Learn about telomerase, the enzyme responsible for adding telomeres to the ends of chromosomes. Discover what telomerase is made of and how it functions to protect the chromosome.
Intro to Chromosome Structure
DNA codes for genetic information are located in all of our cells. When our cells multiply from one to two cells, the DNA within must also replicate. Double-stranded DNA is packaged linearly into chromosomes, most of which form the shape of an X. The ends of these chromosomes are secured against degradation by a protective cap at their distant ends called a telomere (think of ‘tele’ meaning ‘distant’).
Picture a telomere as the cap on a marker, placed there to protect the marker from spoiling.Telomeres don’t exist in all cells, but are always found in more complex cells called eukaryotes. Eukaryotes can be found in single-celled organisms or larger organisms, such as fish or humans. Within each eukaryote cell is a nucleus that envelopes its DNA.
The nucleus also encases other complex cell machinery in containers called organelles.Typically, eukaryotic DNA is a linear double-stranded sequence. However, at the telomere end of each chromosome there’s a single-stranded segment.
This single-stranded DNA is important for DNA replication because it is where the DNA replication process starts. DNA replication is handled by an enzyme called DNA polymerase. However, this enzyme cannot initiate the replication process. Instead, an RNA primer begins the DNA replication process, and it requires single-stranded DNA to work properly.
The single-stranded DNA at the telomere is added to the regular eukaryotic DNA by an enzyme called telomerase. Telomerase adds a few short single-stranded DNA repeats to the ends of chromosomes. Just as a ruler has a 1-inch mark on one end and a 12-inch mark on the other end, DNA ends are also labeled. One end is called the 3-foot end, and the other the 5-foot end.
Telomerase specifically serves to add DNA base pairs to the 3-foot end of the telomere.DNA is comprised of four bases: guanine (G), adenine (A), thymine (T), and cytosine (C). The short DNA sequence of the telomere region has noticeably more guanine (G) bases than the other DNA bases. It turns out that the end of the telomere DNA can fold upon itself. When it does, these guanines are placed together, typically in groups of four.
The guanines form a bond called Hoogsteen base pairing or G-quartet, and this phenomenon is particularly seen in the telomere.
Telomerase is integrated with an RNA molecule called guide RNA. Guide RNA is part of the enzyme and has a short RNA sequence. This sequence serves as a complementary template for the DNA base pairs to match.
As a result, these matching DNA base pairs are those that the telomerase adds. This process is repeated, resulting in identical repeating DNA segments being added to the telomere. Once these DNA repeats are added, DNA replication can begin with the assistance of DNA polymerase.
Let’s review what we’ve learned. Eukaryotic DNA is double stranded and forms an X-shaped chromosome structure. All eukaryotes, which are organisms that can be either single- or multi-celled, contain telomeres and their chromosomes. Telomeres are the end caps that protect the chromosome and contain short repeats of single-stranded DNA and are mostly made up of the DNA base of guanine (G).
Four of these bases come together when the telomeres fold on themselves and create something known as a Hoogsteen base pair. Telomerase, on the other hand, is the enzyme responsible for adding telomeres to the ends of the chromosome. Telomerase has a single-stranded RNA segment, which serves as a template for a single-stranded DNA. These single-stranded DNA fragments are repeated and added to the 3-foot end of the chromosome. It’s also important to remember the telomerase incorporates a single-stranded RNA within its enzyme, called guide RNA.