Introduction

Standard karyotyping is a cytogenetic technique that arranges and photographs chromosomes to study their number and structure, particularly for detecting chromosomal abnormalities. For accurate karyotype analysis, cells must be arrested at the mitotic stage where chromosomes are maximally condensed and clearly distinguishable under a light microscope, which is typically achieved at metaphase.​

Correct option: Metaphase (option c)

During metaphase of mitosis, chromosomes are highly condensed and aligned at the equatorial plate, making their size, shape and number easy to observe and photograph. In standard karyotyping, mitotic inhibitors such as colchicine are used to arrest cells in metaphase so that chromosomes can be spread, stained and imaged for photomicroscopic analysis.​

Because metaphase chromosomes are short, thick and well separated, individual homologous pairs can be identified, ordered and analyzed for structural changes such as deletions, duplications, translocations and aneuploidies. Most clinical cytogenetics laboratories therefore standardize their protocols to block the cell cycle at metaphase before preparing karyotypes.​

Why other options are incorrect

Interphase (option a)

Interphase is not a phase of mitosis; it is the preparatory stage of the cell cycle during which DNA replication and cell growth occur. In interphase, chromatin is decondensed and dispersed in the nucleus, so individual chromosomes cannot be clearly resolved or counted under a light microscope for standard karyotyping.​

Although interphase nuclei can be analyzed using specialized molecular cytogenetic methods such as interphase FISH, conventional banded karyotypes used in routine cytogenetics are not prepared from interphase cells.​

Prophase (option b)

In prophase, chromosomes start to condense and become visible, but they are still elongated and often overlapping, with the nuclear envelope only beginning to break down. At this stage, the degree of condensation is insufficient for optimal banding resolution, and individual chromosomes may not be adequately separated for precise measurement and classification in a karyotype.​

Some specialized protocols may use prometaphase or late prophase preparations to obtain higher band resolution, but the “standard” routine karyotype for diagnostic purposes is classically prepared from metaphase-arrested cells.​

Anaphase (option d)

Anaphase begins when sister chromatids separate at the centromere and migrate toward opposite poles of the cell. During anaphase, chromatids are moving and become spatially separated, making it difficult to capture a complete, well-organized set of chromosomes from a single pole for standard karyotype construction.​

Because chromatids are stretched and displaced during anaphase, the morphology is not ideal for reliable comparison of homologous pairs or for detecting subtle structural abnormalities, which is why this stage is not used in standard karyotyping by photomicroscopy.​

Summary for exam perspective

For MCQ-type questions such as “At which mitotic stage can standard karyotyping be performed using photomicroscopy of cells?”, always choose metaphase because this is when chromosomes are maximally condensed, aligned and routinely arrested in diagnostic cytogenetics. Interphase, prophase and anaphase either lack sufficient condensation or have inappropriate chromosome positioning, making them unsuitable for standard karyotype preparation.​