# Chromosome structure

> CIE A-Level Biology · 9700
> Source: https://www.owlsprep.com/study/cie-9700-u5-chromosome-structure/

This sub-topic covers hierarchical DNA packaging into eukaryotic chromosomes, key structural terminology, and the role of proteins in organizing DNA for cell division. You will learn to identify chromosome features across all stages of the cell cycle.

**Prerequisites:** [Basic DNA structure](https://www.owlsprep.com/study/cie-9700-u4-dna-structure/); [Overview of the cell cycle](https://www.owlsprep.com/study/cie-9700-u5-cell-cycle-overview/)

## Learning objectives

- Describe the hierarchical packaging of eukaryotic DNA into chromosomes
- Distinguish between key structural terms (chromatin, chromatid, chromosome, centromere)
- Explain the role of histone proteins and nucleosomes in DNA organization

## DNA Packaging and Chromatin Structure

**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.

**Worked example:** Explain why histone proteins are positively charged, and how this aids DNA packaging.

1. 1. Recall that the backbone of a DNA molecule contains phosphate groups.
2. $$\text{Phosphate groups have a negative charge at physiological pH}$$
3. 2. Positively charged histone proteins form stabilizing ionic bonds with negatively charged DNA.
4. 3. 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 — this is a common marking point in CIE exams.

## Condensed Chromosome Structure in Cell Division

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.

**Worked example:** 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. 1. Recall that S phase is when DNA replication occurs, and chromosomes are counted by the number of centromeres, not chromatids.
2. 2. After replication, each chromosome still has only one centromere, so total chromosome number does not change: 46 chromosomes.
3. 3. Each replicated chromosome has 2 sister chromatids, so total chromatids = 46 × 2 = 92 chromatids.
4. 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.

## Key Terminology Distinctions

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 |

**Exam command terms**

Common command terms for this topic and their exam expectations:

- **Describe** — Give a structured account of structure, including key components and their functions *(Describe the structure of a nucleosome)*

- **Distinguish** — Explicitly state the key differences between two terms/structures *(Distinguish between a chromatid and a chromosome)*

**Check your understanding**

Test your understanding of core terminology:

1. How many centromeres are in a replicated chromosome with 2 sister chromatids?

   - 0
   - 1
   - 2
   - 4

   *Why:* Correct! Each chromosome has one centromere, regardless of replication status. The two sister chromatids share a single centromere until anaphase.

2. What is the main function of histone proteins?

   - Catalyse DNA replication
   - Protect chromosome ends
   - Package DNA to fit in the nucleus
   - Attach spindle microtubules

   *Why:* Incorrect. Telomeres protect chromosome ends, centromeres attach spindles, histones are packaging proteins.

## Common pitfalls

- **Wrong:** Counting chromosomes by the number of chromatids after replication
  - Why it fails: Chromosome number is defined by the number of centromeres, not chromatids. Replication does not change chromosome number.
  - Correct: Count one chromosome per centromere, whether the chromosome has one or two chromatids.
- **Wrong:** Claiming prokaryotes have chromatin
  - Why it fails: Prokaryotes do not have histones or chromatin: their DNA is circular and 'naked' (not complexed with histone proteins).
  - Correct: Chromatin (DNA + histone complex) is unique to eukaryotic nuclei.
- **Wrong:** Stating centromeres are only present in replicated chromosomes
  - Why it fails: The centromere is a permanent structural region of all chromosomes, even when unreplicated.
  - Correct: Centromeres exist in all chromosomes across all stages of the cell cycle.
- **Wrong:** Claiming nucleosomes only exist in condensed chromosomes
  - Why it fails: Nucleosomes are the basic building block of all chromatin, regardless of condensation level.
  - Correct: Nucleosomes are present in both uncondensed interphase chromatin and condensed mitotic chromosomes.

## 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 |

## 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.

- [Stages of the cell cycle](https://www.owlsprep.com/study/cie-9700-u5-stages-of-the-cell-cycle/)
- [Mitosis and cytokinesis](https://www.owlsprep.com/study/cie-9700-u5-mitosis-and-cytokinesis/)
- [Cell cycle control and cancer](https://www.owlsprep.com/study/cie-9700-u5-cell-cycle-control-and-cancer/)

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