The central dogma of molecular biology is a fundamental concept that explains how genetic information is transferred within a biological system. This principle, articulated by Francis Crick in 1958, outlines the flow of genetic information from DNA to RNA to protein, encapsulated by the phrase "DNA makes RNA, and RNA makes protein." Here's an in-depth look at five key concepts associated with this process:
1. Transcription
Transcription is the first step in gene expression, where the information encoded in a segment of DNA (a gene) is copied into RNA. This RNA, known as messenger RNA (mRNA), carries the genetic code from the DNA in the nucleus to the ribosomes in the cytoplasm. The process involves:
- Initiation: RNA polymerase binds to the promoter region of the DNA.
- Elongation: RNA polymerase synthesizes a strand of RNA by adding complementary nucleotides to the template DNA strand.
- Termination: The RNA polymerase reaches a termination sequence, and transcription ends, releasing the newly formed mRNA.
🧬 Note: mRNA is not the only type of RNA produced; others include tRNA (transfer RNA) and rRNA (ribosomal RNA), which play essential roles in translation.
2. RNA Processing
Before mRNA can be used for protein synthesis, it undergoes several modifications:
- Capping: A 5’ cap is added to the mRNA to protect it from degradation and assist in ribosome binding.
- Polyadenylation: A poly-A tail is added to the 3’ end to help transport the mRNA out of the nucleus and stabilize it.
- Splicing: Introns (non-coding regions) are removed from the primary transcript, and exons (coding sequences) are joined together to produce mature mRNA.
| Process | Description | Function |
|---|---|---|
| Capping | Addition of a 7-methylguanosine cap | Protection and Ribosome Binding |
| Polyadenylation | Addition of poly-A tail | Stability and Export from Nucleus |
| Splicing | Removal of introns | Formation of functional mRNA |
3. Translation
The next step after transcription is translation, where the mRNA’s genetic code is read and translated into a chain of amino acids, forming proteins. Here’s how it works:
- Initiation: The ribosome binds to the mRNA at the start codon (AUG), with the initiator tRNA.
- Elongation: tRNAs bring amino acids to the ribosome, and as the codons are read, the amino acids are linked together by peptide bonds.
- Termination: When a stop codon is encountered, the ribosome releases the completed polypeptide chain, and translation ends.
4. Reverse Transcription
Reverse transcription is an exception to the central dogma where genetic information flows from RNA back to DNA, a process carried out by retroviruses like HIV. Here’s a brief overview:
- The RNA genome of the virus is converted into DNA by reverse transcriptase enzyme.
- This DNA is then integrated into the host genome, where it can be transcribed and translated like host DNA.
🔍 Note: Reverse transcription and transposition (RNA to DNA) are significant exceptions where genetic information flows in the opposite direction.
5. Post-Translational Modifications
After translation, proteins often undergo modifications to reach their functional form:
- Proteolytic Cleavage: Proteins are cleaved into smaller, functional units.
- Phosphorylation: Addition of phosphate groups to modify protein activity.
- Glycosylation: Attachment of carbohydrate groups to proteins for stabilization and signaling.
- Ubiquitination: Addition of ubiquitin tags to mark proteins for degradation.
These five concepts highlight the dynamic and complex process of how genetic information in DNA is expressed as functional proteins, allowing for life's intricate cellular functions. Understanding these steps provides insights into mechanisms of disease, genetic engineering, and the development of new therapies. Each step is a control point where cellular or external factors can influence the final protein product, illustrating the flexibility and adaptability of genetic expression.
Why is transcription necessary?
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Transcription is necessary because it allows the cell to use the DNA template to produce RNA copies, specifically mRNA, which can then be exported from the nucleus to the cytoplasm for protein synthesis. This separation of genetic material allows for controlled gene expression and regulation.
What happens if RNA processing goes wrong?
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If RNA processing, particularly splicing, goes wrong, it can lead to the production of abnormal proteins. These can contribute to diseases like muscular dystrophy or spinal muscular atrophy, where the correct protein function is crucial for cell health.
Can the central dogma be altered?
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While the central dogma outlines the primary flow of genetic information, there are exceptions like reverse transcription and RNA editing, where the flow is altered or expanded. Additionally, mechanisms like non-coding RNA affecting gene expression without changing the dogma itself show its complexity and adaptability.
