Mutations & Disease (DP IB Biology: HL)

• A mutation is a change in the sequence of base pairs in a DNA molecule
• They occur randomly and continuously to create new alleles of a gene
  • Often only with a very small number of differences in the base sequence

• As the DNA base sequence determines the sequence of amino acids that make up a polypeptide, mutations in a gene can sometimes lead to a change in the polypeptide that the gene codes for
  • If the change is significant, this could be harmful for the organism (mutations are rarely beneficial). It may affect the ability of the protein to perform its function
  • For example:
    • If the shape of the active site on an enzyme changes, the substrate may no longer be able to bind to the active site
    • A structural protein (like collagen) may lose its strength if its shape changes

• Most mutations are neutral because they do not alter the polypeptide or only alter it slightly so that its structure or function is not changed
  • This is because the genetic code is degenerate 

• Mutations in body cells can lead to cancer. These mutations are often eradicated when the individuals dies
• Mutations of cells which are involved in gamete production can be inherited by the next generation

Mutagenic agents

• There are natural mechanisms that take place within cells to ensure the accuracy of DNA replication
  • These mechanisms involve proofreading and repairing damaged DNA

• When the mutation rate of a cell rises to above a normal (usually low) rate then these mechanisms have become ineffective
• Mutagenic agents are environmental factors that increase the mutation rate of cells
• Radiation can cause chemical changes in DNA, this includes:
  • High-energy radiation such as UV light
  • Ionising radiation such as X-rays, gamma rays and alpha particles

• Chemical substances can also caused changes to DNA, examples include
  • Benzo[a]pyrene and nitrosamines found in tobacco smoke
  • Mustard gas used as a chemical weapon in World War I

Ionising radiation can cause mutations in DNA

Chernobyl Nuclear Disaster

• Chernobyl Nuclear Power Plant is in Ukraine
• In 1986 an incident at the plant caused an explosion and fire in the nuclear reactor core
• A large amount of radioactive material was released from the plant and went into the air
  • Radioactive isotopes of xenon, krypton, iodine, caesium, and tellurium were released as well as large amounts of small particles of uranium

The Chernobyl Disaster is probably the worst nuclear disaster in history

• The effects of the Chernobyl powerplant explosion were significant:
  • The total number of radiation related deaths to date has reached 4000
  • Large areas of pine forest turned brown and died in the weeks afterwards
  • Agricultural animals died due to thyroid damage caused by radioactive iodine and the consumption of contaminated meat e.g. lamb, was banned
  • Milk produced contained high levels of iodine in areas where waterways had been contaminated
  • Bioaccumulation of radioactive materials occurred in nearby waterways affecting fish in countries thousands of kilometres away and contaminating drinking water for many species
  • Thousands of cases of thyroid cancer were recorded as a direct result of the radioactive iodine, including over 4000 in children and adolescents
• However, despite all this, there is no significant evidence of an increase in solid cancers or leukaemia even in the most affected population
• With no human habitation within the exclusion zone since the explosion, other wild animals and many species of plants have moved in and colonised the area
  • Despite the high radiation levels, the life expectancy of these organisms has not been shortened

Nuclear bombing of Hiroshima and Nagasaki

• Two atomic bombs were dropped in Japan towards the end of World War II. One on Hiroshima and one on Nagasaki
• Between 150 000 and 200 000 people died as a direct result of the bombs
  • Half of these people died on the day the bombs were detonated and the rest died in the months immediately afterwards as a result of burns, radiation sickness, injuries or through illnesses and malnutrition

• Huge studies were carried out on other survivors, compared to a control group, to build a bigger picture of the longer term impact of high exposure to radiation
  • Incidence of cancer were much higher in the survivors studied compared to the control group
  • An increase in leukaemia cases was seen in both cities after a 2 year delay, which reached a peak around 6 years after the bombings
  • Those who were closer to ground zero seemed to be more seriously affected
  • There were also thousands of recorded cancerous tumours in the groups being monitored, although due to confounding factors, only around 800 could be formally attributed to the effects of radiation

• A large study was also carried out into the effects on babies pre and post birth
  • It was expected that there would be high numbers of mutations resulting in subsequent stillbirths or deformities, however, the numbers of incidence recorded were not significant
  • There was no evidence to suggest that babies conceived by survivors of the bombings were more likely to be born with birth defects

• There were many more social impacts associated with the bombs
  • The survivors were labelled 'Hibakusha', meaning 'the explosion affected people' and the associated stigma lead to widespread discrimination
    • There were concerns about whether the Hibakusha were contagious or whether the illnesses that they experienced were heritable. Sterilisation programs were even considered
    • As a result, many survivors struggled to find employment or marry