Topic 2: The Discovery of DNA

The scientific method

A systematic approach used in scientific investigation that involves making observations, forming hypotheses, conducting experiments, and drawing conclusions to test theories.

Step 1: Observation

The first stage in the scientific method where scientists gather information and notice phenomena to formulate questions.

Step 2: Forming a hypothesis

The second stage in the scientific method where scientists propose an explanation about a phenomena. Based on obesrvations and assumptions, scientists create a testable statement that can be evaluated through experimentation.

Step 3: Experiments

The third stage in the scientific method where scientists design and conduct tests to validate or invalidate the hypothesis. This involves collecting data and analyzing results to draw conclusions. E

Experimental group

A group in an experiment that receives the treatment or condition being tested, allowing for comparison with the control group.

Control group

A group in an experiment that does not receive the treatment or intervention, serving as a baseline for comparison with the experimental group.

Positive control group

A group in an experiment that is exposed to a treatment known to produce a positive result, ensuring that the experimental setup is functioning as intended.

Negative control group

A group in an experiment that is not exposed to the treatment or intervention, ensuring that any observed effects can be attributed to the treatment rather than external factors.

Independent variable

The variable that is manipulated or changed in an experiment to observe its effects on the dependent variable.

Dependent variable

The variable that is measured or observed in an experiment to assess the impact of the independent variable.

Step 4: Analyze data

In this step, researchers interpret the collected data to determine if there are significant effects or patterns that support or refute the hypothesis. This involves using statistical methods to evaluate the results and may include visual representations like graphs or charts.

Step 5: Draw conclusion

In this step, researchers summarize the findings of the experiment, determining whether the results support the hypothesis or suggest alternative explanations. Conclusions may lead to further research questions or modifications to the original hypothesis. Includes a summary of the results, the significance, and future research.

Theories in science

Broader in scope when compared to a hypothesis, typically supported by a large body of evidence, and generally leads to new testable hypotheses.

Questions that can’t be answered by science

A hypothesis that can not be tested

Scientists initially thought protein was the genetic material

This was because protein had a complex structure of 20 amino acids, and DNA was simpler with 4 nucelotide bases.

Miescher isolates DNA for the first time (1869)

Friedrich Miescher was a Swiss chemist who first extracted nucleic acids from the cell nuclei of white blood cells, identifying what he termed "nuclein," which we now know as DNA. He determined that it was made up of hydrogen, oxygen, nitrogen and phosphorous. He originally thought it was just for storing phosphorous.

Importance of nucleus (1887)

The nucleus was a source of hereditary information. Observations of the chromosomes during processes such as mitosis and meosis identfiied chromosomes as the likely cellular structures carrying genetic information.

Griffith’s discovery of the transforming principle (1928)

Frederick Griffith conducted experiments with Streptococcus pneumoniae, demonstrating that a non-virulent strain could be transformed into a virulent strain when exposed to heat-killed virulent bacteria. Griffith's experiments revealed that a non-virulent strain of Streptococcus pneumoniae could become virulent when mixed with heat-killed virulent bacteria, indicating that DNA was the transforming principle carrying genetic information.

Avery, MacLeod, and McCarty identify DNA as the transforming principle (1944)

Avery, MacLeod, and McCarty conducted experiments building on Griffith's work, demonstrating that DNA extracted from the heat-killed virulent bacteria was responsible for transforming the non-virulent strain into a virulent one. This pivotal discovery established DNA as the molecule carrying genetic information. This was done by showing that only DNA, and not proteins or RNA, could induce transformation in the non-virulent strain by isolating and treating each component. Only the one that was DNA treated produced a lack of viral strains, confirming that DNA is the genetic material.

Hershey and Chase demonstrate that DNA is genetic material in bacteriophage (1952)

In 1952, Alfred Hershey and Martha Chase conducted experiments using bacteriophages to show that DNA, not protein, is the genetic material. Their work involved labeling DNA and protein with radioactive isotopes and demonstrating that only the DNA entered bacterial cells, thus confirming its role in heredity. Radioactivity was found within the pellet, where solid material such as cells end up, for DNA tests, not the superlatant, the liquid part above, where the radiocativity was found for the protein tests.

Chargaff discovers regularity in base ratios of DNA (1948)

Chargaff discovered that the amounts of adenine and thymine, as well as cytosine and guanine, in DNA are always in a 1:1 ratio, leading to the formulation of Chargaff's rule, G=C and A=T.

Watson, Crick Franklin, and Wilkins discover DNA as a double helix

This lead to the discovery of the struture of DNA.