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What Carries Genetic Information?
Before scientists knew that DNA carried genetic information, they had some important constraints on whatever the genetic material was. That is, they knew that the genetic material had to have certain characteristics. It needed to contain a lot of information --- because one tiny egg and an even tinier spermatozoa somehow held all the information needed to make an entire person. It needed to be able to change. That's because people had known for thousands of years that it was possible to breed a species to change their traits --- everything from dogs to bananas are the result of human intervention. But they also knew that the genetic material must copy itself very well --- otherwise offspring would be completely different from their parents.
Closing in on DNA
From the late 1920s through 1950 more and more evidence was gathered that showed DNA might be the molecule that carried genetic information and passed it from generation to generation. There were a handful of now-classic experiments that showed that with DNA traits were passed on from one generation to the next, while without DNA no traits were passed on. But there were serious objections for two reasons: how would the simple DNA molecule carry enough information, and how would it be able to protect that information from being messed up in the environment of the body?
Answering the Questions
Finally, in the early 1950s, James Watson and Francis Crick were the first to win the race to reveal the structure of DNA. This was important because the structure showed how DNA could be protected from getting messed up. DNA is a twisted double strand, kind of like a spiral staircase. The information-carrying part is in the steps. The outer railings protect the steps from being damaged. Then, when it's time to make a copy the two strands of the DNA separate --- like splitting a spiral staircase down the middle. In sexual reproduction one strand from each parent gets put together in the offspring. If the copy wasn't made right, the child would have none of the traits of either parent. Because the copy is made right, the offspring shares characteristics of both parents.
More Copying
That's half the reason DNA needs to make a good copy: to pass traits from one generation to the next. But the other part of DNA's job is to carry all the information necessary to create a living being. It does that in a process called transcription. The details are too much to go into here, but living beings are built from proteins. Proteins hold them together and do all the jobs needed to keep the body running. Those proteins are built from the pattern contained in DNA molecules. A copy of DNA is in just about every one of the billions of cells in your body, for example. That copy is responsible for your liver cells making a working liver, your hair growing, your brain thinking. If DNA didn't copy itself, and copy itself accurately, your cells would not be able to perform their functions, and you would not live.