Exam II (lecture 9-13) Flashcards
Eukaryotic nuclear chromosomes
Chromosoomes are linear and usually very long
- many origins per chromosomes
- slower replication
- shorter okazaki fragments
Histones
Centrpmeres
Linear chromosomes have “end replication problem”
DNA replication takes place during what phase?
S phase, which is part of the interphase of the cell cycle.
Two identical chromosomes are produced, attached at the centromer
Yeast origin
ARS (autonomous replicating sequence)
Creating replication forks at ARS
- origin selection - formation of prereplicative complex (pre-RC) during G1 of cell cycle
- Origin activation - at the beginning of S phase
Step 1 origin selection:
- origin (or replicator) - ARS (in yeast) (autonomously replicating sequence
- Eukaryotic initator - heteohexamer ORC (origin recognition complex)
- ORC - (trans acting factors) recognize and bind to ARSs (cis acting elements). ATP is required for ORC binding.
ORC vs ARS
ORC - trans acting factors
ARS - cis acting elements
Formation of a prereplication complex (pre-RC) in G1.
- binding of initator (ORC)
- Binding of helicase loaders (Cdc6 and Cdt1)
- “Loading of helicases” (Mcm2-7)
Pre-RC consists of
ORC, Cdc6, Cdt1, Mcm2-7
Formation and activation of preRC is controlled by
Cdk (cyclin dependent kinase)
PreRC (origin) activation
S phase
- Cdk and Ddk (cyclin dependent kinases - inactive in G1) activate PreRCs
- Phosphorylation of several proteins leads ro DNA melting and protein recruitment.
- Phosphorylation by S-phase cyclin kinases is necessary for replication fork assembly and confines the initiation of replication to S phase.
Cdk activity low
G1 phase
Pre RC formation allowed
Cdk activity high
S phase
Existing PreRC activation
CDK phosphorylates helicase loaders, preventing them to bind to ORC
When are histones synthesized
only during S phase and are added as replication proceeds
Some histone parts are “inherited” some are new
The spacing of histones every 200nt might be the reason for shorter okazaki fragments in eukaryotes and the slower speed of replication
The “end replication problem” (in eukaryotes)
both ends will be affected!
Results in incompletely replicated DNA due to okazaki fragments.
When completing DNA replication, the lagging strand will have okazki fragments, and the RNA primer fragments have to be removed from the strand by polymerase alpha. This leads to a shorter strand in the end of the newly synthesized DNA.
https://www.youtube.com/watch?v=wf6QiIlGxSg
Telomeres are the ends of the chromosomes
- Protect ends
- Maintain length
Telomere sequence identified as
Simple tandem repeats: TTGGGG
Tetrahymena Thermophila: Single celled pond animal with 40,000 short, linear chromosomes
Telomere repeats in humans
TTAGG
Telomere shortening leads to
cell death or cell senescence
Telomerase reverse transcriptase (TERT)
carry a tighlty bound, noncoding telomerase RNA (TR)
Telomerase:
TERT-TR holoenzyme (10ptn subunits + RNA of 451 nucleotides)
what type of protein is telomerase
ribonucleoprotein
- RNA component from 150-1300 nt long. Serves as a template (3’AAUCCCAAU…5’)
- Protein component has catalytic activities
1. DNA polymerase - adds dNTPs to 3’ end of a linear DNA
2. Reverse transcriptase, that carries its own RNA template
3. DNA/RNA helicase activity
DNA polymerase adds
dNTPs to 3’end of a linear DNA
Reverse transcriptase
carries its own RNA template
DNA/RNA
helicase activity