Biology · DNA, Replication & Repair

Evidence that DNA carries hereditary information

Finding DNA in a chromosome is suggestive, but chromosomes also contain proteins. How could researchers distinguish the material that transfers a hereditary difference from material merely present beside it? The useful question concerns what changes when a candidate molecule is separated, tracked or disrupted.

Use the earlier ideas of nucleotides, complementary molecules, enzymes, inheritance and mutation. A nucleotide is a building block of a nucleic acid. A template is an existing sequence that guides construction of a complementary sequence. The symbols 5′ and 3′ are read five prime and three prime; they identify chemically different strand ends. Every short sequence and numerical example here is constructed for explanation unless identified as historical evidence.

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By the end, you can

Explain how experiments distinguish a molecule that transmits hereditary information from molecules that accompany it.

Look for a transmitted difference

A hereditary material must be capable of storing differences, being copied and transmitting those differences. Its presence in a cell is not enough to demonstrate those roles. An experiment is more informative when it connects a particular material to a change that persists in descendants, while testing alternative explanations.

Transformation is a change produced when a cell takes up genetic material from its surroundings. It gives a possible test: can material isolated from one bacterial type transfer a stable characteristic to another? Persistence matters. A temporary coating on the recipient's surface would be a different explanation from a change reproduced when the recipient lineage grows.

An experimental treatment also needs controls. If a treatment destroys every recipient cell, loss of transformation tells us little about the molecule carrying information. Researchers must examine the activity of the preparation, the specificity of treatments and whether the comparison can still detect a transformed recipient.

Separate a preparation, then challenge its activity

In 1944, Oswald Avery, Colin MacLeod and Maclyn McCarty reported that a DNA-rich preparation from pneumococci could induce a stable change in capsule type. Chemical analysis, purification and enzyme comparisons supported DNA as the active transforming material. The authors also acknowledged the detection limits of their chemical methods.

Protein-digesting enzymes and ribonuclease did not abolish the activity in the reported comparisons. Preparations that broke down DNA did. These treatments narrowed the explanation because destroying a candidate material could remove an activity that survived treatments directed at other materials. The original work involved several lines of analysis, rather than one perfectly pure tube proving every aspect of inheritance.

The schematic presents the logic of those comparisons without reproducing experimental quantities. Trace contaminants and treatment specificity are reasonable questions to ask of the evidence. A strong conclusion becomes stronger when other experiments address the same issue with different methods.

Challenge a transforming preparation

Original schematic of selected historical evidence. It does not depict experimental amounts, perfect purity, a home procedure or a conclusion about every virus.

Read the diagram in words

A common DNA-rich, activity-bearing preparation divides into three treatment branches. Each branch leads to a recipient test. Protein-directed and RNA-directed treatments retain reported transforming activity, whereas DNA-degrading treatment removes it. A separate control strip names treatment specificity and a usable recipient test. This is the qualitative logic of selected Avery–MacLeod–McCarty comparisons, rather than their measured data or a complete laboratory protocol.

DNA-rich preparation: Transforming activity was isolated and chemically examined.
Protein-directed treatment: Reported tested proteases did not abolish the activity.
RNA-directed treatment: Reported ribonuclease treatment did not abolish the activity.
DNA-degrading treatment: DNA-depolymerising preparations removed the activity.
Activity retained: A stable recipient change remained detectable.
Activity retained: A stable recipient change remained detectable.
Activity lost: The transforming effect was no longer detected.

Challenge a transforming preparation
From	To	Relationship
DNA-rich preparation	Protein-directed treatment	Compare treatment
DNA-rich preparation	RNA-directed treatment	Compare treatment
DNA-rich preparation	DNA-degrading treatment	Compare treatment
Protein-directed treatment	Activity retained	Test activity
RNA-directed treatment	Activity retained	Test activity
DNA-degrading treatment	Activity lost	Test activity

Check treatment specificity and whether the recipient test remains usable

Qualitative comparison; no measured quantities

Track different components through infection

In 1952, Alfred Hershey and Martha Chase studied T2 bacteriophages, viruses that infect bacteria. Separate radioactive labels tracked the phage's DNA and protein coat. After infection and separation of attached coats from cells, much of the DNA label was associated with cells, while most coat label could be removed. This supported different roles for DNA and the outside coat.

The separation was not perfectly absolute. The original paper reported remaining coat label and some DNA label outside the cells, and limited what could be inferred about untracked material. Its value is the contrasting behaviour of independently labelled components, considered with other observations. The choice of labels depended on this phage system; it is not a claim that proteins can never contain phosphate.

Together with later molecular evidence, such studies established DNA as the hereditary material of cellular organisms. Some viruses instead have RNA genomes. DNA also needs cellular machinery to be copied and used: identifying the information-bearing molecule does not make proteins, RNA or the environment irrelevant to inheritance.

Worked example

Separate an observation from its explanation

Imagine an activity-bearing preparation gives a persistent recipient change. After a DNA-directed treatment, that change is absent. After a protein-directed treatment, it remains. These invented qualitative observations favour a DNA-dependent activity, provided the treatments worked as intended and the recipient test remained usable.

Now suppose the DNA-directed treatment also killed all recipients. The same absence of a change could follow from cell death. A recipient-survival control therefore tests an alternative explanation. Even good controls establish the conclusion within the tested system; further evidence is needed before generalising to every organism or every virus.

Check your understanding

Why is a stable change in a recipient lineage more informative than a temporary change in its appearance?
Show reasoning
A stable difference reproduced by descendants connects the transferred material to heredity. A temporary appearance could result from material coating or briefly affecting the recipient.
Does loss of transformation after a DNA-directed treatment prove DNA's role if the treatment also kills all recipients?
Show reasoning
No. Cell death supplies an alternative explanation. Treatment specificity, recipient survival and the ability of the test to detect transformation must be checked.
Does DNA's hereditary role in cellular organisms establish that every virus has a DNA genome?
Show reasoning
No. Some viruses use RNA genomes. The scope of the cellular conclusion and the particular T2 experiment must remain clear.

What to carry forward

Hereditary evidence is strongest when material is linked to a transmitted difference and competing explanations are tested by independent methods.

References

Resources supporting this lesson’s explanations and examples.