Chapter 7 of Cellular Biochemistry Foundations: Replication, Transcription & Translation. Macromolecules and metabolism, protein structure and function, the c…

Chapter 7: Replication, Transcription & Translation

DNA has two jobs: copy itself, and get read out into protein. This chapter covers both — DNA replication and its lab cousin PCR, then transcription (DNA → mRNA) and translation (mRNA → protein), the two halves of the central dogma's readout step.

Replication vs. PCR

Replication is DNA copying itself in vivo (inside a living cell), using the cell's own enzyme machinery. PCR (polymerase chain reaction) is DNA copying done in vitro — outside the body, in a PCR machine — and it's one of the most widely used tools in molecular diagnostics.

The three stages of one PCR cycle A repeating cycle of three stages: denaturation, which opens the double helix; annealing, where short primers bind; and extension, where new DNA strands are built, after which the cycle repeats. Denaturation open double helix Annealing primers bind Extension new strand built repeats each cycle — n cycles ≈ 2ⁿ × starting DNA
Each PCR cycle repeats three stages: denaturation (heat opens the double helix), annealing (short primers bind the single strands), and extension (DNA polymerase builds new complementary strands). Repeated for n cycles — commonly around 30 — the DNA quantity roughly doubles each time, reaching about 2ⁿ times the starting amount.

RT-PCR: how PCR tests an RNA virus

Standard PCR only copies DNA — so testing for an RNA virus like SARS-CoV-2 needs one extra step first. RT-PCR (reverse-transcription PCR) adds a reverse transcription step that converts the viral RNA genome into DNA (the reverse of transcription), which can then be amplified by ordinary PCR. This is exactly the diagnostic principle behind COVID-19 RT-PCR testing: RNA → (reverse transcription) → DNA → (PCR amplification) → detectable signal.

Testing DNA: with or without amplification

  • Without amplification — e.g. the Southern blot method (the DNA analog of a Western blot for protein, or a Northern blot for RNA). Requires a relatively larger amount of intact DNA, and takes longer.
  • With amplification — the PCR method. Needs only a small amount of DNA, doesn't require it to be fully intact, and gives a rapid result. This speed and sensitivity is why PCR-based testing has displaced non-amplified methods for most clinical and forensic uses.

A DNA electrophoresis result is read the same way as the protein electrophoresis in Chapter 4: a size ladder (marker lane, "M") of known base-pair sizes runs alongside sample lanes, and each sample's band position is compared against the ladder to read off its fragment size.

Transcription: DNA to mRNA

Transcription converts DNA into mRNA. In eukaryotic cells this happens in the nucleus. The immediate product is pre-mRNA, which still contains both the coding sequences (exons) and the non-coding sequences (introns) copied from the DNA template. Splicing then cuts out the introns and joins the exons together — mature mRNA is, in the end, nothing but a string of exons joined end to end.

Splicing: pre-mRNA to mature mRNA Pre-mRNA made of alternating exon and intron segments; the introns loop out and are cut away, leaving mature mRNA built only from the joined exons. pre-mRNA exon 1 intron exon 2 intron exon 3 splicing — introns removed mature mRNA exon 1 exon 2 exon 3 AAAA... (poly-A tail)
Pre-mRNA is transcribed with introns still in place. Splicing loops out and removes each intron, joining the exons directly together. Mature mRNA is exons only, capped at the 3' end with a long stretch of adenine nucleotides (the poly-A tail) that helps identify and stabilize it.

Translation: mRNA to protein

Translation converts mRNA into protein, and takes place in the ribosome. Three components are involved: mRNA (the template), tRNA (the adaptor), and the ribosome itself (the machine).

  • mRNA carries the codon — a sequence of three nucleotides — and the sequence of codons determines the sequence of amino acids in the finished protein.
  • tRNA carries the complementary anticodon, and delivers the specific amino acid that codon calls for.
Codon-anticodon pairing during translation An mRNA strand divided into three-nucleotide codons, with a tRNA molecule base-pairing its anticodon to one codon and carrying the matching amino acid. mRNA AUG GCU UAC codon CGA anticodon (tRNA) Start codon (AUG) = methionine always the first amino acid Protein synthesis ends at a stop codon
Each mRNA codon (3 nucleotides) is read by a tRNA carrying the complementary anticodon — base-pairing follows the usual rules (A pairs with U, since this is RNA; C pairs with G). The tRNA arrives already charged with the one amino acid that anticodon specifies. Translation always starts at an AUG codon (methionine) and ends at a stop codon.

Translation itself proceeds through the same three sub-phases as transcription and, more broadly, protein synthesis as a whole: initiation, elongation, and termination (the same terms covered for antibiotic inhibitors of protein synthesis in Chapter 4 — those drugs work by blocking one of these three phases).

Key teaching point

Replication, transcription, and translation are three different copying jobs on three different templates, but they share one grammar: DNA is always read by base-pairing against a complementary strand or adaptor — polymerase reading a template strand in replication and transcription, tRNA reading an mRNA codon in translation. Once you see that shared mechanism, PCR (an artificial replication) and RT-PCR (transcription run in reverse, then replicated) stop looking like separate lab tricks and start looking like the same biology, just redirected for a diagnostic purpose.

Chapters

  1. Macromolecules & Metabolism Overview
  2. Protein Structure, Peptide Bonds & Function
  3. The Cell Membrane
  4. Plasma Proteins — Albumin & Globulin
  5. Nucleotides & the DNA Backbone
  6. Genes, Chromosomes & the Genome
  7. Replication, Transcription & Translation