Eukaryotic genes and viruses Flashcards
What is chromatin?
A complex of negative DNA and its associated positive proteins (histones)
What are nucleosomes and linker DNA in chromatin?
Nucleosomes are the DNA coiled around histones
Linker DNA is the uncoiled regions between them
What are nucleosomes?
The basic unit of chromatin
How does chromatin condensation affect gene expression
Condensed chromatin is transcriptionally inactive as it limits the access of proteins required for transcription
Relaxed chromatin is active as proteins required for transcription can access the DNA
What is the most condensed state of DNA?
Metaphase chromosome
What are exons and introns
Regions of DNA in pre-mRNA
Exons are coding regions and introns are non-coding regions
How are introns removed from pre-mRNA?
- through splicing
- spliceosome cuts them out
What is the function of topoisomerases I and II?
relieve torsional stress ahead of fork
What is the function of DNA helicase?
Unwinds parental strands by ATP hydrolysis
What happens in DNA replication in eukaryotes?
- replication originates at the autonomous replication sequences that recruits a complex of proteins known as the ORC
- DNA helicases unwind the DNA
- replication protein A binds and stabilises the DNA at the fork
What happens in replication priming?
- an RNA primer is required to initiate synthesis
- a complex of DNA polymerase a and primase creates a hybrid RNA/DNA primer
RNA polymerase adds a short stretch of RNA nucleotides which are then extended by DNA pol A
What are the roles of DNA polymerases alpha, delta and epsilon?
Alpha: extends the RNA primer with DNA but lacks proofreading
Delta: lagging strand DNA polymerase
Epsilon: leading strand DNA polymerase
What is processivity?
The ability of DNA pol to continuously synthesise DNA without frequent dissociation
What is the role of the clamp loader and PCNA clamp?
Clamp loader recruits the PCNA clamp at the function between the RNA/DNA primer and the template strand.
The clamp then forms a ring that displaces pol alpha and recruits delta or epsilon
How are the RNA primers removed on the lagging strand
Pol delta elongates the Okazaki fragments
When it encounters an RNA primer is displaces it to form a flap
FEN1 cuts the flap at the branch point
DNA ligase I seals the gap
What is the hayflick limit?
It isnt possible to add or remove RNA primers at telomeric ends and so the chromosome end shortens after each round of replication. The hayflick limit is where the telomeres become too short and the cell stops dividing
How does telomerase increase the telomere repeat length?
Telomerase RNA provides a template for extending DNA template strand.
Telomerase reverse transcriptase extends the DNA template
How does telomerase affect aging and cacner?
Telomerase is regulated in stem cells but inactivated during development. Telomere shortening is the sign of aging cells
In cancer, telomerase is inappropriately reactivated