GATE BT

Gene expression

Replication, transcription, translation and regulation of gene expression

GATE Biotechnology study notes on DNA replication (prokaryotic and eukaryotic, telomeres), transcription, RNA processing, translation, prokaryotic operons (lac genetics, trp attenuation) and eukaryotic regulation including miRNA, siRNA, RNAi and other non-coding RNAs, with comparative tables, worked numerical examples and 7 original GATE-style practice questions.

21 previous-year questions · 20 solved · 7 practice questions

DNA Replication: Mechanism and Enzymes

DNA replication is semi-conservative: each daughter duplex keeps one parental strand and one new strand. The classic density-shift experiment showed this: cells were grown in heavy nitrogen (15N) and then moved to 14N. After one generation all DNA was of hybrid density, and after two generations it was half hybrid and half light. New DNA is always made 5'→3', reading the template 3'→5'. The two template strands are antiparallel, so at each fork one new strand (the leading strand) is made continuously. The other (the lagging strand) is made discontinuously, as Okazaki fragments. Replication starts at defined origins: a single origin, oriC, in E. coli and most bacteria, and many origins on each eukaryotic chromosome. It is usually bidirectional, so two forks move away from each origin.

Proteins at a bacterial (E. coli) replication fork and what each does

DnaA binds the DnaA boxes in oriC and melts the neighbouring AT-rich region, opening a replication bubble.
DnaB helicase is loaded by DnaC. It encircles the lagging-strand template, moves 5'→3' along it and unwinds the duplex at each fork.
SSB (single-strand binding protein) coats the exposed single strands. This stops them re-annealing and forming secondary structure.
DNA gyrase (a type II topoisomerase) acts ahead of the fork and relieves the positive supercoils that unwinding creates. Topoisomerase IV separates (decatenates) the linked daughter circles at the end of replication.
Primase (DnaG) makes a short RNA primer of about 10–12 nt: once at the start of each leading strand, and once for every Okazaki fragment.
DNA polymerase III holoenzyme extends each primer. Its core has the α subunit (polymerase) and the ε subunit (3'→5' proofreading exonuclease). The β sliding clamp, loaded by the γ/τ clamp loader, makes it highly processive. The lagging-strand template loops out, so one holoenzyme can make both strands at a fork (the 'trombone' model).
DNA polymerase I removes each RNA primer with its 5'→3' exonuclease and fills the gap with DNA at the same time (nick translation).
DNA ligase seals the remaining nick with a phosphodiester bond, joining adjacent fragments. It uses NAD+ in E. coli and ATP in eukaryotes.
Termination: the two forks meet in a terminus region opposite oriC. There, Tus protein bound to ter sites stops fork movement in one direction only.

Replicative and associated DNA polymerases compared

Enzyme	Role	3'→5' proofreading exonuclease	5'→3' exonuclease
DNA Pol III (bacteria)	Main replicative polymerase; extends primers on both strands	Yes	No
DNA Pol I (bacteria)	Removes RNA primers and fills the gaps (nick translation); repair	Yes	Yes
DNA Pol II (bacteria)	DNA repair and restart of stalled replication	Yes	No
Pol α–primase (eukaryotes)	Starts every strand and every Okazaki fragment. Primase subunits make ~10 nt of RNA; Pol α adds a short stretch of DNA	No	No
DNA Pol δ (eukaryotes)	Main lagging-strand polymerase; works with the PCNA clamp	Yes	No
DNA Pol ε (eukaryotes)	Main leading-strand polymerase	Yes	No

Eukaryotic replication: what differs from bacteria

Many origins per chromosome. In G1, each origin is 'licensed': the origin recognition complex (ORC), Cdc6 and Cdt1 load the MCM helicase. Each origin then fires only once per S phase.
The active helicase is the CMG complex (Cdc45–MCM–GINS).
PCNA is the sliding clamp, the counterpart of the bacterial β clamp. It is loaded by RFC (replication factor C).
Eukaryotic polymerases have no 5'→3' exonuclease. Instead, Pol δ displaces the primer into a flap, which FEN1 (flap endonuclease) cuts off, and RNase H2 can degrade RNA in RNA–DNA hybrids. DNA ligase I then seals the nick.
Okazaki fragments are about 100–200 nt long, compared with about 1000–2000 nt in bacteria.

OKAZAKI FRAGMENTS PER COMPLETE ROUND OF REPLICATION

N_{Okazaki} \approx \frac{G}{ℓ}

$G$ = genome (or region) length in bp and $ℓ$ = average Okazaki fragment length in nt. Every base pair is copied by exactly one fork, and each fork makes one strand continuously and the other discontinuously. So the total lagging-strand DNA equals $G$ nt, whatever the number of origins. Primer lengths and end effects at origins and termini are ignored.

WORKED EXAMPLE

A 60,000 bp region of a eukaryotic chromosome is replicated bidirectionally from a single origin at its centre. The average Okazaki fragment length is 200 nt. How many Okazaki fragments are made? Ignore primer lengths and end effects.

Solution

Each fork copies 30,000 bp. At each fork one strand is lagging, so each fork makes 30,000 nt of lagging-strand DNA.
Total lagging-strand DNA = 2 × 30,000 = 60,000 nt, which equals the region length $G$ , as the formula says.
Number of fragments = 60,000 / 200 = 300.

Answer: 300 Okazaki fragments

End-replication problem and telomerase. On a linear chromosome, the RNA primer at the very 5' end of each lagging strand is removed, and no polymerase can fill the gap it leaves, because polymerases only extend a 3'-OH. Each round of replication therefore leaves a 3' single-stranded overhang, and without compensation the chromosome ends shorten. Telomerase is a ribonucleoprotein reverse transcriptase. Its RNA component is the template, and its catalytic protein (TERT) uses it to add repeats to the 3' end of the G-rich strand (TTAGGG in vertebrates). The complementary strand is then filled in by Pol α–primase. Telomerase is active in germ cells, stem cells and most cancer cells, and low or absent in most somatic cells.

EXAM TIP

Primer handling differs between the two domains. In bacteria, Pol I removes primers with its 5'→3' exonuclease. In eukaryotes, the replicative polymerases lack that activity: primers are removed by strand displacement plus FEN1 and RNase H2.

Transcription: RNA Polymerases, Promoters and Termination

Transcription makes RNA 5'→3' from ribonucleoside triphosphates, reading the template (antisense) strand 3'→5'. It needs no primer, unlike DNA polymerase. The RNA has the same sequence as the coding (sense) strand (with U in place of T), so gene sequences are written as the coding strand, 5'→3'. RNA polymerase proofreads only a little, by backtracking and cleaving the transcript, so its error rate is much higher than that of DNA replication. Bacteria have a single RNA polymerase: a core enzyme (α2ββ'ω) plus a σ factor that recognises promoters. Eukaryotes have three main nuclear RNA polymerases, each making a different class of RNA. Plants also have Pol IV and Pol V, which make small RNAs for silencing.

Eukaryotic nuclear RNA polymerases (α-amanitin sensitivity as in mammalian cells)

Polymerase	Product(s)	α-amanitin sensitivity
RNA Pol I (nucleolus)	45S pre-rRNA, processed into 28S, 18S and 5.8S rRNA	Insensitive
RNA Pol II	pre-mRNA (hnRNA), U1/U2/U4/U5 snRNAs, most miRNA precursors, lncRNAs	Highly sensitive (inhibited at low concentrations)
RNA Pol III	tRNA, 5S rRNA, U6 snRNA, 7SL RNA and other small RNAs	Inhibited only at high concentrations

Stages of bacterial transcription

Initiation. The σ factor (σ70 for most genes) guides the core enzyme to the promoter by recognising the -35 element (consensus TTGACA) and the -10 element (consensus TATAAT). The polymerase melts about 12–14 bp around the start site (+1), forming an open complex.
Abortive initiation and promoter escape. The enzyme often makes and releases several short transcripts before it escapes the promoter. σ is usually released after about 10 nt, and the core enzyme then continues.
Elongation. The polymerase unwinds DNA ahead of it and rewinds it behind, extending the RNA 5'→3'. The RNA–DNA hybrid in the transcription bubble is about 8–9 bp long.
Rho-independent (intrinsic) termination. A GC-rich inverted repeat forms a stable hairpin in the RNA, followed by a run of U's. The weak rU:dA hybrid then lets the transcript and polymerase dissociate.
Rho-dependent termination. Rho, a hexameric ATP-dependent helicase, binds a C-rich rut site on the nascent RNA, moves 5'→3' along it and unwinds the RNA–DNA hybrid at a paused polymerase.

Many, but not all, eukaryotic Pol II core promoters carry a TATA box (consensus TATAAA) about 25–30 bp upstream of the start site. It is recognised by the TATA-binding protein (TBP) subunit of TFIID. TBP is also recruited to promoters that lack a TATA box. The general transcription factors (TFIIA, B, D, E, F and H) assemble with Pol II into the pre-initiation complex. TFIIH has helicase activity, which melts the promoter. It also has kinase activity: it phosphorylates Ser5 of the repeated heptad (YSPTSPS) in the C-terminal domain (CTD) of Pol II's largest subunit, which triggers promoter clearance. The phosphorylated CTD then recruits the capping, splicing and polyadenylation machinery.

EXAM TIP

Do not confuse the bacterial -10 element (TATAAT) with the eukaryotic TATA box (TATAAA). Both are AT-rich and both help position the start site, but different machinery reads them (σ factor vs TBP/TFIID) and they lie at different distances from +1.

RNA Processing: Capping, Polyadenylation and Splicing

In eukaryotes, the primary Pol II transcript (pre-mRNA) is processed in the nucleus before export:

Capping. Once the RNA is about 20–30 nt long, a 7-methylguanosine cap is added to its 5' end through an unusual 5'–5' triphosphate linkage.
Cleavage and polyadenylation. The 3' end is cleaved about 10–30 nt downstream of an AAUAAA signal. Poly(A) polymerase then adds a tail of up to about 200 A residues in mammals. Replication-dependent histone mRNAs are the classic exception: they end in a stem-loop and have no poly(A) tail.
Splicing removes the introns.

Bacterial mRNA has no 5' cap and no stabilising eukaryote-type poly(A) tail; where bacteria do polyadenylate RNA, the tail promotes its decay. Bacterial genes also almost never contain spliceosomal introns. Bacteria have no nucleus, so their transcription and translation are coupled.

Splicing mechanism (two sequential transesterifications)

U1 snRNP base-pairs with the 5' splice site (GU). U2 snRNP base-pairs around the branch point, leaving the branch-point adenosine bulged out.
The U4/U6·U5 tri-snRNP joins. After rearrangements, U1 and U4 are released, and U2 and U6 snRNAs form the catalytic core. The spliceosome is therefore a ribozyme.
First transesterification. The 2'-OH of the branch-point adenosine attacks the phosphate at the 5' splice site. This forms a 2'–5' phosphodiester (the lariat intermediate) and releases the 5' exon with a free 3'-OH.
Second transesterification. The 3'-OH of the 5' exon attacks the phosphate at the 3' splice site (the AG at the end of the intron). This joins the two exons and releases the intron as a lariat, which is then debranched and degraded.
No phosphodiester bonds are gained or lost overall. The ATP used by splicing drives the spliceosome's assembly and rearrangements, not the chemical steps.

Processing events compared

Event	Location on transcript	Key factor(s)	Purpose
5' capping	5' end	RNA triphosphatase, guanylyltransferase and guanine-N7 methyltransferase	Protects against 5'→3' exonucleases; needed for export, splicing of the first intron and cap-dependent translation initiation
3' cleavage and polyadenylation	3' end: CPSF binds AAUAAA, CstF binds a downstream GU/U-rich element	CPSF, CstF, cleavage factors, poly(A) polymerase, poly(A)-binding protein	mRNA stability, nuclear export, translation efficiency
Splicing	Exon–intron junctions (GU…AG) and branch point	Major spliceosome (U1, U2, U4, U5, U6 snRNPs)	Removes introns; alternative splicing yields several mRNAs from one gene

EXAM TIP

GU–AG is the dominant splice-site rule for nuclear pre-mRNA introns (GT–AG in the DNA), but not the only mechanism. A minor spliceosome (U11, U12, U4atac, U6atac and U5) removes rare U12-type introns, some of which have AU–AC ends. Group I and group II introns splice themselves without any spliceosome.

Translation: The Genetic Code and Protein Synthesis

The genetic code maps 64 triplet codons to 20 amino acids plus stop signals. Its properties:

It is read 5'→3' on the mRNA, in one continuous frame, with no overlap and no gaps (non-overlapping and commaless).
It is unambiguous: each codon specifies only one amino acid.
It is degenerate: 18 of the 20 amino acids have more than one codon, and the codons for one amino acid usually differ at the third position. Met (AUG) and Trp (UGG) have one codon each.
AUG is also the start codon. Bacteria occasionally start at GUG or UUG, which are still read as fMet.
It is nearly universal. The known exceptions are in mitochondria and a few organisms; for example, UGA encodes Trp in vertebrate mitochondria.

CODON COMBINATORICS

4^{3} = 64

4 bases in 3 positions give 64 codons: 61 sense codons (including AUG) and 3 stop codons (UAA, UAG, UGA).

ORF LENGTH FROM POLYPEPTIDE LENGTH

L_{ORF} = 3 (N_{aa} + 1)

$L_{ORF}$ = nucleotides from the first base of the start codon to the last base of the stop codon, inclusive. $N_{aa}$ = number of amino acids in the primary translation product, including the initiator Met. The +1 is the stop codon, which is not translated. If the initiator Met is removed after translation, add it back: a mature chain of $n$ residues gives $N_{aa} = n + 1$ . Any signal peptide or propeptide that is cleaved off must be added back in the same way.

WORKED EXAMPLE

An mRNA has an ORF of 1,203 nt, counted from the A of AUG to the last base of the stop codon. How many amino acids are in the primary translation product, and how many remain if only the initiator Met is removed?

Solution

Number of codons = 1,203 / 3 = 401.
One of these is the stop codon, which encodes no amino acid, so $N_{aa}$ = 401 − 1 = 400.
Check with the formula: 3(400 + 1) = 1,203.
If only the initiator Met is removed, the mature protein has 399 residues.

Answer: 400 amino acids as synthesised; 399 after removal of the initiator Met

Charging tRNAs and the energy cost

Each of the 20 aminoacyl-tRNA synthetases attaches one amino acid to its tRNAs in two steps. First, amino acid + ATP → aminoacyl-AMP + PPi. Second, the aminoacyl group is transferred to the 3'-terminal A of the tRNA. Many synthetases proofread (edit) wrongly activated amino acids.
Charging uses ATP → AMP, which costs 2 high-energy phosphate bonds. Elongation uses 1 GTP to deliver the aminoacyl-tRNA (EF-Tu) and 1 GTP to translocate (EF-G). So each peptide bond costs 4 high-energy phosphate bonds, not counting initiation, termination or proofreading.

Stages of translation

Initiation (bacteria). The 30S subunit, with IF1, IF2·GTP and IF3, binds the mRNA. The Shine–Dalgarno sequence, a purine-rich element a few nucleotides upstream of the start codon, base-pairs with the 3' end of 16S rRNA. IF2 places fMet-tRNA(fMet) on the AUG in the P site. The 50S subunit then joins to form the 70S initiation complex.
Initiation (eukaryotes). eIF4F binds the 5' cap and recruits the 43S complex: the 40S subunit with eIF1, eIF1A, eIF3 and eIF2·GTP·Met-tRNAi. The complex scans 5'→3' to the first AUG in a good Kozak context (a purine at −3 and G at +4). The 60S subunit then joins to form the 80S ribosome.
Elongation. EF-Tu·GTP (bacteria) or eEF1A·GTP (eukaryotes) delivers each aminoacyl-tRNA to the A site. The peptidyl transferase centre, formed by rRNA (23S in bacteria, 28S in eukaryotes), catalyses peptide-bond formation, so the ribosome is a ribozyme. EF-G·GTP (bacteria) or eEF2·GTP (eukaryotes) then translocates the ribosome by one codon. The chain grows from its N terminus to its C terminus.
Termination. A release factor enters the A site at a stop codon: in bacteria, RF1 (UAA, UAG) or RF2 (UAA, UGA); in eukaryotes, eRF1 (all three). It triggers hydrolysis of the peptidyl-tRNA bond, which releases the polypeptide. RF3 (bacteria) and eRF3 (eukaryotes) are GTPases that assist, and the ribosome is then recycled.

Common inhibitors of transcription and translation

Inhibitor	Target	Effect
Rifampicin	β subunit of bacterial RNA polymerase	Blocks transcription initiation (extension of the first few nucleotides)
Actinomycin D	DNA (intercalates at GC pairs)	Blocks transcription elongation in both bacteria and eukaryotes
α-amanitin	Eukaryotic RNA Pol II (Pol III at high concentrations, mammals)	Blocks elongation
Streptomycin	Bacterial 30S subunit	Interferes with initiation; causes misreading
Tetracycline	Bacterial 30S subunit (A site)	Blocks binding of aminoacyl-tRNA
Chloramphenicol	Bacterial 50S subunit	Inhibits peptidyl transferase
Erythromycin	Bacterial 50S subunit (exit tunnel)	Blocks elongation of the growing chain
Puromycin	A site of both 70S and 80S ribosomes (aminoacyl-tRNA analogue)	Accepted as an amino acid, then releases the chain early
Cycloheximide	Eukaryotic 60S subunit	Blocks translocation (80S ribosomes only)
Diphtheria toxin	eEF2	ADP-ribosylates eEF2 and stops translocation

EXAM TIP

Wobble. The first (5') base of the anticodon pairs with the third (3') base of the codon, and this pairing is less strict than Watson–Crick pairing. Anticodon G pairs with C or U; anticodon U pairs with A or G; anticodon I (inosine) pairs with U, C or A. So one tRNA can read several codons for the same amino acid, which is why degeneracy is concentrated at the third position.

Regulation of Gene Expression in Prokaryotes: The Operon

In bacteria, genes for related functions are often transcribed together, from one promoter, as a single polycistronic mRNA. This unit is an operon. Regulation acts mainly on transcription initiation: repressors and activators bind sites at or near the promoter. In some biosynthetic operons there is also attenuation, which works only because transcription and translation are coupled in bacteria.

lac operon genes and classic mutants (gene symbols written plainly)

Structural genes: lacZ (β-galactosidase), lacY (lactose permease), lacA (thiogalactoside transacetylase). They are transcribed together from the lac promoter; the operator overlaps the promoter.
lacI encodes the repressor. It has its own promoter and is expressed constitutively.
The natural inducer is allolactose, a lactose isomer made by β-galactosidase. IPTG is a gratuitous inducer: it induces the operon but is not metabolised.
lacI⁻ (no functional repressor): expression is constitutive. It is recessive, because a lacI⁺ copy anywhere in the cell supplies repressor in trans and restores inducibility.
lacOᶜ (operator-constitutive, cannot bind repressor): the genes on the same DNA molecule are expressed constitutively. It is cis-dominant: it has no effect on a second copy of the operon.
lacIˢ (super-repressor, cannot bind inducer): the operon cannot be induced. It is trans-dominant over lacI⁺.

lac operon vs trp operon

Feature	lac operon	trp operon
Type of control	Inducible (catabolic genes, switched on when the substrate is present)	Repressible (biosynthetic genes, switched off when the product is abundant)
Repressor without effector	Active; binds the operator and blocks transcription	Inactive (aporepressor); cannot bind the operator
Effector and its action	Allolactose (inducer) binds the repressor and inactivates it	Tryptophan (corepressor) binds the aporepressor and activates it to bind the operator
Positive control	The CAP–cAMP complex binds upstream of the promoter and activates transcription when glucose is low	Not a major feature
Additional fine control	No attenuation; glucose also acts through inducer exclusion (glucose uptake inhibits lactose permease)	Attenuation: early termination in the leader region, set by how fast the ribosome moves over the leader's Trp codons

trp operon attenuation, step by step

The leader (trpL) encodes a 14-amino-acid leader peptide with two adjacent Trp codons in region 1. The leader RNA has four regions that can pair in alternative ways: 1:2, 2:3 or 3:4. The 3:4 hairpin, followed by a run of U's, is a rho-independent terminator. The 2:3 hairpin is the antiterminator.
Trp plentiful (charged tRNA-Trp available): the ribosome translates through region 1 and stops at the leader's stop codon, where it covers region 2. Region 3 then pairs with region 4, the terminator forms, and transcription stops before the structural genes.
Trp scarce (charged tRNA-Trp limiting): the ribosome stalls at the Trp codons and so covers region 1. Region 2 is free and pairs with region 3 (the antiterminator), so the 3:4 terminator cannot form and RNA polymerase continues into the structural genes.
No translation of the leader: regions 1:2 and 3:4 pair, the terminator forms, and transcription stops.
Repression by the Trp–repressor complex and attenuation act together, giving a wide range of control.

EXAM TIP

The lac operon is expressed strongly only when both of two conditions hold: the repressor is inactivated (inducer present) and CAP–cAMP is bound (glucose low). These two controls act independently. Together with inducer exclusion, they explain why E. coli uses glucose before lactose (diauxic growth).

Eukaryotic Gene Regulation and Non-coding RNA (miRNA, siRNA, RNAi)

Eukaryotic gene expression is controlled at many levels beyond the bacterial operon.

Chromatin. Histone acetylation by histone acetyltransferases generally opens chromatin; deacetylation by histone deacetylases closes it. Methylation of cytosine in CpG dinucleotides at promoters (CpG islands) is associated with stable silencing.
Transcription factors. Many factors bind in combination to elements near the promoter.
Enhancers and silencers. These act at long range, largely independent of orientation and distance. DNA looping brings them into contact with the promoter, often through the Mediator complex, and insulators limit their reach.

Levels at which eukaryotic expression is controlled

Chromatin: histone modifications, DNA methylation and nucleosome remodelling.
Transcription initiation: activators, repressors, enhancers, silencers and Mediator.
RNA processing: alternative splicing and alternative polyadenylation.
Export and mRNA stability: for example, nonsense-mediated decay of mRNAs that carry a premature stop codon.
Translation: for example, phosphorylation of eIF2α reduces translation initiation overall; small RNAs repress translation of specific mRNAs.
After translation: protein modification, targeting and degradation.

RNA interference (RNAi) pathway

Long double-stranded RNA, from a virus, a transposon or an experimental source, is cut by the endonuclease Dicer into siRNA duplexes. Each strand is about 21–23 nt long, with about 19 bp paired, 2-nt 3' overhangs and 5' phosphates.
One strand, the guide strand, is loaded into the RNA-induced silencing complex (RISC), whose core is an Argonaute protein. The other strand, the passenger strand, is cleaved and discarded.
The guide strand pairs with a complementary mRNA. With extensive complementarity, including the central region, the Argonaute 'slicer' cuts the mRNA, and the pieces are degraded.
RNAi knocks down expression: it reduces the mRNA while the gene itself stays intact, unlike a knockout. In mammalian cells, long dsRNA triggers an interferon response, so experiments use synthetic ~21-nt siRNAs or short hairpin RNAs (shRNAs) expressed from vectors.

siRNA vs miRNA

Feature	siRNA	miRNA
Origin	Long exogenous or endogenous dsRNA, fully paired	Genome-encoded hairpin transcript (pri-miRNA), usually made by Pol II
Processing	Dicer cuts long dsRNA directly into siRNA duplexes	In the nucleus, Drosha–DGCR8 (the Microprocessor) crops the pri-miRNA to a ~60–70 nt pre-miRNA hairpin. Exportin-5 (with Ran-GTP) exports it, and Dicer in the cytoplasm cuts it to a ~22-nt duplex
Target complementarity	Usually full complementarity to a specific target	In animals, usually partial: the seed region (nt 2–8 of the miRNA) pairs with sites often in the 3' UTR, so one miRNA can regulate many mRNAs. Plant miRNAs usually pair almost fully
Typical outcome	mRNA cleavage by Argonaute	In animals, translational repression and mRNA decay (deadenylation); in plants, mostly cleavage

Major classes of non-coding RNA

rRNA and tRNA: the core of the translation machinery.
snRNA: splicing (the RNA components of the spliceosome).
snoRNA: guide chemical modification of rRNA (2'-O-methylation and pseudouridylation).
miRNA (~22 nt) and siRNA (~21–23 nt): post-transcriptional silencing through Argonaute.
piRNA (~24–31 nt): made without Dicer and bound by Piwi-clade Argonautes. They silence transposons in the germline.
lncRNA (longer than 200 nt, not translated into protein): regulate chromatin and transcription. For example, Xist RNA coats one X chromosome in female mammalian cells and silences it.

EXAM TIP

Keep the regulatory non-coding RNAs (miRNA, siRNA, piRNA, lncRNA) separate in your recall from the 'housekeeping' ones (rRNA, tRNA, snRNA, snoRNA). The key contrasts between siRNA and miRNA are where they come from (long dsRNA vs a genome-encoded hairpin), how fully they pair with their target, and whether the result is cleavage or repression.

PREVIOUS-YEAR QUESTIONS

21 questions from the BT papers

Try each question first, then open the answer. One shows the official answer only. We placed each question under a topic ourselves; the exam paper does not label topics.

See these papers in full: GATE BT 2026 · GATE BT 2025 · GATE BT 2024 · GATE BT 2023 · GATE BT 2022 · GATE BT 2020 · GATE BT 2019

GATE 2026 BT Q.20

GATE 2026 · BTQ.20 · 1 mark · MCQ

Which one of the following antibiotics blocks protein chain elongation by preventing the action of peptidyl transferase ?

(A)
Bleomycin
(B)
Rifampicin
(C)
Chloramphenicol
(D)
Tetracycline

Answer and solution

GATE 2026 BT Q.20: answer and solution

See the question in the paper

Gene expression

DNA Replication: Mechanism and Enzymes

Transcription: RNA Polymerases, Promoters and Termination

RNA Processing: Capping, Polyadenylation and Splicing

Translation: The Genetic Code and Protein Synthesis

Regulation of Gene Expression in Prokaryotes: The Operon

Eukaryotic Gene Regulation and Non-coding RNA (miRNA, siRNA, RNAi)

21 questions from the BT papers

7 questions to test yourself