Transcription & RNA Processing

Transcription for the MCAT

Part 1 of 4 — From Gene to RNA: Polymerases, Promoters & Strands

The Two Strands Problem (Most-Missed Vocabulary on the Exam)

For any gene, only ONE strand is copied:

• Template (antisense) strand: the strand RNA polymerase actually reads, 3' to 5'.
• Coding (sense) strand: the OTHER strand — identical in sequence to the mRNA (with T in place of U). Genes are written out as their coding strand.

So: mRNA = coding strand with U substituted; mRNA = reverse complement of the template strand. Different genes on one chromosome can use different strands as template — "the" template strand is defined per gene, not per chromosome.

RNA Polymerase vs DNA Polymerase

No primer requirement is why primase (an RNA polymerase) can start replication — and low proofreading is tolerable because a flawed transcript is disposable while a mutation is forever.

Finding the Start: Promoters

• Prokaryotes: one RNA polymerase; its sigma factor subunit recognizes the promoter, then releases after initiation.

• Sigma reads two promoter elements, at -10 (TATAAT, the Pribnow box) and -35. Alternative sigmas re-point the polymerase at new gene sets (heat shock).

• Eukaryotes: three polymerases — Pol I (rRNA, in the nucleolus), Pol II (mRNA + many regulatory RNAs; the one all mRNA questions mean), Pol III (tRNA, 5S rRNA). Pol II cannot find promoters alone: general transcription factors bind the promoter's TATA box (~-25) and recruit it, and enhancer-bound activators (Part 3) tune its output.

• The TATA-binding protein (TBP) is a subunit of the factor TFIID; the assembled factors plus Pol II form the preinitiation complex.

• Numbering convention: +1 = transcription start site; negative numbers = upstream. The promoter is upstream, on the same molecule, and is NOT transcribed into the mRNA.

• Termination: prokaryotes use rho-independent hairpins (GC-rich stem-loop + poly-U slip) or the rho helicase; eukaryotic Pol II termination is coupled to cleavage at the poly-A signal (Part 2).

Amanitin & Rifampin — Inhibitors That Double as Tools

• Alpha-amanitin (death cap mushroom): potent inhibitor of eukaryotic Pol II — mRNA synthesis dies first; hepatotoxicity follows.
• Rifampin: blocks initiation by bacterial RNA polymerase — selective toxicity, and a passage favorite for "which process stops first" kinetics.

• Actinomycin D intercalates into DNA, blocking transcription in ALL cells — a lab tool for "freeze transcription and watch mRNA decay."

Worked Example — Strand Bookkeeping Under Pressure

Passage-style problem. A gene's template strand contains the sequence 3'-TACGGATCT-5' spanning the transcription start region.

Question 1 — Write the mRNA. Pair each template base with its RNA complement, building 5' to 3' as polymerase does: T pairs A, A pairs U, C pairs G, G pairs C, G pairs C, A pairs U, T pairs A, C pairs G, T pairs A. Result: 5'-AUGCCUAGA-3'. Note it begins with AUG — a start codon, which is how the exam signals you oriented correctly.

Question 2 — Write the coding strand. Same sequence as the mRNA with T for U: 5'-ATGCCTAGA-3'. No pairing arithmetic needed — this shortcut (coding strand = mRNA with T) saves time and errors. Conversely, if a passage GIVES you "the gene sequence," it is giving the coding strand, and you can read codons straight off it.

Question 3 — A point mutation changes the template strand's third base (C, reading from its 3' end) to A. What happens to the mRNA? The third mRNA base was G (paired to that C); now it pairs with template A, becoming U. The mRNA's first codon changes from AUG to AUU — the start codon is destroyed. Translation initiation fails at this site entirely (scanning may find a downstream AUG, yielding a truncated or garbage protein). A one-base change with catastrophic effect — position matters more than count.

Question 4 — The trap to anticipate. If the question had instead said the CODING strand's third base (G) changed to T, the mRNA would change identically at that position, G to U (coding strand mirrors mRNA) — no complementation step. Misapplying the complement to the coding strand is the single most common error on strand questions. Always annotate the given strand as template or coding BEFORE touching the bases.

Polymerases, Promoters & Strands 🎯

Key Takeaways — Part 1

• Template strand: read 3' to 5' by polymerase; coding strand: equals mRNA with T for U — label the given strand before any base arithmetic, and remember templates are per gene, not per chromosome
• RNA polymerase: 5' to 3' synthesis, no primer (starts de novo), minimal proofreading — acceptable because transcripts are disposable
• Prokaryotes: one polymerase + a sigma factor that finds the promoter
• Eukaryotes: Pol I = rRNA (nucleolus), Pol II = mRNA (TATA box + general transcription factors), Pol III = tRNA and 5S
• Promoters are upstream, untranscribed, cis-acting: mutations change amount, not protein sequence, and affect only the adjacent gene
• Inhibitor toolkit: alpha-amanitin (euk Pol II), rifampin (bacterial RNA pol — selective toxicity)
• After a transcription block, each protein decays on its mRNA's half-life — expect kinetics questions built on that lag

• Low-yield extras: sigma reads the -10 (TATAAT) and -35 elements, and alternative sigmas redirect whole gene programs; termination is by hairpin or rho in bacteria, and by poly-A cleavage for Pol II; TBP (in TFIID) binds the TATA box and the factors assemble a preinitiation complex; actinomycin D intercalates and blocks all transcription

Transcription for the MCAT

Part 2 of 4 — Eukaryotic mRNA Processing

Three Modifications, All Before Export

The primary transcript (pre-mRNA, hnRNA) is processed co-transcriptionally inside the nucleus; only the finished product leaves through nuclear pores.

1. 5' cap: a 7-methylguanosine added backwards — via an unusual 5'-to-5' triphosphate linkage — almost as soon as the transcript emerges. Jobs: protects against 5' exonucleases, and is the docking site for translation initiation (ribosome scanning starts here).

• The cap-binding initiation factor is eIF4E.

1. 3' poly-A tail: after the transcript is cleaved, poly-A polymerase adds ~200 adenosines without any template. Jobs: stability (tail length is a decay timer), export, and translation efficiency.

• Cleavage is signalled by the sequence AAUAAA, ~10-30 nt upstream of the cut. Histone mRNAs are the famous exception: they have no tail.

1. Splicing: introns are removed and exons are joined. The joined exons leave the nucleus as mRNA; the introns are degraded.

Splicing Mechanics

• Carried out by the spliceosome: snRNPs ("snurps," small nuclear RNA + protein) assembled on the intron.

• The spliceosome's snRNPs are U1, U2, U4, U5 and U6 (U4 and U6 work as a pair).

• Landmarks: intron begins with GU (5' splice/donor site), ends with AG (3' splice/acceptor site), with a branch-point A upstream of the 3' end.
• The intron is cut out as a looped lariat, and the two exons are joined.

• The chemistry is two transesterifications: the branch A's 2'-OH attacks the 5' splice site, forming the lariat; the freed exon's 3'-OH then attacks the 3' splice site, joining the exons and releasing the lariat intron.

• Some RNAs self-splice (group I/II introns) — RNA acting as an enzyme, i.e., a ribozyme (the spliceosome's catalytic core and rRNA's peptidyl transferase are also RNA catalysis — a recurring MCAT theme).

Why Bother? Alternative Splicing & One Gene, Many Proteins

• ~20,000 human genes produce far more proteins because exons can be included or skipped combinatorially — tissue-specific splicing factors decide.
• Consequences for analysis: one gene can yield several mRNA sizes (northern blot bands) and several protein isoforms (western bands) from a single locus.

• RNA editing (rare, e.g., ApoB): base changes post-transcriptionally create tissue-specific variants — ApoB-100 (liver) vs ApoB-48 (intestine, where editing creates a premature stop).

When Processing Breaks — Clinical Anchors

• Beta-thalassemia: many alleles are splice-site mutations in the beta-globin gene — a destroyed GU donor site forces the spliceosome to use a cryptic splice site, yielding a frameshifted or intron-retaining mRNA and little functional globin.
• Intron retention usually shifts the reading frame or introduces premature stop codons.

• Such transcripts are often destroyed by nonsense-mediated decay, a surveillance pathway that flags stop codons upstream of the final exon junction.
• Systemic lupus erythematosus: autoantibodies against snRNPs (anti-Smith) — spliceosome components as autoantigens.

Worked Example — A Splicing Mutation Passage

Passage-style problem. A patient with severe anemia has normal beta-globin promoter and coding sequences, but sequencing reveals a G-to-A change at the first base of intron 1. Northern blot of reticulocyte RNA shows: healthy control — one sharp mRNA band; patient — a faint band of normal size plus a prominent LARGER band. A western blot shows drastically reduced beta-globin protein.

Question 1 — Why does this single intronic base matter when introns are discarded anyway? The first two intron bases are the invariant GU donor site the U1 snRNP recognizes. G-to-A destroys it: the spliceosome cannot initiate splicing of intron 1 at the correct position. "Introns don't matter" is true of their middles, not their boundaries — boundaries are functional sequence.

Question 2 — Interpret the larger northern band. RNA larger than normal mRNA from the same gene means intron retention (or use of a cryptic splice site adding partial intron). The retained intron adds length AND, almost inevitably, premature stop codons or a frameshift — hence protein loss on the western even though the gene is still being transcribed. Transcription is fine; PROCESSING is broken: the northern/western combination localizes the lesion between synthesis and translation.

Question 3 — Why does a faint normal-size band persist? Not from this allele: the G at the first intron position is invariant, so changing it abolishes correct splicing (a beta-zero allele). The faint normal band comes from the patient's other beta-globin allele, a milder mutation that still allows some correct splicing (a beta-plus allele). This is beta-thalassemia's clinical spectrum: severity tracks how much correct splicing survives.

Question 4 — Predict the effect of the same G-to-A change at the LAST base of the intron. The acceptor AG would become AA — also destroyed, same class of outcome (skipped acceptor, cryptic 3' site or exon skipping). Donor GU and acceptor AG are equally mutable Achilles heels; the branch-point A completes the trio of vulnerable positions.

mRNA Processing 🎯

Key Takeaways — Part 2

• Processing trio, all nuclear and co-transcriptional: 7-methylguanosine cap (5'-to-5' triphosphate linkage; exonuclease shield + translation-initiation docking), poly-A tail (untemplated ~200 A; stability timer), splicing (introns out, exons joined)
• Spliceosome: snRNPs (snRNA + protein); invariant GU donor, branch-point A, AG acceptor; the intron leaves as a lariat
• RNA can catalyze: self-splicing introns, the spliceosome core, and rRNA peptidyl transferase are ribozymes
• Alternative splicing multiplies the proteome — same DNA, tissue-specific isoforms differing by whole exons
• Splice pathology: boundary mutations cause intron retention, exon skipping, or cryptic-site use (beta-thalassemia)
• Blot logic: larger-than-normal mRNA + reduced protein = processing defect downstream of transcription, upstream of translation — localize lesions by which measurement first deviates

• Low-yield extras: the snRNPs are U1, U2, U4/U6 and U5; splicing is two transesterifications, closing the lariat with a 2'-5' bond only RNA's 2'-OH can make; eIF4E binds the cap; AAUAAA signals cleavage; histone mRNAs lack a tail; ApoB is RNA editing (a created stop), not splicing; nonsense-mediated decay removes premature-stop transcripts; anti-snRNP (anti-Smith) antibodies mark lupus; a deep intronic mutation matters when it CREATES splice signals (a pseudo-exon)

Transcription for the MCAT

Part 3 of 4 — Regulation: Operons, Enhancers & Epigenetics

Prokaryotic Logic: The Operon

An operon = one promoter driving several genes onto ONE polycistronic mRNA — coordinated control of a whole pathway.

The lac operon (inducible, catabolic) — two inputs, an AND gate:

• Negative control: the LacI repressor sits on the operator (DNA between promoter and genes) until the inducer allolactose pulls it off.
• Positive control: low glucose raises cAMP; cAMP-CAP binds upstream and recruits polymerase. Both conditions must be right — use lactose only when the better fuel is gone.

The trp operon (repressible, anabolic): the repressor is INACTIVE alone; tryptophan is a corepressor that activates it. Product shuts off its own synthesis — end-product feedback at the gene level.

Attenuation adds a second, translation-coupled brake on the trp operon.

Classify any operon question fast: catabolic pathways are inducible (substrate turns them ON); anabolic pathways are repressible (product turns them OFF).

Cis vs Trans — The Genetics of Regulation

• Cis-acting elements (promoter, operator, enhancer): DNA sites serving only the genes on their OWN molecule. An operator mutation (O$^c$) that cannot bind repressor makes its own operon constitutive — a second normal copy elsewhere cannot fix it.
• Trans-acting factors (repressors, activators, sigma): diffusible proteins acting on ANY copy. A lacI$^-$ mutation is rescued by a good lacI gene on a plasmid. Merodiploid (F') puzzles all reduce to: elements = cis, products = trans.

Eukaryotic Layers

1. Enhancers/silencers: cis elements that work at huge distances, in either orientation, upstream or downstream — DNA looping brings their bound activators to the promoter's mediator complex. Contrast with the fixed, close-range prokaryotic operator.
2. Chromatin: histone acetylation (HATs) neutralizes lysine's positive charge, loosening DNA-histone grip → transcription up; HDACs reverse it. DNA methylation of CpG islands (promoter regions) silences — stable, heritable through division = epigenetics (genomic imprinting, where a gene is silenced according to which parent it came from; X-inactivation/Barr body).
3. Combinatorial control: each gene's promoter integrates many transcription factors; tissue identity = the combination of factors present. No polycistronic mRNAs — each eukaryotic mRNA is monocistronic (one gene, one message).

Worked Example — Merodiploid Operon Genetics

Passage-style problem. E. coli strains carry the chromosomal lac region plus an F' plasmid with a second lac copy. Beta-galactosidase (LacZ) activity is measured with and without inducer (glucose absent throughout). Genotype notation: I = lacI (repressor), O = operator, Z = lacZ; superscript + is wild type, minus is loss of function, O$^c$ is an operator that cannot bind repressor.

Strain 1: I$^+$ O$^c$ Z$^+$ / F' I$^+$ O$^+$ Z$^-$. Result: enzyme made WITHOUT inducer (constitutive).

Analysis. Ask of each Z$^+$ gene: what controls MY operator? The functional Z$^+$ sits downstream of O$^c$, which cannot bind any repressor — from either lacI copy, because repressor acts in trans but the operator only protects its own DNA (cis). So Z$^+$ is always on. The plasmid's normal operator dutifully represses... a dead Z$^-$. Constitutive expression confirmed: O$^c$ is cis-dominant.

Strain 2: I$^-$ O$^+$ Z$^+$ / F' I$^+$ O$^+$ Z$^-$. Result: inducible (normal regulation).

Analysis. The chromosome makes no repressor, but the plasmid's I$^+$ produces diffusible repressor that finds and binds the chromosomal O$^+$, restoring control of Z$^+$. I$^+$ is trans-dominant over I$^-$ — the defining demonstration that repressor is a diffusible product.

Strain 3: I$^s$ (super-repressor that cannot bind allolactose) O$^+$ Z$^+$ / F' I$^+$ O$^+$ Z$^-$. Result: NO enzyme even WITH inducer.

Analysis. I$^s$ repressor binds operators normally but ignores the inducer — it never lets go. Because it diffuses, it clamps every O$^+$ in the cell, and wild-type repressor cannot displace it (an empty operator is instantly rebound). I$^s$ is trans-dominant negative — uninducible.

The algorithm: (1) inventory every functional Z; (2) examine only the operator physically attached to it; (3) let all repressor genes contribute one shared repressor pool; (4) remember O$^c$ = cis-constitutive, I$^-$ = trans-recessive.

And I$^s$, the super-repressor, = trans-dominant uninducible.

Gene Regulation 🎯

Key Takeaways — Part 3

• Operon = one promoter, polycistronic mRNA, coordinated pathway control; catabolic operons are inducible (lac), anabolic are repressible (trp)
• lac needs BOTH: repressor off (allolactose) AND cAMP-CAP on (low glucose); glucose present = catabolite repression regardless of lactose
• trp: tryptophan is a corepressor that arms its repressor — end product silences its own synthesis
• Cis vs trans by connectivity, not distance: operators/promoters/enhancers serve their own molecule (O-c is cis-dominant constitutive); repressors/activators diffuse (I-plus rescues I-minus in trans)
• Merodiploid algorithm: for each functional structural gene, consult only ITS operator; pool all repressors
• Eukaryotes: enhancers act far away via looping (still cis); histone acetylation opens chromatin (HAT up, HDAC down); CpG methylation silences stably
• Epigenetics = heritable expression states without sequence change: maintenance methylation copies marks at replication (X-inactivation, imprinting)

• Low-yield extras: the I-s super-repressor is trans-dominant and uninducible; attenuation is trp's second, translation-coupled brake

Transcription for the MCAT

Part 4 of 4 — Reading Transcription Experiments & MCAT Integration

The Question Behind Every Technique

Transcription passages hand you data from a small toolkit. Know what QUESTION each answers:

Steady state is a balance: an mRNA's level (northern/qPCR) = synthesis MINUS decay. A rise in mRNA alone does not prove the gene is transcribed faster — the mRNA could simply be decaying more slowly.

• Nuclear run-on asks whether regulation is at transcription itself or at mRNA stability: it labels only the RNAs being actively transcribed at that moment. If a treatment doubles mRNA but run-on shows unchanged transcription, the treatment stabilized the mRNA — a stability story wearing a transcription costume.

Reporter Dissection — The Standard Puzzle Format

Fuse candidate DNA fragments to luciferase; measure light:

• Full promoter region: 100% activity
• Delete segment A: activity drops to 5% — segment A contains an essential element (or the core promoter)
• Delete segment B: activity RISES to 300% — segment B contains a silencer
• Move segment C 2 kb away or flip it: activity unchanged — orientation/distance independence marks C as an enhancer

Integration Rules for Passage Day

1. Locate the level: DNA element (cis) vs protein factor (trans) vs chromatin state vs RNA stability — each has a diagnostic experiment (reporter, expression rescue, ChIP for histone marks, measures of synthesis rate).
2. In-vitro binding is not in-vivo function: EMSA shows CAN bind; ChIP shows IS bound; the reporter shows binding DOES something. Strong passages stack all three — track which claim each figure supports.
3. Controls are answers: the no-antibody ChIP lane, the empty reporter vector, the scrambled probe in EMSA — when a question asks "why include X," name the artifact X rules out.

• Kinetics order the cascade: after a stimulus, transcription factors activate in minutes (pre-existing protein, phosphorylation), immediate-early mRNAs rise in ~15-30 min, protein products follow, and secondary target genes rise only after those proteins accumulate. A gene needing new protein synthesis to respond (blocked by the translation inhibitor cycloheximide) is a SECONDARY response gene.

Worked Example — A Hormone-Response Passage, Assembled

Passage-style problem. A steroid hormone induces gene G in liver cells. Researchers gather three results:

• Fig 1 (RT-qPCR): G mRNA rises 20-fold within 60 minutes of hormone.
• Fig 2 (ChIP with anti-receptor antibody): hormone treatment causes the receptor to be recovered with a DNA region 3 kb upstream of G; no recovery without hormone, and none at a control locus.
• Fig 3 (reporter): the 3 kb-upstream fragment fused to luciferase confers hormone responsiveness; deleting a 15 bp palindrome within it abolishes the response.

Question 1 — What does each figure contribute that the others cannot? Fig 1: the transcript accumulates (but cannot say why). Fig 2: the receptor is physically at the upstream site in vivo, hormone-dependently. Fig 3: that site is FUNCTIONAL — it converts hormone into transcription, and the 15 bp palindrome is the responsible element (a hormone response element; palindromic because the receptor binds as a dimer). Amount, occupancy, function — three claims, three techniques.

Two more results, for students going deeper:

• Fig 4 (nuclear run-on): transcription of G increases 20-fold with hormone — matching the mRNA rise. This adds MECHANISM: run-on isolates the synthesis term, so the mRNA rose because synthesis increased, not because decay slowed.
• Fig 5 (RT-qPCR with cycloheximide): the rise in G mRNA persists when cycloheximide (a translation inhibitor) is present.

Primary or secondary response gene? Primary. Cycloheximide blocks synthesis of NEW protein; since induction survives it (Fig 5), everything required (the receptor) pre-existed. Secondary response genes — needing a freshly made transcription factor — would be silenced by cycloheximide. One drug cleanly cuts the cascade in two.

Question 2 — Predict: receptor mutant that binds hormone but cannot dimerize. ChIP recovery lost (weak/no binding to the palindromic HRE), reporter response lost, G induction lost — but the mutant is trans-acting, so supplying wild-type receptor restores everything. Conversely, deleting the HRE in the genome is cis: no amount of good receptor rescues gene G, though other hormone targets respond normally.

The template generalizes: every regulation passage is a few of these claims (amount, occupancy, function) in some order. Name which claim a figure makes before answering anything about it.

Transcription Experiments 🎯

Key Takeaways — Part 4

• Map technique to claim: northern/RT-qPCR = how much RNA (steady state); reporter = element function; ChIP = in-vivo occupancy; EMSA = in-vitro binding
• Steady state = synthesis minus decay: a rise in mRNA alone does not prove faster transcription
• Reporter deletion logic: activity down = removed activator/core element; activity up = removed silencer; distance/orientation independence = enhancer
• Binding is three separate claims — CAN bind (EMSA), IS bound (ChIP), MATTERS (reporter) — never let one stand in for another
• Controls name artifacts: no-antibody ChIP (nonspecific pulldown), control locus (site specificity), empty vector (baseline), cold competitor (binding specificity)
• Cis lesions (deleted response elements) cannot be rescued in trans; factor mutations can — the rescue experiment assigns the level of every defect

• Low-yield extras: nuclear run-on measures synthesis rate directly — mRNA up with flat run-on means stabilization, not activation; cycloheximide splits cascades: induction that survives it = primary response (pre-existing factors), induction it blocks = secondary response (needs a newly made protein, often the primary gene's product)