BIOL1020 Chap.5 DNA Replication and the Cell Cycle
DNA Replication and the Cell Cycle
DNA replication duplicates genetic material before division, while mitosis and cytokinesis distribute that material into daughter cells. This chapter follows 5-prime-to-3-prime synthesis through leading and lagging strands, then tracks chromosome state through G1, S phase, G2 and mitosis. It distinguishes DNA amount from chromosome condensation and uses checkpoints to connect molecular errors with cell phenotypes.
Binary fission provides a prokaryotic comparison, showing the shared need to copy and segregate DNA without the eukaryotic spindle and nuclear cycle.
What this chapter covers
- 01
DNA polymerase direction
- 02
Leading and lagging strands
- 03
Okazaki fragment processing
- 04
Interphase and mitotic phase
- 05
Cell-cycle checkpoints
- 06
Mitosis and cytokinesis
Worked example · free
Predicting the result of inactive DNA ligase
- +1DNA polymerase can still extend primers in the 5-prime-to-3-prime direction.
- +1The lagging strand is produced as multiple Okazaki fragments, while the leading strand is mostly continuous.
- +1After primer replacement, nicks remain between adjacent lagging-strand fragments.
- +1Ligase normally seals those covalent backbone breaks, so the new strand remains discontinuous and vulnerable.
Key terms
- Replication Fork
- The region where parental DNA strands separate and new complementary strands are synthesised.
- Leading Strand
- The new strand synthesised continuously toward an opening replication fork.
- Lagging Strand
- The new strand synthesised discontinuously away from the fork as short fragments.
- Sister Chromatids
- Replicated chromosome copies that remain associated until separation during cell division.
- Cytokinesis
- The physical division of a cell's cytoplasm following chromosome segregation.
- Checkpoint
- A control point that blocks cell-cycle progression until a required condition is satisfied.
DNA Replication and the Cell Cycle FAQ
Why is there a leading and lagging strand?
The parental strands are antiparallel, while DNA polymerase can extend only in the 5-prime-to-3-prime direction. One new strand can therefore follow the opening fork continuously, but the other must be built away from it in short fragments that are later joined.
When is DNA copied in the cell cycle?
DNA is replicated during S phase of interphase. G1 supports growth and normal activity before copying, while G2 prepares for division and checks completion. Mitosis separates the replicated chromosome copies; it does not perform the original duplication.
What is the difference between mitosis and cytokinesis?
Mitosis organises and separates replicated chromosomes into equivalent nuclear sets. Cytokinesis divides the cytoplasm and completes formation of separate daughter cells. Animal cells use an actin-based cleavage furrow, while plant cells assemble a cell plate between the nuclei.
Why does DNA strand direction matter during replication?
DNA polymerase adds a nucleotide to an available three-prime end, so every new strand grows five-prime to three-prime. Because parental strands are antiparallel, one template supports continuous synthesis toward the fork while the other requires repeated synthesis away from it. Direction explains the asymmetry.
Why does DNA polymerase require a primer?
The polymerase extends an existing nucleotide chain rather than starting one from nothing. Primase supplies a short RNA segment with a free three-prime end, and DNA polymerase continues from that point. The primer is later removed, replaced with DNA and joined into the continuous backbone.
What do cell-cycle checkpoints protect?
Checkpoints delay progression when required conditions are not met. They assess signals such as cell readiness, completion and integrity of replicated DNA, and correct chromosome attachment. A failed checkpoint can let damaged or unequally attached material enter division, increasing mutation or chromosome-number errors in descendant cells.
How do Okazaki fragments become one strand?
Each fragment begins at a separate primer and is extended until it approaches the preceding fragment. After RNA primers are removed, DNA fills the resulting gaps. DNA ligase then seals remaining breaks in the sugar-phosphate backbone, converting discontinuous synthesis into a covalently continuous daughter strand.
Why must replication finish before chromosome separation?
Each future daughter cell needs a complete copy of every chromosome. Unreplicated regions, unresolved damage or incomplete attachment can create missing, duplicated or broken genetic material after division. Cell-cycle order and checkpoints reduce that risk by making completion and integrity prerequisites for the later mechanical steps of packaging and separation.
Exam move
Draw one replication fork with template directions, primers and synthesis arrows, then explain why only one strand needs repeated restarting. Memorise enzyme jobs as a sequence of physical actions: open, stabilise, prime, extend, replace and seal. For the cell cycle, make a table with phase, DNA state, chromosome appearance and control question.
Practise counting DNA molecules at G1, after S phase and after chromatid separation without relying on an X-shaped cartoon. When given a checkpoint failure, state what should have been verified and what abnormal material enters the next phase. Contrast mitosis with bacterial binary fission using both similarities and architectural differences.
Reverse one template direction and determine which synthesis labels must change to preserve polymerase chemistry.
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