Lecture 1 - The Nucleus & The Nuclear Envelope Flashcards
what compartmentalises the DNA from the cytoplasm in eukaryotic cells?
the nucleus which increases control of processes but creates a problem as large mRNAs have to move in and out of the nucleus
what does the compartmentalisation of the eukaryotic cell allow?
compartmentalisation of the eukaryotic cell allows the cells to become much larger and more complex than prokaryotic cells, in part through focusing different activities in distinct internal membrane contained compartments
how specifically is DNA contained in eukaryotes?
in eukaryotes, DNA is enclosed by two CONCENTRIC membranes - a double membrane that forms the nuclear envelope
outer-nuclear membrane:
the outer nuclear membrane is continuous with endoplasmic reticulum (ER), so the space between the inner and outer nuclear membranes is directly connected with the lumen of the ER
inner nuclear membrane function:
the inner nuclear membrane carries nuclear specific proteins such as the membrane proteins that organise the nuclear lamina, a fibrous network that provides structural support to the nucleus (without out the membrane will fragment)
evolution of the nuclear envelope and ER:
may have evolved through invagination of the plasma membrane
why is transport across the nuclear membrane essential:
- mRNA has to be exported from the nucleus to be translated into protein
- transcription and replication requires enzymes and these proteins have to be imported into the nucleus
nucleus membrane studding;
the nuclear membrane is studded with nuclear pore complexes that are the sole channels through which polar molecules and macromolecules pass through the nuclear envelope
structure of the nuclear pore:
- nuclear pores are large multi-protein complexes composed of about 30 different proteins
- eight-fold symmetry organised around a large central channel
- protein fibrils protrude from both sides of the complex and on the nuclear side they form a basket like structure
- also contains unstructured regions that form a jumbled meshwork that fills the channel to ensure no proteins over 60kD can travel through
nuclear lamina:
fibrous network that provides structural support to the nucleus, preventing fragmentation
how do proteins over 60kD in size enter the nucleus?
they can only enter the nucleus via active transport
nuclear localisation signals (NLS):
nuclear localisation signals are proteins take that identifies proteins destined for the nucleus (like little agents)
nuclear localisation signals (NLS) structure:
the tag will typically exist of one or two short sequences containing positively charged Lysine residues
importins:
structures that carry proteins into the nucleus
exportins:
structures that carry proteins out of the nucleus
nuclear transport receptors:
repeated a-helices that stack into large arches or snail-shaped coils that escort large molecules through he nuclear pore
what drives nuclear transport?
the energy supplied by GTP hydrolysis
what causes cargo release?
Ran-GTP binding leads to cargo release
how does the active process of transport occur in the nucleus?
through the GTP hydrolysis of a protein called “Ran”
where does Ran-GTP only accumulate?
Ran-GTP only accumulates in the nucleus where it will cause a displacement of the desired cargo protein through binding to the nuclear import receptor in a different position to where the desired cargo protein is causing the desired protein to be released into the nucleus
what happens once the nuclear transport receptor has swapped its original protein for the Ran-GTP in the nucleus?
once displaced, the nuclear transport receptor will re-enter the cytoplasm where the attached Ran-GTP will hydrolyse into Ran-GDP and a phosphate group allowing the nuclear transport receptor to be empty and once again ready for transport
what is Ran-GTP?
Ran-GTP is a compound made of two subunits: ‘Ran’ which is a small proteins and ‘GTP’ which can hydrolyse itself in order to free the nuclear transport receptor
process that is the source of energy in nuclear cargo transport?
the hydrolysis of GTP in the Ran-GTP complex
where do cargo proteins and Ran-GTP bind on the nuclear transport receptor?
cargo proteins and Ran-GTP bind to different arches on the nuclear transport receptor
why is it important that Ran-GTP covers the loop on the nuclear transport?
this is important as in covering the loop Ran-GTP allows for NLS binding (which in turn recruits proteins destined for the nucleus)
what can promote export through the pore?
Ran-GTPase binding to exportins with cargo promotes export through the pore
what always puts the GTP onto Ran?
Ran-GEF found in the nucleus
what is nuclear import and export described as?
dynamic
most prominent structure in the nucleus and its function:
the electron dense nucleolus which is the site of ribosome biogenesis
ribosome structure:
the ribosome is a very large & complex strucutre, composed of two-thirds RNA and one-third protein
two small sub-units which acts as a platform where tRNAs are matched to the codons of the mRNA
one large subunit which catalyses the formation of the peptide bonds linking amino acids into a polypeptide chain
in higher eukaryotes, the ribosome contains four types of ribosomal RNAs (rRNAs):
5S, 5.8S, 18S & 28S
how are rRNA genes arranged in the genome?
rRNA genes are arranged in ‘tandem arrays’ where each transcription unit is separated by a non-transcribed spacer
how are the 5.8S, 18S & 38S rRNAs transcribed?
they are transcribed as a single 45S pre-rRNA that is sequentially cleaved to give rise to three mature rRNAs
site of rRNA processing:
nucleolus
ribosomes are assembled in the nucleus and exported to the cytoplasm:
the proteins are imported into the nucleus and assemble on the pre-rRNA in the nucleolus
as the rRNAs mature, additional ribosomal proteins are added to form pre-ribosomal particles
these particles are exported from the nucleus via nuclear pores to yield active ribosomal subunits
what is the nuclear inner membrane enriched in?
nucleus-specific proteins and organises the nuclear lamina
________ or ________ recognise the localisation signals and transport proteins through the nuclear pore
importins or exportins (both are types of nuclear transport receptors)
nucleolus:
aggregation of rRNA gene clusters what is the site of ribosome assembly
ribosomal proteins assemble on:
pre-rRNAs in the nucleolus
