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About Science MCQs
Science is a core subject in almost every competitive examination, including CSS, PMS, PPSC, FPSC, and NTS. It encompasses the study of the physical world and natural phenomena. Whether you are preparing for a general science paper or a specialized entry test, mastering Science MCQs requires a clear understanding of fundamental concepts and the ability to apply them logically.
Topics Covered in This Category
- Biology: Human anatomy, cell biology, genetics, and ecology.
- Chemistry: Chemical reactions, periodic table, organic and inorganic chemistry.
- Physics: Mechanics, electricity, magnetism, optics, and modern physics.
- Everyday Science: Practical applications of science in daily life, famous inventions, and scientific laws.
How to Prepare for Science Exams
Science is not just about memorizing facts; it is about understanding the "why" and "how" behind natural phenomena. Focus on conceptual clarity rather than rote learning. When practicing MCQs, pay special attention to units of measurement, chemical formulas, and scientific terminologies. Reviewing the explanations for wrong answers is the fastest way to improve your score.
All Questions for: Transcription
Question 1: Transcription is the process of:
- A Synthesizing RNA from a DNA template
- B Synthesizing DNA from an RNA template
- C Synthesizing protein from mRNA
- D Copying DNA into two identical molecules
Explanation: Transcription is the synthesis of an RNA molecule using DNA as a template, catalyzed by RNA polymerase.
Question 2: Which enzyme synthesizes RNA during transcription in bacteria?
- A DNA-dependent RNA polymerase
- B DNA polymerase III
- C Primase
- D Reverse transcriptase
Explanation: Bacterial DNA-dependent RNA polymerase is a multi-subunit enzyme that catalyzes RNA synthesis using a DNA template.
Question 3: The DNA strand that is copied during transcription is called the:
- A Template (antisense) strand
- B Coding (sense) strand
- C Leading strand
- D Lagging strand
Explanation: The template (antisense) strand is read by RNA polymerase to produce complementary RNA. The other strand is called the coding (sense) strand.
Question 4: The coding strand of DNA has the same sequence as which molecule (except T→U)?
- A mRNA (except T→U)
- B tRNA
- C rRNA
- D The template strand
Explanation: The coding (sense) strand is complementary to the template and therefore matches the mRNA sequence, with thymine replaced by uracil.
Question 5: A promoter is best described as:
- A A DNA sequence upstream of a gene where RNA polymerase binds
- B A protein that unwinds DNA
- C The 3' untranslated region of mRNA
- D The origin of DNA replication
Explanation: The promoter is a DNA sequence located upstream of the transcription start site that binds RNA polymerase and transcription factors to initiate transcription.
Question 6: In bacteria, the -10 sequence of the promoter is also known as the:
- A Pribnow box
- B TATA box
- C CAAT box
- D GC box
Explanation: The -10 sequence (consensus TATAAT) is called the Pribnow box. It is recognized by the sigma factor of RNA polymerase.
Question 7: In bacteria, the -35 sequence of the promoter is primarily recognized by:
- A The sigma factor of RNA polymerase
- B The rho termination factor
- C The TATA-binding protein
- D DNA gyrase
Explanation: The sigma factor (σ) of bacterial RNA polymerase recognizes both -10 and -35 promoter elements, enabling specific initiation of transcription.
Question 8: The role of the sigma factor in bacterial transcription is:
- A Direct RNA polymerase to the promoter for initiation
- B Catalyze elongation of RNA
- C Terminate transcription
- D Splice introns from mRNA
Explanation: Sigma factor directs RNA polymerase to promoters, initiates transcription, and is released after elongation begins. Different sigma factors allow responses to environmental changes.
Question 9: In eukaryotes, which RNA polymerase transcribes messenger RNA (mRNA)?
- A RNA polymerase II
- B RNA polymerase I
- C RNA polymerase III
- D RNA polymerase IV
Explanation: RNA polymerase II transcribes protein-coding genes into mRNA (and many non-coding RNAs). RNA polymerase I makes rRNA, and III makes tRNA and 5S rRNA.
Question 10: In eukaryotes, which RNA polymerase transcribes ribosomal RNA (rRNA)?
- A RNA polymerase I
- B RNA polymerase II
- C RNA polymerase III
- D RNA polymerase IV
Explanation: RNA polymerase I is dedicated to transcribing the large rRNA precursor in the nucleolus, which is processed into 18S, 5.8S, and 28S rRNAs.
Question 11: In eukaryotes, which RNA polymerase transcribes transfer RNA (tRNA)?
- A RNA polymerase III
- B RNA polymerase I
- C RNA polymerase II
- D RNA polymerase IV
Explanation: RNA polymerase III transcribes tRNA genes, 5S rRNA, and other small non-coding RNAs.
Question 12: The TATA box is a promoter element found in:
- A Many eukaryotic promoters
- B All bacterial promoters
- C The ribosome binding site in mRNA
- D Introns only
Explanation: The TATA box is a conserved AT-rich sequence in many eukaryotic promoters, typically about 25–30 bp upstream of the transcription start site.
Question 13: General (basal) transcription factors in eukaryotes function to:
- A Assemble with RNA polymerase II at the promoter to initiate transcription
- B Terminate transcription at specific sites
- C Splice mRNA introns
- D Synthesize poly-A tails
Explanation: Basal transcription factors (TFIIA, TFIIB, TFIID, TFIIE, TFIIF, TFIIH) assemble with RNA polymerase II at the promoter to form the preinitiation complex and enable transcription.
Question 14: The 5' cap added to eukaryotic mRNA during processing is:
- A 7-methylguanosine linked via a 5'-5' triphosphate bridge
- B A chain of ~200 adenine nucleotides
- C A chain of ~200 thymine nucleotides
- D A short peptide tag
Explanation: The 5' cap is a 7-methylguanosine linked via a 5'-5' triphosphate bridge. It protects the mRNA from degradation and helps ribosomes recognize the transcript.
Question 15: The poly-A tail added to eukaryotic mRNA is composed of:
- A Adenine nucleotides
- B Guanine nucleotides
- C Cytosine nucleotides
- D Uracil nucleotides
Explanation: The poly-A tail is a stretch of ~150–250 adenine residues added post-transcriptionally to the 3' end of mRNA, increasing stability and translation efficiency.
Question 16: Introns in primary RNA transcripts are:
- A Intervening non-coding sequences removed during splicing
- B Protein-coding sequences retained in mRNA
- C Sequences that encode the 5' cap
- D Signals that initiate transcription
Explanation: Introns are intervening non-coding sequences removed from the primary transcript during RNA splicing. Exons are retained and joined to form mature mRNA.
Question 17: Exons are best described as:
- A Coding sequences retained in mature mRNA after splicing
- B Non-coding sequences removed during splicing
- C Promoter sequences only
- D Terminator sequences only
Explanation: Exons are the coding segments of a gene that are retained in mature mRNA after introns are spliced out.
Question 18: The spliceosome is a complex that:
- A Catalyzes intron removal from pre-mRNA
- B Synthesizes RNA from DNA
- C Adds the poly-A tail to mRNA
- D Translates mRNA into protein
Explanation: The spliceosome is a large RNA-protein complex (containing snRNAs and many proteins) that catalyzes intron removal and exon joining during mRNA processing.
Question 19: Alternative splicing of a pre-mRNA allows a single gene to produce:
- A Multiple protein isoforms from one gene
- B Only one protein isoform
- C Only non-coding RNAs
- D DNA from RNA
Explanation: Alternative splicing varies which exons are retained in the mature mRNA, so one gene can generate multiple distinct protein isoforms.
Question 20: Rho-dependent termination in bacteria requires:
- A The Rho protein (an ATP-dependent helicase)
- B The sigma factor
- C A poly-A tail
- D The TATA box
Explanation: Rho-dependent termination uses the Rho protein, an ATP-dependent helicase that binds the emerging RNA and disrupts the RNA-DNA-RNA polymerase complex.
Question 21: Rho-independent termination in bacteria involves:
- A A GC-rich hairpin in the RNA followed by a run of uridines
- B The Rho protein
- C The sigma factor
- D A 5' cap structure
Explanation: Rho-independent (intrinsic) termination depends on a GC-rich inverted repeat that forms a stable stem-loop followed by a run of uridines in the RNA, causing polymerase pausing and release.
Question 22: Reverse transcriptase synthesizes:
- A DNA from an RNA template
- B RNA from a DNA template
- C Protein from mRNA
- D Lipids from DNA
Explanation: Reverse transcriptase is an RNA-dependent DNA polymerase that makes DNA from an RNA template — the reverse of normal transcription.
Question 23: A polycistronic mRNA:
- A Encodes multiple proteins from one transcript
- B Encodes one protein from many transcripts
- C Contains no coding sequences
- D Is unique to eukaryotes
Explanation: Polycistronic mRNAs contain multiple coding regions (cistrons) under a single promoter, allowing several proteins to be translated from one transcript — common in bacteria and absent from most eukaryotic mRNAs.
Question 24: An operon is:
- A A cluster of co-transcribed genes regulated by a single promoter
- B A eukaryotic enhancer element
- C A type of intron
- D The origin of DNA replication
Explanation: An operon is a cluster of prokaryotic genes controlled by a single promoter and regulatory sequences, allowing coordinated expression.
Question 25: Enhancers are DNA elements that:
- A Increase transcription from a distance, independent of orientation
- B Terminate transcription
- C Encode the poly-A tail
- D Direct splicing of introns
Explanation: Enhancers are distal DNA elements that increase transcription of a target gene when bound by specific transcription factors — often from a distance and independent of orientation.