Semi-Conservative DNA Replication – Part 1

Here is a molecule of DNA. 

Now, for DNA replication to take place, these two DNA strands need to separate

And, in order for the two strands to separate, the base pairs need to be broken apart. 

These complementary base pairs are held together by…

Complementary base pairs are held together by hydrogen bonds

And at the beginning of DNA replication, these hydrogen bonds are broken by an enzyme called DNA helicase.

DNA helicase moves along DNA, breaking the hydrogen bonds between complementary base pairs…

And exposing the bases on both strands. 

Next, as well as enzymes and a DNA molecule, the nucleus also contains DNA nucleotides floating around

And, once bases on both strands are exposed, each of these floating DNA nucleotides is  attracted to a complementary base.

For example, this nucleotide containing guanine is attracted to this exposed cytosine…

This nucleotide containing thymine is attracted to this exposed adenine.

While this nucleotide containing cytosine is attracted to this exposed guanine.

And once the DNA nucleotides reach the complementary base on the DNA strand, hydrogen bonds form between the bases.

So now, DNA nucleotides containing which bases will be attracted to these exposed bases?

Cytosine is complementary to guanine…

And thymine is complementary to adenine

So, these nucleotides will be attracted to these bases…

And once these DNA nucleotides reach the exposed bases, hydrogen bonds form between them.

Now, since the order of the exposed bases on each DNA strand determines which new DNA nucleotides are added…

Biologists say that each of these DNA strands acts as a template for a new strand. 

However…these aren’t quite strands yet.

So far, these DNA nucleotides are attached to the DNA strand…but they’re not attached to each other.

So we’ll look at how these nucleotides are joined together, next!

To sum up for now, in the first step of DNA replication…

In the first step of DNA replication, DNA helicase breaks the hydrogen bonds between DNA bases…

Breaking the two strands apart and exposing the bases on each strand. 

Next…

Next, DNA nucleotides floating around in the nucleus are attracted to exposed, complementary bases

And once there, hydrogen bonds form between the DNA nucleotides and their complementary bases. 

And since the order of the exposed bases on each DNA strand determines the order of bases on the new strands that form

Biologists say that each DNA strand acts as a…

Biologists say that each DNA strand acts as a template