Skin cancer risk and the DNA repair plate lab

Project 3.2.2 notebook entry (sample)

ABCDE, and what each letter asks:

A, asymmetry: fold the spot in half in your mind. Do the halves match?

B, border: is the edge clean, or ragged and blurred into the skin?

C, color: one shade, or several browns, blacks, or reds in one spot?

D, diameter: is it wider than a pencil eraser, about 6 mm?

E, evolving: has it changed over weeks or months? This is the letter people skip, and change over time is the strongest single signal.

What the check is: a screening rule that decides who gets looked at by a clinician. It does not diagnose anything.

Skin cancer types:

Type | Starts in | How likely to spread

Basal cell carcinoma | Basal layer of the epidermis | Rarely spreads, locally destructive

Squamous cell carcinoma | Keratinocytes above the basal layer | Can spread, usually late

Melanoma | Melanocytes | Spreads early, and is the one that kills

My risk table:

Cannot change | Already do | Would have to start

Skin that burns before it tans | Sunglasses most days | Sunscreen on cloudy days, not only sunny ones

A family history of melanoma | Shade at midday in summer | A yearly skin check

Many moles across my back | | Reapplying after swimming, not once in the morning

Experimental procedure, written before I opened anything:

Independent variable: UV exposure time, at 0, 10, 20, and 40 seconds.

Dependent variable: percent of the plate covered by growth at 48 hours.

Plates in my set:

WT unexposed. Wild type, no lamp. Shows the strain grows and the plate was not contaminated.

RM unexposed. Repair mutant, no lamp. Shows the mutant is not simply a sick strain to begin with. Without its own unexposed plate I could not separate repair failure from poor growth.

WT exposed, one plate at each time point.

RM exposed, one plate at each time point.

Held constant: both strains go under the lamp together, same shelf distance, same seconds, lids off, from the same overnight culture at the same dilution. The only thing allowed to differ between the exposed plates is the strain.

Safety the bench version would need: UV at this wavelength gives the eye no warning, so the lamp stays interlocked and off whenever hands are inside, with a face shield and gloves, and every yeast plate is autoclaved before disposal.

Percent coverage, wild type:

Exposure | Reading 1 | Reading 2 | Mean

0 s | 88 | 90 | 89

10 s | 84 | 86 | 85

20 s | 76 | 74 | 75

40 s | 61 | 65 | 63

Percent coverage, repair mutant:

Exposure | Reading 1 | Reading 2 | Mean

0 s | 87 | 85 | 86

10 s | 52 | 56 | 54

20 s | 24 | 20 | 22

40 s | 6 | 4 | 5

Claim my plate set supports: the two strains start together and separate as soon as UV is applied. Unexposed, they are within 3 points of each other, at 89 and 86. By 20 seconds wild type reads 75 percent coverage and the repair mutant reads 22. The mutant is not being struck more often; it is less able to undo the damage, and it is the pair of unexposed plates that lets me say that rather than blaming a weak strain.

The ceiling on that claim:

Percent coverage is percent of the plate covered by visible growth. It is not percent of cells that survived, and no arithmetic converts one into the other. A cell that is alive but dividing too slowly to raise a visible colony in 48 hours scores exactly the same as a dead one. Measuring survival would mean counting separated colonies from a dilution series and dividing by the count from the unexposed plate.

Seconds are not a dose. Dose is the lamp's intensity multiplied by time, so the same 20 seconds on a different shelf, or with an older tube, is a different experiment. My numbers do not transfer to another setup.

I read coverage by eye, so a 75 and a 78 are not distinguishable in my data, and I should not argue from a 3 point difference anywhere in these tables.

This set says nothing about wavelength. It was run at 254 nm, which is near the peak where DNA absorbs. A lamp near 365 nm or 470 nm would not be a weaker version of this experiment, because those wavelengths power photolyase, which takes the lesion apart again.

5-fluorouracil: a chemotherapy drug used as a cream on sun-damaged skin. It blocks a step in making DNA, so the cells dividing fastest take the heaviest hit, and on sun-damaged skin those are the damaged ones. The skin goes red and raw while it works, which is the drug reaching its target rather than a reaction against it.