the central dogma describes the normal direction of information flow (DNA → RNA → Protein), exceptions exist. Retroviruses like HIV use reverse transcriptase to copy RNA back into DNA, which is then integrated into the host genome. The central dogma is a powerful organizing principle but not an absolute law — it describes the default pathway. Treating it as inviolable is explicitly listed as a common misconception for this topic.
Question 5 Short Answer
How does the regulated pipeline model of gene expression explain why a liver cell and a neuron — with identical DNA — function so differently?
Think about your answer, then reveal below.
Model answer: Gene expression is controlled at multiple levels: which genes are transcribed (transcriptional regulation by transcription factors), how pre-mRNA is spliced (alternative splicing), which mRNAs are stabilized or degraded (post-transcriptional regulation), how efficiently mRNAs are translated (translational regulation), and how the resulting proteins are modified and targeted (post-translational modifications). The cell's developmental context determines which regulatory factors are active, so the same DNA template produces very different outputs in different cell types.
This multi-level regulation is what makes differentiation possible and what makes gene expression a pipeline rather than an automatic readout. Understanding a cell requires more than knowing its genome — you need to know which genes are expressed, how transcripts are processed, and how proteins are modified and localized. The liver cell and neuron are the same genome running very different programs.