8 Exceptions to The Central Dogma of Molecular biology

The Central Dogma of molecular biology, proposed by Francis Crick, states that genetic information flows from DNA to RNA to protein. However, there are several known exceptions to this rule: 

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1. Reverse Transcription: Certain viruses, such as retroviruses (HIV is a well-known example), use an enzyme called reverse transcriptase to convert their RNA genomes into DNA. This is against the Central Dogma, which states that information cannot flow from RNA back to DNA.

8 Exceptions to The Central Dogma of Molecular biology

2. RNA Viruses: Some viruses, like the influenza virus and coronavirus, store their genetic information in RNA, not DNA. They replicate by a process called replication, which involves the synthesis of a new RNA strand from an RNA template using an RNA dependent RNA polymerase enzyme.

3. Prions: Prions are infectious proteins that can transmit information, despite having no nucleic acid genome; either DNA or RNA. They cause several neurodegenerative diseases in animals, including Creutzfeldt-Jakob disease in humans, mad cow disease and scrapie disease in sheep. Prions replicate by converting normal proteins into the infectious prion form with a different structure, bypassing the need for nucleic acids entirely.

4. Viroid: are infectious single stranded circular RNAs that cause several diseases in plants like Potato spindle tuber disease. These infectious RNAs can replicate by itself without the need of a protein of DNA.

5. RNA Editing: In some organisms, the RNA transcript can be altered by enzymes after it has been synthesized. This can result in a protein that is not directly encoded by the DNA sequence. There may be addition or deletion of nucleotide in mRNA transcript leading to the production of different related proteins. Another mechanism is alternative splicing in eukaryotes, where exons in mRNA are joined in different combinations leading to the synthesis of many related proteins from a single gene without changing the DNA sequence.

RNA molecules like rRNA and tRNA

6. rRNA (ribosomal RNA): rRNA is a component of the ribosome, the protein manufacturing machinery of all living cells. The Central Dogma states that the flow of genetic information in a cell is from DNA to RNA to protein. However, rRNA does not code for proteins. Instead, it forms the core of the ribosome’s structure and catalyzes protein synthesis.

7. tRNA (transfer RNA): tRNA is responsible for bringing amino acids to the ribosome during the process of translation. Like rRNA, tRNA does not code for proteins. Instead, it serves as the link between the mRNA codon and the amino acid. Each tRNA molecule has an anticodon that pairs with a specific mRNA codon, and it carries the corresponding amino acid.

8. Epigenetic Changes:  Epigenetic changes involve modifications to DNA or associated proteins, without changing the DNA sequence, that alter gene activity. Examples include DNA methylation and histone modification. DNA methylation is the addition of methyl group to cytosine residues causing chromatin condensation and suppresses gene expression. Histone acetylation is the addition of acetyl groups to lysine residues of the histone causing chromatin relaxation and often enhances gene expression.

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