Transcriptome analysis

Transcriptome analysis

In Run-off transcription assay, the intensity of bands on gel reflects:

In run-off transcription assays, a linear DNA template is incubated with RNA polymerase and radiolabeled or fluorescently tagged ribonucleotides. Transcription proceeds to the end of template, generating transcripts of uniform length dictated by restriction cut position. These products are electrophoresed on denaturing polyacrylamide gels and detected by autoradiography. Band intensity correlates proportionally with the number of transcription events and overall transcriptional activity under experimental condition, reflecting promoter efficiency, factor presence, or inhibitor effect. It does not report DNA purity, mRNA degradation, or RNase activity directly.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Transcriptome analysis

Which of the following is not an in vivo transcriptome study method?

In vivo transcriptome methods aim to examine RNA within living biological context or preserve spatial and physiological conditions, such as direct RNA sequencing from freshly harvested organisms, FISH performed on fixed but morphologically intact specimens retaining tissue architecture, and CAGE capturing authentic capped transcripts from living material. Microarray technology requires RNA extraction, purification, labeling, and hybridization to synthetic probes on an artificial substrate outside the organism. This intermediate in vitro processing disconnects RNA from native context, making microarray primarily an ex vivo analytic platform rather than direct in vivo observation.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Transcriptome analysis

Which in vitro assay measures the rate of transcription from a cut DNA template?

Run-off transcription is an in vitro biochemical assay designed to study promoter strength and transcription factor requirements. A DNA template containing a promoter followed by a defined sequence is linearized downstream with restriction enzymes, causing RNA polymerase to transcribe and then fall off at the cut site, producing a transcript of fixed length. Reactions include labeled UTP and nuclear extracts or purified factors. Quantity of RNA synthesized is measured by autoradiography. Northern blot detects steady-state RNA, SAGE quantifies tags, and ChIP assesses protein occupancy, not transcription rate from truncated template.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Transcriptome analysis

A hybridization-based method that uses mRNA extracted from colonies on a membrane is:

Colony hybridization involves transferring bacterial colonies or phage plaques from agar plates onto nitrocellulose or nylon membranes, lysing them to release nucleic acids, and immobilizing DNA or RNA onto membrane surface. Labeled probes specific to target mRNA or genes are hybridized to filter, allowing screening of many clones simultaneously. This method was pivotal for library screening before high-throughput sequencing. FISH performs in situ hybridization on chromosomes, northern blot involves gel electrophoresis, and CAGE captures 5′ caps, none employing colony lysis on membranes for mRNA detection from colonies.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Transcriptome analysis

Which technique would best identify more than 1000 differentially expressed genes in one test?

A transcriptome microarray, also called expression array, contains probes for all annotated genes or entire transcriptome represented as tens of thousands of spots on a chip. Single hybridization of labeled cDNA from experimental and control samples allows parallel assessment of expression differences. Statistical analysis identifies hundreds to thousands of differentially expressed genes by fold change and p-value. RAPD assesses DNA polymorphism, genome sequencing determines DNA sequence rather than expression levels, and ChIP assesses protein-DNA interactions, making them unsuitable for simultaneous high-throughput quantification of over a thousand expression changes.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Transcriptome analysis

Which technique enables study of gene copy number and chromosomal aberrations in tumors?

Comparative Genomic Hybridization was developed to survey genome-wide copy number alterations in tumor samples where karyotyping is challenging. By competitively hybridizing fluorescently labeled tumor DNA and normal reference DNA to normal metaphase chromosomes or microarray probes, gains and losses are visualized as altered green-to-red ratios. Amplifications of oncogenes and deletions of tumor suppressor loci are revealed across chromosomes. CAGE maps TSS, DNA microarrays measure expression, and run-off assays measure transcription rates, none providing direct genome-wide copy number and structural aberration assessment inherent to CGH design.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Transcriptome analysis

What feature allows rapid screening of Down syndrome in M-FISH?

M-FISH uses combinatorial fluorescence strategy where each chromosome is painted with a unique combination of fluorochromes, creating a distinct spectral signature decoded by software. Down syndrome results from trisomy 21. In M-FISH, chromosome 21 probes labeled with a specific fluorochrome combination appear thrice per metaphase spread instead of twice, readily identified by extra same-colored chromosome and automated image analysis. Color-coded fluorophores thus enable rapid whole-karyotype screening without chromosome-specific conventional G-banding expertise. Probe size, RNA tagging, and secondary antibody use are not primary discriminators for aneuploidy detection speed.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Transcriptome analysis

Which of the following detects tandem repeats poorly?

Interphase FISH uses decondensed nuclear chromatin, where spatial resolution and signal coalescence are suboptimal. Tandem repeat clusters, being repetitive and often closely spaced, produce merged or diffuse fluorescent signals that are hard to quantitate and enumerate in interphase nuclei. Metaphase chromosomes provide condensed, linearly ordered targets where repeat arrays are linearly resolved. Microarray and SAGE interrogate sequence content differently. Hence, interphase FISH suffers reduced sensitivity and specificity for enumerating tandem repeats compared to metaphase FISH, which offers superior cytogenetic resolution of repeated sequences and breakpoints.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Transcriptome analysis

What is the basic principle behind DNA microarrays?

DNA microarray performance depends on Watson-Crick base pairing between immobilized probes and fluorescently labeled target nucleic acids. Under controlled temperature and salt concentration, complementary sequences form stable duplexes, while mismatched sequences are removed during stringent washes. The resulting hybridization signal strength reflects thermodynamic stability and target concentration, permitting quantitative comparison across samples. PCR amplification, competitive binding unrelated to nucleic acid complementarity, and northern transfer principles describe separate techniques. Specificity of hybridization ensures that even single-base mismatches reduce signal, enabling expression and genotyping applications.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Transcriptome analysis

In DNA Microarrays, what does each spot represent?

A DNA microarray contains ordered microscopic spots, each printed with multiple identical probes complementary to a single transcriptional unit. Typically, probes correspond to a specific gene, EST, or predicted open reading frame. When fluorescently labeled cDNA or cRNA derived from samples hybridizes, intensity at each coordinate correlates with abundance of that gene's transcripts. Although a gene may contain multiple exons, array design aggregates exons under one gene identifier or multiple spots per exon in exon arrays. Spots do not represent individual primers, proteins, or isolated exons in conventional expression arrays.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Transcriptome analysis

Which of the following techniques detects RNA directly in intact tissues or cells?

In situ hybridization preserves spatial context by detecting nucleic acids within fixed cells or tissue sections. Labeled antisense RNA or DNA probes penetrate permeabilized cells and hybridize to endogenous mRNA targets, which are then visualized enzymatically or fluorescently, revealing tissue-specific and subcellular expression patterns. Northern blot, real-time PCR, and CGH all require RNA or DNA extraction, destroying morphology and precluding localization. Because developmental biology, neurobiology, and pathology rely on knowing which cells express a transcript, in situ methods uniquely enable direct RNA detection in intact biological architecture.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Transcriptome analysis

Which invention is credited to Patrick O. Brown?

Patrick O. Brown, at Stanford University, introduced the concept of printing defined DNAs onto glass slides via robotic spotting in 1995. His laboratory demonstrated that differently labeled cDNAs from two samples could competitively hybridize to such arrays to quantify differential gene expression genome-wide. This innovation founded modern transcriptomics, enabling simultaneous monitoring of thousands of genes, shifting paradigm from single-gene northern blots. Fluorescence In Situ Hybridization, RNA sequencing, and CGH were developed by different groups focusing on cytogenetics and sequencing, not array-based expression profiling credited to Brown.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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