What is Annealing? - Definition, Biology & Process

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  • 0:04 Background on DNA
  • 1:17 Annealing Definition
  • 2:26 Annealing Uses
  • 4:11 Lesson Summary
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Lesson Transcript
Instructor: Bridgett Payseur

Bridgett has a PhD in microbiology and immunology and teaches college biology.

Scientists know a lot about how DNA behaves, including that annealing describes how two strands of DNA join up to form one molecule. What does DNA look like, how does it behave, and why do we want to know?

Background on DNA

You may have heard DNA referred to as 'the building block of life' in one of those movies about dinosaurs being resurrected to live in a theme park for tourists to come to see (before, of course, the dinosaurs escape and go on a rampage). So how does deoxyribonucleic acid, what scientists call DNA, build life? The answer to this question exists in the structure of DNA itself.

DNA is made up of many, many small individual units, called nucleotides, strung together. The strands are paired up, forming a spiraled structure called a double helix. The common analogy is to picture a rope ladder, and twist it up.

The parts making up the rungs of the ladder are the nitrogenous bases. There are four bases in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). These bases like to pair up. For example, if one strand of DNA has an adenine, the other strand will have a thymine at the same spot. The two bases are connected by a special attraction called a hydrogen bond. Basically, small charges match up on the bases so the two strands hold together.

Annealing Definition

The hydrogen bonds are based on attraction between opposite charges. This means the two strands of DNA aren't physically connected to each other. Instead, the nitrogenous base pairs, the rungs of the ladder, have an electrostatic attraction. They like each other and try to stay close, allowing DNA to maintain its overall shape.

When DNA is copied, the strands have to separate. Because this looks like a zipper unzipping, we call the process ''unzipping''. More scientifically, the process of DNA strands separating is called denaturation, because it's no longer in its natural state. Heat can disrupt the DNA's hydrogen bonds and lead to denaturation. Because heat is involved, this process of turning double-stranded DNA into single strands is sometimes referred to as ''DNA melting.''

However, the strands can't stay separated. They have to rejoin once all the new pieces have been made. The process of two strands of DNA rejoining is called annealing. Annealing happens when temperatures drop or return to a level where DNA can be in its natural state.

Annealing Uses

Why does it matter whether we describe DNA as either together as one molecule or as two split-up strands? We have found a way to use DNA annealing for our own benefit. Because we know that heat can cause DNA to denature, and cooling it down can cause it to anneal, we can control what state the DNA is in.

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