Molecular Basis Of Inheritance

Step 1: Chargaff's Rule states that in DNA, and . Also, the total percentage of purines equals pyrimidines ( ).
Step 2: If , then must also be . Together, .
Step 3: The remaining must be . Since , then and .
Step 4: Uracil is not present in DNA; it is only found in RNA. Therefore, Uracil is .

Step 1: Understanding the Concept:
The Lac operon is a system for the regulated expression of genes involved in lactose metabolism in E. coli.
Step 2: Detailed Explanation: Substance A is the Inducer.
In the absence of an inducer, the repressor protein (produced by the 'i' gene) binds to the operator region and prevents RNA polymerase from transcribing the structural genes.
When the inducer (Lactose) is present, it binds to the repressor.
This binding causes a conformational change in the repressor, making it inactive and unable to bind to the operator.
This allows RNA polymerase access to the promoter, and transcription proceeds (the operon is switched "ON").
Step 3: Final Answer:
The function of substance A is to inactivate the repressor so that the operon can be transcribed.

Step 1: Understanding the Concept:
The structural genes in the Lac operon code for three different enzymes required for lactose metabolism.
Step 2: Detailed Explanation:
1. Label B (from 'z' gene): This codes for -galactosidase. This enzyme hydrolyzes lactose into glucose and galactose.
2. Label C (from 'y' gene): This codes for Permease. This enzyme increases the permeability of the cell to -galactosides (lactose).
3. Label D (from 'a' gene): This codes for Transacetylase. It transfers an acetyl group from acetyl-CoA to -galactosides.
Step 3: Final Answer:
B is -galactosidase, C is Permease, and D is Transacetylase.

Step 1: Understanding the Concept:
The regulation of the Lac operon is an energy-saving mechanism; the cell only produces enzymes when the substrate is available.
Step 2: Detailed Explanation:
If the inducer (lactose) is not present:
1. The 'i' gene constitutively synthesizes the repressor mRNA, which is translated into an active repressor protein.
2. This active repressor binds firmly to the operator region (o) of the operon.
3. The bound repressor physically blocks the RNA polymerase from moving forward from the promoter to the structural genes.
4. Consequently, transcription of genes does not occur, and no enzymes are produced.
Step 3: Final Answer:
The operon would remain in a repressed (OFF) state, and no enzymes for lactose metabolism would be synthesized.

Step 1: Understanding the Concept:
By 1952, scientists knew that chromosomes carried genetic information, but they debated whether that information was stored in DNA or Protein.
Hershey and Chase used bacteriophages (viruses that infect bacteria) because they consist only of a DNA core surrounded by a protein coat.
Step 2: Key Formula or Approach:
The experimental approach relied on labeling the two components with specific radioactive tracers that do not overlap.
- DNA contains Phosphorus ( ) in its phosphate backbone but lacks Sulfur ( ).
- Proteins contain Sulfur ( ) in amino acids like Cysteine and Methionine but lack Phosphorus ( ).
Step 3: Detailed Explanation:
The experiment was conducted in three logical stages:
1. Infection:
- Batch 1: Phages were grown with . Their DNA became radioactive.
- Batch 2: Phages were grown with . Their protein coats became radioactive.
- These phages were allowed to infect E. coli bacteria.
2. Blending:
- The mixture was agitated in a blender. This mechanical force sheared off the viral "ghosts" (empty protein shells) from the surface of the bacteria without killing the bacteria.
3. Centrifugation:
- The mixture was spun at high speed. The heavy bacteria formed a "pellet" at the bottom, while the lighter viral parts remained in the "supernatant" (liquid) at the top.
4. Results:
- In the batch, radioactivity was found in the pellet (inside the bacteria).
- In the batch, radioactivity was found in the supernatant (outside the bacteria).
Step 4: Final Answer:
This proved that DNA was the material that actually entered the cell and directed the production of new viruses, confirming DNA as the genetic material.

Step 1: Understanding the Concept:
The genetic code is the "dictionary" used by the ribosome during translation.
Since there are 4 nucleotides and 20 amino acids, a single (4) or doublet (16) code is insufficient; a triplet code ( ) provides more than enough combinations.
Step 2: Detailed Explanation:
The "nature" of the code is defined by several unique properties:
1. The Triplet Codon: Each "word" in mRNA is a sequence of three bases. Of the 64 codons, 61 code for amino acids.
2. Unambiguous: A specific codon always codes for the same amino acid (e.g., GUG always codes for Valine). There is no confusion.
3. Degenerate: Many amino acids are coded by more than one codon (e.g., Serine is coded by 6 different codons). This provides a buffer against mutations.
4. Comma-less: The code is read sequentially without any gaps or overlapping of bases. If one base is deleted, the entire "reading frame" shifts.
5. Universal: The code is nearly identical across all life forms, from a simple E. coli to a complex Human. This is strong evidence for a common ancestor.
6. Start and Stop Signals:
- AUG is the start codon (it also codes for Methionine).
- UAA, UAG, UGA are stop codons (nonsense codons) that do not code for any amino acid and signal the end of translation.
Step 3: Final Answer:
The genetic code is a highly organized, universal system that ensures the accurate translation of genetic information into functional proteins.