Structure and function of the genome

Chromosomes

The normal human genome is diploid and consists of 46 human chromosomes in 23 pairs. Chromosome abnormalities may be re­lated to the number (aneuploidy) or structure of chromosomes.

Meiosis is the cell division process in germline cells, resulting in the production of haploid gametes (ova and sperm). The process of meiosis generates four haploid cells, which can participate in fertil­ization. Mitosis is a process of the cell cycle in somatic cells resulting in the formation of two diploid daughter cells with identical gen­omes to that of the parental cell.

Genes and gene expression

A gene is a sequence of nucleotides in deoxyribonucleic acid (DNA) which codes for a protein. The sequence of nucleotides determines the amino acid sequence of the protein and its function. Each gene is represented twice (alleles) in the complement of genes known as the genome. Genes contain information that determines phenotype.

Genes are made up of exons and introns. Exons code for the pro­tein and introns are spliced out during processing to messenger ribonucleic acid (mRNA). The length of the introns is far greater than that of the exons. The exact function of the introns is unclear. Although the exon sequence is highly conserved between individ­uals, the intron sequence is not.

Gene expression is the process by which information from a gene is used in the synthesis of a protein or another gene product such as transfer RNA (tRNA) or functional RNA. This process involves transcription of RNA from a DNA template and translation of mRNA into protein.

Although all cells in an organism have the same information in their DNA, only 3-5% of genes are active in a cell. Most of the genome is suppressed, a characteristic of gene expression.

Changes in gene regulation result in the expression of various gene products and the suppression of others. Methods for meas­uring RNA to evaluate gene expression include northern blot, ribo­nuclease protection assay, in situ hybridization, reverse- transcription quantitative polymerase chain reaction (PCR), and spotted comple­mentary DNA arrays. Genome-wide methods for profiling gene expression include oligonucleotide arrays (microarrays) and tran­scriptome sequencing.

Epigenetics

Epigenetics is the study of heritable genome modifications in gene expression that are not due to alterations in DNA sequences. DNA methylation and histone modification (acetylation, methylation) are common epigenetic changes and can affect the process of transcrip­tion or silencing of gene expression.

Other epigenetic changes include modifications in non-coding RNAs and telomere length. Influences of environmental factors and epigenetic changes in the development of diseases such as cancer have been investigated in recent years. Such factors may include drugs, ultraviolet light, infection, and diet. Geographic differences in the incidence of autoimmune diseases have also been studied (1). Ageing and development of disease is another area of epigenetics involvement.

Molecular biology techniques

Molecular diagnostic tools in clinical genetics are applied for genotyping, detection of mutations, and assessment of chromo­somal structural variants.

PCR technology is used to identify mutations. It requires prior knowledge of the DNA sequence of the fragment to be amplified. Real-time PCR allows the simultaneous detection and quantifica­tion of a DNA molecule and selection of mutant DNA. Deletion and insertion mutations can be identified using this technique.

PCR can detect organisms such as human immunodeficiency virus (HIV), methicillin-resistant Staphylococcus aureus, as well as chromosomal translocations associated with cancers.

Cytogenetic karyotype analysis by chromosomal banding, fluor­escence in situ hybridization (FISH) on metaphase or interphase nuclei, or array comparative genomic hybridization (CGH) can identify structural variations. The resolution improves from karyo­typing to interphase FISH and to array CGH. New sequence variants continue to be discovered with methods that allow analysis of entire genes or genomes.