WHAT IS CYSTIC FIBROSIS?

Cystic Fibrosis is a highly common genetic disease. It most often appears in early childhood and is identified by difficulty in breathing, faulty or incomplete digestion and large losses of salts via sweat.

Cystic Fibrosis is caused by a mutation in the CFTR gene present on chromosome 7. The CFTR gene is expanded to the Cystic Fibrosis Transmembrane conductance regulator gene. This gene is mainly responsible for controlling a membrane protein which functions as a chloride ion channel. The chloride ion channel is mostly involved in the secretion of seat, mucus and digestive juices.

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A nonsense base substitution mutation occurs in the recessive allele of the CFTR gene and leads to this disease. Nonsense base substitution mutations are detrimental mutations that are caused by the substitution of one of the four nitrogenous bases (Adenine, Guanine, Cytosine, Thymine) in a nucleotide (the monomeric unit of a DNA molecule). 


In this case, the base sequence on the sense strand of the DNA which originally formed a code CAG (Cytosine, Adenine, Guanine) is changed to the code TAG (Thymine, Adenine, Guanine). The substitution of the nitrogenous base Cytosine with Thymine causes the entire code to change. Hence, for protein synthesis, while transcription and translation occurs, the mRNA codon which was originally CAG changes to UAG (as RNA contains Uracil instead of Thymine). UAG is a stop codon and it triggers the halting of the protein synthesis. As a result, instead of synthesizing a protein that contains 1,480 amino acids, a protein with only 493 amino acids is synthesized. The result is chloride ion channels that do not function properly.

The improper functioning of the chloride ion channels causes the production of sweat with excessive NaCl (sodium chloride). On the other hand, digestive juices and mucus are secreted with less NaCl than normal. Due to this, the water potential and tonicity of the secretions are different. Not enough water moves into the secretions by endosmosis. Hence, it results in very viscous secretions. This causes the mucus to build up in the lungs and the pancreatic duct, which further leads to infections and prevents digestive enzymes from reaching the small intestine, thus resulting in incomplete digestion.

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