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M03.01.01 Central Dogma of Molecular Biology
An organism must be able to store and preserve its genetic information, pass that information along to future generations, and express that information as it carries out all the processes of life. The major steps involved in handling genetic information are illustrated by the central dogma of molecular biology.
Central Dogma of Molecular Biology
The central dogma of molecular biology describes the flow of genetic information from DNA to RNA to protein.
- DNA: Stores genetic information.
- Transcription: DNA is transcribed into RNA.
- Translation: RNA is translated into protein.
- Replication: DNA makes a copy of itself.
- Reverse Transcription: RNA can be reverse-transcribed into DNA, especially in retroviruses.
Genetic Information Storage
Genetic information is stored in the base sequence of DNA molecules. During gene expression, this information is used to synthesize all the proteins made by an organism.
Classical Definition of a Gene
A gene is a unit of DNA that encodes a particular protein or RNA molecule. Although this definition has evolved with an increased understanding of gene expression, it remains useful as a working definition.
Gene Expression and DNA Replication
Gene expression and DNA replication are key processes in handling genetic information.
Gene Expression
- Transcription: The first stage in gene expression, involves the transfer of information from a double-stranded DNA molecule to the base sequence of a single-stranded RNA molecule. If the RNA is a messenger RNA (mRNA), translation follows.
- Translation: Converts the information in the RNA base sequence into the amino acid sequence of a protein.
DNA Replication
When cells divide, each daughter cell must receive an accurate copy of the genetic information. DNA replication is the process by which each chromosome is duplicated before cell division.
Comparison of Gene Expression and DNA Replication
| Aspect | Gene Expression | DNA Replication |
|---|---|---|
| Purpose | Produces all the proteins an organism requires | Duplicates the chromosomes before cell division |
| Process | Transcription: RNA copy of a small section of a chromosome (10⁴–10⁵ nucleotide pairs) | DNA copy of entire chromosome (10⁸ nucleotide pairs) |
| Location and Timing | DNA copy of an entire chromosome (10⁸ nucleotide pairs) | Occurs during S-phase |
| Translation | Occurs in the cytoplasm throughout the cell cycle | Replication occurs in the nucleus |
The Cell Cycle
The cell cycle describes the timing of events in a eukaryotic cell. It consists of the M phase and interphase.
Phases of the Cell Cycle
- M Phase (Mitosis): The cell divides to form two daughter cells.
- Interphase: The time between two cell divisions, subdivided into G1, S, and G2 phases.
Interphase Subdivisions
- G1 Phase (Gap 1): Cellular growth preceding DNA synthesis. Cells that have stopped cycling, such as muscle and nerve cells, are in a special state called G0.
- S Phase (DNA Synthesis): DNA replication occurs. Each chromosome doubles its DNA content, forming two identical sister chromatids linked at the centromere.
- G2 Phase (Gap 2): Cellular growth after DNA synthesis but before mitosis. Replicated DNA is checked for errors before cell division.
Control of the Cell Cycle
Control of the cell cycle is accomplished at checkpoints by strategic proteins such as cyclins and cyclin-dependent kinases. These checkpoints ensure that cells do not enter the next phase of the cycle until the molecular events in the previous phase are concluded.
Clinical Relevance: Chemotherapeutic Agents
Many chemotherapeutic agents function by targeting specific phases of the cell cycle:
- S-phase: Methotrexate, 5-fluorouracil, hydroxyurea
- G2 phase: Bleomycin
- M phase: Paclitaxel, vincristine, vinblastine
- Non-cell-cycle specific: Cyclophosphamide, cisplatin
Reverse Transcription
Reverse transcription produces DNA copies from an RNA template. This process is commonly associated with the life cycles of retroviruses, which replicate and express their genome through a DNA intermediate (an integrated provirus). It also occurs to a limited extent in human cells, amplifying certain highly repetitive DNA sequences.
Key Points to Remember
- Storage: Genetic information is stored in DNA.
- Transmission: Genetic information is passed to future generations during cell division.
- Expression: Genetic information is expressed through transcription and translation.
Bibliography
- Watson, J.D., et al. (2017). Molecular Biology of the Gene. Pearson.
- Alberts, B., et al. (2014). Molecular Biology of the Cell. Garland Science.