Curious Kids: how do scientists read a person’s DNA

Africa Studio/Shutterstock

Mark Lorch, University of Hull

How do scientists read a person’s DNA? – James, aged 11, Thame, UK

DNA (which stands for deoxyribonucleic acid) contains all the information needed to make your body work займы по паспорту на карту. It is also surprisingly simple.

DNA is made from four chemical building blocks, which are arranged one after each other. This sequence is the instruction manual for your body. The building blocks are called adenine, thymine, guanine and cytosine, but we usually just call them A, T, G and C.

Curious Kids is a series by The Conversation that gives children the chance to have their questions about the world answered by experts. If you have a question you’d like an expert to answer, send it to and make sure you include the asker’s first name, age and town or city. We won’t be able to answer every question, but we’ll do our very best.

The information in your DNA is bunched into sections called genes. The genes are like sentences in an instruction manual.

Most genes control the everyday running of your body – how it grows hair, digests food or carries oxygen around. So 99.9% of your DNA is exactly the same as everyone else on the planet. The rest is what makes you unique. For example, if you have blue eyes, then a few of the letters in some genes will be different from someone with brown eyes.



As we grow, cells in our body divide. One cell becomes two. Every time this happens each of the new cells needs a full copy of DNA. DNA makes this easy, because it’s made of two strands. When a cell divides the strands split up, and a new copy is made of each one. Let’s look at how this is done.

The letter A on one strand is always opposite the letter T on the other, and G is always opposite C. So a short double strand of DNA might looks like this:

Illustration of matching pairs aligned vertically.
Illustration of a double strand of DNA.
Mark Lorch, Author provided

Let’s see what happens with one of the strands in a dividing cell. First a T is added opposite the first A to make:

DNA illustration
T pairs with A.
Mark Lorch, Author provided

Then an A gets attached to the T like this:

DNA illustration
A pairs with T.
Mark Lorch, Author provided

Next, C get placed opposite G:

DNA illustration
C pairs with G.
Mark Lorch, Author provided

And so on until a whole new double-stranded piece of DNA is made.

Reading DNA

We can use this knowledge of how DNA copies itself to read a person’s DNA.

To do this, a scientist puts the DNA into four tubes. Then they add all the machinery that the cell uses to copy DNA, and lots of extra As, Ts, Cs and Gs into each of the tubes.

Next, they add some DNA letters that have been changed so they can’t join with the next letter in the sequence. You can think of them like pieces of Lego, but with flat tops so you can’t add a brick on top of it. Let’s call these special DNA letters A*, T*, G* and C*.

Each of our four tubes gets some of the special DNA letters added to it: A*s in the first tube, T*s in the second, G*s in the third and C*s in the fourth.

Let’s imagine what happens with our DNA sequence in the tube containing A*.

First, just like before, T is added opposite the first A to make:

DNA illustration
T pairs to A.
Mark Lorch, Author provided

Next, though, an A* might get added to make this:

DNA illustration
An A* is added.
Mark Lorch, Author provided

If this happens, then the next letter can’t get attached to the A*. This is as long as this stretch of DNA gets.

But there’s plenty more DNA and letters in the tube and in some cases a normal A will have been added at that point, followed by two Cs to make this:

DNA illustration
Two Cs are added.
Mark Lorch, Author provided

Next there is a choice again. If an A* gets added the DNA sequence will look like this:

DNA illustration
Then an A*.
Mark Lorch, Author provided

Every time we reach the point where we need to add an A there is a chance an A* might get added, which stops the DNA getting any longer. So in the end these DNA strands get made:

DNA illustration
Different lengths of DNA.
Mark Lorch, Author provided

The scientist reading the DNA knows that each strand in this tube ends in an A*. She then counts how many pairs are in each strand of DNA. By doing this, she can work out that the 2nd, 5th and 7th letters are all As.

She then does exactly the same thing with the other tubes and works out that the 1st and 6th letters are Ts, the 3rd, 4th and 9th letters are Cs and the 7th is a G. By putting that all together, she can read the whole DNA sequence.

Reading DNA is useful because sometimes the letters in the genes aren’t quite right, like a misspelled word in a set of instructions. This might cause some of your cells to not work properly.

For example, just one wrong letter in one particular gene might mean someone is more likely to become diabetic, or get cancer when they are older. Reading someone’s DNA allows doctors to spot and treat these diseases before they become too bad.The Conversation

Mark Lorch, Professor of Science Communication and Chemistry, University of Hull

This article is republished from The Conversation under a Creative Commons license. Read the original article.

By February 22, 2022 25 comments chemical education

Great Explanations – Crowdfunded science anthology

Excuse the slight off (chemistry) topic post, but I wanted to let folks know about a crowdfunded science anthology I’ve launched via Unbound.

You don’t need a PhD in horology to know that there are piles of great popular science books out there and not enough time to read them all. So I’ve collected a bunch of the best established and emerging science writers – people working at the cutting edge of their own particular disciplines – and asked them to distil their passions into just one chapter each.

The result is Great Explanations, an anthology of the most pressing, fascinating and sometimes just plain overlooked topics from the far reaches of science, engineering and maths topped with a smattering of the philosophy and history of science. These are the subjects that working scientists are most passionate about, or interested in, or surprised by, in their own disciplines – the things they think curious general readers really ought to know.

We’ve also teamed up with Sense about Science, which promotes the public interest in sound science and evidence. Some of the contributing authors are part of their Voice of Young Science network and 15% of the book’s profits will be donated to the charity.

Contributors include:

A whole load more fabulous scientists and writers will be revealed over the next few weeks; stay tuned to our updates to be among the first to find out more.

Of course, if it doesn’t get funded the book will never exist, so pop over to Unbound and pledge!

By September 5, 2021 8 comments science events, science news

The Periodic Dinner table

Chemistry built the modern world, from the materials that make up the everyday objects around us, the batteries in our devices and cleaning products that help to maintain sanitation. All this and much more besides are examples of chemistry in everyday life.

To illustrate this and have a bit of fun along the way we (Phil Bell-Young, the Salter’s Institute for Chemistry and I) put together a demonstration packed show called ‘The Periodic Dinner Table’.
It is a cross between demo lecture, comedy sketch and a game of bingo played on a periodic table. Just watch the video, and when you spot us interacting with an element cross it off on your periodic table (here’s one specially adapted for the show).

And in case you want the answers, you can find them on this video or in these teachers’ notes.

Hope you enjoy the show!

By August 9, 2021 10 comments chemical education, entertainment, fun

Biochemistry – A Very Short Introduction

My new book – Biochemistry – A Very Short Introduction is available for pre-order now! LoansCashNet

A here’s a sneak preview…

From the simplest bacteria to humans, all living things are composed of cells of one type or another. Amazingly, no matter where on the evolutionary tree they perch, those organisms all have fundamentally the same chemistry. This chemistry must provide mechanisms that allow cells to interact with the external world, a means to power the cell, machinery to carry out all the varied processes, a structure within which everything runs, and of course some sort of governance. Cells, in many ways, are like communities, but controlled and governed through a web of interlocking chemical reactions. Biochemistry is the study of those reactions, the molecules that are created, manipulated, and destroyed as a result of them, and the massive macromolecules (such as DNA, cytoskeletons, proteins and carbohydrates) that form the chemical machinery and structures on which these biochemical reactions take place.

Or, put more succinctly by the great physicist Erwin Schrödinger,

In biology .. a single group of atoms .. produces orderly events marvellously tuned in with each other and the environment according to the most subtle laws.’

Biochemistry is then the endeavour to understand those subtle laws governing those finely tuned orderly events, it is the study of biological molecules and their interactions, and so aims to reveal the molecular basis of life.

Of course, life in all its glory is so much more than just single cells. Cells come together to form multi-cellular organisms which then require a means for individual cells to communicate and ‘trade’ with one another. The organisms, in turn, interact to form the complex webs that are our eco-systems. And all of those interactions are modulated and facilitated through biochemical means. For example, consider the rhodopsin molecules that respond to photons of light, and so act as the first stage of a predator spotting its next meal. Or the olfactory proteins that bind a few minuscule molecules, which trigger a cascade of biochemical reactions that result in prey being alerted to the predator’s presence. Or the antibodies that act as the first guards, recognising the foreign molecules of an invading parasite and triggering the army that is the immune response. All of these processes fall within the realm of biochemistry.

It didn’t take long for an understanding of the chemistry of life to turn into a desire to manipulate it. Drugs and therapies all aim to modify biochemical processes for good or ill: Penicillin, derived from a mould, stops bacteria making their cell walls. Aspirin, with its origins in willow bark, inhibits enzymes involved in inflammatory responses. A few nanograms of botulinum toxin (botox), can kill by preventing the release of neurotransmitters from the ends of nerves and so leads to paralysis and death. Alternatively, the same botulinum toxin administered in tiny quantities results in a wrinkle free forehead. This is all biochemistry.

Detailed description of these topics could easily have made it into this book, and some readers may feel I was remiss in neglecting them and other topics as fundamental as vitamins, hormones, chromosomes, and numerous biochemical techniques. But this is after all a very short introduction, and so I had to draw the line somewhere. As a result, for much of the book I’ve focussed on some of the chemistry that occurs within cells. For therein lie the fundamental chemical processes that all life shares.

Finally, the boundaries of biochemistry are ill defined; it overlaps with genetics, molecular biology, cell biology, biophysics and biotechnology. And so I finish with a pair of chapters which explore how fundamental discoveries in biochemistry are influencing these fields and society at large.

By April 19, 2021 20 comments Uncategorized