Molecular Biology, Gene Technology & Biosafety – Units 07 & 08 (G.C.E. A/L Biology Grade 13)

300 English question-and-answer flashcards covering DNA structure, replication, gene expression, mutations, biotechnology tools, PCR, sequencing, GMOs, biosafety, and related concepts from Units 07 and 08 of the G.C.E. (A/L) Biology Grade 13 syllabus.

What are the two rails of the DNA double helix composed of?

Alternating deoxyribose sugars and phosphate groups.

Which bases pair together in DNA and with how many hydrogen bonds?

Adenine pairs with thymine via two H-bonds; guanine pairs with cytosine via three H-bonds.

Who proposed the DNA double helix model?

James Watson and Francis Crick (using Rosalind Franklin’s X-ray data).

In prokaryotes, where is the DNA located?

In the nucleoid region of the cytoplasm.

What is a eukaryotic chromosome primarily composed of?

A single linear double-stranded DNA molecule associated with histone and non-histone proteins.

Define chromatin.

The DNA–protein complex that packages eukaryotic DNA inside the nucleus.

Differentiate euchromatin from heterochromatin.

Euchromatin is loosely packed and transcriptionally active; heterochromatin is tightly packed and largely inactive.

What is a nucleosome?

DNA wrapped ~1.7 turns around a histone octamer, forming the basic unit of chromatin.

Approximate diameter of the 10 nm ‘beads-on-a-string’ fiber?

About 10 nanometres.

What diameter does the chromatin fiber reach after solenoid/zig-zag folding?

Roughly 30 nm.

Name the four levels of eukaryotic DNA compaction.

(1) Nucleosomes (10 nm) (2) 30 nm fiber (3) Radial loop domains (~300 nm) (4) Mitotic chromosome (700 nm chromatid).

What additional genetic elements may be present in prokaryotes besides the chromosome?

Plasmids (extra-chromosomal circular DNA).

Define DNA replication.

The semi-conservative process by which a double-stranded DNA molecule is copied to produce two identical DNA molecules.

Why is DNA replication essential before mitosis and meiosis?

To ensure each daughter cell or gamete receives the correct genetic information and chromosome number.

What is the specific DNA sequence where replication begins?

Origin of replication (Ori).

Name the continuously synthesized strand.

Leading strand.

What are Okazaki fragments?

Short DNA fragments synthesized discontinuously on the lagging strand.

Function of DNA helicase.

Unwinds and separates the two DNA strands by breaking hydrogen bonds.

Role of single-strand binding proteins (SSB).

Stabilize separated strands and prevent re-annealing during replication.

Function of topoisomerase.

Relieves torsional strain ahead of the replication fork by introducing transient breaks.

What enzyme synthesizes the RNA primer?

Primase (an RNA polymerase).

Which enzyme extends DNA from the primer?

DNA polymerase (adds dNTPs in 5′→3′ direction).

Which enzyme replaces RNA primers with DNA in prokaryotes?

DNA polymerase I.

Name the enzyme that seals nicks between adjacent DNA fragments.

DNA ligase.

What is proofreading activity?

DNA polymerase’s 3′→5′ exonuclease function that removes incorrectly paired nucleotides.

Define mutation.

A heritable change in the nucleotide sequence of DNA.

Difference between gene mutation and chromosomal mutation.

Gene mutation alters nucleotide sequence of a single gene; chromosomal mutation changes chromosome number or large structural segments.

What is a silent mutation?

A substitution that does not alter the encoded amino acid.

What is a missense mutation?

A base substitution that changes one amino acid to another.

Define nonsense mutation.

A substitution converting an amino-acid codon into a stop codon, prematurely terminating translation.

Frameshift mutation results from what?

Insertion or deletion of nucleotides not in multiples of three, altering the reading frame.

Give an example of a human disease caused by a missense mutation.

Sickle-cell anaemia (Glu → Val in β-globin).

Define aneuploidy.

Presence of one or a few chromosomes more or less than the normal diploid number (e.g., 2n ± 1).

What chromosomal abnormality causes Down syndrome?

Trisomy of chromosome 21 (47, +21).

Klinefelter syndrome karyotype?

47, XXY.

Turner syndrome karyotype?

45, XO.

Define polyploidy.

Possession of more than two complete sets of chromosomes (e.g., 3n, 4n).

Give an example of a naturally triploid crop.

Banana (3n).

Define gene.

A DNA segment at a specific locus that codes for a functional product (RNA or polypeptide).

What is an operon?

A prokaryotic gene cluster under control of a single promoter and operator, transcribed as one mRNA.

Introns vs exons.

Introns are non-coding sequences removed during mRNA processing; exons remain and code for protein.

Define transcription.

Synthesis of RNA using DNA as template.

Which enzyme catalyzes transcription?

RNA polymerase.

What is a promoter?

DNA sequence where RNA polymerase binds to initiate transcription.

Direction of RNA synthesis.

5′→3′.

Define translation.

Process of decoding mRNA to synthesize a polypeptide at the ribosome.

Start codon and its amino acid.

AUG; codes for methionine.

How many stop codons exist?

Three (UAA, UAG, UGA).

What is meant by ‘genetic code is universal’?

Generally, the same codon specifies the same amino acid in almost all organisms.

Define anticodon.

A tRNA triplet complementary to an mRNA codon.

Name the three ribosomal sites.

A (aminoacyl), P (peptidyl), and E (exit) sites.

What is a polyribosome?

Multiple ribosomes translating a single mRNA simultaneously.

Post-translational modification example.

Addition of phosphate groups (phosphorylation).

Define DNA repair by nucleotide excision.

Removal of damaged segment by nuclease, gap filling by DNA polymerase, sealing by ligase.

What enzyme joins DNA fragments from different sources in cloning?

T4 DNA ligase.

What are restriction endonucleases?

Enzymes that recognize specific DNA sequences and cut both strands at or near those sites.

Give an example of a restriction enzyme and its source.

EcoRI from Escherichia coli.

Sticky ends vs blunt ends.

Sticky ends have single-stranded overhangs; blunt ends are straight cuts without overhangs.

Purpose of agarose gel electrophoresis.

To separate DNA fragments by size using an electric field.

How is DNA visualized in gels?

Staining with ethidium bromide and viewing under UV light.

Define DNA probe.

A labeled single-stranded DNA fragment used to detect complementary sequences by hybridization.

What is Southern blotting used for?

Transferring DNA fragments from gel to membrane for probe hybridization.

Define cloning vector.

Self-replicating DNA molecule used to carry foreign DNA into a host cell for cloning.

Give two common bacterial cloning vectors.

Plasmids and bacteriophage λ derivatives.

Essential features of a plasmid cloning vector.

Origin of replication, selectable marker, and multiple cloning site.

Selectable marker example in plasmids.

Antibiotic resistance gene (e.g., amp^R).

Transformation definition (in genetics).

Uptake of naked foreign DNA by a cell resulting in genetic change.

What is polymerase chain reaction (PCR)?

In vitro technique that amplifies a specific DNA segment exponentially using cycles of denaturation, annealing, and extension.

Heat-stable DNA polymerase commonly used in PCR.

Taq DNA polymerase from Thermus aquaticus.

Typical denaturation temperature in PCR.

About 94–95 °C.

Purpose of primers in PCR.

Provide free 3′-OH ends for DNA polymerase to extend and define the region to be amplified.

How many cycles are common in standard PCR?

30–40 cycles.

Define cDNA.

Complementary DNA synthesized from an mRNA template by reverse transcriptase.

What is a genomic library?

Collection of host cells each carrying a vector with a different fragment of an organism’s entire genome.

What is a cDNA library useful for?

Studying expressed genes without introns and examining tissue-specific transcription.

Name a yeast artificial chromosome feature not in simple plasmids.

Centromere sequence enabling segregation like a chromosome.

Define restriction map.

Diagram showing the positions of restriction sites and distances between them on DNA.

What is DNA sequencing?

Determining the exact order of nucleotides in a DNA molecule.

First-generation sequencing method discovered by?

Frederick Sanger (dideoxy chain-termination method).

Two main applications of DNA sequencing in medicine.

Diagnosis of genetic diseases and guiding personalized therapies.

Define metagenomics.

Sequencing and analysis of pooled DNA from an environmental sample to study microbial communities.

What are Short Tandem Repeats (STRs)?

1–6 bp sequences repeated in tandem, highly polymorphic and used in DNA fingerprinting.

How many STR loci are typically used in forensic profiling?

Thirteen core loci (in many jurisdictions).

Purpose of DNA fingerprinting in paternity testing.

To compare child’s STR alleles with those of alleged parents for inheritance matches.

Bt toxin source organism.

Bacillus thuringiensis.

Main advantage of Bt crops.

Intrinsic resistance to specific insect pests, reducing pesticide use.

Define herbicide-tolerant crop (HTC).

GM crop engineered to survive application of a particular broad-spectrum herbicide.

Trade name of glyphosate-tolerant crops.

Roundup Ready.

Example of disease-resistant GM fruit crop.

Papaya resistant to Papaya ringspot virus.

Golden rice is engineered to produce which nutrient?

β-carotene (pro-vitamin A).

What is an edible vaccine?

A vaccine produced in an edible plant tissue that elicits immunity when consumed.

Define gene therapy.

Clinical technique that introduces, removes, or alters genes in a patient’s cells to treat disease.

Example of a vector for gene therapy.

Adeno-associated virus (AAV).

What is sterile insect technique (SIT)?

Releasing sterile (often GM) males to mate with wild females, reducing pest populations.

Industrial enzyme produced by GM yeast for cheese making.

Chymosin (rennin).

Define biosafety.

Policies and procedures to ensure safe handling and use of GMOs to protect human health and environment.

Cartagena Protocol aligns with which broader treaty?

Convention on Biological Diversity (CBD).

Core aim of the Cartagena Protocol.

Protect biodiversity and human health from potential risks of living modified organisms.

What is Biosafety Clearing-House (BCH)?

An information exchange mechanism under the Cartagena Protocol for LMOs.

Year Sri Lanka ratified Cartagena Protocol.

2004.