Lesson 2: Transcription and Translation

Nucleus: The Headquarters

• The nucleus is the cell’s “undisputed headquarters” because it stores almost all genetic material as deoxyribonucleic acid (DNA).

• DNA in non-dividing cells is relaxed chromatin: messy, “ball-of-wool” appearance.

• When a cell prepares to divide, chromatin condenses into recognizable X-shaped chromosomes.

DNA Packaging Facts

• Diameter of double helix: 2\,\text{nm} (finer than silk).

• Length of DNA in a single cell (if stretched): \approx 2\,\text{m}.

• Total length of all DNA in one human: long enough to reach the Moon and back 6000 times.

• Compaction strategy:

  1. DNA wraps around histone proteins ("Red-Bull-can" model) forming nucleosomes.

  2. Nucleosomes coil into chromatin fibers.

  3. Fibers further fold into chromosomes for mitosis.

• Purpose: Maintain order, protect DNA, enable cell division.

Why Copy DNA?

1. Protein synthesis (continuous; gene-by-gene copying).

2. Mitosis (somatic cell division; whole-genome duplication for daughter cells).

Cellular Demand for Proteins

• Structural proteins: skin, hair, nails, collagen, elastin.

• Enzymes: metabolic & digestive.

• Contractile & transport: muscle fibers, hemoglobin.

• Immune molecules & hormones.

• Protein output underpins homeostasis; deficits require medical supplementation.

Central Dogma: Flow of Genetic Information

DNA \longrightarrow RNA \longrightarrow Amino Acids \longrightarrow Protein

• DNA: double-stranded; bases A, T, C, G (A–T, C–G pairing).

• RNA: single-stranded; bases A, U, C, G (no T; U replaces it).

• Ribosome reads RNA in triplets (codons). Each codon = one amino acid.

  • Example: \text{UCC}\rightarrow\text{Serine}.

  • Methionine (AUG) often serves as START; three dedicated STOP codons terminate synthesis.

  • Degeneracy: multiple codons (e.g., for tyrosine) encode the same amino acid.

Transcription (Occurs in Nucleus)

1. Initiation: RNA polymerase binds promoter (non-coding regulatory sequence).

2. Elongation: DNA locally unwinds; complementary ribonucleotides added, forming pre-mRNA.

3. Processing: Introns (non-coding) excised; exons spliced. Product is shorter than original DNA.

4. Termination: Polymerase disengages at terminator sequence.

5. Export: Mature mRNA exits via nuclear pore to cytoplasm.

Translation (Occurs in Cytoplasm)

1. mRNA threads between small & large ribosomal subunits (type-writer analogy).

2. tRNAs, each carrying a specific amino acid, base-pair via anticodon with successive mRNA codons.

3. Ribosome catalyzes peptide-bond formation, elongating the polypeptide “worm-tape.”

4. A STOP codon triggers release; nascent chain folds/modifies into functional protein.

5. Most active ribosomes stud the rough endoplasmic reticulum (rER).

DNA vs RNA Summary

Transcription vs Translation Comparison

Additional Context & Connections

• Reading: Totora, Principles of Anatomy & Physiology, 3rd ed., Ch. 3 (e-book via Massey Library).

• Discovery history of DNA reserved for later lecture.

• Rough ER appears “rough” due to docked ribosomes (connection to Lecture 1 on cell biology).

• Chromosome behavior during mitosis will be addressed in the next session.

• Questions? Contact lecturer via email or course forum.