If you've read How Cells Work, you know that our cells divide. It's how we maintain ourselves, grow and repair injuries. If you're an adult, you might be surprised to learn that 2 million cells in your bone marrow divide every second to keep enough red blood cells in your blood [source: Becker].
Every one of your new bone marrow cells looks and acts just like the old. Why? Because they have the same genetic instructions in the form of DNA. The old cells take great pains to copy their DNA and hand it down to the new cells. You might think it happens like copying on a copy machine, where old cells keep their old DNA, and new cells get new DNA. But what happens instead is more like sewing.
If you could look inside one of your old bone marrow cells, you would see that DNA is made of two strands "sewn" together by chemical bonds. When the cell divides, a "scissors" enzyme, called helicase, rips apart the two strands. Like little pins, binding proteins hold the two strands apart. DNA polymerase, an enzyme that's like the best tailor in the city, follows the template of the old strands and sews in a new strand made from building blocks in the cell. After the cells split, each has "tailored" DNA made of a new and an old strand. DNA replication is an amazing and intricate process that you can learn about in How DNA Works.
Now that we know how our cells capably and constantly complete this process, let's see how aspiring seamstress scientists compare.