Lecture 1 - The Nucleus & The Nuclear Envelope

Question 1

what compartmentalises the DNA from the cytoplasm in eukaryotic cells?

Answer 1

the nucleus which increases control of processes but creates a problem as large mRNAs have to move in and out of the nucleus

Question 2

what does the compartmentalisation of the eukaryotic cell allow?

Answer 2

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

Question 3

how specifically is DNA contained in eukaryotes?

Answer 3

in eukaryotes, DNA is enclosed by two CONCENTRIC membranes - a double membrane that forms the nuclear envelope

Question 4

outer-nuclear membrane:

Answer 4

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

Question 5

inner nuclear membrane function:

Answer 5

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)

Question 6

evolution of the nuclear envelope and ER:

Answer 6

may have evolved through invagination of the plasma membrane

Question 7

why is transport across the nuclear membrane essential:

Answer 7

• 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

Question 8

nucleus membrane studding;

Answer 8

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

Question 9

structure of the nuclear pore:

Answer 9

• 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

Question 10

nuclear lamina:

Answer 10

fibrous network that provides structural support to the nucleus, preventing fragmentation

Question 11

how do proteins over 60kD in size enter the nucleus?

Answer 11

they can only enter the nucleus via active transport

Question 12

nuclear localisation signals (NLS):

Answer 12

nuclear localisation signals are proteins take that identifies proteins destined for the nucleus (like little agents)

Question 13

nuclear localisation signals (NLS) structure:

Answer 13

the tag will typically exist of one or two short sequences containing positively charged Lysine residues

Question 14

importins:

Answer 14

structures that carry proteins into the nucleus

Question 15

exportins:

Answer 15

structures that carry proteins out of the nucleus

Question 16

nuclear transport receptors:

Answer 16

repeated a-helices that stack into large arches or snail-shaped coils that escort large molecules through he nuclear pore

Question 17

what drives nuclear transport?

Answer 17

the energy supplied by GTP hydrolysis

Question 18

what causes cargo release?

Answer 18

Ran-GTP binding leads to cargo release

Question 19

how does the active process of transport occur in the nucleus?

Answer 19

through the GTP hydrolysis of a protein called “Ran”

Question 20

where does Ran-GTP only accumulate?

Answer 20

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

Question 21

what happens once the nuclear transport receptor has swapped its original protein for the Ran-GTP in the nucleus?

Answer 21

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

Question 22

what is Ran-GTP?

Answer 22

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

Question 23

process that is the source of energy in nuclear cargo transport?

Answer 23

the hydrolysis of GTP in the Ran-GTP complex

Question 24

where do cargo proteins and Ran-GTP bind on the nuclear transport receptor?

Answer 24

cargo proteins and Ran-GTP bind to different arches on the nuclear transport receptor

Question 25

why is it important that Ran-GTP covers the loop on the nuclear transport?

Answer 25

this is important as in covering the loop Ran-GTP allows for NLS binding (which in turn recruits proteins destined for the nucleus)

Question 26

what can promote export through the pore?

Answer 26

Ran-GTPase binding to exportins with cargo promotes export through the pore

Question 27

what always puts the GTP onto Ran?

Answer 27

Ran-GEF found in the nucleus

Question 28

what is nuclear import and export described as?

Answer 28

dynamic

Question 29

most prominent structure in the nucleus and its function:

Answer 29

the electron dense nucleolus which is the site of ribosome biogenesis

Question 30

ribosome structure:

Answer 30

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

Question 31

in higher eukaryotes, the ribosome contains four types of ribosomal RNAs (rRNAs):

Answer 31

5S, 5.8S, 18S & 28S

Question 32

how are rRNA genes arranged in the genome?

Answer 32

rRNA genes are arranged in ‘tandem arrays’ where each transcription unit is separated by a non-transcribed spacer

Question 33

how are the 5.8S, 18S & 38S rRNAs transcribed?

Answer 33

they are transcribed as a single 45S pre-rRNA that is sequentially cleaved to give rise to three mature rRNAs

Question 34

site of rRNA processing:

Answer 34

nucleolus

Question 35

ribosomes are assembled in the nucleus and exported to the cytoplasm:

Answer 35

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

Question 36

what is the nuclear inner membrane enriched in?

Answer 36

nucleus-specific proteins and organises the nuclear lamina

Question 37

________ or ________ recognise the localisation signals and transport proteins through the nuclear pore

Answer 37

importins or exportins (both are types of nuclear transport receptors)

Question 38

nucleolus:

Answer 38

aggregation of rRNA gene clusters what is the site of ribosome assembly

Question 39

ribosomal proteins assemble on:

Answer 39

pre-rRNAs in the nucleolus