Central Dogma

MCAT Biology - Chapter 2 - Section 1.1 - Gene Expression - Central Dogma

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Sample MCAT Question - Central Dogma

Which enzyme can make DNA from an RNA molecule?

a) Integrase

b) DNA polymerase

c) Reverse transcriptase

d) Helicase

Click to Reveal the Answer

C is correct. Reverse transcriptase. An exception to the central dogma of molecular biology is when DNA is formed from RNA. This is possible through the enzyme reverse transcriptase. Answer choice A is incorrect, integrase is a viral enzyme that catalyzes the insertion of virally derived DNA into the host genome. Answer choice B is incorrect, DNA polymerase creates DNA strands from deoxyribonucleotides. Answer choice D is incorrect, helicase is a motor protein that ‘unzips’ double stranded DNA.

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The Central Dogma of Molecular Biology

The central dogma of molecular biology states that the flow of genetic information in an organism goes from DNA to RNA to protein. Going from DNA to RNA is called transcription, and going from RNA to proteins is called translation.

Exceptions to the Central Dogma

There are, however, two exceptions to the central dogma. The first is that RNA can be reversed transcribed to DNA. The enzyme responsible for this is reverse transcriptase. Reverse transcriptase is used by retroviruses, such as HIV, non-retroviruses, such as Hepatitis B, and also by eukaryotic cells as well.

The second exception to the central dogma is that RNA can be copied to RNA. This process does not happen in eukaryotes, but only in some viruses. The viruses use an RNA-dependent RNA polymerase to carry out this process, which means they use RNA as a template to make more RNA.

A real-world example of viruses using an RNA dependent RNA polymerase is SARS-CoV-2, which is a viral strain that causes the respiratory illness COVID-19. SARS-CoV-2 is a positive-sense single-stranded RNA virus, which differs from negative-sense single-stranded RNA viruses and double-stranded RNA viruses. The differences between these different types of viruses are beyond the scope of the MCAT exam. What is interesting to note, however, is that all positive-sense single-stranded RNA viruses use an RNA-dependent RNA polymerase to replicate.