Flashcards in Origin of the nucleus Deck (87)
List 6 major unique components of the eukaryotic cell.
1. Double nuclear envelope
3. Internal lamina
6. Folding of DNA around histones
Is this nuclear envelope a continuous double membrane?
No, it is a series of flattened bags
Is the lumen of the nucleus continuous with the ER?
What is open mitosis?
Whereby the nuclear membrane completely dissembles and reforms by fusion of the ER vesicles at every division
Did LECA have NPCs?
Did LECA have chromatin?
What did the histones an nucleosomes evolve from?
Archaeal structures, e.g. from methanogens
Is eukaryotic RNA polymerase more closely related to archaeal or bacterial polymerase?
Is there much homology between archaea and eukaryotes?
Are the major unique features of eukaryotic cells conserved across all eukaryotes?
Are the major unique features of eukaryotic cells found in LECA?
Are there any intermediary forms between LECA and current eukaryotes?
No, eukaryotes just 'appeared'
Bacteria have a similar structure to the nucleus. What is it called?
The nucleoid that contains the genophore
What is the difference between a eu nucleus and a bacterial nucleoid?
The nucleoid is NOT separated from the cytoplasm as it does not have a membrane
Due to the nucleoid what can bacteria do in DNA replication?
Couple transcription and translation as the ribosomes can freely interact with the genetic material
Why is there an advantage to the bacterial nucleoid not having a membrane?
Because the coupling of transcription/translation allows rapid response to the environment
The nucleus only arose once. True or false?
True, as LECA only arose once
What are the 5 hypotheses for the origin of the nucleus?
1. Autogenous invagination
3. Inside-out hypothesis
4. Viral origin
5. Intron invasion
Which hypothesis does Nick Lane think is most likely?
What is autogenous invagination?
The 'textbook' explanation:
Proto-eukaryotes were phagocytic and the nucleus and ER are derived from an invagination of the plasma membrane
What support is there for the autogenous invagination hypothesis? Give 2 reasons.
1. Morphology is correct; nuclear membrane and ER are continuous and evolutionarily related
2. A phagocyte would need a nucleus to protect the DNA from stretching in phagocytosis
Who found that the nuclear envelope lamina acts as a 'molecular shock absorber'? How?
Dahl et al., 2004:
Nuclear envelope has a network structure that allows elasticity and compressibility. Normally the nucleus is compressed but can expand in phagocytosis.
So the model for autogenous invagination predicts that...
The first eukaryote was phagocytic and endosymbiosis came later
What are the 2 major constraints of the autogenous invagination hypothesis?
1. Necessitates a cytoskeleton, yet most prokaryotes have a cytoskeleton, so why hasn't this happened multiple times?
2. The loss of the cell wall led to the evolution of eus; L-form bacteria exist without cell walls, so why hasn't this happened multiple times?
Who was a big advocate for the autogenous invagination hypothesis? What did they say
Said that the 'catastrophic loss of the cell wall' led to eu evolution
What is the endokaryosis hypothesis?
The nucleus evolved from an endosymbiont like a methanogen that lost its metabolic machinery over time
Basically there were two endosymbiosis events, one to uptake the nucleus and one for the mitochondria
What support is there for the endokaryosis hypothesis?
May explain why information systems (replication, transcription, translation) are archaeal but the membrane systems are bacterial
The nuclear membrane shows homology to the membranes of which group?
What are the constraints of the endokaryosis hypothesis? List 6.
1. No known FL cell with a membrane similar to the nucleus
2. Why was only one endosymbiont uptaken and not a population like the mitochondria?
3. FL cells have ATP-generating physiology and nucleus does not
4. Open mitosis is unique to the nucleus, FL cells don't do that
5. No known FL cell is as permeable as the nucleus due to NPCs
6. Why would all host DNA be transferred to the endosymbiont?