D2.1.5—DNA replication prerequisite
DNA replication in S phase creates sister chromatids that remain attached until their separation during mitosis or meiosis later in division.
- Syllabus
- First assessment 2025
- Objective
- D2.1.5
- Level
- HL
Exam analysis
Chance of appearing2%of analysed past papers
Latest appearanceMay 2023
Most common paperPaper1
Typical marks1
Common command terms
- Distinguish
Scoring notes
- Common mistake
- Counting chromosomes and chromatids as the same thing after DNA replication.
Recent exam appearances
May 2023Paper1 ["HL"] · TZ25[ 1 ]D2.1.5—DNA replication prerequisite
May 2021Paper1 ["HL"] · TZ24[ 1 ]D2.1.5—DNA replication prerequisite
Practice this objective
Coverage 2021–2023 · Updated 16 Jul 2026
DNA Replication Must Precede Nuclear Division
DNA replication occurs before nuclear division so each daughter nucleus can receive a complete chromosome set.
During S phase, each chromosome becomes two sister chromatids joined at a centromere. Division then separates chromatids or homologues according to the process.
Trace: replication; duplicated chromosome; spindle attachment; chromosome separation; daughter nuclei.
If a cell entered mitosis without replicating DNA, one daughter could receive too little genetic material.
Replication doubles DNA amount, not chromosome number in the usual chromosome-counting convention.
DNA replication prerequisite
Assessment in practice
1 marksHow it is assessed
This objective is assessed through structured response, commonly using Distinguish.
Command terms
Distinguish
What earns marks
Build the answer around this relationship: DNA replication occurs in S phase of interphase.
Watch for
Counting chromosomes and chromatids as the same thing after DNA replication.
Representative question
Question 1
Distinguish between the quantity of DNA of the cell at G1 and G2.
Core Cell Division
- Cell division produces daughter cells for growth, repair or reproduction; cytokinesis divides cytoplasm by a contractile ring in animals or a cell plate in plants.
- DNA replication creates sister chromatids joined at centromeres before nuclear division.
- Mitosis preserves chromosome number: chromosomes condense, align, sister chromatids separate and nuclei reform, producing genetically identical nuclei.
- Meiosis follows one replication with two divisions: homologous chromosomes separate in meiosis I and sister chromatids in meiosis II, producing haploid cells.
- Crossing over, independent orientation and random fertilization generate allele combinations.
- Non-disjunction is failed chromosome separation and can produce aneuploid cells, including trisomy 21.
- Identify stages in micrographs from chromosome condensation, equatorial alignment, separation and nuclear-envelope cues.
Concept essentials
- DNA replication occurs in S phase of interphase.
- After replication, each chromosome consists of two sister chromatids.
- Sister chromatids remain joined at the centromere before separation.
- DNA amount doubles from G1 to G2 before division.