5. • Telomeres are repetitive nucleotide sequencesTelomeres are repetitive nucleotide sequences
located at the termini of linear chromosomeslocated at the termini of linear chromosomes
of most eukaryotic organisms.of most eukaryotic organisms.
• Its name is derived from the Greek nounsIts name is derived from the Greek nouns
telos ='end' and merοs = 'part’.telos ='end' and merοs = 'part’.
• Most prokaryotes do not have telomeres.Most prokaryotes do not have telomeres.
6. Repetitive DNA sequenceRepetitive DNA sequence
(TTAGGG in vertebrates)(TTAGGG in vertebrates)
Specialized proteinsSpecialized proteins
Form a 'capped' end structureForm a 'capped' end structure
7. • Telomere specific proteins, eg. TRF1 & TRF2Telomere specific proteins, eg. TRF1 & TRF2
bind to the repeat sequence and protect thebind to the repeat sequence and protect the
• Without these proteins, telomeres are actedWithout these proteins, telomeres are acted
upon by DNA repair pathways leading toupon by DNA repair pathways leading to
chromosomal fusions.chromosomal fusions.
8. • While replicating DNA, the eukaryotic DNAWhile replicating DNA, the eukaryotic DNA
replication enzymes cannot replicate thereplication enzymes cannot replicate the
sequences present at the ends of thesequences present at the ends of the
chromosomes .chromosomes .
• Hence, these sequences and the informationHence, these sequences and the information
they carry may get lost.they carry may get lost.
• Telomeres "cap" the end-sequences andTelomeres "cap" the end-sequences and
themselves get lost in the process of DNAthemselves get lost in the process of DNA
10. THE END REPLICATION PROBLEMTHE END REPLICATION PROBLEM
RNA primer nearRNA primer near
end of theend of the
chromosome onchromosome on
lagging strand can’tlagging strand can’t
be replaced withbe replaced with
DNA since DNADNA since DNA
polymerase mustpolymerase must
add to a primeradd to a primer
• Telomeric sequence is maintained by a specialTelomeric sequence is maintained by a special
enzyme called telomerase.enzyme called telomerase.
• SpecializedSpecialized reverse transcriptase.reverse transcriptase.
• Carries its own template RNA , which isCarries its own template RNA , which is
complementary to telomeric repeat sequences.complementary to telomeric repeat sequences.
14. • In most multicellular eukaryotic organisms,In most multicellular eukaryotic organisms,
telomerase is active only in germ cells, stemtelomerase is active only in germ cells, stem
cells, and certain white blood cells.cells, and certain white blood cells.
18. Removal of the RNARemoval of the RNA
primer leaves an overhanging 3' end ofprimer leaves an overhanging 3' end of
chromosomal DNA, which canchromosomal DNA, which can
form loops at the ends of eukaryoticform loops at the ends of eukaryotic
20. •Many experiments haveMany experiments have
shown that there is a directshown that there is a direct
relationship betweenrelationship between
telomeres and aging, and thattelomeres and aging, and that
telomerase has the ability totelomerase has the ability to
prolong life and cell division.prolong life and cell division.
21. • Most somatic cells do not have sufficientlyMost somatic cells do not have sufficiently
high levels of telomerase to maintain thehigh levels of telomerase to maintain the
length of their telomeres for an indefinitelength of their telomeres for an indefinite
number of cell divisions.number of cell divisions.
• Consequently, telomeres gradually shorten asConsequently, telomeres gradually shorten as
cells age.cells age.
• This shortening eventually leads to cell deathThis shortening eventually leads to cell death
or senescence.or senescence.
22. • Cancer cells express abnormally high levels ofCancer cells express abnormally high levels of
telomerase, allowing them to continue dividingtelomerase, allowing them to continue dividing