Central Dogma - Chromosomal basis of Inheritance | Class 12th BIOLOGY NCERT | NEET 2026 скачать в хорошем качестве

Central Dogma - Chromosomal basis of Inheritance | Class 12th BIOLOGY NCERT | NEET 2026

DNA Replication Meselson and Stahl Experiment (1958) – “The most beautiful experiment in biology” Aim: To prove the mode of DNA replication. Method: They grew E. coli in 15N (heavy nitrogen) medium → DNA became heavy. Then transferred bacteria to 14N (light nitrogen) medium. DNA was extracted after 1 generation and 2 generations. DNA was separated by density gradient centrifugation (CsCl centrifugation). Observation: After 1st generation: DNA showed intermediate (hybrid) density (one old heavy strand + one new light strand). After 2nd generation: Two bands → 50% hybrid DNA + 50% light DNA. Conclusion: DNA replicates in a semiconservative manner → each daughter DNA has one parental strand and one new strand. Definition: DNA replication is the process of making an identical copy of DNA before cell division. Model: Watson & Crick proposed semiconservative replication → each daughter DNA molecule has one parental (old) strand and one newly synthesized strand. Steps: Initiation → DNA double helix unwinds at the origin with the help of helicase, forming replication fork. Elongation → DNA polymerase adds nucleotides to the 3' end (new strand synthesized in 5' → 3' direction). Leading strand: continuous synthesis. Lagging strand: discontinuous synthesis in fragments (Okazaki fragments), later joined by ligase. Termination → replication completes, producing two identical DNA molecules. DNA Replication DNA replication is the process by which a cell makes an exact copy of its DNA before cell division. It is semiconservative, meaning each new DNA molecule consists of one old (parental) strand and one newly synthesized strand. Steps in DNA replication: Initiation – The double helix unwinds with the help of helicase, forming a replication fork. Elongation – DNA polymerase adds complementary nucleotides (A–T, G–C) to each parental strand. Leading strand: synthesized continuously. Lagging strand: synthesized in fragments (Okazaki fragments) joined by ligase. Termination – Two identical DNA molecules form, each with one parental and one new strand.