Key Points: Molecular Structure of DNA
1. DNA structure was first studied by Rosalind Franklin (1953); later explained by Watson and Crick, who proposed the double helix model (Nobel Prize, 1962).
2. DNA is a macromolecule made of two complementary strands twisted into a double helix.
3. Each strand is made up of nucleotides, which include phosphate, sugar (pentose), and a nitrogenous base.
4. There are four nitrogenous bases:
- Adenine (A) pairs with Thymine (T) (2 hydrogen bonds)
- Guanine (G) pairs with Cytosine (C) (3 hydrogen bonds)
5. The two strands form a ladder-like structure, with bases as rungs and sugar-phosphate as the backbone.
Key Points: Structure of Eukaryotic Chromosome (Packaging of DNA)
- In eukaryotes, chromosomes are gene carriers, and each chromosome consists of a single long DNA molecule associated with histone proteins (unineme model).
- DNA is packaged into repeating units called nucleosomes, where DNA is wrapped around a histone octamer made of H₂A, H₂B, H₃, and H₄ proteins.
- Nucleosomes form a 10 nm beaded fibre, which coils further into a 30 nm solenoid structure with the help of histone H₁.
- Higher-level folding condenses chromatin into looped domains attached to a protein scaffold, finally forming metaphase chromosomes with the help of non-histone chromosomal proteins.
- Chromatin exists as euchromatin (loosely packed, transcriptionally active) and heterochromatin (densely packed, transcriptionally inactive).
Key Points: Properties of Genetic Material
- DNA is the primary genetic material in most organisms, while RNA acts as genetic material in some viruses.
- A genetic material must be capable of replication, which both DNA and RNA can achieve through base pairing.
- DNA is chemically and structurally more stable than RNA because it lacks the reactive 2′-OH group and contains thymine instead of uracil.
- Both DNA and RNA can undergo mutations, but RNA mutates faster due to its unstable nature, leading to rapid evolution in RNA viruses.
- DNA stores genetic information efficiently, whereas RNA helps in expression and transmission of genetic information through protein synthesis.
Key Points: Mechanism of DNA Replication
- Initiation at Origin: DNA replication begins at a specific site called the origin of replication; prokaryotes have a single origin, while eukaryotes have multiple origins (replicons).
- Unwinding of DNA: The DNA double helix is unwound and strands are separated by helicase, while topoisomerase (DNA gyrase) relieves supercoiling, forming replication forks.
- Primer Formation: A short RNA primer is synthesized by the enzyme primase to provide a free 3′-OH end for DNA synthesis.
- Elongation of New Strands: DNA polymerase adds nucleotides in the 5′ → 3′ direction using parental strands as templates; synthesis is continuous on the leading strand and discontinuous on the lagging strand forming Okazaki fragments.
- Completion and Ligation: RNA primers are removed, gaps are filled with DNA, and Okazaki fragments are joined by DNA ligase to form complete daughter DNA molecules.
Key Points: Transcription Unit
| Component |
Location |
Function |
| Promoter |
At the 5′ end of the structural gene |
Provides binding site for RNA polymerase and initiates transcription |
| Structural Gene |
Between promoter and terminator |
Contains genetic information to be transcribed |
| Template Strand |
DNA strand with 3′ → 5′ polarity |
Serves as template for RNA synthesis |
| Coding Strand |
DNA strand with 5′ → 3′ polarity |
Does not code directly; used as reference strand |
| Terminator |
At the 3′ end of the coding strand |
Signals the end of transcription |
Key Points: Mutations and Genetic Code
- Mutation studies help explain the relationship between genes and DNA; large mutations cause loss or gain of genes, while point mutations affect single base pairs.
- A point mutation in the β-globin gene (glutamate replaced by valine) causes the genetic disorder sickle-cell anaemia.
- Insertion or deletion of one or two bases shifts the reading frame of codons, producing frameshift mutations.
- Insertion or deletion of three or multiples of three bases removes or adds whole codons, so the reading frame remains unchanged.
Key Points: The Lac Operon
- The operon model, proposed by Jacob and Monod (1961), explains coordinated regulation of gene expression in prokaryotes at the transcriptional level.
- An operon consists of a promoter, operator, and a group of structural genes that are regulated together and transcribed as a single polycistronic mRNA.
- In the lac operon of E. coli, three structural genes—z (β-galactosidase), y (permease), and a (transacetylase)—are involved in lactose metabolism.
- The regulator gene (lac i) produces a repressor protein that binds to the operator and prevents transcription in the absence of lactose.
- When lactose is present, it inactivates the repressor, allowing RNA polymerase to transcribe the structural genes (induction).
- Catabolite repression occurs when glucose is present; lac operon expression is suppressed and later activated by CAP–cAMP when glucose is depleted.
Key Points: Human Genome Project
- The Human Genome Project (HGP) was proposed in 1986 to map and sequence the entire human genome.
- It was an international effort involving about 100 laboratories from nearly 18 countries.
- The main goals included identifying all human genes, sequencing three billion base pairs, and storing data in databases.
- Two major approaches were used: Expressed Sequence Tags (ESTs) and whole genome sequencing with annotation.
- DNA fragments were cloned using BAC and YAC vectors in bacteria and yeast for sequencing.
- The human genome contains about 3.2 billion nucleotide bases and approximately 20,000–30,000 genes.
- Less than 2% of the genome codes for proteins, while repetitive DNA forms a large portion.
- About 99.9% of DNA sequences are identical among all humans; variations include SNPs and CNVs.
- HGP has applications in disease diagnosis, genetic counselling, gene therapy, and understanding human evolution.
- Ethical concerns include misuse of genetic data for discrimination, insurance denial, and attempts at genetic manipulation.
Key Points: DNA Fingerprinting
- DNA fingerprinting is a technique used to identify individuals based on unique patterns in their DNA sequences.
- It is based on repetitive DNA sequences called VNTRs (Variable Number Tandem Repeats), which show high polymorphism among individuals.
- These repetitive sequences are part of satellite DNA, including minisatellites and microsatellites.
- The technique was developed by Alec Jeffreys, and even a very small DNA sample can be used.
- The main steps include DNA extraction, PCR amplification, restriction digestion, gel electrophoresis, Southern blotting, hybridisation, and autoradiography.
- The resulting banding pattern is unique for each individual, except in identical (monozygotic) twins.
- DNA fingerprinting is widely used in forensic science, paternity testing, and personal identification.
- It is also useful in studying genetic diversity, population structure, and diagnosing certain inherited diseases.
