19. TELOMERES Flashcards
- What are Telomeres?
- they are the ends of Eukaryotic Chromosomes
- What do Telomeres consist of?
- they consist of the following repetitive sequence:
- (TTAGGG)n
- this is bound to several proteins
- What is the role of Telomeres?
THEY PROTECT THE CHROMOSOMAL ENDS FROM:
- erosion
- degradation
- recombination with other chromosomes
THEY POSTPONE THE SHORTENING OF DNA MOLECULES:
- they do this by postponing the erosion of the genes
near the ends of the DNA molecules
- they cannot prevent the shortening
- What is the shortening of Telomeres causes by?
- it is caused by cellular ageing
- this is followed by chromosomal ageing
- What is End-Replication problem?
(the telomere replication problem)
- there is no Okazaki fragment for the replication of the
3’ end of the chromosomes - this will result in generations of 3’ projections at the
two 3’ ends
- What happens with the telomere during DNA Replication?
IN EVERY DNA REPLICATION ROUND (S-PHASE):
- a small region of the telomere cannot be replicated
IN EACH CELL DIVISION:
- some base pairs are lost
- about 50-200 base pairs
- Does this diagram make sense?
- yes
- What is Telomerase?
- this is a Ribonucleic enzyme
- it conserves the Telomere length
- it prevents the shortening of the telomeres
- What is dependent on the length of the Telomeres?
- the Replication Capacity of the cells
- What is the role of Telomere shortening?
- it protects cells from Carcinogenesis
- it does this by limiting the number of cell divisions
- this is due to Hayflick’s Limit
- What is Hayflick’s Limit?
- is the number of times a normal somatic,
differentiated human cell population will divide before
cell division stops
NB:
- this limit does not apply to Stem Cells
- What causes the shortening of the Telomerase?
- this shortening is less than 4-7kb
- it is caused by incomplete replication
- this results in the chromosome end leading to
Replicative Senescence
(this is also known as cellular ageing)
(this leads to the ageing of the organism)
- What is Senescence?
- this is permanent cell cycle arrest
- this happens after a cell has reached a certain number
of cell divisions - this is known as Hayflick’s limit
- What size should a Telomere be?
- it should be 15-20 kb
- How does Telomerase prevent the shortening of the Telomere?
- it synthesises the repetitive sequence: TTAGGG
- this is the sequence of the telomere
- telomerase synthesises the repetitive sequence by
using a template RNA Primer - it uses this primer PLUS an enzyme called
Reverse Transcriptase
- Can RNA ever be converted to DNA?
- no
- What are the 2 components that Telomerase consists of?
- hTERT
- this is a reverse transcriptase
- TEMPLATE RNA PRIMER
- there is a absence of active telomerase (hTERT) in
human somatic cells
- there is a absence of active telomerase (hTERT) in
- Where do we find Activated Telomerase?
IT IS FOUND ONLY IN:
- germ cells
- stem cells
- hair follicle cells (epithelial cell type)
- activated lymphocytes
- several type of cancer cells
- What does Telomerase protect with regards to gametes?
- it protects the chromosomes of the gametes
- In which three cells is there regulation of Telomerase Activity?
- Normally Active in Germ Cells
- Normally Inactive in most types of Somatic Cells
- Abnormally active in some cancer cells
- Why is it normal that Telomerase is active in Germ cells?
IF THE CHROMOSOMES OF GERM CELLS BECAME SHORTER IN EVERY CYCLE:
- the essential genes would eventually be missing from
the gametes that they produce
NB:
- sperm and egg cells need a good chromosomal
structure
- Why is it normal for Telomerase to be inactive in most types of Somatic Cells?
EXCEPT FOR:
- stem cells
- hair follicle cells
- activated lymphocytes
- Why is it Abnormal for Telomerase to be active in some cancer cells?
- this may allow for the cancer to persist
- this will cause continuous proliferation
- Does this diagram make sense?
- yes
