NDA Biology · Cell Biology

Cell Division and DNA Replication

Before a cell divides it copies its DNA (replication); the loose chromatin then coils into rod-shaped chromosomes. Eukaryotes divide by mitosis/meiosis, but prokaryotes divide differently — by binary fission.

All Cell Biology notes NDA Biology strategy

Why this matters

Small (2 PYQs) but both are MODERATE statement questions that reward careful reading. Two ideas are tested: the sequence of replication (DNA copies when chromatin is open, then condenses into chromosomes for division) plus the prokaryote-vs-eukaryote difference, and ploidy in double fertilization (zygote 2N, endosperm 3N). Both MODERATE.

Concept 1 of 2

DNA replication and cell division — the sequence

Intuition

A cell can only copy its DNA while the chromatin is loosely unwound — replication needs access to the strands. Once copied, the chromatin condenses into compact rod-shaped chromosomes so they can be cleanly separated during division. And how a cell divides depends on its type.

Definition

The correct statements the bank tests:

DNA replication takes place when the chromatin is opened up (uncoiled) — TRUE.
Chromatin then organises into rod-shaped chromosomes before division — TRUE.
Prokaryotes and eukaryotes do NOT use the same division process — prokaryotes divide by binary fission, eukaryotes by mitosis / meiosis — so 'same process' is FALSE.

Statement	Correct?
DNA replicates when chromatin is opened up	True
Chromatin forms rod-shaped chromosomes before division	True
Prokaryotes and eukaryotes divide by the same process	False Prokaryotes = binary fission; eukaryotes = mitosis/meiosis. NOT the same.

Practice this conceptself-check · 3 quick reps

Try it yourself

Which statements are correct? 1. DNA replication takes place when chromatin is opened up. 2. Chromatin organises into rod-shaped chromosomes before division. 3. Prokaryotes and eukaryotes share the same division process.

Practice — Level 1 (3 reps)

Quick reps to lock in the method. Try each, then check.

1.
Is the chromatin open or condensed during DNA replication?
2.
How do prokaryotes divide?
3.
Do prokaryotes and eukaryotes divide by the same process?

From the bank · past-year question

Example 1Cell BiologyMODERATE

Consider the following statements : 1. DNA replication takes place when chromatin is opened up. 2. Chromatin organises itself into rod-shaped chromosomes before cell division. 3. Both prokaryotes and eukaryotes have the same process for cell division. Which of the statements given above is/are correct ?

[Q82 · Apr · 2024]

Prokaryote and eukaryote division differ

A statement claiming 'both prokaryotes and eukaryotes have the same process for cell division' is FALSE. Prokaryotes split by binary fission; eukaryotes use mitosis (body cells) or meiosis (gametes).

Concept 2 of 2

Ploidy and double fertilization (N, 2N, 3N)

Intuition

Ploidy just counts chromosome sets. A gamete carries one set (N, haploid); a normal body cell carries two (2N, diploid). In flowering plants, 'double fertilization' makes two products with different ploidy: a 2N zygote and a 3N endosperm.

Definition

Ploidy in plant double fertilization:

One set of chromosomes = N (haploid); a normal body cell = 2N (diploid).
Zygote = sperm (N) + egg (N) = 2N.
Endosperm = sperm (N) + two polar nuclei (N + N = 2N) = 3N (triploid).
So for a diploid plant, the zygote and endosperm have 2N and 3N sets respectively.

Double fertilization products

\text{zygote} = N + N = 2N \qquad \text{endosperm} = N + 2N = 3N

None set of chromosomes (haploid)
2Ntwo sets (diploid) — the zygote
3Nthree sets (triploid) — the endosperm

Worked example

In a plant whose one chromosome set is N, what is the ploidy of the egg cell, and of the endosperm formed in double fertilization?

The egg is a gamete, so it carries a single set: N (haploid).
The endosperm forms from one sperm (N) fusing with the two polar nuclei (N + N = 2N).
N + 2N = 3N, so the endosperm is triploid (3N).

Answer:Egg = N; endosperm = 3N.

Practice this concept3 quick reps

Practice — Level 1 (3 reps)

Quick reps to lock in the method. Try each, then check.

1.
What is the ploidy of a zygote in a diploid plant?
2.
What is the ploidy of the endosperm?
3.
How many chromosome sets does a gamete carry?

From the bank · past-year question

Example 2Cell BiologyMODERATE

If one set of chromosomes for a given plant is represented as

N

; in case of double fertilization, the zygote and the endosperm nucleus of a diploid plant would have how many sets of chromosomes respectively?

[Q87 · Sep · 2018]

Endosperm is 3N, not 2N

The zygote is 2N (one sperm + egg). The endosperm is 3N because the second sperm fuses with TWO polar nuclei (N + 2N = 3N). Don't give both products the same ploidy.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Ploidy and double fertilization (N, 2N, 3N)
Double fertilization products
$\text{zygote} = N + N = 2N \qquad \text{endosperm} = N + 2N = 3N$

Reference tables (1)

DNA replication and cell division — the sequence3 rows

Statement	Correct?
DNA replicates when chromatin is opened up	True
Chromatin forms rod-shaped chromosomes before division	True
Prokaryotes and eukaryotes divide by the same process	False Prokaryotes = binary fission; eukaryotes = mitosis/meiosis. NOT the same.

Watch out for (2)

Prokaryote and eukaryote division differ
→ DNA replication and cell division — the sequence
Endosperm is 3N, not 2N
→ Ploidy and double fertilization (N, 2N, 3N)

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