Mistakes in DNA
Genetic messages are found on the chromosomes, and we can think of them as genetic codes that are made up of three letter words. For example a very simple genetic message might be: thecathitthedoganddidnotseethemat
Electron micrograph of a human chromosome
Electron micrograph rather than a diagrammatic version showing a human chromosome.
A genetic message might be an instruction to assemble amino acids into a sequence to make a protein. These genetic messages are often thought of as blueprints for the cell. The genetic messages are found on DNA (deoxyribonucleic acid), and the DNA segments that carry these messages are called genes. Genes are found on chromosomes.
The DNA found in the chromosomes is arranged like a twisted ladder. The first clues to the arrangement of the molecules in this twisted ladder was provided with an X-ray diffraction image of DNA that was taken by Rosalind Franklin in 1952 when she was working in London.
Cytogeneticists studying chromosomes were able to identify the different parts of the DNA molecule. Scientists had identified that there were particular types of chemicals called bases that were found in DNA. These bases were Adenine (abbreviated as A), Thymine (T), Guanine (G) and Cytosine (C). These bases are found in the centre of a DNA molecule.
DNA bases
DNA molecules showing the arrangement of the bases that make up the rungs of the twisted ladder.
However, scientists were not sure how they would be arranged in a DNA molecule. It was the evidence provided by Rosalind Franklin in 1952 that helped James D Watson and Francis Crick to build their model of a DNA molecule. Subsequent research has shown that this is indeed the code that the cell uses to provide information to other parts of the cell to make proteins.
The nucleic bases appear like the rungs of a twisted ladder (double helix) where the sides are made of sugars and phosphate groups.
Nature of science
Often theories change in science. There is an explanation that some people are resistant to HIV because they have a mutation that makes them less susceptible. One theory is that this mutation may have been carried by people in Europe who survived bubonic plague in the 14th century. Lately, another theory has been put forward that it may be due to those who have survived small pox.
Mutations
When there is a mistake in the copying of the genetic message that is permanent, a mutation has occurred. UV light can cause mutations, as the DNA molecules are good absorbers of UV. Two of the bases in DNA (Cytosine and Thymine) are the most vulnerable, and when this happens, they may pair with each other or themselves and the message is changed.
DNA molecule
Diagram of a DNA molecule looking like a twisted ladder.
Let’s use the message above – thecathitthedoganddidnotseethemat – and read it using the three letter code: the cat hit the dog and did not see the mat.
Now let’s provide a mistake in the message, for example, repeating one of the letters in the message – thecatthitthedoganddidnotseethemat. If you read this message using the three letter code, it doesn’t make sense: the cat thi tth edo gan ddi dno tse eth ema t.
This is what happens to the dividing cells of the basal epidermis, and these mutations provide the wrong messages to the cell that may result in a cell becoming cancerous, for example, a malignant melanoma cell. For example the Thymine bases (coloured yellow in the diagram) that are next to each other in a DNA molecule may bond with each other rather than across the DNA ladder which makes a bulge.
UV photon altering the DNA message
An incoming UV photon can produce a mutation in the base pairing.
This means that the changed DNA molecule does not function properly.
Activity ideas
The activity ideas below help cover some of the big ideas in science:
