5.1.4 The Role of Telomeres

• The ends of the chromatids in chromosomes are ‘sealed’ with protective structures called telomeres
• They are made of non-coding DNA (DNA that does not contain genes) that is made up of short base sequences that are repeated many times (multiple repeat sequences)
• In telomeres, one strand is rich in the base guanine (G) and the other strand is rich in the complementary base cytosine (C)
• The main function of telomeres is to ensure that the very ends of the DNA molecules are included in DNA replication during mitosis (the copying enzyme responsible for DNA replication is unable to run right to the very end of the DNA molecule and stops a little short of the end)
• If this end part of the DNA molecule contained an important gene, that piece of genetic information would be lost during DNA replication
• In each subsequent cell division, a little more genetic information would be lost
• Telomeres therefore act as a ‘buffer’ region of non-essential DNA and ensure that no important coding sections near the ends of the DNA molecules are left out of the replication process
• This ensures no genes are lost during cell division (the loss of vital genes can even result in cell death) and allows for continued replication of a cell
• To avoid the risk of losing genes most cells have an enzyme called telomerase that adds additional bases at each end (the telomeres)
• Some cells (generally specialised cells) do not have telomerase to ‘top up’ their telomeres and therefore after a certain number of cell divisions the cell dies, this has been connected with the ageing process

A telomere with multiple repeat sequences