DNA Structure, Replication, and Cell Division

Human Cell Numbers & Single-Cell Origin

• Approximate number of cells in an adult human body: 30{,}000{,}000{,}000{,}000 (30 trillion)

  • Value is an estimate; literature shows slight variations.

  • Gut bacteria expelled during defecation can temporarily reduce total cell count—illustrates the dynamic nature of cellular composition.

• Developmental perspective

  • Every person begins as one fertilized egg (zygote).

  • Sequence of doublings: 1 \rightarrow 2 \rightarrow 4 \rightarrow 8 \rightarrow 16 \rightarrow \dots \rightarrow 30{\,}\text{trillion} via repeated cell divisions.

  • Nearly every somatic cell retains an exact genomic copy of that original zygotic DNA—key to organismal unity.

• • .Two Fundamental Modes of Eukaryotic Cell Division

    • Mitosis ("body" cell division)

      • Occurs in somatic cells.

      • Input: 1 diploid parent cell.

      • Output: 2 identical diploid daughter cells.

      • Mnemonic: “Mitosis has I & T → Identical Twins.”

      • Purposes: growth, tissue repair, routine replacement of old or lost cells.

    • Meiosis ("sex" cell division)

      • Occurs in germ-line cells to generate gametes (sperm & eggs).

      • Input: 1 diploid parent cell.

      • Output: 4 non-identical haploid gametes.

      • Each gamete carries half the chromosome number (haploid) so that fertilization restores the diploid state

  • Mnemonic: “Meiosis has E & S → Eggs & Sperm.”

  • Biological rationale: prevents exponential chromosome doubling across generations.

The Cell Cycle & Details of Mitosis

• Interphase (not part of mitosis proper)

  • Cell grows, performs normal functions, duplicates DNA.

  • Briefly contains twice the usual DNA quantity just before division.

• Ordered mitotic phases (constant across divisions)

  1. Prophase

  2. Metaphase

  3. Anaphase

  4. Telophase

  • Followed by Cytokinesis (cyto = cell, kinesis = cutting/splitting): physical separation into two cells.

• Instructor-coined mnemonic: “I Pee More After This Coke”

  • I = Interphase, P = Prophase, M = Metaphase, A = Anaphase, T = Telophase, C = Cytokinesis.

DNA Replication – The Indispensable Prelude

• Core takeaway: Every time a cell divides, it must replicate its DNA so each daughter inherits a complete genome.

• Term synonyms: DNA replication, DNA synthesis.

Historical Path to DNA’s Structural Discovery

• Early 20th century: Uncertainty whether DNA or protein carried hereditary information.

• Mid-20th century breakthroughs

  • Rosalind Franklin collected critical X-ray crystallography images of DNA fibers.

  • Technique: bombard crystal with X-rays → analyze diffraction pattern → infer 3-D shape.

  • James Watson & Francis Crick (1953) synthesized existing data, including Franklin’s, to propose the double-helix model.

  • Nobel Prize awarded to Watson, Crick & Wilkins; Franklin’s contribution historically undervalued—raises ethical questions about recognition in science.

Molecular Architecture of DNA

• Polymer class: nucleic acid; monomers: nucleotides.

• Each nucleotide =

  1. Pentose sugar (deoxyribose)

  2. Phosphate group

  3. Nitrogenous base (the information-bearing component)

• Only four bases in DNA (“four-letter alphabet”)

  • Adenine (A)

  • Thymine (T)

  • Cytosine (C)

  • Guanine (G)

• DNA is double-stranded with strands running antiparallel and twisting into a right-handed double helix.

Complementary Base Pairing Rules

• A pairs with T, C pairs with G — consistent, exclusive hydrogen-bonding pattern.

  • Classroom mnemonic: “All Tigers Can Growl.”

• Resulting rungs = base pairs (bp).

  • Human genome size ≈ 3 \times 10^{9} bp per haploid set.

• Biological significance

  1. If one strand’s sequence is known, the complementary strand can be reconstructed automatically—foundation for DNA repair & replication.

  2. Allows accurate copying with minimal instructions; biochemical simplicity yields enormous informational capacity.

Replication Mechanism (Preview)

• During interphase, enzymes unzip the helix, read each strand, and assemble complementary nucleotides, yielding two identical DNA molecules.

• This self-templating property is what makes the precise 30-trillion-to-one genome identity possible across somatic cells.

Integrative & Real-World Connections

• Links to earlier lectures

  • Central Dogma: DNA → RNA → Protein; genes (DNA segments) ultimately determine protein synthesis and phenotypic traits.

• Medical relevance

  • Errors in replication or cell-cycle regulation underpin cancer, framing upcoming discussions on oncogenesis.

• Philosophical note

  • A single fertilized cell, guided by molecular rules and accurate DNA copying, orchestrates the complexity of a human body—highlights the elegance of biological self-organization.

• Ethical dimension

  • Rosalind Franklin’s under-recognition serves as a cautionary tale about equity and credit in collaborative scientific endeavors.