Micro finishing chem lecture

DNA Backbone & Directionality

  • DNA’s backbone is made of repeating deoxyribose sugars and phosphates; the sugars provide the “directionality.”

    • Each sugar ring is numbered 1′!→5′1'!\to5'.

    • Bonds form between the 3′3' carbon of one nucleotide and the 5′5' carbon of the next.

  • “Reading” DNA requires the instructor (or an enzyme) to specify the orientation:

    • 3′!→5′3'!\to5' example: T C A J (as spoken in class).

    • 5′!→3′5'!\to3' example: G C T (reverse interpretation of same strand).

    • Without a stated direction, the code is ambiguous.

DNA-Dependent Enzymes & Replication Constraints

  • DNA polymerases (the enzymes that copy DNA) travel only 3′!→5′3'!\to5' along the template, synthesizing new DNA 5′!→3′5'!\to3'.

    • They cannot move the opposite way, enforcing a biological “arrow of time.”

Antiparallel Double Helix

  • Two strands run side-by-side in opposite directions (antiparallel):

    • If one strand is 3′!→5′3'!\to5', the complementary strand is 5′!→3′5'!\to3'.

  • Antiparallel geometry is mandatory for proper base-pair hydrogen bonding.

Complementary Base Pairing & Hydrogen Bonds

  • Purine ↔︎ pyrimidine pairing keeps the helix width constant (a “sterile distance”).

  • Specific pairs & bond counts:

    • Adenine (A) ↔︎ Thymine (T) with 2 H-bonds.

    • Cytosine (C) ↔︎ Guanine (G) with 3 H-bonds.

  • Wrong pairings fail because the required number/geometry of H-bonds cannot form.

  • Consequences:

    • Knowing one strand’s sequence predicts the other’s with certainty.

    • Breaks in complementarity = mutations ➔ can lead to cancer.

Why Complementarity Matters

  • Foundation of genetics: accurate replication, transcription, and translation.

  • Mutations = “oopsies” in the code.

    • Single or multiple mutations can accumulate; clusters of mutations in growth-control genes drive cancer.

  • Molecular explanation of hydrogen-bond limit (student Q&A):

    • A–T lacks an extra hydrogen donor/acceptor; therefore capped at two H-bonds.

DNA Sequence ➔ Protein Sequence ➔ Phenotype

  • Central dogma reminder: DNA sequence determines amino-acid sequence (primary structure) in proteins.

  • Even a one-base change can be catastrophic:

    • Sickle-cell disease: one A→T point mutation converts an isoleucine codon to valine ➔ red-blood-cell shape changes from “pillowy donut” to sharp sickle.

    • Demonstrates magnification of a tiny genetic change through all structural levels of a protein.

Overview of RNA Types (Historical Perspective)

  • Early discovery: only messenger RNA (mRNA) known.

  • Subsequent findings: rRNA, tRNA, mtRNA, microRNA, siRNA, snRNA, scaffolding RNA, etc.

  • Instructor focus: mRNA (with brief mention of tRNA during translation).

Four Core RNA Principles (Which RNAs They Apply To)

  1. Ribose sugar – universal.

  2. Uracil replaces thymine – universal.

  3. Single-stranded – mainly mRNA (others can fold into stems/loops).

  4. Very unstable/short-lived – characteristic of mRNA (and some small RNAs).

Functional Rationale for mRNA Instability

  • mRNA lifetime: seconds → hours; never days or years (contrast with DNA longevity – some strands are older than you).

  • Allows cells to produce a finite number of protein copies, then stop.

  • Persistent mRNA would cause relentless protein overproduction ➔ toxic gain-of-function disorders.

    • Some diseases arise from constitutive expression where the body must “over-tax” itself clearing excess proteins.

Clarifying “Short-Lived” (Student Question)

  • Short-lived = chemically degraded quickly, not merely short-acting.

  • Decay mechanisms (not detailed in lecture): exonucleases, deadenylation, etc.

Real-World Anecdote: Coxsackie (Hand-Foot-Mouth) Disease

  • Viral infection causes blistering on hands, feet, mouth (and in the lecturer’s child’s case, the “booty”).

  • Serves as a humanizing segue before switching topics.

Ethical & Practical Implications

  • Understanding DNA/RNA chemistry underlies genetic counseling, cancer diagnosis, and therapeutic gene editing.

  • Complementary base pairing fidelity is “the base of life” – errors can harm individuals and future generations.

Cross-Links to Previous Lectures / Foundational Principles

  • Builds on prior lessons covering nucleotide chemistry and the central dogma.

  • Sets stage for upcoming units on transcription, translation, and prokaryotic molecular biology.