DNA Polymerases - Online Tutor, Practice Problems & Exam Prep

DNA polymerases are essential enzymes that synthesize new DNA strands during replication, always extending from the 5' to 3' direction. In prokaryotes, DNA polymerase III is the primary enzyme responsible for this elongation. Two critical requirements for DNA polymerases are a template strand, which serves as a guide, and a primer, a short RNA molecule that provides a free 3' hydroxyl group necessary for strand extension. The RNA primer is synthesized by primase and later replaced by DNA, ensuring accurate DNA replication.

DNA Polymerases

In this video, we're going to begin our lesson on DNA Polymerases. It turns out that the primary enzyme responsible for building new DNA strands is actually these DNA polymerases. You can see the function of this enzyme here in its name. Anything that ends in -ase is going to be an enzyme, as we discussed in our previous lesson videos when we first introduced enzymes. Therefore, these are going to be enzymes that polymerize or build DNA. DNA polymerases are going to be the primary enzyme responsible for building new strands of DNA.

Organisms tend to contain multiple types of DNA polymerases, and these different types of DNA polymerases will have slightly different functions. Moving forward, we're not going to talk about all the different types of DNA polymerases. We're only going to focus on the most important DNA polymerases involved directly with DNA replication.

New DNA strands that are built by these DNA polymerases are always going to be built in the same direction: from the 5' end to the 3' end of the DNA molecule. Thus, the new DNA strands are always elongated from the free 3' hydroxyl group. Recall from our previous lesson videos that the 3' end of the DNA strand has the free hydroxyl group, which is required to elongate the DNA strand. We'll talk more about this requirement in our next lesson video.

Below, we're showing you this image, which is essentially a little cartoon to help you remember that new DNA strands are always elongated from 5' to 3'. Notice that in this image, the boss is saying, "Hey, Pol or Polymerase, can you work an extra shift today?" The worker, the DNA polymerase, is saying, "Boss, you know I only work from 5' to 3'. I don't work any other shifts." Hopefully, this creative image can help you remember that new DNA strands are always built from 5' to 3' and never in the opposite direction from 3' to 5'.

To remind you a little bit of DNA structure, remember that the DNA structure consists of two strands of nucleotides that are antiparallel with respect to each other. They go in opposite directions in terms of their 5' and 3' ends. Notice that the 5' end is going to have a free phosphate group for each of them, and the 3' end has the free hydroxyl group. It is the free hydroxyl group that is required to elongate the new DNA strands. The new DNA strands can only be built in this direction, from 5' to 3'.

This here concludes our brief introduction to DNA polymerases, and we'll continue to talk more about DNA polymerases and their requirements as we move forward in our course. So I'll see you all in our next video.

DNA Polymerase Requirements

In this video, we're going to talk about some DNA polymerase requirements. In prokaryotic organisms, we know that there are multiple DNA polymerases that can have slightly different functions from our previous lesson videos. We're not going to talk about all of the different types of DNA polymerases, but what you should know is that in prokaryotes, it's specifically DNA polymerase III, written with a Roman numeral III, is actually the primary enzyme that's for elongating or building new DNA strands. Notice in our image down below here, we're showing you DNA Polymerase III.

It turns out that all DNA polymerases have 2 central requirements that are necessary for them to operate. We've got these 2 central requirements listed down below, number 1 and number 2 right here. The first central requirement for all DNA polymerases is a template. All DNA polymerases require a template, and the template is just referring to the old or parental DNA strand that's going to act as a guide for building the new strands. If you take a look at our image down below, notice that the template strand is the strand that's down below right here. This is the old DNA template strand.

The second requirement that all DNA polymerases require is a primer. A primer is really just a small RNA molecule that acts as the starting point for DNA polymerase. DNA polymerase requires a free 3' hydroxyl group and can only extend existing strands. It cannot actually build brand new DNA molecules from scratch. It requires, again, number 1, a template, and requires a primer as a starting point so that it can provide the free 3' hydroxyl group that's needed for DNA polymerase to elongate or extend the new DNA strand.

The primer is going to be built by the enzyme primase, and primase is going to be the enzyme that builds the RNA primer. When we take a look at our image down below, notice that we're showing you the primase enzyme up here, and its job is to build this primer, this RNA primer. The RNA primer acts as a starting point for DNA Polymerase III. That's because the RNA primer provides a free 3' hydroxyl group, and this free 3' hydroxyl group is what's needed for DNA polymerase III to extend the strand. As it extends the strand, it's going to be base pairing those free nucleotides that come in with the template strand, and that is how it knows what nucleotides replace the RNA primer in this molecule. Ultimately, the RNA primer is going to be converted to DNA, and the DNA will be part of the newly built DNA strand.

We'll talk more about this a little bit later, but the enzyme that replaces or converts the RNA to DNA is going to be DNA polymerase I, a different DNA polymerase. We'll talk more about that later in our course. But for now, this here concludes our lesson on DNA polymerase requirements that DNA Polymerase III is the primary enzyme that builds these new DNA strands, and it has two central requirements. It requires a template strand, and it requires a primer, so the primer provides the starting point for it to elongate the new DNA strand.

We'll be able to get some practice applying these concepts and learn more about DNA replication as we move forward in our course. So I'll see you all in our next video.

Here’s what students ask on this topic:

What is the primary function of DNA polymerases?

DNA polymerases are essential enzymes responsible for synthesizing new DNA strands during replication. They add nucleotides to a growing DNA strand, always extending from the 5' to 3' direction. This process ensures that genetic information is accurately copied and passed on during cell division. In prokaryotes, DNA polymerase III is the primary enzyme for this elongation. DNA polymerases require a template strand to guide the synthesis and a primer to provide a free 3' hydroxyl group necessary for strand extension.

Why do DNA polymerases require a primer?

DNA polymerases require a primer because they cannot initiate the synthesis of a new DNA strand from scratch. The primer, a short RNA molecule synthesized by the enzyme primase, provides a free 3' hydroxyl group. This free 3' hydroxyl group is essential for DNA polymerases to add nucleotides and elongate the new DNA strand. Without a primer, DNA polymerases would not have a starting point for DNA synthesis, making the replication process impossible.

What are the two central requirements for DNA polymerases to function?

The two central requirements for DNA polymerases to function are a template strand and a primer. The template strand is the old or parental DNA strand that serves as a guide for building the new DNA strand. The primer is a short RNA molecule that provides a free 3' hydroxyl group, which is necessary for DNA polymerases to add nucleotides and elongate the new DNA strand. These requirements ensure accurate and efficient DNA replication.

In which direction do DNA polymerases synthesize new DNA strands?

DNA polymerases synthesize new DNA strands in the 5' to 3' direction. This means that they add nucleotides to the free 3' hydroxyl group of the growing DNA strand. The 5' end of the new strand corresponds to the phosphate group, while the 3' end has the hydroxyl group. This directionality is consistent across all DNA polymerases and is crucial for the accurate replication of genetic material.

What role does DNA polymerase III play in prokaryotic DNA replication?

In prokaryotic DNA replication, DNA polymerase III is the primary enzyme responsible for elongating or building new DNA strands. It adds nucleotides to the growing DNA strand in the 5' to 3' direction, using a template strand to guide the synthesis. DNA polymerase III requires a primer to provide a free 3' hydroxyl group, which is essential for the addition of nucleotides. This enzyme ensures the accurate and efficient replication of the prokaryotic genome.