Unit 3
Genetics
Chapter 3.1
Genetic MaterialsClass 12 Biology – Genetic Materials Notes PDF
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Introduction
In most organisms the genetic material is DNA. Some viruses use RNA. This chapter focuses on the composition, structure and function of DNA and RNA, DNA replication and the basic concept of the genetic code.
1. Nucleic Acids and Nucleotides
DNA and RNA are nucleic acids made of repeating units called nucleotides. Each nucleotide contains three components:
- A pentose sugar.
- A phosphate group.
- A nitrogenous base.
DNA contains deoxyribose sugar and the bases adenine (A), guanine (G), cytosine (C) and thymine (T). RNA contains ribose sugar and uses uracil (U) instead of thymine.
Fig. 1 – General Structure of a Nucleotide
2. DNA – Deoxyribonucleic Acid
Composition
A DNA nucleotide contains deoxyribose sugar, phosphate and one of four bases: A, T, G or C.
Structure of DNA
- DNA usually consists of two polynucleotide strands.
- The two strands run in opposite directions and are described as antiparallel.
- The sugar-phosphate backbone lies outside; nitrogenous bases face inward.
- Adenine pairs with thymine through two hydrogen bonds.
- Guanine pairs with cytosine through three hydrogen bonds.
- The paired strands form a double helix.
Fig. 2 – DNA Double Helix and Base Pairing
Functions of DNA
- Stores hereditary information.
- Replicates before cell division.
- Provides information for synthesis of RNA and proteins.
- Transmits genetic information from one generation of cells or organisms to the next.
3. RNA – Ribonucleic Acid
RNA is generally a single-stranded nucleic acid containing ribose sugar, phosphate and the bases A, G, C and U.
Main Types and Functions
mRNA
Messenger RNA carries genetic information copied from DNA to ribosomes for protein synthesis.
tRNA
Transfer RNA carries specific amino acids and contains an anticodon that recognizes a codon on mRNA.
rRNA
Ribosomal RNA combines with proteins to form ribosomes and participates in protein synthesis.
General Role
RNA is central to expression of genetic information and can also serve structural and catalytic roles.
Fig. 3 – Simplified tRNA Structure
4. Difference Between DNA and RNA
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, G, C | A, U, G, C |
| Strands | Usually double-stranded | Usually single-stranded |
| Main role | Long-term storage and transmission of genetic information | Expression of genetic information and protein synthesis |
| Stability | More chemically stable | Generally less stable |
5. DNA Replication
Replication is described as semi-conservative because each daughter DNA molecule contains one parental strand and one newly synthesized strand.
Main Steps
- The double helix unwinds and the two strands separate.
- Each parental strand acts as a template.
- Complementary nucleotides are added according to base-pairing rules.
- New DNA strands are synthesized in the 5′ → 3′ direction.
- Two DNA molecules are produced, each with one old and one new strand.
Fig. 4 – Semi-Conservative DNA Replication
Fig. 5 – Simplified Replication Fork
6. Genetic Code
Important Features
- Triplet: each codon contains three nucleotides.
- Degenerate: more than one codon may specify the same amino acid.
- Unambiguous: a particular codon specifies one amino acid or stop signal.
- Nearly universal: the same codons generally specify the same amino acids across organisms, with limited exceptions.
- Non-overlapping: codons are normally read in sequence without sharing bases.
- Start codon: AUG usually initiates translation and codes for methionine.
- Stop codons: UAA, UAG and UGA signal termination.
Fig. 6 – From DNA Information to Codons
7. Quick Revision & Exam Points
- Define genetics and genetic material.
- Draw and label a nucleotide.
- Describe the composition and structure of DNA.
- Explain complementary base pairing.
- Write the main functions of DNA.
- Describe RNA and its main types.
- Differentiate DNA and RNA.
- Define DNA replication and explain why it is semi-conservative.
- Draw a simple replication fork.
- Define genetic code and list its important features.
- Nucleotide = sugar + phosphate + base.
- DNA sugar = deoxyribose.
- RNA sugar = ribose.
- DNA uses thymine.
- RNA uses uracil.
- A pairs with T in DNA.
- G pairs with C.
- DNA strands are antiparallel.
- Replication is semi-conservative.
- Genetic code is triplet.
- AUG is the usual start codon.
- UAA, UAG, UGA are stop codons.
Source handling: The original handwritten PDF remains embedded above. The typed section is a syllabus-aligned reconstruction designed to follow the same chapter scope while providing clearer searchable text and mobile-friendly labelled diagrams.
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