Replication 4

The End Replication Problem:Telomeres shorten with each S phase

OriDNA replication is bidirectional

Polymerases move 5' to 3'

Requires a labile primer

3'5'

3'5'

5'

5' 3'3' 5'

Each round of DNAreplication leaves

50-200 bp DNA unreplicatedat the 3' end

Telomere Structure

Telomeres 'cap' chromosome ends

The structure of mammalian telomeres

Telomere Length (humans)

Number of Doublings

20

10

Cellular (Replicative) Senescence

Normal Somatic Cells

(Telomerase Negative)

Telomere also provide a means for "counting" cell division: telomeres shorten with each cycle

Telomeres shorten from 10-15 kb(germ line) to 3-5 kb after 50-60 doublings

(average lengths of TRFs)

Replicative senescence is triggered whencells acquire one or a few critically short telomeres.

Telomerase prevents telomere shortening

3'5'

DNA Replication

DNA Replication

Telomerase

DNA Polymerase

Telomere shortening:

•telomeric chromosome fusions•chromosome instability•replicative senescence•cell death

Telomere length maintenance:

•essential for replicative immortality

Synthesis of telomeric sequences1) Recognition

2) Elongation

3) Translocation

DNA substrate binding to hTERT and RNA template

Addition of nucleotides

DNA substrate and enzyme repositioning

CAAUCCCAAUC

3’

5’

hTERT

hTR5’- GGTTAGGGTTAGGGTTAG3’- CCAAT

GGTTAGGGTTAGGGTTAG

4) Repeated translocation and elongation=repeat addition processivity

Te

lom

ere length (humans)

Number of Doubling

20

10

Cellular (Replicative) Senescence

Normal Somatic Cells

(Telomerase Negative)

Germ Cells (Telomerase Positive)

+ Telomerase

Telomere Length and Cell Division Potential

Telomere related diseasewerner syndrome