In linear DNA fragments, the 5' end becomes shortened in each daughter DNA molecule. This is because, when the primer required between Okazaki fragments is removed, the DNA polymerase cannot fill the gap.

Repeated DNA replication results in a shorter DNA molecule. If this is not corrected, eukaryotes would become extinct so they have developed a mechanism to preserve the ends of chromosomes:

When linear chromosomes are replicated, the internal gaps between bases are filled once the primer is degraded but not the terminal gap. There is an overhang on the 3' end as the 5' end is too short. So, on the end of the 3' strands there is an overhang which contains contain a highly repeated sequence. For example, in humans, the sequence TTAGGG is repeated in tandem 100 to over 1000 times. This repeated sequence is known as a telomere and matching DNA bases can bind to it which lengthens the short 5' end.

A special RNA containing enzyme called telomerase can make additional copies of the repeated sequence on the 3' end, replacing those lost during replication. The RNA acts as a template for DNA synthesis. Telomerase is a specialised reverse transcriptase.

The strand extended by telomerase acts as a template for lagging-strand synthesis. The action of telomerase and the lagging strand machinery restores the telomere length to that of before replication.

	Created by Emma Lloyd over 8 years ago

Genetics I Note on DNA replication in eukaryotes , created by Emma Lloyd on 07/04/2018.