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Telomeres: Definition & Function

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  • 0:00 Telomere Definition
  • 0:44 Cell Division & DNA…
  • 2:19 Telomeres & Aging
  • 3:32 Lesson Summary
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Lesson Transcript
Instructor: Shannon Compton

Shannon teaches Microbiology and has a Master's and a PhD in Biomedical Science. She also researches cancer and neurodegenerative diseases.

Every time your cells divide you lose some DNA. Does this mean that you are losing genetic information during cell division? No, cells have telomeres that protect genetic information. Read on to learn more about telomeres.

Telomere Definition

Telomeres are short tandem repeats of nucleotides at the ends of chromosomes. Let's break this definition down a bit.

Nucleotides are the building blocks of DNA, which is the carrier of genetic information. A tandem arrangement is when things are placed one behind another. So, we have short pieces of DNA that are copies of one another and are stuck to the ends of a chromosome.

Telomere Function

Telomeres function to protect the ends of chromosomes. They prevent one chromosome from binding to another (DNA is sticky). They also don't have any genetic information. This comes in handy during DNA replication because we lose a bit of DNA with each round of cell division, so the telomeres protect the chromosomes so they are not lost.

Cell Division and DNA Replication

Before a cell divides, it must replicate its DNA so that each daughter cell gets its own complete copy of DNA. To put telomeres in context, let's review some important points of DNA replication.

DNA is an antiparallel double helix. Both strands are replicated. Both strands have polarity (3'-5' and 5'-3'). So, one strand runs 3'-5' and the other runs 5'-3' (antiparallel). They are bound to each other and twist around each other in a double helix.

DNA polymerase copies DNA, which makes a new strand of DNA. This creates a big problem, however. DNA polymerase can only add nucleotides to a preexisting 3' end. So, one template strand is easy to copy (leading strand) and requires only one primer. The other strand (the lagging strand) must be copied in short segments (Okazaki fragments), and each has its own primer.

The primers are made of RNA, not DNA. Therefore, they have to be replaced because we can't have any RNA in our DNA. The last primer on the lagging strand does not have a free 3' end. As a result, it is removed but not replaced with DNA. This leads to a progressive shortening of the DNA ends, or telomeres. After a single round of replication, the lagging strand is already shorter than the parent strand. After a second round of replication, both the leading and lagging strands are significantly shorter than the template strands.

Telomeres and Aging

With each round of cell division and DNA replication, telomeres shorten. After about 20 rounds of cell division, the telomeres are used up and actual genetic information is lost. When this happens, a cell will stop dividing and actually dies. So, the length of a telomere determines how many times a cell can divide.

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