7.2 Applications and skills

7.2.2 Evidence for theories of inheritance and epigenetics

  • Many prominent scientists were not convinced that DNA was the genetic material until the 1950s.
  • The ‘blender’ experiments, performed by Alfred Hershey and Martha Chase in 1952, provided convincing evidence that DNA was responsible for inheritance.

The Hershey-Chase experiments

  • Bacteriophage are small viruses that infect bacteria and transform them into virus-producing factories.
  • They are made of a protein coat and a DNA core.

Figure 7.2.2a – Structure of a generalised bacteriophageFigure 7.2.2a – Structure of a generalised bacteriophage

  • Viruses inject their genetic material into cells, but the non-genetic part of the virus remains outside the cell.
  • Hershey and Chase wanted to know if the protein or the DNA was the chemical that transforms bacteria.
  • In two separate experiments, Hershey and Chase infected E. coli bacteria with radioactively labelled phage.
  • In the first experiment, viral proteins were labelled with radioactive sulphur-35.
  • In the second experiment, viral DNA was labelled with radioactive phosphorus-32.
  • The virus was left to infect the bacteria for a number of hours.
  • After infection, the cultures were agitated in a blender to detach the virus from the bacteria.
  • The mixture was then centrifuged in order to separate the components by density.

Figure 7.2.2b – Method and results of the blender experimentsFigure 7.2.2b – Method and results of the blender experiments

Skill: Analysing results of the blender experiments

1. Describe briefly why the virus was labelled with radioactive isotopes.

Radioactive isotopes are unstable, and slowly break down to release radiation. Radiation can be detected with UV light.

2. Explain why two separate experiments were necessary.

Proteins contain sulphur, but not phosphorus. DNA contains phosphorus, but not sulphur.

3. Evaluate the conclusion that DNA is the genetic material.

The conclusion is valid. Bacterial cells are larger and more dense than viruses – after centrifugation they would be found in the pellet. When viral DNA was labelled, radioactivity was found in the pellet, meaning the viral DNA was injected into the bacterial cells. When viral proteins were labelled, they were found in the supernatant, indicating that proteins were not injected into bacterial cells.

Nature of Science: Trends and patterns in epigenetics

  • Epigenetics is any modification that is heritable through cell division but is not related to a change in the underlying DNA sequence.
  • A factor that may be passed on in this way is called an epigenetic tag, or an epigenetic factor. Examples of epigenetic tags include DNA methylation patterns and histone modification.
  • There is mounting evidence that the environment can trigger heritable changes in epigenetic factors.

Skill: Analysing methylation patterns

  • Drone bees are haploid males, while queen and worker bees are diploid females.
  • The queen and worker bees are genetically identical, but display very different phenotypes.
  • A study of methylation patterns in bee brain DNA yielded the results shown in Figure 7.2.1c.

Figure 7.2.2c – Honeybee methylation patternsFigure 7.2.2c – Honeybee methylation patterns

Source: Lyko et al (2010) ‘The Honeybee Epigenome: Differential methylation of brain DNA in queens and workers’. PLOS. Retrieved from: journals.plos.org

1. Compare the percentage of methylation of workers and queens at the gene GB14848.

The queen and the workers have no methylation at loci 1 and 2. There is more methylation in worker bee DNA than in queen bee DNA at loci 3, 4 and 5.

2. Determine which gene has the highest methylation rate for all types of bees.

GB1806

3. Explain the effect of methylation on gene expression and predict how this affects gene expression in worker bees.

Methyl groups are added to cytosine bases on the DNA molecule. This causes chromatin to become more condensed, and gene expression is reduced or switched off. It is likely that many worker bee genes are silenced, due to the high rate of methylation compared to the queen bee.

4. Determine which gene is most active.

GB15132

Figure 7.2.2d – Hershey and ChaseFigure 7.2.2d – Hershey-Chase
Alfred Hershey and Martha Chase in 1953

Course links

  • Watson and Crick’s model of DNA structure suggested a mechanism for replication. Review the evidence for semi-conservative replication in 2.2.6.
  • Review the fundamentals of gene expression regulation in 7.1.2.

Figure 7.2.1e – Phage infecting bacteriaFigure 7.2.2e – Phage infecting bacteria

TOK

The relative importance of innate qualities versus those acquired through experiences is still under discussion. Is it important for science to attempt to answer this question?

Figure 7.2.2f – HoneybeesFigure 7.2.2f – Honeybees
Genetically identical honeybees, Apis mellifera: queen bee (right) and worker bee (left)

Further reading

  • In 1944, Oswald T. Avery published results of his bacterial transformation experiments, which he claimed proved DNA is the genetic material. Why wasn’t the scientific community convinced?

www.theguardian.com

  • Read about genomic imprinting and other epigenetic phenomena:

learn.genetics.utah.edu