Part 1

The DNA Book

Introduction Life is the greatest show on Earth, and we humans have the extraordinary privilege of having brains large enough to begin to understand it. Some life- forms are microscopic, while others are so big they can be seen from space. There are plants that eat animals, and bacteria that live in scalding-hot water. Some animals spend their lives entirely in the dark, while many others live underwater. The secret of all this amazing diversity is DNA. DNA is the molecule that holds the code for life, from bats to beetles, and from mushrooms to mammoths. We all rely on it to grow, survive, and reproduce. Read on to find out what DNA is and what it can do—and what happens when it gets mutated! Professor Alison Woollard Dr. Sophie Gilbert

What is DNA? DNA stands for deoxyribonucleic acid (dee-ox-ee-ry-boe-new-klay-ik a-sid). It's the chemical that makes up your genes, which contain the instructions that make you who you are, from the color of your eyes to the size of your feet. DNA DNA is made up of three different things: sugar, phosphate, and bases. These are joined together to make one really long chain that forms a shape called a double helix. DNA has a strong structure and can store information for a long time. Double helix The twisted shape of DNA is called a double helix. Bases Pairs of bases run along the inside of DNA. Genome A complete set of your DNA is called your genome. The genomes of different organisms, or living things, can be very different sizes. Genes Certain sections of DNA make up your genes, which are responsible for all your characteristics. We have about 20,000 genes—and so do worms! FRIEDRICH MIESCHER DNA was discovered by a Swiss scientist named Friedrich Miescher. He was investigating white blood cells, which he collected from pus-covered bandages at a nearby hospital. From these he extracted a substance he called "nuclein." Nuclein later became nucleic acid, and today it is known as deoxyribonucleic acid (DNA). Miescher discovered DNA in the late 1860s, but he didn't know what it did in the body. DNA code lives forever! Even when you're gone, your DNA code will live forever in your family. Backbone Sugar and phosphate units make up the outer backbone of DNA. Bases are the code of life The order, or sequence, of the bases makes up the information in the DNA. There are four different bases in DNA, called A, T, C, and G. They make up the code for all life on Earth. When the two halves of the DNA helix zip together, A always pairs with T, and C with G.

Where is DNA in my body? Each tiny cell in your body contains a complete copy of your genome. Amazingly, if you stretched out your genome it would be 6 ft 6in (2 m) long, so it has to be packed up very tightly to fit inside your cells. Tracking down your DNA We need to delve deep inside your cells to find the DNA. It is kept inside a special compartment of the cell called the nucleus. Here, your DNA is wound up very tightly to keep it safe. Cells We have about 40 trillion cells in our bodies—that's 40,000,000,000,000! You can fit 8,000 of these cells on the head of a pin. Nucleus The nucleus is the command center of the cell because it contains your genome. Chromosomes Your genome is shared out into separate packages, called chromosomes. Humans have 23 different chromosomes. There are two copies of each, so that's 46 in total. Gene A gene is a length of DNA that contains the code for a particular characteristic, such as hair color. Are there cells without DNA? Unusually, red blood cells that carry oxygen around your body don't have a DNA-containing nucleus. They live for only four months and have to be constantly replaced. DNA in food Most of our food comes from living things. When you eat a tomato, for example, you're also eating all of the DNA in its cells. Your body chops up this DNA during digestion. DNA Surprisingly, 98 percent of DNA isn't used for genes! Scientists haven't yet worked out exactly what it all does. IS ALL OUR DNA IN THE NUCLEUS? No! Some of our DNA lives outside the nucleus in mitochondria, which are tiny batteries that power cells. Plant cells also have DNA inside their chloroplasts, which they use to change sunlight into food. Mitochondria There are lots of mitochondria in animal cells, but only one nucleus. Chloroplasts Plant cells have chloroplasts, which contain a chemical that makes them green.

What does DNA do? DNA is a code that makes all life on Earth—from humans to fish, and daffodils to dinosaurs. The main purpose of the DNA code is to make proteins. All living things use proteins to make and repair themselves. Different codes Living things on Earth vary a lot because of differences in their DNA code. Protists are single-celled organisms. They contain a nucleus, like the cells of fungi, plants, and animals. Mushrooms, molds, and yeasts are all fungi. Most fungi feed on the remains of dead plants and animals. WHERE DID DNA COME FROM? DNA probably developed from a molecule called RNA, which may have made up life billions of years ago. RNA looks a bit like DNA and can also do some of the things that proteins do. Some viruses still use RNA to encode their genes. There are about 400,000 different species of plant. Most of them are green. There are more than a million known species of animal, with many new ones discovered each year! IS DNA BORING? Even after the discovery of DNA, scientists thought it was boring for a long time. This was because DNA is made of only three things: sugar, phosphate, and bases. People thought that it was too simple to hold the code for life. I'm NOT boring! Inheritance Before the discovery of DNA, people were puzzled about how certain characteristics were passed on to the next generation. They set out to find a molecule that could be passed on, or inherited. People thought the molecule would be a protein— but they were wrong! In 1944, scientists discovered that if DNA from DNA can change characteristics smooth bacteria was mixed with rough bacteria, the rough bacteria became smooth. Only DNA could pass on this characteristic, not proteins. This was the proof that DNA was the important stuff!

Let's experiment! Would you like to try and see DNA for yourself? Perform this fun experiment, which you can do at home. You'll be getting DNA out of strawberries. Can I see DNA? It's very hard to know what something is like unless you can see it. DNA is very tiny and hidden in our cells, but we have some tricks for getting it out and taking a look at it! WARNING! Rubbing alcohol contains a lot of alcohol. It is used as an antiseptic to kill germs, but it's also poisonous, so you must never drink it. It can also easily catch fire. Use it only with the help of an adult in an airy or well-ventilated area, and make sure you do not breathe in its fumes. YOU WILL NEED » 2 beakers or plastic cups » 2 tsp dishwashing liquid » 1 tsp salt » ½ cup water » 2 strawberries » 1 resealable plastic bag » Strainer or coffee filter » ½ cup cold rubbing alcohol » Tweezers » 1 scientist (that's you!) 1 Preparing the DNA-extraction mixture In a beaker, mix together 2 teaspoons of dishwashing liquid, 1 teaspoon of salt, and ½ cup of water. 2 Breaking open the cells Put the strawberries into the plastic bag, seal the bag, and crush the berries well with your fingers. Add 2 teaspoons of the DNA-extraction mixture, reseal the bag, and continue crushing for another minute. 3 Separating the DNA Strain the strawberry liquid into the other clean beaker. Next, gently add the rubbing alcohol, pouring it down the side of the beaker. Use an amount equal to the amount of strawberry liquid. DO NOT mix or stir. 4 Seeing the DNA Look for a white, cloudy layer at the top of the mixture. You can tilt the cup and pick up this layer using the tweezers. That's the DNA! Seeing chromosomes It's often hard to see DNA in cells because the chromosomes are all spread out. When cells get ready to divide, however, the chromosomes pack up and copy themselves (see Copying your DNA, page 24). UNDER THE MICROSCOPE We still haven't seen the DNA in much detail, have we? Very powerful microscopes can help us here… Viral DNA An electron microscope uses a beam of incredibly tiny particles to show microscopic objects, such as this DNA (blue) from a virus. Bacterial DNA This DNA (pink) from a bacterium has been made visible by an atomic force microscope, which builds up an image of the DNA by probing it with a minute instrument.