Molecular Basis of Inheritance

The molecular basis of inheritance focuses on how genetic information is stored, organised, and transmitted from one generation to the next. This study involves understanding how genes, which are specific segments of DNA found on chromosomes, determine the traits we inherit. In simple terms, it looks at how DNA directs the synthesis of RNA and proteins, allowing living organisms to grow, develop, and reproduce.

Genes and Chromosomes

Genes are the functional units of heredity. They code for specific proteins or RNA molecules that perform various functions in the body. Chromosomes are thread-like structures composed of DNA and proteins. In eukaryotes (such as plants and animals), chromosomes are found in the nucleus, while in prokaryotes (such as bacteria), they are present in the cytoplasm.

DNA: Structure and Key Features

DNA, short for deoxyribonucleic acid, is the main hereditary material in most living organisms and some viruses. It was first identified as an acidic substance by Friedrich Miescher. Later, James Watson and Francis Crick proposed the double-helical model of DNA.

1. Composition

• Nucleotides are the building blocks of DNA. Each nucleotide consists of:

• A phosphate group

• A five-carbon sugar (deoxyribose)

• A nitrogenous base (Adenine, Thymine, Guanine, or Cytosine)

2. Double-Helical Structure

• DNA consists of two polynucleotide strands twisted around each other in a helix.

• The sugar-phosphate backbones of the strands lie on the outside, while the nitrogenous bases project towards the inside.

• Adenine (a purine) pairs with Thymine (a pyrimidine), and Guanine (a purine) pairs with Cytosine (a pyrimidine). This complementary base pairing is held together by hydrogen bonds.

• The two strands are antiparallel, meaning one strand runs in the 5′ to 3′ direction and the other in the 3′ to 5′ direction.

For more details, see our DNA Structure page.

Central Dogma

Francis Crick put forward the concept of the central dogma, which describes the flow of genetic information:

• DNA → RNA → Protein

In typical cells, DNA is first transcribed into RNA, and then RNA is translated into proteins. However, in retroviruses, the flow of information occurs in reverse (RNA → DNA → mRNA → Protein), primarily because they use an enzyme called reverse transcriptase.

DNA Packaging

In eukaryotic cells, DNA is organised with the help of proteins called histones. DNA, which is negatively charged, wraps around a positively charged histone octamer to form a structure known as a nucleosome. A series of these nucleosomes coils further to form chromatin. During cell division, chromatin condenses to form distinct chromosomes.

DNA Replication

One remarkable aspect of DNA is its ability to replicate itself (self-replication). This ensures that each daughter cell receives an identical copy of genetic material.

• Semi-Conservative Nature: During replication, the double helix unwinds, and each parent strand acts as a template. New nucleotides pair with the exposed bases, forming two new DNA molecules, each with one old (parent) strand and one newly synthesised strand.

• Replication Fork and Direction: Replication proceeds at sites called replication forks. DNA synthesis always occurs in the 5′ to 3′ direction.

• Time Frame: In prokaryotes, replication can occur within minutes, while in eukaryotes, it typically takes longer (several hours) and happens during the S-phase of the cell cycle.

For more information, see our DNA Replication page.

RNA and Its Importance

Ribonucleic acid (RNA) is another vital nucleic acid found in all living cells. It often serves as an intermediary between DNA and proteins, but it can also function as genetic material in certain viruses.

1. Structure

• RNA contains ribose sugar instead of deoxyribose.

• It uses the base Uracil (U) instead of Thymine (T).

2. Types of RNA

• Messenger RNA (mRNA): Carries coding information from DNA to ribosomes for protein synthesis.

• Transfer RNA (tRNA): Brings specific amino acids to the ribosome during protein synthesis.

• Ribosomal RNA (rRNA): Forms the core of ribosomes, which are the sites of protein synthesis.

• Other types include small nuclear RNA (snRNA), microRNA (miRNA), etc.

See our Structure of RNA page to learn more.

The Genetic Code

The genetic code is a set of rules that defines how a sequence of nucleotides in mRNA is translated into a sequence of amino acids in a polypeptide. Each group of three nucleotides (codon) corresponds to a specific amino acid or a “stop” signal.

1. Characteristics

• Triplet: Three nucleotides form a codon.

• Degenerate: More than one codon can code for the same amino acid.

• Universal: Almost all organisms use the same code (with a few exceptions).

• Non-Overlapping and Comma-Less: Codons are read in a continuous, non-overlapping manner.

• Non-Ambiguous: Each codon specifies only one amino acid (or a stop signal).

2. Codon Usage

• Out of 64 possible codons, 61 code for amino acids and 3 act as stop codons.

• The frequency of each codon’s usage varies among different organisms.

Human Genome Project

The Human Genome Project was an international effort aimed at identifying and mapping all the genes in the human genome. The primary goals included:

1. Sequencing the Genome

• Providing a complete sequence of approximately 3 billion base pairs.

2. Comparative Genomics

• Sequencing the genomes of model organisms like mice and fruit flies to aid in medical research.

3. Data Accessibility

• Developing tools to analyse and share the data widely.

4. Future Prospects

• Better understanding of genetic disorders, personalised medicine, and advanced diagnostic tools.

Additional Unique Insights

Introns and Exons

• Introns: Non-coding regions within a gene that are removed during RNA splicing.

• Exons: Coding regions that remain in the final mRNA to be translated into proteins.

Telomeres and Telomerase

• Telomeres: Repetitive nucleotide sequences at the ends of eukaryotic chromosomes, protecting them from damage.

• Telomerase: An enzyme that helps in adding telomeric repeats to chromosomes in certain cells, allowing them to divide indefinitely (e.g., stem cells).

Quick Quiz

1. Which enzyme synthesises DNA from an RNA template in retroviruses?
A. DNA polymerase
B. Reverse transcriptase
C. RNA polymerase
D. Helicase
Answer: B. Reverse transcriptase

2. Which of the following statements about the genetic code is correct?
A. It is overlapping.
B. It is non-ambiguous.
C. There are 64 amino acids.
D. All codons are stop codons.
Answer: B. It is non-ambiguous

3. Which structure is formed when DNA wraps around histone proteins?
A. Chromosome
B. Nucleosome
C. Chromatid
D. Centromere
Answer: B. Nucleosome