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D3.7 · Describe discoveries that advanced molecular genetics

Learn to describe discoveries that advanced molecular genetics through clear examples and targeted practice.

Ontario Grade 12 Biology

Molecular Genetics

How evidence revealed what genes are made of and how genetic information is copied

In SBI3U, you learned that genes are inherited and that they influence traits. Molecular genetics asks what genes are made of and how their information is stored and passed on. Scientists answered these questions through experiments. Each discovery narrowed the possibilities. Their conclusions became stronger when different kinds of evidence supported the same explanation.

What you will learn

  • Describe key discoveries that shaped molecular genetics.
  • Explain how evidence supported the conclusion that DNA carries hereditary information.
  • Connect the structure of DNA to the idea of accurate copying.
  • Distinguish DNA replication from transcription and translation.

From inheritance to molecules

A trait is a feature of an organism, such as a particular flower colour. Inheritance is the passing of information from parents to offspring. A gene is a section of hereditary information that can influence a trait. In the early twentieth century, scientists knew that chromosomes carry genes, but they still had to identify which chromosome molecule carried the information.
Chromosomes contain DNA and proteins. DNA is a molecule that can store hereditary information. Proteins are molecules built from smaller units called amino acids. Both DNA and proteins seemed complex enough to be candidates. Researchers needed evidence that could distinguish between them.
  • Genes are hereditary information found on chromosomes.
  • Identifying the information-carrying molecule required experiments that compared DNA and protein.

Evidence that DNA carries hereditary information

In 1928, Frederick Griffith studied bacteria that caused pneumonia in mice. One bacterial type had a smooth outer coating and caused disease. A second type lacked the coating and did not cause disease. Griffith found that heat-killed disease-causing bacteria could change living harmless bacteria into disease-causing bacteria when the two were mixed. He called the change transformation.
Griffith's results suggested that some substance from the dead bacteria changed the living ones. His experiment did not identify that substance. In 1944, Oswald Avery, Colin MacLeod, and Maclyn McCarty tested material from the disease-causing bacteria. They treated samples to destroy different kinds of molecules. Transformation still occurred when proteins were destroyed, but it did not occur when DNA was destroyed. They concluded that DNA was the transforming material in their system.
In 1952, Alfred Hershey and Martha Chase used viruses that infect bacteria. A virus is a tiny infectious particle; the viruses in this experiment contained DNA and protein. They marked viral DNA and protein separately so they could track each one. After infection, the marked DNA was found inside the bacteria, while most marked protein stayed outside. This supported the conclusion that DNA, rather than protein, carried the information needed to direct the production of new viruses.
Together, these experiments built a strong case for DNA. Each experiment had limits: Griffith did not identify the substance, and Hershey and Chase studied a particular virus. The findings were persuasive because the experiments used different approaches and pointed to the same conclusion.
  • Griffith discovered transformation but did not identify its cause.
  • Avery, MacLeod, and McCarty found that destroying DNA stopped transformation.
  • Hershey and Chase found that viral DNA entered bacteria during infection.

DNA structure and copying

In the early 1950s, several lines of evidence helped scientists describe DNA's structure. Erwin Chargaff found patterns in the amounts of DNA bases: the amount of adenine was similar to thymine, and the amount of guanine was similar to cytosine. Bases are the parts of DNA that form its information-bearing sequence.
Rosalind Franklin and Maurice Wilkins used X-ray diffraction, a method that produces patterns when X-rays interact with a molecule, to study DNA fibres. The patterns provided evidence that DNA had a regular, helical shape. In 1953, James Watson and Francis Crick proposed a double-helix model. It had two strands with paired bases: adenine with thymine, and guanine with cytosine. Franklin's and Wilkins's evidence, Chargaff's findings, and other work helped make the model possible.
The paired structure suggested a way for DNA to be copied. If the two strands separate, each strand can guide the formation of a matching strand because the bases pair in a specific way. In 1958, Matthew Meselson and Franklin Stahl provided evidence about DNA replication, the process of copying DNA. Their results supported a model in which each new DNA molecule contains one original strand and one newly made strand.
This model concerns replication, not the other steps of gene expression. Transcription is the making of an RNA copy from a DNA sequence. Translation is the use of RNA information to build a protein. These processes are related, but they are not interchangeable.
A−T,  G−C\mathrm{A-T,\;G-C}
  • Chargaff's findings and X-ray diffraction evidence helped reveal DNA's structure.
  • Complementary base pairing explains how one strand can guide copying.
  • Meselson and Stahl's experiment supported a copying model with one old and one new strand in each DNA molecule.

How to judge a discovery

A scientific discovery is more than a memorable name or date. To describe one, state the question, the evidence, and the conclusion the evidence supports. Keep the conclusion within the experiment's limits. For example, Griffith showed that a change could pass between bacteria under his experimental conditions; he did not show that DNA caused the change.
Later experiments strengthened the DNA explanation by identifying DNA as the transforming material and tracking DNA into infected bacteria. Structural work then explained how DNA could hold information and be copied. The history is a chain of connected findings, not one experiment that answered every question.
The symbols for paired DNA bases summarize a pattern, not a complete DNA molecule. The order of bases along a strand contains information. Pairing describes which base is matched across the two strands.
  • Separate an observation from the conclusion drawn from it.
  • Describe what an experiment supports without claiming more than it tested.
  • Several independent findings can strengthen a scientific model.

Worked example

Interpreting Griffith's result

Griffith mixed heat-killed disease-causing bacteria with living harmless bacteria. The mice became ill, and disease-causing bacteria were later found. What conclusion is supported, and what remains unknown?
  1. State the observation
    Living harmless bacteria were associated with the appearance of disease-causing bacteria after mixing with the heat-killed disease-causing type.
  2. Limit the conclusion
    The result supports the idea that a transferable substance changed the living bacteria. Griffith called the process transformation, but his experiment did not identify the substance.
Answer: Griffith showed that transformation occurred in his experiment. He did not establish that DNA caused it.
Check: Avery, MacLeod, and McCarty later tested which kind of molecule was necessary for transformation.

Worked example

Connecting the Avery experiment to its conclusion

A sample that could transform harmless bacteria was treated to destroy its protein. Transformation continued. A second sample was treated to destroy its DNA, and transformation stopped. Which molecule is the best-supported transforming material?
  1. Compare the treatments
    Destroying protein did not stop transformation, so protein was not required for the observed change in this test.
  2. Use the second result
    Destroying DNA stopped transformation. This indicates that DNA was necessary for the transforming activity in the tested material.
Answer: DNA is the best-supported transforming material in this experiment.
Check: The conclusion comes from the difference between destroying protein and destroying DNA, not from the name of the researchers.

Worked example

Explaining how base pairing supports copying

A short DNA strand has the base sequence adenine, guanine, thymine. Use complementary pairing to identify the matching sequence on the other strand. Explain why this matters for DNA replication.
  1. Match each base
    Adenine pairs with thymine, and guanine pairs with cytosine. Apply the pairing rule to each base in order.
    A−G−T  ↔  T−C−A\mathrm{A-G-T \;\leftrightarrow\; T-C-A}
  2. Connect the pattern to replication
    When strands separate, each can serve as a guide for a matching strand. The sequence of the original strand therefore helps determine the sequence of its partner.
Answer: The matching strand has the sequence thymine, cytosine, adenine. Complementary pairing provides a simple basis for understanding how DNA can be copied.
Check: The example describes DNA replication, not transcription or translation.

Common mistakes and how to avoid them

Saying Griffith proved that DNA is genetic material.
Correction: Griffith observed transformation but did not identify the transforming substance.
Saying the Avery experiment showed that protein was the transforming material.
Correction: Transformation continued after protein was destroyed and stopped after DNA was destroyed. The evidence supported DNA.
Treating DNA replication, transcription, and translation as the same process.
Correction: Replication copies DNA. Transcription makes an RNA copy from DNA. Translation uses RNA information to build a protein.
Claiming that a single experiment settled every question about genes.
Correction: Different experiments answered different questions. Their combined evidence strengthened the molecular model.

Lesson summary

  • Griffith discovered bacterial transformation but did not identify its cause.
  • Avery, MacLeod, and McCarty found evidence that DNA was the transforming material.
  • Hershey and Chase found evidence that viral DNA entered bacteria during infection.
  • Chargaff's findings and X-ray diffraction evidence helped scientists develop a model of DNA structure.
  • Meselson and Stahl provided evidence for DNA replication in which each copy contains one original strand and one new strand.

Check your understanding

Question 1

What did the Avery, MacLeod, and McCarty experiments support?
  1. Protein was required for bacterial transformation.
  2. DNA was the transforming material in their experiments.
  3. Griffith's experiment identified the structure of DNA.
  4. RNA entered bacteria in the Hershey-Chase experiment.
Show answer and explanation
DNA was the transforming material in their experiments.
Transformation stopped when DNA was destroyed, while destroying protein did not stop it.

Question 2

Which statement best describes the role of Chargaff's findings in the DNA story?
  1. They showed that adenine amounts were similar to thymine amounts, and guanine to cytosine.
  2. They proved that proteins enter bacteria during viral infection.
  3. They showed that DNA is copied by making RNA first.
  4. They identified the transforming substance in Griffith's experiment.
Show answer and explanation
They showed that adenine amounts were similar to thymine amounts, and guanine to cytosine.
Chargaff's base patterns helped inform the paired-base model of DNA.

Question 3

Which process makes a copy of DNA?
  1. Translation
  2. Transformation
  3. Transcription
  4. Replication
Show answer and explanation
Replication
DNA replication is the process of copying DNA. Transcription makes RNA from DNA, and translation uses RNA information to build a protein.

Key terms

Gene
A section of hereditary information that can influence a trait.
Transformation
A change in bacteria caused when a substance from other bacteria alters their characteristics.
Base
A component of DNA; the order of bases carries information.
Double helix
The twisted, two-strand shape of DNA.
Complementary base pairing
The matching pattern in DNA in which adenine pairs with thymine and guanine pairs with cytosine.
DNA replication
The process of copying DNA.
Transcription
The process of making an RNA copy from a DNA sequence.
Translation
The process of using RNA information to build a protein.

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