Mnemonics: Molecular Basis of Inheritance | Biology Class 12 - NEET PDF Download

1. Key Experiments for Genetic Material

4 Experiments: Griffith’s Transformation, Avery-Mac Leod-Mc Carty’s Biochemical Characterization, Hershey-Chase’s Bacteriophage Experiment, Meselson-Stahl’s Semiconservative Replication.
Mnemonic: "Griffith Transforms, Avery Finds, Hershey Chases, Meselson Proves"
Breakdown:

• Griffith Transforms → Griffith’s 1928 experiment with Streptococcus pneumoniae showed transformation (R to S strain via a “transforming principle”).
• Avery Finds → Avery-MacLeod-McCarty (1933-44) identified DNA as the transforming principle by showing DNase inhibits transformation.
• Hershey Chases → Hershey-Chase (1952) used radioactive phosphorus (DNA) and sulfur (protein) to prove DNA enters bacteria during bacteriophage infection.

The Hershey-Chase experiment

• Meselson Proves → Meselson-Stahl (1958) confirmed semiconservative replication using heavy (15N) and light (14N) nitrogen in E. coli.

Meselson and Stahl’s Experiment

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2. Polysaccharides (in Griffith’s Experiment)

Concept: Polysaccharide coat distinguishes smooth (S) and rough (R) strains of Streptococcus pneumoniae in Griffith’s transformation experiment.
Mnemonic: "Smooth Coats Kill, Rough Don’t"
Breakdown:

• Smooth → S strain has a polysaccharide (mucous) coat, making it virulent (kills mice).
• Coats → The polysaccharide coat is key to the S strain’s virulence.
• Kill → S strain causes pneumonia, leading to death in mice.
• Rough Don’t → R strain lacks the polysaccharide coat, is non-virulent, and doesn’t kill mice.

3. DNA Replication in Prokaryotes

Concepts: Semiconservative replication, replication fork, DNA-dependent DNA polymerase, continuous/discontinuous synthesis, DNA ligase, occurs in cytosol.
Mnemonic: "Fork Opens, Poly Builds Fast"
Breakdown:

• Fork → Replication fork where DNA strands separate.

• Opens → Strands act as templates (3’→5’ for continuous, 5’→3’ for discontinuous synthesis).

• Poly → DNA-dependent DNA polymerase catalyzes 5’→3’ polymerization.

• Builds → Semiconservative replication produces two daughter DNAs (one parental, one new strand).

• Fast → Rapid process in the cytosol.

Replicating Fork

4. DNA Replication in Eukaryotes

Concepts: Semiconservative replication, replication fork, DNA polymerase, occurs in nucleus during S-phase, coordinated with cell cycle.
Mnemonic: "Forks Move, Nucleus Works Slow"
Breakdown:

• Forks → Replication forks form where strands separate.

• Move → Strands are templates for semiconservative replication.

• Nucleus → Replication occurs in the nucleus at S-phase.

• Works → DNA polymerase synthesizes new strands (5’→3’).

• Slow → Slower due to larger genome and cell cycle coordination.

5. Transcription in Prokaryotes

Concepts: Single RNA polymerase, promoter, template strand, polycistronic mRNA, no processing, coupled with translation, cytosolic.
Mnemonic: "One Pen Copies, mRNA Goes"
Breakdown:

• One → Single DNA-dependent RNA polymerase transcribes all RNAs.

• Pen → Promoter (5’-end) binds polymerase to start transcription.

• Copies → Template strand (3’→5’) is copied into RNA (A-U, G-C pairing).

• mRNA → Polycistronic mRNA codes for multiple proteins.

• Goes → mRNA is functional immediately, used in cytosolic translation (coupled).

Process of Transcription in Bacteria

6. Transcription in Eukaryotes

Concepts: Three RNA polymerases, nuclear transcription, monocistronic hnRNA, splicing, capping, tailing, transport to cytoplasm.
Mnemonic: "Three Pens Slice, Cap, Send"
Breakdown:

• Three → Three RNA polymerases: I (rRNAs), II (hnRNA), III (tRNA, 5S rRNA, snRNAs).

• Pens → Promoter-driven transcription in the nucleus.

• Slice → Splicing removes introns, joining exons in hnRNA.

• Cap → 5’-methyl guanosine cap and 3’-poly-A tail added.

• Send → Mature, monocistronic mRNA is transported to cytoplasm.

Process of Transcription in Eukaryotes

7. Translation in Prokaryotes

Concepts: 70S ribosomes, initiator tRNA (AUG), polycistronic mRNA, coupled with transcription, cytosolic, peptide bond formation.
Mnemonic: "Small Ribs Read, Proteins Form"
Breakdown:

• Small → 70S ribosomes (50S + 30S).

• Ribs → Ribosomes read mRNA codons, with rRNA as a ribozyme (peptide bonds).

• Read → Initiator tRNA recognizes AUG, starting translation.

• Proteins → Polycistronic mRNA produces multiple proteins.

• Form → Peptide bonds form in initiation, elongation, termination (stop codons: UAA, UAG, UGA).

Translation

8. Translation in Eukaryotes

Concepts: 80S ribosomes, initiator tRNA (AUG), monocistronic mRNA, cytoplasmic, peptide bond formation, UTRs.
Mnemonic: "Big Ribs Scan, Proteins Build"
Breakdown:

• Big → 80S ribosomes (60S + 40S).

• Ribs → Ribosomes read mRNA, with rRNA as a ribozyme.

• Scan → Initiator tRNA recognizes AUG, with UTRs aiding initiation.

• Proteins → Monocistronic mRNA codes for one protein.

• Build → Peptide bonds form in initiation, elongation, termination (stop codons).

9. Lac Operon

Concepts: Regulatory gene (i), structural genes (z, y, a), promoter, operator, repressor, inducer (lactose), negative regulation.
Mnemonic: "Lactose Frees O, Genes Make Milk"
Breakdown:

• Lactose → Lactose acts as the inducer, binding the repressor.

• Frees O → Repressor (from i gene) is inactivated, freeing the operator.

• Genes → Structural genes z (beta-galactosidase), y (permease), a (transacetylase) are transcribed.

• Make → RNA polymerase binds promoter, transcribes genes for lactose metabolism.

• Milk → Refers to lactose (milk sugar) breakdown into galactose and glucose.

The lac Operon