Molecular Basis of Inheritance


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What is the Structure of the DNA?

Structure of nucleotideThe double helical structure of DNA was cracked by Watson and Crick based on the X-ray crystallography results. DNA is a double helical structure. Each strand of a DNA helix is composed of repeating units of nucleotides. Nucleotide consists of 3 components: ribose or deoxyribose sugar, nitrogenous base (purines or pyrimidines) and phosphate.

DNA is negatively charged due to the presence of negatively charged phosphate groups. This negatively charged phosphate groups are stabilized by basic proteins known as histone.

DNA as a Genetic Material

Griffith performed an experiment known as transforming experiment. He used two strains of Pneumococcus. These two different strains were used to infect the mice. The two strains used were type III-S (smooth), that contains outer capsule made up of polysaccharide and type II-R (rough) strain do not contain capsule. The capsule protects the bacteria from the host immune system.

The Griffith experiment is explained below-

  • Rough strain of Pneumococcus is injected in mouse. The mouse is alive.

  • Griffith experimentSmooth strain of Pneumococcus is injected in mouse. The mouse dies.

  • When heat killed smooth strain of Pneumococcus is injected into mouse, the mouse is alive.

  • In the last set of experiment, rough strain and heat killed smooth strain is injected into mouse. The mouse dies.

This proves that there is some transforming substance present in heat killed S strain that is converting or transforming the rough strain into virulent strain that is responsible for the death of the mouse. This transforming substance is DNA.

Later on, Avery, McLeod McCarty and Hershey and Chase confirms that DNA is a genetic material. 


Q1. What is Central Dogma of Molecular Biology? 

It is an explanation about how the flow of genetic information occurs in a biological system. This explains how DNA replicates and then gets converted into messenger RNA (mRNA) via transcription. Then this mRNA is translated to form proteins.

  Central dogma of molecular biology


Q2. What is DNA Replication?

DNA Replication is a process of producing two identical copies of DNA from a single DNA molecule. It is a process of biological inheritance. DNA is a double helix in which two strands are complementary to each other. These two strands of a helix separate at the time of replication to form two new DNA molecules. Out of the two strands of DNA formed, one is identical to one of the strand and another strand is complementary to the parent strand. This form of replication is known as semi-conservative replication. Before the cell enters the mitosis, the DNA is replicated in S phase of interphase.

DNA polymerase in the most important enzyme involved in DNA replication.

Q3. What is Transcription?

It is a process of formation of RNA such as messenger RNA from DNA before gene expression or protein synthesis occurs. During transcription, one of the strand of DNA acts as template for mRNA formation. The synthesis of mRNA occurs via RNA polymerase enzyme. Transcription usually occurs for a particular DNA segment which is required further for gene expression. Other than the messenger RNA, other forms of RNA such as ribosomal RNA, micro RNA, small nuclear RNA can also be transcribed in the similar manner.

Some viruses have a property of reverse transcription. They are able to convert RNA template into DNA. The enzyme used is known as reverse transcriptase.

For Example: Human immunodeficiency virus that causes “AIDS”.

Q4. What is Translation?

This is the process of gene expression or protein synthesis that occurs in cytosol. Ribosomes are the cell organelles that are involved in protein synthesis. The messenger RNA formed by the process of transcription is decoded by ribosomes to form a polypeptide made up of amino acids. Messenger RNA is composed of polymer of nucleotides or codon. Each codon consists of 3 nucleotides that will code for a single amino acid. There are some important components that are involved in protein synthesis- ribosomes, messenger RNA and transfer RNA (tRNA).

Transfer RNA is involved in physically linking mRNA and the amino acid sequence of proteins.

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