Chromosome structure
CIE A-Level BiologyΒ· 5.1 The Mitotic Cell CycleΒ· 15 min read
1. DNA Packaging and Chromatin Structureβ β ββββ± 5 min
Chromatin
The uncondensed form of DNA found in the interphase nucleus, consisting of DNA complexed with packaging proteins. It allows access for transcription and DNA replication.
Eukaryotic DNA is extremely long: a human diploid cell contains ~6 billion base pairs of DNA that stretches to ~2 meters when fully extended. This must be tightly packaged to fit inside a nucleus that is only ~10 micrometers in diameter.
Nucleosome
The basic repeating unit of chromatin, consisting of DNA wrapped around a core of 8 positively charged histone proteins.
Example:
A single human nucleosome contains ~147 base pairs of DNA wrapped around a histone octamer.
Explain why histone proteins are positively charged, and how this aids DNA packaging.
- 1
- Recall that the backbone of a DNA molecule contains phosphate groups.
- 2
- 3
- Positively charged histone proteins form stabilizing ionic bonds with negatively charged DNA.
- 4
- This neutralizes DNA's negative charge, eliminating electrostatic repulsion that would prevent tight packing.
Exam tip:
Always mention the charge interaction between histones and DNA when explaining packaging β this is a common marking point in CIE exams.
2. Condensed Chromosome Structure in Cell Divisionβ β β βββ± 6 min
When cell division (mitosis or meiosis) begins, chromatin undergoes further higher-order condensation to form short, thick chromosomes. This condensation prevents tangling and breakage during chromosome separation.
Sister Chromatid
Two identical copies of a replicated chromosome, produced during S phase of the cell cycle, joined together at the centromere. Each chromatid contains one complete DNA molecule.
Two key regions define a metaphase chromosome: the centromere is the attachment point for sister chromatids and spindle microtubules, while telomeres are repetitive DNA sequences at the ends of chromosomes that protect coding DNA from degradation.
A diploid human cell has 46 chromosomes in G1 phase. How many chromosomes, chromatids, and DNA molecules are present after S phase, before mitosis?
- 1
- Recall that S phase is when DNA replication occurs, and chromosomes are counted by the number of centromeres, not chromatids.
- 2
- After replication, each chromosome still has only one centromere, so total chromosome number does not change: 46 chromosomes.
- 3
- Each replicated chromosome has 2 sister chromatids, so total chromatids = 46 Γ 2 = 92 chromatids.
- 4
- Each chromatid contains one DNA molecule, so total DNA molecules = 92.
Exam tip:
Never count chromatids to get chromosome number: always count centromeres. This is the most frequent exam mistake on this topic.
3. Key Terminology Distinctionsβ β ββββ± 4 min
CIE exams regularly test your ability to distinguish between commonly confused terms in this topic. The table below summarizes the key differences:
Term | Cell Cycle Stage | Key Description |
|---|---|---|
Chromatin | Interphase | Uncondensed, active for transcription/replication |
Chromosome | Mitosis/Meiosis (M phase) | Highly condensed, inactive for transcription |
Chromatid | S phase to anaphase | One copy of a replicated DNA molecule |
Centromere | All stages | Region joining chromatids, spindle attachment site |
Test your understanding of core terminology:
How many centromeres are in a replicated chromosome with 2 sister chromatids?
0
1
2
4
Reveal answer
1 βCorrect! Each chromosome has one centromere, regardless of replication status. The two sister chromatids share a single centromere until anaphase.
What is the main function of histone proteins?
Catalyse DNA replication
Protect chromosome ends
Package DNA to fit in the nucleus
Attach spindle microtubules
Reveal answer
Package DNA to fit in the nucleus βIncorrect. Telomeres protect chromosome ends, centromeres attach spindles, histones are packaging proteins.
4. Common Pitfalls
Wrong move:
Counting chromosomes by the number of chromatids after replication
Why:
Chromosome number is defined by the number of centromeres, not chromatids. Replication does not change chromosome number.
Correct move:
Count one chromosome per centromere, whether the chromosome has one or two chromatids.
Wrong move:
Claiming prokaryotes have chromatin
Why:
Prokaryotes do not have histones or chromatin: their DNA is circular and 'naked' (not complexed with histone proteins).
Correct move:
Chromatin (DNA + histone complex) is unique to eukaryotic nuclei.
Wrong move:
Stating centromeres are only present in replicated chromosomes
Why:
The centromere is a permanent structural region of all chromosomes, even when unreplicated.
Correct move:
Centromeres exist in all chromosomes across all stages of the cell cycle.
Wrong move:
Claiming nucleosomes only exist in condensed chromosomes
Why:
Nucleosomes are the basic building block of all chromatin, regardless of condensation level.
Correct move:
Nucleosomes are present in both uncondensed interphase chromatin and condensed mitotic chromosomes.
5. Quick Reference Cheatsheet
Term | Key Feature | Rule |
|---|---|---|
Chromatin | Uncondensed, DNA + histones, interphase | N/A |
Chromosome | Condensed, M phase | Count by number of centromeres |
Unreplicated chromosome | 1 chromatid, 1 DNA molecule | 1 chromosome = 1 DNA |
Replicated chromosome | 2 sister chromatids, 2 DNA molecules | 1 chromosome = 2 DNA |
Nucleosome | 147bp DNA + 8 histones | Basic unit of chromatin |
Centromere | Joins chromatids, spindle attachment | 1 per chromosome |
Telomere | Repeats at chromosome ends | Protects coding DNA |
6. Frequently Asked
What is the difference between a chromatid and a chromosome?
A chromatid is one identical copy of a replicated chromosome, joined to the other copy at the centromere. A non-replicated chromosome consists of a single chromatid, and a replicated chromosome has two sister chromatids sharing one centromere.
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 Β· 12
Describe eukaryotic chromosome structure
- 2023 Β· 21
Compare chromatin and chromosome structure
Going deeper
What's Next
Understanding chromosome structure is a foundational requirement for learning mitosis, where replicated chromosomes are separated to produce genetically identical daughter cells. It also provides the base for understanding meiosis and genetic variation, as well as chromosomal mutations that arise from errors in chromosome segregation. This topic builds on your knowledge of DNA structure and underpins all subsequent topics in genetics and cell division for CIE A-Level Biology.