Cytogenetics, the study of genetic material at the level of the chromosomes, is now a routine part of the investigation of many cancers as well as in the detection of inherited chromo - some disorders such as Down syndrome (trisomy 21). However, dramatic advances in molecular techniques over the past two decades have facilitated the extension of cytogenetic studies to probe deeper to the level of the genes themselves, for example by fluorescent in situ hybridisation (FISH) and comparative genomic hybridisation (CGH). This chapter will briefly review the principles of cell division and cell culture in the context of cytogenetic analysis, explore the principles and clinical applications of cytogenetics and molecular cytogenetics, and introduce CGH array concepts.
CONVENTIONAL CYTOGENETIC ANALYSIS
Cytogenetic analysis explores the full chromo - some complement in number and structure. There are two different kinds of cellular division in vivo: meiosis, the specialised division that occurs in germ cells, and mitosis, or somatic (non-reproductive) cell division.
A thorough understanding of the principles of the somatic cell cycle and mitotic cell division, as detailed in Chapter 4, is man - da tory for the understanding of cytogenetics. Between successive divisions, the cell cycle is characterised by four stages: G1 or gap 1 (in the mammalian cell cycle this lasts approxi mately 9 hours), S or synthesis of DNA (5 hours), G2 or gap 2 (3 hours) and M or mitosis (1 hour). Resting cells that are not dividing are said to be in G0. A full cell cycle spans about 18 hours.
Manipulating the cell cycle in vitro to ‘catch’ metaphases.
In order to visualise individual chromosomes the cell must be at the mitosis stage of the cell cycle and in metaphase. This is when chromosomes are the most compact and individualised. When cells do not divide they are said to be in interphase. During this stage the chromatin that forms the chromosomes unfolds to varying degrees, particularly within regions of active gene expression. The chromatin is loose and chromosomes are not visibly individualised. For cytogenetic analysis it is necessary to obtain a number of cells in meta phase, and cell culture is needed to achieve this in most cases.