Our cells use both DNA and RNA to make the various proteins required for daily cellular activities. DNA and RNA are made up of different sequences of nucleotides, or genetic building blocks. Adenine (A), cytosine (C), and guanine (G) are nucleotides found within both DNA and RNA. However, only DNA contains thymine (T), and only RNA contains uracil (U).
Each nucleotide within DNA and RNA has a complementary nucleotide that it can pair with. This is somewhat similar to the idea of complementary colors that work well together: blue always complements orange, just as green works well with red. In DNA, adenine is complementary to thymine, and guanine is complementary to cytosine. In RNA, adenine is complementary to uracil, and guanine remains complementary to cytosine.
Keep in mind that there is no uracil in DNA and no thymine in RNA. However, DNA and RNA can still be complementary to one another even though they may have different nucleotides. For instance, uracil within RNA will still be complementary to an adenine in DNA just as an adenine in RNA will be complementary to a thymine in DNA. In other words, adenine will pair with either thymine in DNA or uracil in RNA, and guanine will always pair with cytosine in both DNA and RNA. Therefore, each nucleotide has a partner that it always associates with depending on if it's in a molecule of DNA or RNA. The following figure demonstrates the complementary nature of both DNA and RNA nucleotides.
Overview of DNA Replication
Now that we understand how complementarity works in DNA and RNA, we can begin to understand how our cells make copies of DNA when necessary. Our cells age and constantly need to be replaced by new cells. For this to happen, the cell must first make a copy of the DNA before a new one is produced through cell division. This ensures that the new cell will have the full set of DNA found in every other cell. DNA replication is the term for the process that copies our DNA before a new cell is made. During this process, the DNA must first be unwound or unzipped from its complementary strand. Then each strand is used as a template to build a new complementary piece of DNA. In the figure seen here, the red strands of DNA represent the original pieces of DNA, and the pink strands signify the new complementary pieces of DNA that are built during replication.
At the end of replication, each set of DNA consists of one original strand and one new strand.
The Role of RNA Primers
During DNA replication, two pieces of DNA are separated and used to build new complementary strands of DNA. DNA polymerase is the enzyme responsible for assembling the new strands of DNA. However, DNA polymerase can only bind to the original DNA if a second strand is present. This presents a problem since the original strands have been separated from one another resulting in single strands of DNA. So, before DNA polymerase can start synthesizing new DNA, a primer must be made. In general, a primer is a short segment of DNA or RNA that is required for DNA synthesis.
In DNA replication, the primer is a complementary strand of RNA, shown in blue, that binds to the original strand of DNA shown in red. By creating a small segment of complementary nucleotides, DNA polymerase now has a site that it can bind to. After the RNA primer has been made, DNA polymerase will bind to it and start creating a new strand of DNA by adding complementary nucleotides to the primer. The figure here shows the original DNA, RNA primers, and the new complementary DNA.
After DNA polymerase finishes building the new DNA, another enzyme will come in and remove the RNA primers before replacing them with complementary DNA nucleotides. At the end of this process, all that remains are two copies that lack any RNA.
Let's review. Both DNA and RNA are made up of certain nucleotides that act as the building blocks of the genetic code. Each nucleotide has another complementary nucleotide that it pairs with. In DNA, adenine pairs with thymine, and guanine pairs with cytosine. In RNA, adenine pairs with uracil, and guanine pairs with cytosine. During DNA replication, two original complementary DNA strands are separated, and each is used as a template to create new DNA. DNA polymerase is the enzyme responsible for building new complementary DNA. However, it requires the presence of an RNA primer, which serves as a starting point for replication. Each primer is a short piece of RNA that is complementary to the original strand of DNA. Without a primer, DNA polymerase cannot copy the DNA. In short, RNA primers serve as a start site for DNA polymerase when DNA needs to be copied. Once the DNA has been copied, the primers aren't needed anymore and are, therefore, removed.
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