Students explore the relationship between DNA and proteins, using a model to investigate how DNA’s structure allows it to make copies of its instructions for building specific proteins. These proteins in turn determine an organism’s traits.
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A Whale Named Isabella
In 1998, a group of scientists took pictures and genetic samples of a blue whale that was swimming near the Galapagos Islands. Then in 2006, another team of scientists took pictures and genetic samples of a whale they saw off the coast of Chile. When they compared their samples, they realized it was the same whale. The scientists named this whale Isabella.
Blue whales are the largest animals that have ever lived on Earth, larger even than any dinosaur. Blue whales can grow to be about 30.5 meters (100 feet) long and can weigh 160 tons. They are so large that their heart is the size of a Volkswagen Beetle and pumps 10 tons of blood through the whale’s body. The size of the whale is a trait—a behavioral or physical characteristic of an organism.
The reason scientists knew that Isabella was the same whale that was studied in both 1998 and 2006 was that the DNA matched in both samples. DNA stands for deoxyribonucleic (pronounced dee-ox-ee-rye-bo-new-clay-ick) acid. In a multicellular organism such as Isabella, almost every cell in its body contains the same DNA.
Scientists took samples from Isabella’s skin cells, and found that the DNA was the same in 1998 as it was in 2006. This confirmed for the scientists that the whale near the Galapagos Islands in 1998 and the whale off the coast of Chile in 2006 was Isabella because an organism’s DNA is unique to that organism.
Studying DNA
Scientists who want to understand why an organism looks or behaves the way it does often begin with that organism’s DNA. This is because DNA holds the instructions for making all of the proteins needed to keep the organism functioning. Remember that proteins are large, complex molecules that play many important roles in the body. Scientists have identified more than 100,000 proteins, which have a wide range of functions. Everything that takes place in an organism is either made of proteins or is the result of an action caused by a protein.
For example, like humans, blue whales have an aorta, which is its main blood vessel. The aorta of the blue whale is so large that a human can crawl through. The aorta and all other blood vessels are made up of connective tissues. These connective tissues have highly elastic proteins that allow the tissues to regain their shape after they’ve been stretched or contracted. Elasticity is important for blood vessels because they are constantly stretched as blood flows through them. Hemoglobin is another complex protein that transports oxygen in blood.
Keratin is a structural protein that makes up your hair and nails. Keratin also makes up the filter-feeder system inside the mouths of blue whales that allows it to filter large amounts of water and food. Hormones are another group of proteins that regulate various functions. They do this by sending messages between cells to regulate the cell’s life functions. Still other proteins allow parts of cells to move and cause muscles to contract so animals can move from place to place.
All of your cells are constantly building and using proteins. And the instructions for making each protein are held in your DNA. This is why DNA has been called life’s instruction manual. DNA is a polymer (a large molecule made up of many smaller molecules bonded together in a repeating chainlike pattern). These smaller molecules are called nucleotides. A nucleotide is itself made up of three types of molecules: a phosphate, a 5-carbon sugar, and a nitrogen-containing base.
DNA gets its name from the sugar molecule, which is called deoxyribose. There are four nitrogen bases in DNA: adenine (A), thymine (T), guanine (G), and cytosine (C). DNA has two strands that wind around each other, which makes the DNA molecule twist like a spiral staircase. This shape is why DNA is called a double helix. The sugar and phosphate molecules make up the sides of the staircase, and the nitrogen bases pair up to form the rungs.
The base pairs follow a predictable pattern: A always links to T, and C always links to G. Because the bases are complementary, if you know the sequence of nucleotides on one side, you know the sequence of nucleotides on the other side. The base pairs are held together by hydrogen bonds.
There are several steps between the gene and the protein. Making proteins is called protein synthesis. It is a multi-step process that involves many parts of the cell. In order to understand these steps, it is important to remember that the cell is a dynamic place, and there is constant activity happening. Each organelle has a specific role to play in keeping the cell functioning properly.
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