Learning Objective: Explain the flow of genetic information, compare gene expression with DNA replication, and describe the structure and function of nucleosides and nucleotides within the context of the eukaryotic cell cycle.

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Gene Expression and DNA Replication

Genetic information is stored in the base sequence of DNA. During gene expression, this information is ultimately used to synthesize all proteins required by an organism. This process is summarized by the central dogma of molecular biology:

DNA → RNA → Protein
(With reverse transcription in some viruses)

A gene is classically defined as a segment of DNA that encodes a functional RNA or protein. Although modern discoveries introduce more complexity, this definition remains practical for foundational understanding.

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Comparison of Gene Expression and DNA Replication

Both processes involve reading DNA, but they occur at different times in the cell cycle and produce different outcomes.

Table 1: Comparison of Gene Expression and DNA Replication

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Eukaryotic Cell Cycle

The cell cycle describes events leading to cell division.

Phases

• M phase (mitosis): The Cell physically divides to produce two daughter cells.
• Interphase: Period between mitotic divisions. Includes:
  • G1 phase: Cell growth before DNA synthesis.
    • Non-cycling cells (e.g., neurons, skeletal muscle) enter G0.
  • S phase: DNA synthesis; each chromosome becomes two identical sister chromatids joined at the centromere.
  • G2 phase: Growth and DNA quality check before mitosis.

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High-Yield Pharmacology Connection

Many chemotherapy drugs target specific cell-cycle phases:

• S phase: methotrexate, 5-fluorouracil, hydroxyurea
• G2 phase: bleomycin
• M phase: paclitaxel, vincristine, vinblastine
• Non-specific: cyclophosphamide, cisplatin

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Nucleosides and Nucleotides

Nucleosides

A nucleoside = nitrogenous base + sugar
(Base attached to the 1′ carbon of ribose or deoxyribose)

Nucleotides

A nucleotide = nucleoside + phosphate group(s)
(Phosphate attached to the 5′ carbon)

Nucleoside di- and triphosphates (e.g., ATP, GTP) contain high-energy phosphate bonds that release energy when hydrolyzed.

Examples

• UMP – uridine monophosphate
• ATP – adenosine triphosphate
• dGMP – deoxyguanosine monophosphate

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Table 2: Bases, Nucleosides, and Nucleotides

Note: Thymine almost always occurs with deoxyribose, so “deoxy-” is often omitted.

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