Getting started

We would like to start the course by summarising some of the key terms that you will meet over the next few weeks.

The glossary below provides a list of key terms used throughout the course. You do not need to read them all now; we’ll be linking back to this step wherever these terms appear, so you may refer back to this list if you are unsure of the terminology being used:

Term Definition
Alignment The process of matching reads back to their original position in the reference genome.
Allele An allele is one of a number of alternative forms of the same gene or genetic locus. We inherit one copy of our genetic code from our mother and one copy of our genetic code from our father. Each copy is known as an allele.
Array CGH Microarray based genomic comparative hybridisation. This is a technique used to detect chromosome imbalances by comparing patient and control DNA and comparing differences between the two sets. It is a useful technique for detecting small chromosome deletions and duplications which would not have been detected with more traditional karyotyping techniques.
Base A unit of DNA. There are four bases which form the cross links (or rungs) of the DNA double helix: adenine (A), thymine (T), guanine (G) and cytosine (C).
Capture see Target enrichment.
Cell differentiation The process by which a cell becomes specialized in order to perform a specific function.
Centromere The point at which the sister chromatids are joined.
Chromosome A structure located in the nucleus all living cells, comprised of DNA bound around proteins called histones. The normal number of chromosomes in each human cell nucleus is 46 and is composed of 22 pairs of autosomes and a pair of sex chromosomes which determine gender: males have an X and a Y chromosome whilst females have two X chromosomes.
Chromatid Two identical copies from the replication of a single chromosome. Therefore a sister chromatid refers to either of the two identical copies.
Clinical exome sequencing Differs per laboratory, but usually involves sequencing of all genes known to be associated with human disease.
Coverage The number of reads giving information about the base present at a set position in the reference sequence.
Crystallography The experimental science of determining the arrangement of atoms in crystalline solids.
DNA Deoxyribonucleic acid. DNA is a molecule consisting of two long chains of nucleotides twisted together to form a double helix. Genes are made from DNA.
DNA sequencing The process of determining the order of the bases – adenine, guanine, cytosine and thymine – in a molecule of DNA.
Epigenome Chemical marks on the DNA, regulating whether the gene is turned “on” or “off”.
Exome The coding portion of the genes. The exome constitutes 1-2 % of the genome.
Gene A portion of DNA that serves as the basic unit of heredity.
Genome The entirety of an individual’s genetic material including ≅20 000 genes and the genetic material between genes.
Gene panel A collection of genes to be sequenced together, which are usually linked by common biological pathways, or known disease associations.
Germline mutations Where a genetic error occurs in the egg or sperm pre fertilization. Therefore, the genetic mutation is passed onto the offspring.
Haploid A cell having a single set of unpaired chromosomes.
Histones Histones are basic proteins which function as spools for thread-like DNA to wrap itself around, allowing DNA to be packaged efficiently.
Homologous chromosomes A pair of chromosomes which have the same genes at the same loci.
Indel An insertion or deletion.
Karyotype The number and appearance of the chromosomes when viewed down a microscope.
Mapping See Alignment.
Meiosis Occurs in the production of sperm and eggs. Four daughter cells are produced from the original parent cell. There are two stages to meiosis, meiosis 1 and meiosis 2. At the end of meiosis, each daughter cell has only 23 chromosomes.
Missense mutation A missense mutation describes where a base substitution or change results in a codon which causes the insertion of a different amino acid into a protein. [see non-synonymous change]
Mitosis Occurs at the end of normal cell division. Two daughter cells are produced from one parent cell, both with the same number of chromosomes (46) as the parent cell.
Mosaicism Where a genetic error occurs after fertilisation, resulting in two distinct genetic cell lines.
Next Generation sequencing High-throughput DNA sequencing where millions of DNA bases are sequenced in parallel.
Non-pathogenic Not disease-causing.
Non-synonymous change A non-synonymous change describes where a base substitution or change results in a codon which causes the insertion of a different amino acid into a protein. [see missense mutation]
Nucleotide A nucleotide is composed of a DNA base, a phosphate and a pentose sugar.
Pathogenic Disease-causing.
Pharmacogenomics The branch of genetics concerned with determining the likely response of an individual to therapeutic drugs.
Phenotype The set of observable characteristics of an individual as a result of their genotype/environment.
Pull down see Target enrichment.
Read A computer generated sequence of bases representing the sequenced code from an original DNA fragment.
Reference sequence/genome An assembled version of a genome that can be used to make comparisons to the genomes from other individuals.
RNA Ribonucleic acid, a nucleic acid present in all living cells. Its principal role is to act as a messenger carrying instructions from DNA for controlling the synthesis of proteins.
Single nucleotide polymorphism A single base substitution occurring at high frequency (more than 1%) in the general population.
Somatic mosaicism The existance of genetically distinct cells lines in the body of an individual, but not the sex cells (germline)
Spindle Fibres which draw the chromosomes to either end of the poles during mitosis and meiosis.
Target enrichment A method for selecting a specific portion of the genome to undergo sequencing.
Transcription Transcription is the first step of gene expression, in which a particular segment of DNA is copied into RNA by the enzyme RNA polymerase.
Translation Translation is the step after transcription, in which cellular ribosomes use the RNA to produce a specific protein.
Variant of uncertain significance An alteration to the DNA sequence where it is unclear (on the basis of the available evidence) whether it is disease-causing or not.
Whole (human) exome sequencing Sequencing the portion of the genome which codes for proteins.
Whole (human) genome sequencing Sequencing of the entire length of the human genome.

We thank Miranda Shanks for her help in developing this glossary.

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This article is from the free online course:

The Genomics Era: the Future of Genetics in Medicine

St George's, University of London

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