DNA sequencing in a laboratory
In the 1950s, scientists using early lab techniques to look at chromosomes discovered that Downs Syndrome was caused by the presence of an additional copy of chromosome 21 in the baby’s DNA. This was the very beginning of genetic testing as we know it today.
Over the years, the field of pregnancy genetic testing and screening has evolved from these earliest methods of chromosome counting and sorting to become some of the most advanced technical analyses in the sphere of medical science.
Here we look briefly at the development of this important branch of medicine.
Every human being has a unique karyotype (an individual set of chromosomes).
Karyotyping is the name given to the laboratory process which first enabled an image of a person’s chromosomes to be created. The process, developed in the mid-1950s, used microscopes and medical dyes to enable the isolation and photography of individual chromosomes. Scientists could then arrange the images in numerical order and look for abnormalities and mutations.
In 1958, Dr Jerome Lejeune, identified the cause of Trisomy 21 (then known as Mongolism, now known as Down’s Syndrome) through karyotyping when he observed an extra chromosome in pair 21.
In 1959 his findings were published and, for the first time, a link was established between an intellectual disability and a chromosomal abnormality. Perhaps most importantly, it established that Trisomy 21 was not hereditary.
Dr Lejeune became the first Professor of Fundamental Genetics at the Faculty of Medicine in Paris and is often described as the father of modern genetics.
DNA molecules are made up of four nucleotides linked together in what can be described as a DNA sentence. These “sentences” within each cell contain the instructions for building the proteins and other molecules that the cell needs to carry out its daily work.
DNA sequencing technology was developed by Frederick Sanger in the 1970s. Also known as the “chain determination method, it enables the identification of the nucleotide sequence in DNA (the order that the nucleotides appear in the chain of DNA molecules) which allows scientists to “read” the gene sequence of an individual.
In Sanger sequencing, the process of DNA replication is recreated. Firstly, copies are made of the DNA strands being examined, then as replication occurs the scientists can see which order the nucleotides are added during the sequence.
Due to its overall efficiency and low radioactivity, Sanger sequencing became the primary technology used in the early days of commercial and laboratory genetic sequencing.
In the early days, DNA sequencing was a very lengthy process. Now, new methods are used which have advanced Sanger’s original method and reduced the time in which DNA can be sequenced. By using flurophores (detection reagents) and computers, a human’s entire DNA can now be sequenced in a few days rather than the 13 years it took the original Human Genome Project.