What is the difference between chromatin and a chromosome?

Chromatin is the loose, uncoiled complex of DNA and proteins found during interphase, while a chromosome is the highly condensed version of this complex visible during cell division. Chromatin allows for DNA access, whereas chromosomes are optimized for physical movement.

How do sister chromatids differ from homologous chromosomes?

Sister chromatids are genetically identical copies of a single chromosome produced during DNA replication. Homologous chromosomes are a pair of chromosomes (one maternal, one paternal) that have the same genes at the same loci but may have different versions (alleles) of those genes.

When does a chromosome transition from having one chromatid to having two sister chromatids?

This transition occurs during the S phase (Synthesis phase) of interphase. During this time, the DNA molecule is replicated, resulting in two identical strands joined at the centromere.

What is a common misconception regarding the shape of chromosomes throughout the cell cycle?

A common error is believing chromosomes are always X-shaped. They only appear as the familiar X-shape after replication in the S phase and before they are pulled apart during anaphase of mitosis.

Why is it incorrect to say that telomeres prevent DNA shortening?

Telomeres do not stop the DNA from shortening during replication; instead, they ensure that the shortening occurs within non-coding, repetitive regions. This prevents the loss of essential genes that would otherwise occur if the DNA shortened at coding regions.

What mistake is made when confusing the centromere with the centrosome?

The centromere is the DNA region that holds sister chromatids together, while the centrosome is an organelle in the cytoplasm that organizes microtubules. Confusing them leads to a misunderstanding of how chromosomes are physically structured versus how they are moved.

Define 'Histones' and their primary chemical characteristic.

Histones are globular proteins that DNA wraps around to form chromatin. They are characterized by a strong positive charge, which allows them to attract and bind to the negatively charged phosphate groups of DNA.

What are telomeres and where are they located?

Telomeres are protective, non-coding caps consisting of repetitive DNA sequences. They are located at the very ends of linear chromosomes in eukaryotic cells.

Define the 'Centromere'.

The centromere is the constricted region of a chromosome that joins two sister chromatids together. It is the site where the kinetochore forms and spindle microtubules attach during cell division.

Why must DNA be associated with proteins in eukaryotic cells?

DNA is extremely long (meters) and must fit into a microscopic nucleus (micrometers). Proteins like histones provide the structural framework to coil and fold the DNA into a compact, organized volume.

Chromosome Structure | Cambridge International Examinations AS-Level Biology

Revision Notes

AS-Level

Cambridge International Examinations

Biology

5. The Mitotic Cell Cycle

Chromosome Structure

Summary

Chromosome structure represents the highly organized packaging of genetic material within eukaryotic nuclei. By associating long DNA molecules with specialized histone proteins, cells can condense their genome into manageable units that ensure stable inheritance and precise segregation during cell division.

1. Definition & Core Concepts

Chromosomes are thread-like structures located inside the nucleus of eukaryotic cells, consisting of a single, extremely long molecule of DNA associated with proteins.
The primary proteins involved are histones, which are large, positively charged globular proteins that facilitate the tight coiling and organization of the negatively charged DNA backbone.
The complex formed by the association of DNA and histone proteins is known as chromatin, which exists in varying states of condensation depending on the stage of the cell cycle.
This structural organization is essential for fitting approximately 2 meters of DNA into a microscopic nucleus while maintaining accessibility for transcription and replication.

Diagram showing DNA coiling around histone proteins to form chromatin, and the resulting structure of a replicated chromosome with sister chromatids and a centromere.

2. Anatomy of a Replicated Chromosome

Following the S phase of interphase, each chromosome consists of two identical DNA molecules known as sister chromatids.
These chromatids are held together at a specialized, narrow region called the centromere, which serves as the attachment point for spindle fibers during mitosis.
The presence of the centromere is critical for the equal distribution of genetic material, as it ensures that one copy of each chromatid is pulled to opposite poles of the dividing cell.
A chromosome is considered a single unit whether it consists of one chromatid (pre-replication) or two sister chromatids (post-replication).

3. The Role of Telomeres

Telomeres are protective caps located at the extreme ends of eukaryotic chromatids, composed of non-coding, repetitive DNA sequences (e.g., TTAGGG in humans).
Their primary function is to act as a buffer zone, preventing the loss of vital protein-coding genes during DNA replication.
Because DNA polymerase cannot replicate the very end of a linear DNA molecule, a small portion of the telomere is lost with each cell division instead of essential genetic information.
Telomeres also prevent the ends of chromosomes from being recognized as damaged DNA, which would otherwise trigger cell cycle arrest or inappropriate DNA repair mechanisms.

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions