Things may still seem a little dark right now but December 21 was the longest night of the year. Each day since the amount of daylight has slowly increased and will continue to do so until June 20. In songs, paintings, weather reports, and vacation photos sunshine is celebrated – and for good reasons. It’s the foundation of most food webs, keeps our planet comfortably warm, and helps us and other animals see! It also has clear health benefits. Sunshine helps us produce vitamin D, fall asleep easily, and feel calm and happy! But sunshine also has a dark side – read on to discover how it can also damage our DNA.
Sunshine is a leading carcinogen – a substance that causes cancer in living tissues. Other top carcinogens include radon, asbestos, and formaldehyde which do not have the same positive reputation in our culture, to say the least! Scientists study the interactions between the sun and our cells in order to understand how exposure leads to cancer and how to best avoid this risk while still enjoying the benefits.
Every hour about around 430 quintillion (430,000,000,000,000,000,000) joules of solar energy hit the earth. This is a lot of energy. In fact, it’s very close to the amount of energy that the global human population uses in a whole year! Energy from the sun (sunshine) is a mixture – or spectrum – of different light types. These types can be roughly grouped into three categories: infrared energy which we feel as heat, visible light which is what allows us to see, and ultraviolet light which we can’t feel or see but which packs a large energy punch.
Of the three subtypes of sunlight, ultraviolet, or UV, light possesses the largest cancer risks because its short wavelengths have the most energy. At its most energetic UV light can cause electrons to break away from their atoms. This process, known as ionization, changes the chemical properties of an atom and often causes old chemical bonds to break and new ones to form. Ionizing UV light is called UVC. However, UVC is all but absent on Earth because our atmosphere absorbs and reflects higher energy wavelengths like gamma rays, x-rays, and UVC rays.
Instead, the UV light reaching the Earth’s surface is mostly in the form of UVA and UVB. These two types of ultraviolet light have slightly longer wavelengths and thus less energy. They can’t remove electrons. However, they can still cause reactions by temporarily moving electrons to higher energy states. For example fluorescence occurs when UVA or UVB light causes electrons in certain materials to become excited and then, gradually return to a lower energy level. During the latter process, the material emits part of the absorbed light as visible light. This is why a yellow safety vest appears so bright in sunlight and also why black lights can make images pop out of specially printed paper.
The energy in UVA and UVB also affects cells. In mammalian cells, both UV light types trigger the synthesis of vitamin D. Less helpfully, both UV types also cause Vitamin A and collagen in cells to degrade. This causes skin cells to lose their elasticity and is a major reason for photoaging – the premature wrinkling and sagging of the skin due to repeated sun (or artificial tanning) exposure. Finally, UVB light can also affect cells by damaging their genetic code. This can lead to serious health consequences like cell death, mutations, and cancer.
DNA is a self-replicating thread-like chain of nucleotides that contain the genetic instructions for life. When this molecule is exposed to the high energy packaged in UVB rays multiple things can happen. Around 99.9% of the time what happens is …. nothing. The molecular structure of DNA is highly stable and adept at handling UV radiation. In these cases, DNA molecules absorb the UV energy, convert it to heat, and then release the heat back into the environment.
This catch-and-release process is fast, but still dangerous because it temporarily puts a DNA molecule into a higher and more reactive energy state.
When DNA is reactive new bonds can form and key structural bonds can break. For example, following UV exposure, DNA nucleotides that usually bond horizontally with other nucleotides on the opposite strand can instead bind with the nucleotides above or below them. This creates a new structure in the DNA called a pyrimidine dimer.
Less frequently, strand breaks can occur. These happen when the bonds between the phosphate group and the sugar group of two neighboring nucleotides disconnect. Single strand breaks or nicks are when only one side of a DNA’s double helix is damaged. In certain situations, these breaks actually help DNA molecules unwind themselves for copying which is why cells sometimes intentionally create them and why they can often recover from them. Much more damaging and difficult to recover from are double strand breaks. These are when breaks form on both sides of the DNA helix close to each other.
New bonds and breaks both change the shape of DNA molecules and interfere with one of their primary functions- being accurately copied again and again and again. Before a cell divides, the DNA within that cell must be copied so that each daughter cell has the genetic instructions it needs to function. During this process DNA unwinds and its two strands separate. Next, an enzyme called DNA polymerase travels along each single strand adding complementary nucleotides and eventually creating two double stranded molecules that are the exact copy of each other. A similar process happens when DNA is used as the template to create mRNA during transcription – the first step in protein synthesis. However, breaks and extra bonds derail these processes either by stopping polymerase in its tracks or by causing this enzyme to make copy errors.
Copy errors are changes in the nucleotide sequence of a DNA strand. When these changes occur in protein coding sections they can result in abnormal proteins. Ehen these abnormal proteins are involved in cell growth, DNA repair, or tumor suppression, the error may eventually lead to cancer. Cancerous cells grow and divide continuously (even when they are unneeded, old or damaged) and create daughter cells that do the same. If left unchecked, the resulting mass of cancerous cells – called malignant tumors – can invade surrounding tissues, spread to other parts of the body, and can eventually be fatal. In the case of UV light exposure, the major cancers are basal cell carcinoma, squamous cell carcinoma, and melanoma.
Luckily, most cases of UV exposure do not lead to cancer. Instead, cells have evolved to prevent, repair, or isolate damaged DNA. Defenses start with the resilient structure of DNA and its careful packaging within cells. Additional protection is also provided when cells produce pigments that act like chemical bodyguards for DNA. When breaks, bonds, or copy errors do occur DNA can identify these abnormalities and quickly repair them. In fact, cells are constantly running repair operations that either reverse the chemical changes created by UV energy or that remove and replace DNA regions flagged as potentially damaged and dangerous.
However, in some cases too much repair is needed. When this happens a cell may begin a process of apoptosis or programmed cell death. This is particularly common in multicellular organisms where replacing a single cell is less energy intensive than repairing lots of damaged DNA and much less risky! If you’ve ever had a sunburn then you’ve experienced apoptosis first hand. During a sunburn many damaged cells in your skin die. The death of so many cells triggers inflammation which causes the skin to become red, hot, itchy and sensitive. However, these unconformable symptoms are also a powerful defense against cancer because they allow the body to get rid of cells that are now mostly a liability.
Ready to safely bring some sunshine into the classroom in 2021. Think about these experiments.
- Investigation 10: Energy Dynamics – Explore the very positive role of the sun – providing the energy that makes life possible – in this fun and free simulation of a meadow ecosystem.
- 957: Blinded by the Light – Directly observe the effects of UV light on DNA by running a time series test comparing UV exposed plasmid samples and examining their results using electrophoresis. This experiment recieved a major upgrade this year and now incorporates sunscreen testing!
- 381: Break-though! – Use quantitative PCR amplification and analysis to assess the effect of either UV damage or DNAse I digestion on the integrity of plasmid DNA. This experiment requires that you have access to a qPCR thermocycler.