Statement A — TRUE ✓
The Watson-Crick double helix of B-DNA (the most common form) has a right-handed helical structure. The helix coils clockwise when viewed from above (like a conventional screw). This is one of the key features of the secondary structure proposed by Watson and Crick in 1953.
Statement B — FALSE ✗
The two strands of DNA are ANTI-PARALLEL, not parallel. One strand runs 5'→3' and the complementary strand runs 3'→5'. This antiparallel arrangement is essential for proper base pairing and is a fundamental feature of the double helix.
Statement C — TRUE ✓
Adenine (A) pairs with Thymine (T) by exactly 2 hydrogen bonds. This AT base pair is less stable than GC pairs. Remember: A=T (2 bonds), G≡C (3 bonds). The double and triple bond symbols indicate the number of H-bonds.
Statement D — TRUE ✓
Guanine (G) pairs with Cytosine (C) by exactly 3 hydrogen bonds. The extra H-bond makes GC pairs more stable and requires more energy (higher temperature) to separate. DNA with high GC content has a higher melting temperature (Tm).
In 1953, James Watson and Francis Crick proposed the double helix model of DNA, based on X-ray crystallography data from Rosalind Franklin and Erwin Chargaff's base pairing rules. The model explained how genetic information is stored and replicated. The key features of B-DNA (the most common form found in cells):
📌 Double-stranded helix — two polynucleotide chains
📌 Right-handed helix (coils clockwise when viewed from top)
📌 Antiparallel strands: one 5'→3', other 3'→5'
📌 Sugar-phosphate backbone on outside; bases inside
📌 Bases held by hydrogen bonds (A=T: 2 H-bonds; G≡C: 3 H-bonds)
📌 Stacking interactions between adjacent bases also stabilise helix
📌 Pitch (one complete turn) = 3.4 nm, contains 10 base pairs
📌 Diameter of helix ≈ 2 nm
DNA is made of nucleotide monomers. Each nucleotide has three parts: a deoxyribose sugar (5-carbon), a phosphate group, and a nitrogenous base. The four nitrogenous bases in DNA are Adenine, Guanine (purines — double ring), Thymine, and Cytosine (pyrimidines — single ring). Nucleotides are linked by 3'-5' phosphodiester bonds between adjacent nucleotides to form the polynucleotide chain.
Purines (double ring): Adenine (A), Guanine (G)
Pyrimidines (single ring): Thymine (T), Cytosine (C) — in DNA
Pyrimidines in RNA: Uracil (U) instead of Thymine
Base pairing: A=T (2 H-bonds), G≡C (3 H-bonds)
Erwin Chargaff discovered that in any DNA sample, [A] = [T] and [G] = [C] (molar ratios). This is called Chargaff's rule, and it provided crucial evidence for complementary base pairing. It follows directly from the double helix structure — every A on one strand pairs with a T on the other, and every G pairs with a C. Importantly, [A+G] = [T+C] (purines = pyrimidines) but [A+T] ≠ [G+C] (varies between species).
📌 DNA: deoxyribose sugar; RNA: ribose sugar (extra OH at 2' position)
📌 DNA: Thymine (T); RNA: Uracil (U) — U has no methyl group
📌 DNA: usually double-stranded; RNA: usually single-stranded
📌 DNA: found mainly in nucleus; RNA: nucleus and cytoplasm
📌 DNA: more stable (no 2'-OH for hydrolysis); RNA: less stable
📌 Base pairing in RNA: A-U (2 H-bonds), G-C (3 H-bonds)
mRNA (messenger RNA): Carries genetic information from DNA to ribosomes. Typically single-stranded. Contains codons (triplets of bases) that specify amino acids. About 5% of total cellular RNA.
tRNA (transfer RNA): Brings amino acids to the ribosome during translation. Has cloverleaf secondary structure with anticodon loop that pairs with mRNA codon. About 15% of cellular RNA. Smallest RNA (~75-90 nucleotides).
rRNA (ribosomal RNA): Forms the structural and catalytic core of ribosomes along with ribosomal proteins. Most abundant RNA (~80% of cellular RNA). Forms complex secondary structures with extensive base pairing within the single strand.
Meselson and Stahl's experiment (1958) proved DNA replication is semiconservative — each daughter DNA has one original (parental) strand and one new strand. Enzymes involved: Helicase (unwinds and separates strands), DNA Polymerase (synthesises new strand in 5'→3' direction only), Primase (makes RNA primer), DNA Ligase (joins Okazaki fragments on lagging strand). Replication is bidirectional from origins of replication.
DNA → (Transcription) → RNA → (Translation) → Protein
Reverse transcription (in retroviruses): RNA → DNA
The central dogma states that genetic information flows from DNA to RNA to protein. Transcription makes mRNA from DNA template. Translation uses mRNA codons and tRNA anticodons to assemble proteins at ribosomes. Genetic code is triplet (3 bases = 1 codon), degenerate (multiple codons per amino acid), and universal (same in almost all organisms).
Mutations are changes in DNA sequence. Point mutations: substitution of one base for another (may be silent, missense, or nonsense). Frameshift mutations: insertion or deletion of bases (shifts reading frame, usually more severe). Transitions: purine-purine or pyrimidine-pyrimidine change. Transversions: purine-pyrimidine change. Sickle cell anaemia is caused by a single point mutation (A→T) in the beta-globin gene, converting Glu to Val at position 6.