Biotechnology: Bacterial Transformation

AP Biology Required Lab: Bacterial Transformation

This lab is really about two things at once. First, it shows you that foreign DNA can be taken up by bacterial cells and actually expressed as a protein. Second, it gives you a living, visible example of gene regulation in action. When you see glowing or antibiotic-resistant colonies on a plate, you are watching gene expression happen in real time.

Why This Lab Matters for the AP Exam

The AP exam will ask you to connect transformation results to gene expression, not just to "bacteria got new DNA." You need to be able to explain why a colony grows or glows in terms of transcription, translation, and regulation. Free-response questions often use transformation scenarios to test whether you understand the full path from DNA to phenotype, including what controls whether a gene gets expressed at all.

CED Connections

This lab connects directly to Unit 6: Gene Expression and Regulation.

Topic 6.2 (DNA Replication) - LO 6.2.A / EK 6.2.A.1

When transformed bacteria divide, they replicate their entire genome, including the new plasmid DNA. That replication follows the same rules: synthesis runs 5' to 3' direction, helicase unwinds the double helix, DNA polymerase builds new strands, and the result is semiconservative replication. Every daughter cell that inherits the plasmid can express the gene it carries.

Topic 6.4 (Translation) - LO 6.4.A / EK 6.4.A.1, 6.4.A.2, 6.4.A.3

Once the plasmid is inside the bacterial cell, the gene on it gets transcribed into mRNA and then translated into a polypeptide. Because bacteria are prokaryotes, transcription and translation happen simultaneously in the cytoplasm. The ribosome reads codons on the mRNA in triplets, starting at AUG, and builds a protein through initiation, elongation, and termination. The protein produced (often a fluorescent protein or an enzyme that breaks down an antibiotic) is what you actually observe on your plates.

Topic 6.5 (Regulation of Gene Expression) - LO 6.5.A / EK 6.5.A.1, 6.5.A.3 / LO 6.5.B / EK 6.5.B.1

The gene on the plasmid does not just automatically run at full blast. It is controlled by a promoter and, in many versions of this lab, an inducible system. Regulatory proteins interact with regulatory sequences on the plasmid to turn transcription on or off. This is the same logic as a prokaryotic operon. The phenotype you observe (glowing, growing on antibiotic plates) is a direct result of which genes are expressed and at what level, connecting to EK 6.5.A.3.

What You Need to Be Able to Do

• Design controls - identify which plates serve as negative controls and explain what each one rules out
• Interpret plate results - connect colony growth or fluorescence to successful transformation and gene expression
• Trace the path from DNA to phenotype - explain how a gene on a plasmid becomes a visible protein product
• Apply the genetic code - use a codon chart to connect mRNA sequences to amino acid sequences (you do not need to memorize the chart, but you need to use it)
• Distinguish transformation from expression - a cell can take up DNA without expressing it; you need to know what conditions allow expression
• Make a claim with evidence and reasoning (CER) - use your plate data to support or refute a claim about whether transformation occurred and whether the gene was expressed

Core Concepts

Bacterial transformation is the process by which a bacterial cell takes up foreign DNA from its environment and incorporates it. In this lab, the foreign DNA is a circular piece of DNA called a plasmid. The plasmid carries one or more genes that give the bacteria a new trait.

Prokaryotic cells (like E. coli, the bacteria used in this lab) are single-celled organisms without a membrane-bound nucleus. Their DNA floats in the cytoplasm, which means transcription and translation can happen at the same time in the same place.

Gene regulation refers to the control of when, where, and how much a gene is expressed. Bacteria regulate genes using sequences near the gene that interact with regulatory proteins. Some genes are always on (constitutively expressed). Others are part of an inducible system, where a signal molecule triggers expression.

mRNA is the messenger molecule transcribed from DNA. It carries the instructions for building a protein. In bacteria, ribosomes start translating the mRNA almost immediately after transcription begins.

Codons are three-nucleotide sequences on mRNA that each specify one amino acid. The genetic code is the set of rules that maps codons to amino acids. Nearly all living organisms use the same genetic code, which is strong evidence for common ancestry.

Elongation is the phase of translation where the ribosome moves along the mRNA, reading codons one at a time, and adds amino acids to the growing polypeptide chain. This continues until a stop codon is reached.

Regulatory proteins are proteins that bind to specific DNA sequences and either promote or block transcription. In the context of this lab, the promoter on the plasmid controls whether the inserted gene gets transcribed.

Epigenetic changes (like DNA methylation or histone modification) affect gene expression without changing the DNA sequence itself. While bacteria do not use histone-based epigenetics, the concept of reversible regulation connects to the broader Unit 6 picture of how gene expression is controlled.

Housekeeping genes are genes expressed in almost all cells at all times because they code for proteins needed for basic cell survival. The antibiotic resistance gene on a plasmid often behaves like a housekeeping gene once it is expressed, since the bacteria need it to survive on antibiotic plates.

Cell differentiation in eukaryotes happens because different genes are expressed in different cell types. Transformation is a useful comparison point: you are essentially giving bacteria a new gene to express, which changes their phenotype, just like differential gene expression changes cell identity in multicellular organisms.

How the Lab Works

The core logic is simple: you introduce a plasmid carrying a reporter gene into bacteria, then check whether the bacteria express that gene.

The plasmid typically carries two genes. One is an antibiotic resistance gene, which lets transformed bacteria survive on plates containing that antibiotic. The other is often a gene for a fluorescent protein (like GFP, green fluorescent protein), which makes transformed colonies glow under UV light. Both genes are under the control of promoter sequences on the plasmid.

To get the plasmid inside the bacteria, you make the cells temporarily permeable using a chemical treatment and a brief heat shock. This is not a natural process in the lab setting, but it mimics how bacteria can take up environmental DNA in nature. Once inside, the plasmid replicates along with the bacterial chromosome every time the cell divides.

After transformation, you spread bacteria onto different types of plates. Some plates have no antibiotic (a control). Some have the antibiotic. By comparing which plates have colonies and whether those colonies fluoresce, you can determine whether transformation happened and whether the gene is being expressed.

The key insight is that colony growth on an antibiotic plate is not just evidence of transformation. It is evidence of gene expression. The bacteria had to transcribe the resistance gene into mRNA, translate that mRNA into a functional enzyme, and use that enzyme to survive. That is the full central dogma playing out on a petri dish.

Data and Analysis Moves

Setting up your plate comparisons

You will typically have four plate conditions. Here is how to read them:

If you see growth on the antibiotic plate with plasmid bacteria, transformation and expression both occurred. If you see no growth there, either transformation failed or the gene was not expressed.

Calculating transformation efficiency

Transformation efficiency tells you how many cells were successfully transformed per microgram of plasmid DNA used.

$\text{Transformation Efficiency} = \frac{\text{Number of colonies on antibiotic plate}}{\text{Mass of plasmid DNA used (in micrograms)}}$

The units are colonies per microgram (colonies/µg). A higher number means the transformation worked more efficiently.

Connecting colony count to gene expression

Each colony on an antibiotic plate started from a single transformed cell. That cell expressed the resistance gene, survived, and divided. So the number of colonies is a proxy for how many cells successfully expressed the gene, not just how many took up the DNA.

Fluorescence as a second reporter

If your plasmid also carries a fluorescent protein gene, you can check colonies under UV light. A colony that grows on antibiotic plates AND fluoresces has expressed both genes. This is useful because it confirms that the plasmid is intact and that the regulatory sequences controlling the fluorescent gene are functional.

Variables to identify

• Independent variable: presence or absence of plasmid (or presence/absence of the inducer, if your version uses an inducible system)
• Dependent variable: number of colonies, presence of fluorescence
• Controlled variables: temperature, volume of bacteria plated, concentration of antibiotic, incubation time

Error sources to consider

• Contamination from non-transformed bacteria on antibiotic plates (would give false positives)
• Incomplete heat shock (reduces transformation efficiency)
• Plating too many or too few cells (makes colony counts unreliable)

Common Mistakes

Confusing transformation with expression Taking up a plasmid is not the same as expressing the gene on it. A cell could theoretically have the plasmid but not transcribe it if the regulatory sequences are not functional. On the AP exam, always trace the full path: DNA taken up, gene transcribed to mRNA, mRNA translated to protein, protein produces the observable trait.

Saying bacteria "gained a new trait" without explaining why Vague answers lose points. The bacteria grew on antibiotic plates because the resistance gene was transcribed and translated into a functional enzyme. Say that.

Forgetting that prokaryotes couple transcription and translation In bacteria, the ribosome starts translating the mRNA while it is still being transcribed. This is different from eukaryotes, where transcription happens in the nucleus and translation happens in the cytoplasm. This distinction shows up on the exam.

Misreading the control plates The plate with no plasmid and no antibiotic should have growth. If it does not, something went wrong with the bacteria themselves, not the transformation. Always interpret your controls before drawing conclusions from your experimental plates.

Treating the genetic code as something to memorize You do not need to memorize codons (except AUG = start = methionine). You do need to be able to use a codon chart to read off amino acids. Practice using the chart quickly, because free-response questions sometimes include one.

Mixing up the direction of DNA synthesis DNA is always synthesized in the 5' to 3' direction. When the plasmid replicates inside the bacteria, this rule still applies. New nucleotides are added to the 3' end of the growing strand.

Ignoring the inducible system if your version uses one Some versions of this lab use a promoter that only turns on in the presence of a specific molecule (like arabinose or IPTG). If your lab uses an inducible system, you need to explain that the gene is only expressed when the inducer is present, because the inducer changes the shape or activity of a regulatory protein that controls transcription.

Quick Review Checklist

• You can explain what bacterial transformation is and how it differs from just having DNA present in a cell
• You can trace the path from plasmid DNA to observable phenotype using transcription, translation, and the genetic code
• You know why bacteria are useful for studying gene expression (prokaryotic cells, coupled transcription-translation, fast growth)
• You can calculate transformation efficiency and explain what the number means
• You can correctly interpret all four plate conditions and explain what each one controls for
• You can connect the inducible system on the plasmid to the broader concept of gene regulation in prokaryotes (EK 6.5.B.1)
• You can explain why the universal genetic code used in this lab is evidence for common ancestry of all living organisms (EK 6.4.A.3.iv)
• You can write a complete CER response using plate data as evidence for a claim about transformation and gene expression