Oxford Textbook of Obstetrics and Gynaecology by Sabaratnam Arulkumaran, William Ledger, Lynette Denny, Stergios Doumouchtsis
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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. A karyotype describes the number of chromosomes and major structural abnormalities such as deletions, duplications, or translocations.
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 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.