Molecular Biology

Constitutive genes are expressed continuously at a constant rate without regulation - they're 'housekeeping' genes essential for basic cellular functions like metabolism, structure, and maintenance. Unlike regulated genes that respond to environmental signals, constitutive genes maintain baseline expression levels. Gratuitous inducer genes, repressor genes, and oncogenes are all subject to regulation.

Southern blot begins with DNA isolation from cells - you cannot fragment, separate, or transfer DNA that hasn't been extracted first. After isolation (C), the DNA is cut with restriction enzymes (A), separated by electrophoresis (B), denatured (D), and transferred to a membrane (E). Option C is logically first - you need the DNA material before any processing.

Western blot is used to detect specific proteins, including antibodies against HIV. ELISA (A) is the initial screening test, but Western blot (C) is the confirmatory test that detects antibodies to specific HIV proteins by their molecular weight. FISH (B) is for DNA/RNA localization, Northern blot (D) detects RNA, and Southern blot (E) detects DNA.

Burkitt's lymphoma is caused by over expression of myc gene.

Chronic myelogenous leukemia (CML) is caused by the Philadelphia chromosome, resulting from a reciprocal translocation between chromosomes 9 and 22 - t(9;22)(q34;q11). This creates the BCR-ABL fusion gene with constitutive tyrosine kinase activity. Other malignancies listed have different causes (lung cancer from mutations, CLL from various genetic changes, AML from multiple mutations, bladder cancer from environmental factors).

Antisense therapy uses synthetic DNA/RNA sequences to bind target mRNA and block protein expression. It's been applied to treat acute leukemia (blocking oncogene expression), HIV infection (inhibiting viral replication), and papillomavirus infections. The other options describe different applications: B is DNA fingerprinting, C is genetic testing, D is PCR-based diagnostics, and E is virology techniques.

Creating chimeric DNA (recombinant DNA) begins with isolating and amplifying the specific gene of interest - you need sufficient copies of the target gene before combining it with a vector. Options B, D, and E describe other steps in recombinant DNA technology, while C incorrectly describes annealing without specifying the correct components.

Ribosomes read mRNA in the 5' to 3' direction during translation. As the ribosome moves along the mRNA, it reads codons sequentially and adds amino acids to the growing polypeptide chain. The 3' to 5' option describes the template strand direction during DNA replication, not translation.

Translocation is the process where the ribosome moves exactly 3 nucleotides (one codon) along the mRNA after peptide bond formation. This shifts the tRNAs: the deacylated tRNA moves to the E site, the peptidyl-tRNA moves to the P site, and a new codon enters the A site for decoding.

AUG is the universal start codon in both prokaryotes and eukaryotes. It codes for methionine (or N-formylmethionine in prokaryotes) and signals the initiation of protein synthesis. The other options include stop codons (UGA) or codons for different amino acids.

When lactose is present, some of it is converted to allolactose, which binds to the lac repressor protein. This binding causes the repressor to change shape and release from the operator site, allowing RNA polymerase to transcribe the lac genes. CAP (catabolite activator protein) actually binds to the promoter, not the operator.

When tryptophan is abundant in the environment, it acts as a corepressor by binding to the trp repressor protein. This binding changes the repressor's conformation, allowing it to bind to the trp operator and block transcription of the tryptophan biosynthesis genes. This is negative feedback regulation.

Yeast artificial chromosomes (YACs) can accommodate the largest DNA inserts, ranging from hundreds of kilobases to over 2 megabases. This far exceeds the capacity of BACs (100-200 kb), cosmids (35-45 kb), phages (5-25 kb), or plasmids (typically <15 kb). YACs are essential for cloning large genomic fragments.

Sticky ends are complementary single-stranded overhangs created when restriction enzymes make staggered cuts. When DNA fragments are cut with the same enzyme, they produce identical sticky ends that can anneal through base pairing. This allows DNA ligase to join the fragments covalently, forming recombinant DNA molecules.

DNA probes are labeled single-stranded DNA sequences complementary to the target gene of interest. When added to a genomic library, they hybridize specifically to their complementary sequences through base pairing, allowing researchers to identify and select clones containing the desired gene. This is called colony or plaque hybridization.

Purines (adenine and guanine) have a double-ring structure consisting of a pyrimidine ring fused to an imidazole ring. Pyrimidines (cytosine, thymine, and uracil) have a single six-membered ring. This structural difference is fundamental to DNA and RNA structure and affects base pairing patterns.

In vivo gene therapy delivers genetic material directly into the patient's body. This nasal spray delivers the CFTR gene directly to lung epithelial cells via adenovirus vectors, compensating for the defective gene in cystic fibrosis patients. This contrasts with ex vivo therapy, where cells are modified outside the body before reinfusion.

In the Watson-Crick DNA model, adenine always pairs with thymine through two hydrogen bonds. This complementary base pairing is fundamental to DNA's double-helix structure and replication mechanism.

A nucleotide consists of a nucleoside (nitrogenous base + sugar) plus one or more phosphate groups. When the ribose or deoxyribose sugar of a nucleoside is phosphorylated, it becomes a nucleotide.

FISH (Fluorescence In Situ Hybridization) is the standard screening test for aneuploidy syndromes like Down syndrome, Turner syndrome, etc. It uses fluorescent probes to detect chromosomal abnormalities.

The degeneracy of the genetic code means that most amino acids are specified by more than one codon. For example, leucine has six different codons. This redundancy buffers against mutations.

Dicumarol is a vitamin K antagonist that inhibits the carboxylation of glutamic acid residues in clotting factors II, VII, IX, and X. This post-translational modification is essential for their activity.

DNA melting temperature (Tm) is primarily determined by GC content because G-C pairs form three hydrogen bonds versus two for A-T pairs. Higher GC content means higher Tm due to stronger bonding.