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CH-5 Molecular-Basis-Of-Inheritance

Grade 12 Science | Chapter 5

Molecular Basis of Inheritance

This chapter looks at inheritance at the level of the molecule. It builds the structure of DNA, the proof that DNA is the genetic material, how DNA copies itself, and how its code is read out into RNA and then into protein.

Core Concepts

Key Principles

Worked Examples

Practice Sets

Contents

1.	The Structure of DNA
2.	DNA as the Genetic Material
3.	DNA Replication
4.	Transcription
5.	The Genetic Code and Translation

6.	Gene Expression
7.	Key Reasoning (Principles)
8.	Worked Examples (10)
9.	Practice Sets A to D
10.	Summary and Exam Quick Check

1. The Structure of DNA

DNA is a double helix, worked out by Watson and Crick. Two strands of nucleotides, each a sugar, a phosphate and a base, wind around each other in opposite directions. The strands are held together by base pairing, where adenine pairs with thymine and guanine pairs with cytosine, and the sugar and phosphate form the backbone.

Core idea

The two strands carry the same information twice over, because each base pairs with only one partner, and this is the key to both storing and copying the code.

Diagram 1 – DNA Structure

Fig 1. Two strands wind into a double helix, held by base pairs of A with T and G with C.

2. DNA as the Genetic Material

Careful experiments showed that DNA, not protein, carries the code of life. Griffith found that something could pass a trait from dead to living bacteria, and later work by Hershey and Chase, using viruses, showed clearly that it was the DNA that entered and directed the cell.

3. DNA Replication

When a cell divides, its DNA must be copied. Replication is semiconservative: the two strands separate, and each acts as a template for building a new partner. So every new DNA molecule has one old strand and one new strand, which Meselson and Stahl confirmed by experiment.

Diagram 2 – Replication

Fig 2. The two strands part and each guides the building of a new strand, so each copy keeps one old strand.

4. Transcription

To use a gene, its code is first copied from DNA into messenger RNA, a step called transcription. RNA is single stranded and, importantly, it uses the base uracil in place of thymine, so where the DNA reads A the RNA carries U.

5. The Genetic Code and Translation

The messenger RNA is read in threes. Each triplet, a codon, stands for one amino acid. In translation, a ribosome reads the codons in order and joins the matching amino acids into a protein. The genetic code is nearly the same in all living things, which shows their shared origin.

6. Gene Expression

The flow of information, DNA to RNA to protein, is called the central dogma. Cells do not use every gene at once; they switch genes on and off as needed, a control seen clearly in the lac system of bacteria. Reading the whole human genome has opened a new understanding of our biology.

Diagram 3 – The Central Dogma

Fig 3. Information flows from DNA to RNA to protein, by transcription and then translation.

7. Key Reasoning (Principles)

Principle 1: Base pairing stores and copies the code

Because each base pairs with only one partner, each strand fixes the other, so DNA can hold information and be copied faithfully.

Principle 2: Replication is semiconservative

The strands separate and each templates a new one, so every copy keeps one old strand, which limits errors and preserves the code.

Principle 3: Information flows DNA to RNA to protein

The code is transcribed into RNA and then translated into protein, the central path by which a gene shapes a trait.

8. Worked Examples

Example 1

Q: What is the shape of DNA?

Show Solution

Two strands wound together.

Answer: A double helix.

Example 2

Q: With which base does adenine pair in DNA?

Show Solution

The base pairing rule.

Answer: Thymine.

Example 3

Q: What did Hershey and Chase show?

Show Solution

Using viruses.

Answer: That DNA is the genetic material.

Example 4

Q: What kind of replication does DNA use?

Show Solution

Each copy keeps one old strand.

Answer: Semiconservative.

Example 5

Q: What is the copying of DNA into RNA called?

Show Solution

The first step of using a gene.

Answer: Transcription.

Example 6

Q: Which base does RNA use in place of thymine?

Show Solution

Not found in DNA.

Answer: Uracil.

Example 7

Q: How many bases make a codon?

Show Solution

Read in threes.

Answer: Three.

Example 8

Q: What does a codon stand for?

Show Solution

The building block of protein.

Answer: An amino acid.

Example 9

Q: What is the building of protein from RNA called?

Show Solution

Done by ribosomes.

Answer: Translation.

Example 10

Q: State the central dogma.

Show Solution

The flow of information.

Answer: DNA to RNA to protein.

9. Practice Sets A to D

Set A – Multiple Choice (Basic)

1. DNA is shaped as a: (a) single line (b) double helix (c) sphere (d) sheet

2. In DNA, guanine pairs with: (a) adenine (b) thymine (c) cytosine (d) uracil

3. DNA replication is: (a) conservative (b) semiconservative (c) random (d) none

4. RNA uses in place of thymine the base: (a) adenine (b) uracil (c) guanine (d) cytosine

5. A codon is made of this many bases: (a) one (b) two (c) three (d) four

Reveal Answers

1. (b) double helix.

2. (c) cytosine.

3. (b) semiconservative.

4. (b) uracil.

5. (c) three.

Set B – Short Answer (Understanding)

1. Describe the structure of DNA.

2. How was DNA shown to be the genetic material?

3. Why is replication called semiconservative?

4. What is transcription?

5. What happens in translation?

Reveal Answers

1. Two strands of nucleotides wound into a double helix, held by base pairs of A with T and G with C.

2. Experiments such as those of Hershey and Chase showed that DNA, not protein, enters and directs the cell.

3. Each new molecule keeps one old strand and gains one new one, so half is conserved.

4. The copying of a gene from DNA into messenger RNA, which uses uracil in place of thymine.

5. A ribosome reads the codons of the messenger RNA and joins the matching amino acids into a protein.

Set C – Application and Reasoning

1. Why does base pairing allow faithful copying?

2. Why must the two strands run in opposite directions matter for pairing?

3. Why does semiconservative replication reduce errors?

4. Why is a triplet code needed to name twenty amino acids?

5. Why is the near universal code strong evidence for common ancestry?

Reveal Answers

1. Each base has only one partner, so an old strand exactly fixes the new one that is built against it.

2. The bases can only meet and pair properly when the strands lie in opposite directions along their length.

3. Keeping one proven old strand as a template means the new strand is checked against a correct copy.

4. Pairs of bases give only sixteen combinations, too few, while triplets give sixty four, more than enough for twenty.

5. If very different organisms use the same code, they most likely inherited it from a single shared ancestor.

Set D – Higher Order (Challenge)

1. Explain how the double helix suits DNA to storing and copying information.

2. Explain the experimental logic that DNA is the genetic material.

3. Explain how semiconservative replication was demonstrated.

4. Explain the path from a gene to a protein.

5. Explain why cells switch genes on and off.

Reveal Answers

1. Two paired strands store the code twice and, on separating, each serves as a template to make an exact new partner.

2. By labelling DNA and protein separately and seeing which entered the cell, the DNA was shown to carry the instructions.

3. By following the density of DNA through generations, each copy was shown to keep one old and gain one new strand.

4. The gene is transcribed into messenger RNA, whose codons are then translated by a ribosome into a chain of amino acids.

5. A cell needs different proteins at different times, so it turns genes on only when their products are required.

Chapter Summary

DNA Structure Double helix of two strands, base pairs A to T, G to C.		Genetic Material Shown to be DNA, not protein, by careful experiments.

Replication Semiconservative; each copy keeps one old strand.		Transcription DNA copied into RNA; RNA uses uracil for thymine.

Translation Ribosome reads codons and builds a protein.		Central Dogma Information flows DNA to RNA to protein.

Quantity	Value	Note
DNA pairs	A to T, G to C	two strands
Replication	semiconservative	one old strand kept
Codon	three bases	one amino acid

Eight Point Exam Quick Check

1	DNA is a double helix of two strands with A to T and G to C pairs.

2	Experiments showed DNA, not protein, is the genetic material.

3	Replication is semiconservative; each copy keeps one old strand.

4	Transcription copies DNA into RNA, which uses uracil for thymine.

5	The code is read in triplets called codons, each an amino acid.

6	Translation builds a protein as a ribosome reads the codons.

7	The central dogma is DNA to RNA to protein.

8	Cells switch genes on and off as their products are needed.

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Class 12 Biology Chapter 5: Molecular Basis of Inheritance, Complete Notes and Practice

These free Class 12 Biology notes on the Molecular Basis of Inheritance follow the NCERT syllabus and cover DNA structure, replication, transcription, the genetic code and translation, with worked examples and graded practice, free on SchoolRevise.com.