CourseCourseQuestionQuestionDocumentDocument

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

Coverage 2021–2023 · Updated 16 Jul 2026

Replication Creates Sister Chromatids before Nuclear Division

1

Before S phase, one chromosome contains one DNA molecule. During S phase that DNA is replicated.

An unreplicated chromosome becomes two sister chromatids joined at a centromere and later separates into daughter chromosomes.
2

After replication, the chromosome consists of two nearly identical sister chromatids joined at a centromere. The chromosome count has not doubled, but the DNA amount has.

3

When the sister chromatids separate, each becomes an individual daughter chromosome. Sending one copy to each future nucleus preserves a complete genome.

Do not count chromatids as separate chromosomes while they remain joined at one centromere.

Reconstruct How One Replicated Cell Becomes Two Daughter Cells

DNA replication creates joined sister chromatids → chromosomes condense and attach to the spindle → metaphase alignment checks bipolar attachment → sister chromatids separate → two nuclei re-form → cytokinesis partitions the cytoplasm

Continuity requirement What supplies it
complete genome in each nucleus replication followed by equal chromatid separation
controlled chromosome movement condensation, spindle microtubules and motor proteins
two separate cells animal cleavage furrow or plant cell plate
suitable daughter-cell contents equal or functionally unequal cytoplasmic partition

To identify a stage, prioritize chromosome position and nuclear-envelope state. To explain the outcome, connect those observations to what is being separated.

One DNA Replication Followed by Two Divisions Produces Haploid Nuclei

1

Before meiosis, S phase replicates every chromosome into two sister chromatids. The nucleus is still diploid because both homologues of each pair remain present.

A diploid cell replicates DNA once, separates homologous chromosomes in meiosis I and separates chromatids in meiosis II to form four haploid nuclei.
2

In meiosis I, homologous maternal and paternal chromosomes separate into different nuclei. Chromosome number is reduced from diploid to haploid, although each chromosome still has two chromatids.

3

There is no second DNA replication. In meiosis II, sister chromatids separate, producing four haploid nuclei with one chromosome from each homologous pair.

Reduction prevents chromosome number doubling in every generation: haploid gametes fuse at fertilization to restore the diploid number.

Track What Separates to Explain Reduction, Error and Variation

Event What changes Main consequence
replication before meiosis each chromosome gains a sister chromatid DNA prepared for two divisions
meiosis I homologous chromosomes separate chromosome number is halved
meiosis II sister chromatids separate four haploid products can form
non-disjunction one required separation fails aneuploid gametes may form

Variation is layered: crossing over recombines alleles within chromosomes, random orientation assort homologues independently, and random fertilization combines two independently produced gametes.

When explaining any meiotic outcome, name the stage, the chromosome object that should separate, the chromosome number before and after, and whether the result changes number, allele combination or both.

G1 Builds the Cell, S Copies DNA and G2 Prepares Division

HL only
Interphase stage Main work Chromosome state by the end
G1 cell growth, metabolism, protein and organelle production each chromosome has one chromatid
S DNA replication each chromosome has two sister chromatids
G2 further growth, protein synthesis and preparation for spindle formation replicated chromosomes remain joined

The order matters: the cell first builds capacity, then copies the genome once, then checks and prepares the machinery needed to distribute those copies.

DNA quantity doubles during S phase, but chromosome number does not double because the sister chromatids remain joined at their centromere.

DNA replication prerequisite

[ 1 ]

Distinguish between the quantity of DNA of the cell at G1 and G2.

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.