BIOL10008 · Foundations Of Biology: Life's Machinery
DNA and Cell Division
Before a cell divides it must copy all of its DNA exactly once. This chapter covers how — semi-conservative replication at the fork, where helicase unwinds the duplex, primase lays RNA primers, and polymerase builds new strands only 5′→3′, producing a continuous leading strand and a piecewise lagging strand of Okazaki fragments. It contrasts prokaryotic (circular, single origin) and eukaryotic (linear, many origins) replication, then follows the central dogma — DNA →(transcription) RNA →(translation) protein — through promoters, codons, the ribosome and tRNA adaptors. It closes with the cell cycle (G1·S·G2 interphase + M, gated by Cyclin–CDK checkpoints) and mitosis (PMAT: prophase, metaphase, anaphase, telophase), including the cytokinesis difference between animal (actin contractile ring) and plant (cell plate) cells. The test loves the 5′→3′ rule, the leading-vs-lagging asymmetry, and the metaphase-vs-anaphase distinction.
What this chapter covers
- 01Semi-conservative replication and the antiparallel problem
- 02Mechanism in order: helicase, primase, polymerase, ligase; leading vs lagging
- 03Prokaryote vs eukaryote replication (single vs multiple origins)
- 04The central dogma: transcription and translation
- 05The cell cycle: G1·S·G2·M and Cyclin–CDK checkpoints
- 06Mitosis (PMAT) and cytokinesis: animal vs plant
Worked example: why one strand is continuous and the other comes in pieces
- +1(a) The constraint: polymerase builds only 5′→3′, reading its template 3′→5′. Because the two templates run in opposite directions (antiparallel), the enzyme can run smoothly toward the fork on one template only.
- +1(a) The consequence: on the template oriented the right way, synthesis is one continuous run toward the fork; on the other template, the enzyme must keep jumping back toward the fork and stitching short pieces.
- +1(b) Name them: the continuous strand is the leading strand; the fragmented strand is the lagging strand, built from short Okazaki fragments.
- +1(c) Sealing the gaps: after primers are replaced with DNA, ligase seals the nicks between fragments into a continuous strand.
- +1(c) Why a primer: DNA polymerase can only extend an existing 3′ end, so primase first lays a short RNA primer to give it a starting point — the lagging strand needs a new primer for every fragment.
Key terms
- Semi-conservative replication
- Each new DNA molecule keeps one old strand and one new strand. The intact old strand serves as a reference for proofreading and repair, keeping the mutation rate very low.
- Leading vs lagging strand
- On one template the new strand is built continuously 5′→3′ toward the fork (leading); on the antiparallel template it is built in short Okazaki fragments running away from the fork (lagging), each started by an RNA primer.
- Central dogma
- The one-way flow of genetic information: DNA is transcribed into RNA, and RNA is translated into protein. The protein then does the work.
- Cell cycle
- The ordered loop G1 → S → G2 (interphase) → M (mitosis). Cyclin–CDK checkpoints at G1/S and G2/M verify size, nutrients and DNA integrity before the cell is allowed to advance.
- Mitosis (PMAT)
- Nuclear division in four stages — prophase (condense, spindle forms), metaphase (align at the plate), anaphase (chromatids pulled apart), telophase (two nuclei reform) — producing two genetically identical daughter cells.
DNA and Cell Division FAQ
Why is DNA replication called 'semi-conservative'?
Because each daughter molecule conserves one of the two original strands: one old strand plus one newly built strand. This matters for fidelity — the intact old strand acts as a template for proofreading, which keeps copying errors very low — and for repair: because DNA is double-stranded, a damaged strand can always be rebuilt from its complementary partner.
What is the difference between transcription and translation?
Transcription copies a gene's DNA into a portable mRNA message (RNA polymerase binds a promoter, reads the template 3′→5′ and builds mRNA 5′→3′). Translation reads that mRNA into a protein: the ribosome clamps the mRNA, tRNA adaptors deliver amino acids by matching their anticodon to each three-base codon, and the chain grows until a stop codon (AUG = start = Met). Think recipe (mRNA), waiter (tRNA), kitchen bench (ribosome).
What do the cell-cycle checkpoints do?
They are quality gates run by Cyclin–CDK complexes. At the G1/S and G2/M transitions the cell verifies its size, nutrients and DNA integrity before committing to the next stage; if DNA is damaged or only half-copied, the cycle halts until it is fixed. Failure of these checkpoints is central to uncontrolled, cancerous division — the cell advances when it should have stopped.
How do I avoid mixing up metaphase and anaphase (and mitosis with meiosis)?
For the stages, use the memory hooks: chromosomes aligned single-file at the equator = metaphase (M for Middle); sister chromatids moving apart to opposite poles = anaphase (A for Apart). For mitosis vs meiosis: mitosis makes two identical diploid cells for growth and repair, while meiosis makes four different gametes — don't conflate them. For cytokinesis, a cell plate means a plant cell, a contractile-ring pinch means an animal cell.
Exam move
Get the 5′→3′ rule right first — almost everything in replication follows from it. Be able to list the mechanism in order (helicase → primase → polymerase → ligase) and explain the leading-vs-lagging asymmetry from the antiparallel constraint. For the central dogma, separate the two steps cleanly and name the players in each. For the cell cycle, know what happens in each phase and what the checkpoints check. For mitosis, re-draw PMAT and lock the metaphase-vs-anaphase distinction (aligned vs apart) and the plant-vs-animal cytokinesis trap (cell plate vs contractile ring).
