Question

The central dogma of biochemical genetics is the basic relationship between DNA, RNA, and protein. DNA serves as a template for both its own replication and for the synthesis of RNA, and RNA serves as a template for protein synthesis. How do viruses provide an exception to this flow scheme for genetic information?

Short answer

Viruses provide an exception to the central dogma of biochemical genetics because some viruses, like retroviruses, have RNA as their genetic material instead of DNA. Retroviruses use an enzyme called reverse transcriptase to convert their RNA into DNA, which integrates into the host cell's DNA. Other RNA viruses, such as the common cold or influenza virus, replicate directly from their RNA genome without utilizing DNA synthesis, deviating from the typical flow of the central dogma.

Step-by-step solution

Introduction to Central Dogma

The central dogma of biochemical genetics describes the flow of genetic information in a biological system: from DNA to RNA to proteins. This process involves two main stages: transcription and translation.

Transcription

In transcription, DNA serves as a template for RNA synthesis. The genetic information stored within the DNA molecule is rewritten into a complementary RNA molecule called messenger RNA (mRNA). This process occurs inside the cell nucleus.

Translation

In translation, the mRNA, carrying the genetic code, is read by ribosomes, which are responsible for synthesizing proteins. The ribosomes decode the mRNA sequence into a specific amino acid sequence, assembled into a protein. This process occurs in the cell's cytoplasm.

Viruses as an Exception

Viruses are simple, infectious agents that can only replicate inside the cells of a host organism. They provide an exception to the central dogma because some viruses have RNA as their genetic material instead of DNA (called RNA viruses), and they can also reverse the general flow of genetic information.

Retroviruses

One example of an exception to the central dogma is retroviruses, like the Human Immunodeficiency Virus (HIV). Retroviruses have a single-stranded RNA genome. Instead of transcribing DNA to RNA, these viruses use an enzyme called reverse transcriptase to reverse the process, converting their RNA into DNA, which then integrates into the host cell's DNA.

RNA Replication

Another exception is the replication of RNA viruses such as the common cold virus or influenza virus. These viruses do not use DNA as a template, and their replication process involves direct transcription and translation from their RNA genome. They can make copies of their RNA without going through DNA synthesis, which also deviates from the central dogma's typical flow. In conclusion, while the central dogma of biochemical genetics describes the general flow of genetic information from DNA to RNA to proteins, viruses can provide an exception by reversing or altering this flow, such as through retroviruses with reverse transcriptase or RNA viruses that forego DNA synthesis altogether.

Key concepts

Transcription

Transcription is one of the foundational processes in the central dogma of molecular biology. It occurs within the nucleus, where DNA is transcribed to produce messenger RNA (mRNA). During this process, the enzyme RNA polymerase binds to a specific region of the DNA. This region is known as the promoter, and it marks the start of the gene.

• RNA polymerase unwinds the DNA strand, separating the double helix into two strands.
• Then, it assembles RNA nucleotides into a strand that complementarily matches the DNA template strand.
• The result is a single-stranded RNA copy of the DNA sequence, which can then carry genetic information out of the nucleus.

This newly formed mRNA strand is like a transcript of the genetic instructions written in the DNA, allowing genetic messages to be conveyed from the nucleus to the cytoplasm for the next process: translation.

Translation

Following transcription, translation is the step where the mRNA is read by ribosomes to produce a protein. This occurs in the cytoplasm, where ribosomes—complex molecular machines—decode the mRNA. Each sequence of three nucleotides on the mRNA, called a codon, specifies a particular amino acid.

• Transfer RNA (tRNA) molecules bring the corresponding amino acids to the ribosome.
• These tRNA molecules have anticodons that are complementary to the mRNA codons, ensuring proper amino acid sequence.
• The ribosome assembles these amino acids into a polypeptide chain, which will fold into a functional protein.

Translation effectively turns the genetic code in the mRNA into a physical structure, a protein, that can perform various functions within the cell.

Retroviruses

Retroviruses, like HIV, challenge the traditional central dogma by using reverse transcription. Instead of transcribing DNA to RNA, they start with RNA as their genetic material. These viruses carry an enzyme called reverse transcriptase that allows them to convert their RNA genome into DNA.

• This newly synthesized DNA is then integrated into the host cell's genome.
• Once integrated, the viral DNA can be transcribed and translated using the host cell's machinery, resulting in the production of new viral particles.
• This process allows the virus to persist within the host and evade immune detection.

This reverse process showcases an exception to the central dogma, illustrating how retroviruses can replicate in ways other organisms cannot.

RNA Viruses

RNA viruses provide another interesting twist to the central dogma. These viruses don't follow the typical DNA to RNA to protein pathway. Instead, they can replicate their RNA directly and produce proteins without a DNA intermediary. Examples include the influenza virus and the common cold virus.

• These viruses have RNA genomes that are directly transcribed into new RNA strands.
• Them RNA can also directly serve as mRNA, which is translated into proteins necessary for the virus to assemble and propagate.
• This bypassing of the DNA step allows for rapid replication and adaptation.

RNA viruses illustrate the flexibility of genetic information flows in nature, challenging the central dogma by completely sidestepping DNA.