9.2 Transformation and Viruses
Key Takeaways
Griffith's 1928 mixture of heat-killed smooth pneumococcus and live rough cells killed mice, and live smooth bacteria were recovered.
Avery, MacLeod, and McCarty showed that DNA is the transforming principle because destroying DNA stopped transformation and destroying protein did not.
Hershey-Chase is bacteriophage evidence, not transformation: phosphorus-32 labeled DNA entered bacteria and sulfur-35 labeled protein stayed outside.
A virus is not a cell: it has a DNA or RNA genome in a capsid, sometimes a host-derived envelope, and it replicates only inside a host.
The lytic cycle ends in lysis, the lysogenic cycle copies a prophage such as lambda until induction, and cell-wall antibiotics do not treat viruses such as HIV.
9.2 Transformation and Viruses
Transformation changes a living bacterium when it takes up DNA from outside the cell, and the new traits are then inherited. A virus is not a cell and not a small prokaryote. It is a genome in a protein coat that can replicate only inside a host. CLEP questions often merge three famous experiments. Griffith and then Avery, MacLeod, and McCarty studied transformation. Hershey and Chase studied a bacteriophage.
Bacterial Transformation
Griffith's 1928 Mouse Experiments
Frederick Griffith used Streptococcus pneumoniae, the pneumococcus, in mice. Rough colonies lack a capsule and are nonvirulent. Live rough bacteria did not kill the mice. Smooth colonies make a capsule and are virulent. Live smooth bacteria killed the mice. Heat-killed smooth bacteria injected alone did not kill the mice. The decisive treatment mixed heat-killed smooth cells with live rough cells. That mixture killed the mice, and live smooth bacteria were recovered from the dead animals. Material from the dead smooth cells had transformed live rough cells into the encapsulated virulent form, and the new trait was heritable because the recovered smooth bacteria passed it on.
Avery, MacLeod, and McCarty
In 1944 Oswald Avery, Colin MacLeod, and Maclyn McCarty asked which chemical was Griffith's transforming substance. They treated extracts of heat-killed smooth pneumococcus with enzymes that destroy one type of molecule. Destroying protein did not stop transformation. Destroying RNA did not stop it. Destroying DNA stopped transformation. When DNA remained intact, the extract still converted live rough cells into live smooth cells. DNA is the transforming principle. The design uses naked DNA and living recipient bacteria. It does not use a blender or radioactive sulfur and phosphorus, and it is not a virus experiment.
Transformation, as the term is used after that result, is uptake of free DNA by a competent bacterium and inheritance of what that DNA encodes. Items that make live rough cells lethal on their own, or that make heat-killed smooth cells lethal on their own, contradict Griffith's controls.
Why Hershey-Chase Is Not Transformation
In 1952 Alfred Hershey and Martha Chase studied a T2 bacteriophage, a virus that infects bacteria, not a transforming extract. They labeled phage protein with sulfur-35 because protein contains sulfur and DNA does not. They labeled phage DNA with phosphorus-32 because DNA contains phosphorus and phage protein does not. After the phage attached to E. coli, a blender knocked empty coats off the cells, and centrifugation separated the cells from the coats. Phosphorus-32 labeled DNA entered the bacteria. Sulfur-35 labeled protein stayed outside. This is evidence that the phage injects DNA. It is not evidence that dead smooth pneumococcus transforms live rough pneumococcus.
Warning
A virus is not a small prokaryote, and Griffith's result is not the Hershey-Chase blender experiment. Griffith recovered live smooth bacteria after mixing heat-killed smooth cells with live rough cells. Avery, MacLeod, and McCarty showed that destroying DNA stops transformation and destroying protein does not. Hershey and Chase found phosphorus-32 inside the bacteria and sulfur-35 outside.
Viruses
A Particle, Not a Cell
Viruses are not cells. Each particle holds a genome of DNA or RNA inside a protein capsid. Some also have a membrane envelope taken from a host cell as they leave. The envelope is host-derived membrane, not a cell wall and not a nucleus. A virus has no nucleus and no ribosomes of its own. It cannot build proteins until it uses host ribosomes. Replication is obligate intracellular: outside a host the particle does not grow or divide. Host range is the set of species, and often the cell types, a virus can infect. Bacteriophages infect bacteria with the matching receptor. Animal viruses infect particular animal cells. That limit is why a phage used to label DNA does not infect a mouse.
Lytic Cycle and Lysogenic Cycle
The lytic cycle ends by killing the host. The steps, in order, are attachment, entry, replication, assembly, and lysis. Attachment binds a viral protein to a host receptor. Entry brings the genome in. For many phages the capsid remains outside, which is the material the Hershey-Chase blender removed. Replication copies the viral genome and makes viral proteins with host energy, nucleotides, and ribosomes. Assembly fills new capsids with new genomes. Lysis breaks the cell and releases the particles.
The lysogenic cycle inserts the viral genome and postpones lysis. The inserted genome is a prophage. It is copied with the host chromosome at each cell division, and the host lives. Lambda, a phage of E. coli, is the classic example. Induction, commonly after stress damages the host, excises the prophage and starts the lytic steps of replication, assembly, and lysis. Lysogeny is not transformation. Transforming DNA is naked DNA from the surroundings. It has no capsid and no scheduled lytic program. A prophage entered as a virus and can still be induced.
Retroviruses and Cell-Wall Antibiotics
Retroviruses carry RNA and copy it backward into DNA. Human immunodeficiency virus (HIV) is the standard example. Its enzyme reverse transcriptase makes DNA from the viral RNA after entry. That DNA can integrate into a host chromosome and later be transcribed by host RNA polymerase. Reverse transcriptase is a viral activity. It is not how Griffith's pneumococcus changed, and it is not an enzyme of bacterial cell-wall synthesis.
Antibiotics aimed at bacterial cell walls do not treat viruses. Penicillin-class drugs block peptidoglycan construction. A virus has no peptidoglycan, with or without an envelope, so those drugs do not stop attachment, entry, replication, reverse transcription, assembly, or lysis. A virus is not a tiny prokaryote that a wall-targeting drug could clear.
| Feature | Transformation | Virus |
|---|---|---|
| Nature | Uptake of naked DNA by a living bacterium | Not a cell: DNA or RNA in a capsid, sometimes with a host envelope |
| Evidence | Griffith 1928; Avery, MacLeod, and McCarty 1944: DNA is the transforming principle | Hershey-Chase 1952 is phage evidence, not transformation: phosphorus-32 DNA entered; sulfur-35 protein stayed outside |
| What happens next | The bacterium passes the new trait to daughter cells | Attachment through lysis, or a lambda prophage copied until induction |
| What the newcomer brings | No capsid and no independent replication cycle | No nucleus and no ribosomes; obligate intracellular replication inside a limited host range |
| Medical contrast | A heritable change in a bacterium | Cell-wall antibiotics do not treat viruses. HIV uses reverse transcriptase to make DNA from RNA |
Which result is Griffith's 1928 transformation experiment?
Heat-killed smooth pneumococcus mixed with live rough pneumococcus killed the mice, and live smooth bacteria were recovered.
Phosphorus-32 labeled viral DNA entered bacterial cells, and sulfur-35 labeled protein remained outside after blending.
Live rough nonvirulent pneumococcus killed the mice, and heat-killed smooth pneumococcus killed them as well.
Destroying DNA in a cell-free extract stopped transformation, while destroying protein did not.
Which finding shows that DNA is the transforming principle, rather than the material a bacteriophage injects?
Destroying DNA stopped transformation of rough pneumococcus, and destroying protein did not.
Phosphorus-32 labeled DNA entered E. coli, and sulfur-35 labeled protein stayed outside the cells.
Reverse transcriptase copied a viral protein into the DNA that changed rough cells into smooth cells.
Heat-killed rough cells removed the capsule from live smooth cells and made those smooth cells harmless.
Which description of a virus is accurate?
Griffith's recovery of live smooth bacteria is the same event as the Hershey-Chase blender separation of sulfur-35 protein from bacteria.
A virus is not a cell. Its genome of DNA or RNA sits in a protein capsid, and in the lysogenic cycle a prophage such as lambda is copied with the host chromosome until induction.
In the lytic cycle the virus builds its own nucleus and ribosomes, then remains a prophage without lysing the host.
A virus is a small prokaryotic cell with its own ribosomes and a peptidoglycan wall, so a cell-wall antibiotic clears the infection.
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