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What does a genome look like?

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18/07/2014

By | From the Collections

The human genome has been described in many ways: ‘the book of life’, a set of instructions, a language to be decoded. For many scientists it is a huge data set to be collected and analysed. Besides the metaphors associated with the concept of the genome, visual representations have played an important part in it’s formation.

Inscripitons‘ of the genome created by scientists have changed over the years as a result of a greater a understanding of the science, developments in laboratory techniques and advances in technology. Here are some images collected over the years by Wellcome Images:

B0000249 Down syndrome human karyotype

Karyotype for a human male. This male has a full chromosome complement of 46 plus an extra chromosome no. 21 indicating Down’s syndrome. Credit: Wessex Reg. Genetics Centre. Wellcome Image no. B0000249.

From the 1950s chromosomes became the visible manifestation of the genetic matter transferred from one generation to the next. Visible for a brief period in the cell, they could be stained and photographed under a microscope. When the full complement of human choromosomes – 46 – was determined, these photographs could be cut and pasted and arranged in order of size into a karyotype – a representation of all the human genetic material they contained.

While chromosomes couldn’t tell you about individual genes, they did show abnormalities and damage at the chromosome level. These abnormalities pointed to certain genetic disorders such as Down’s Syndrome, thus making a link between laboratory science and clinical medicine.

B0006064 Separation of DNA fragments by electrophoresis Credit: Guy Tear. Wellcome Images images@wellcome.ac.uk http://wellcomeimages.org Separation of DNA fragments by electrophoresis through an agarose gel. An electric current is passed through the gel and separates the DNA fragments according to size. The mixture of fragments is applied to a well at the top of the gel before the current is started. The smaller fragments travel further and reach the bottom of the gel while the larger ones remain towards the origin. Photograph 2003 Published:  -

Separation of DNA fragments by electrophoresis through an agarose gel. An electric current is passed through the gel and separates the DNA fragments according to size. The smaller fragments travel further, while the larger ones remain near their starting point at the top of the gel. Photograph credit: Guy Tear. Wellcome Image no. B0006064.

Chromosomes can be broken down into fragments of DNA for detailed analysis. With the use of electrophoresis to separate fragments in the 1970s, DNA fragments containing radioactive markers could be identified in the form of autoradioraphs. In 1984 the first genetic fingerprint was created leading to the development of genetics in forensic science.

C0004217 Automated DNA sequencing machines

Some of the vast array of automated DNA sequencing machines used by the Human Genome Project to determine the complete human DNA sequence. Wellcome Image no. C0004217.

By the 1990s techniques for including fluourscent bases in the cloning process used to produce vast quantities of DNA led to the automation of large scale sequencing. Aided by the Human Genome Project, technology such as  the use of a laser to ‘read’ colour coding meant that the results of genome sequencing could be displayed directly as computer output:

B0002668 Automated DNA sequencing output - HGP

The output from an automated DNA sequencing machine. Each vertical lane shows the sequence of thousands of bases in a given stretch of DNA. Each of the four different bases is labelled with one of the four coloured dyes. The order of the bases is analysed by a computer and assembled to give the continuous base sequence of each chromosome. This sequence is part of human chromosome 1. Credit: Sanger Institute, Wellcome Image no. B0002668.

The sequencing could also be represented graphically, with the peaks indicating the occurrence of bases and with further computer analysis this translated into the familiar ‘ACGT’ code shown at the top of the graphical display.

B0002672 Automated DNA sequencing output - HGP

The output from an automated DNA sequencing robot. Each peak shows the presence of a particular base. The sequences of different overlapping traces are ordered by the computer and assembled to give the continuous base sequence of each chromosome (seen along the top of the image). Credit: Sanger Institute, Wellcome Image no. B0002672.

From the photographic karyotypes of discrete chromosmal forms in the 1950s, through schematic ideograms and autoradiographs of fragments of DNA, a representation of the genome as a linear sequence of the DNA contained in all the chromosomes emerged.

While this was a useful reference point for genetic research, it was not particularly useful for medical genetics, where clinicians needed to isolate and identify specific genes or mutations that indicated genetic diseases. For this purpose they produced specific genome maps of pathological organisms such as malaria, and mutations in different cancers.

Malaria genome

A gene map of the malaria genome, Plasmodium falciparum clone 3D7. Each diamond represents a gene. Colour-coding is used to classify the function, biological process and location of the gene products. Wellcome Image no. C0014538.

Nor is this the end of the story. As well as the physical mapping, associating specific genetic traits with ‘landmarks’ on the genome, linkage maps display the relative position of genes on a chromosome, but it is difficult to show this in a linear display of all the chromosomes end to end. Many maps now display the genome in a circular form, with a ring of chromosomes on the outside and concentric rings representing physical and link data as well as associations across chromosomes:

Genome Image credit: Sanger Institute.

Genome in the form of a Circos plot developed by Martin Krzywinski. credit: Sanger Institute.

The DNA double helix has become a symbol for the science of life. Might some depiction of the genome become equally iconic one day?

Reference

Hogan A. The ‘morbid anatomy’ of the human genome. Medical History 2014; 58: 315-336.

Author: Lalita Kaplish is Assistant Web Editor at the Wellcome Library.

 

Lalita Kaplish

Lalita Kaplish is Web Editor at the Wellcome Library. You can also find her on LinkedIn and Twitter @LalitaKaplish.

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