telomeres and eukaryotic genes Flashcards
what are telomeres? what are their main functions?
they are sequences found at the ends of chromosomes and act to maintain the chromosome structure by
- provide a primer for DNA polymerase (solves the end protection problem)
- protect chromosme from exonuclease activity
- distinguish the natural end of the chromosome preventing the initiation of DNA repair mechanism (instead of being recognised as a broken end)
describe the strucutre of a telomere
> it is very GT rich with around 500-2500 tandem repeats of TTAGGG
it has a 3’ overhang on one of the strands with 6-80 copies of this short tandem repeat
bound by the 6 protein complex shelterin which helps to supress dna damage response and stimulate the formation of t-loop/ lasso structure
no genes are encoded here
what happens once the telomere is fully degraded?
telomere function is lost -> dna damage response is initiated and cell becomes senescent
cell eventually dies by apoptosis
what is the role of telomerase?
it maintains the telomere length in germline (not somatic) cells by adding newly synthesized tandem sequences to the newly replicated chromosomes using ddNTPs
> also embryonic/stem cells
> also cancer cells hijack this machinery to become immortal
describe the structural components of telomerase
telomerase is a ribo-protein: RNA - protein complex . it is a RNA transcriptase with its own RNA template
TERC is the RNA component and is always present. it has a sequence complementary to 1.5 of the tandem repeat (TTAGGG = AAUCCCAAU)
TERT is protein component and limited in availability and uses the RNA template to extend telomere length. therefore it is a reverse transcriptase!!
so if we overexpress the TERT gene then the cell can divide continuously and rapidly = immortal
> constituitvely active TERT gene was introduced into culutred cells and shown to be immortal (morales et al 2003)
what is the hayflick limit?
concept to describe cellular aging with regards to the number of cell divisions - normal cells stop dividing after 50-70 cell divisions
describe the medical significance of telomeres
disease
without telomeres, our life expectancy decreases. this is demonstrated by the autosomal dominant disease Dyskeratosis congenita = mutations in TERT or TERC (telomerase)
> telomeres are very short and patients show multiple organ failure and paticualrly in bone marrow (source of stem cells). Disease shows anticipation as the telomeres shorten with each generation reducing the life expectancy as more cells become senescent in a shorter time frame
also cancers can become replicatively immortal
How do cancer cells use telomeres to their advantage?
Cancer cells can divide indefinately/are immortal as the TERT gene is upregulated (due to a SNP mutation)
> so can we target telomerase for an anticancer drug? A: maybe but the enzyme might still be active for another few rounds of cell division
!!!!Reverse transcriptase is required for the transposition of which of the following? DNA transposons DNA transposons and SINEs LINEs only LINEs and SINEs SINEs only
Short and long interspersed transposon elements
transposition means the moving of gene to another place
so probably option A
LINEs and SINEs are both retrotransposons (or RNA transposons) so must be transcribed into RNA and then reversed transcribed back into DNA and inserted at a new site in the genome.
DNA transposons do not undergo transcription during the transposition process so do not need reverse transcriptase.
Which of the following best summarises the human genome and set of proteins it encodes?
A 46 protein-coding genes encoding approximately 20,000 proteins
B Approximately 20,000 protein-coding genes encoding 20,000 proteins
C Approximately 20,000 protein-coding genes encoding 60,000 or more proteins.
D Approximately 35,000 protein-coding genes encoding 35,000 proteins
E Approximately 35,000 protein-coding genes encoding over 1,000,000 proteins
so this is the human genome but we know that because of alternative splicing, we can get waaay more protein products than the actual genes present
so option C as a rough estimate
Mammals have a single Pax6 gene expressed in tissues including brain and pancreas. In zebrafish a gene duplication has generated two related genes, Pax6a + Pax6b. The two genes encode almost identical proteins but Pax6a is expressed mainly in brain and Pax6b mainly in pancreas. This appears to be an example of: A Formation of a retrogene B Formation of a pseudogene C Neofunctionalisation D Subfunctionalisation
option D! - subfunctional as they have SPLIT the function of the original gene between them
how does telomere structure form?
the 3’ extension invades a partially unwound region of the double stranded telomere DNA and basepairs with the lower complimentary strand (see diagram)
describe how to synthesis telomeres
Is it fntp or ddntp??
telomerase intrinsic RNA template (TERC) complementary base pairs with telomere sequence
Reverse transcriptase activity of TERT synthesises a new length of telomere incorporating dNTPs in a 5->3 direction to elongate telomere
Telomerase translocates along the telomere and continues synthesising
telomerase dissociates from the telomere
t-loop reforms
how does shelterin help to enhance function of telomere?
- suppress DNA damage response
>e.g subunit TRF2 helps to supresses ATM-mediated apoptosis signalling which then inhibits the DNA damage response - to stimulate tloop formation and e.g. TIN2 can stabilise the T-loop
> POT1 can increase affinity of ss DNA to ds DNA and shelters the 3’ overhang protecting it from degradation from exonucleases
what is dyskeratosis congenita?
autosomal dominant genetic disease where mutations in telomerase reduce its activity so telomeres are very short
due to haploinsufficiency
its a disease that shows anticipation with each generation (earlier onset and more severe)
