1.1 - Nucleus and Nuclear Import

Question 1

structure of outer nucleus membrane (relating to endoplasmic reticulum)

Answer 1

continuous with ER - space between inner and outer nuclear membranes directly connected with lumen of ER

Question 2

functionality of outer nucleus membrane (relating to endoplasmic reticulum membranes)

Answer 2

functionally similar - differ slightly in protein composition

Question 3

how may nuclear envelope and ER evolve?

Answer 3

through invagination of plasma membrane

Question 4

nuclear lamina

Answer 4

fibrous network that provides structural support to the nucleus

Question 5

what would happen without nuclear lamina?

Answer 5

nuclear membrane will fragment

Question 6

why is protein import into the nucleus required

Answer 6

transcription and replication require enzymes

Question 7

nuclear pore complexes (2)

Answer 7

1. stud nuclear membrane
2. sole channels through which polar molecules and macromolecules can pass through the nuclear envelope

Question 8

nuclear pore protein composition

Answer 8

large multi-protein complexes composed of about 30 different proteins

Question 9

what does negatively stained electron microscopy of purified nuclear pores reveal?

Answer 9

structure with eight-fold symmetry organised around large central channel

Question 10

nuclear pore complex structure (2)

Answer 10

1. protein fibrils protrude from both sides of complex (form basket-like structure on nuclear side)
2. many nuclear pore proteins contain extensive unstructured regions that form a jumbled meshwork that fills centre of channel
   (proteins over 5kD blocked from passively diffusing through)

Question 11

Nuclear localisation signals (NLS) (2)

Answer 11

1. protein tag that identified proteins destined for nucleus
2. tag typically consists of 1/2 short sequences containing positively charged lysine residue

Question 12

importins

Answer 12

carry proteins into nucleus

Question 13

exportins

Answer 13

carry proteins out of nucleus

Question 14

nuclear transport receptors

Answer 14

(importins/exportins)
escort large molecules through the nuclear pore

Question 15

energy supplied for nuclear transport

Answer 15

GTP hydrolysis

Question 16

RAN-GTP role in NLS binding (2)

Answer 16

1. cargo and RAN-GTP bind different arches
2. RAN-GTP covers loop which is important for NLS binding

Question 17

ran-GTP nuclear export (3)

Answer 17

1. nuclear export signal recognised by exportin
2. ran-GTP binds to exporting with cargo promoting export through pore
3. GTP hydrolysed by cytosol GTPase (RAN-GAP) switching to ran-GDP and disociating from cargo

Question 18

cytoplasmic GTPase

Answer 18

RAN-GAP

Question 19

nuclear GTPase

Answer 19

RAN-GEF

Question 20

nucleolus (2)

Answer 20

1. most prominent structure of the nucleus (electron dense)
2. site of ribosome biogenesis where transcribed and processed rRNA are combined with proteins to form ribosomal subunits

Question 21

ribosome (2)

Answer 21

1. translate mRNA -> proteins in the cytoplasm (rough ER/ cytosol
2. very large/complex structure (2/3 RNA and 1/3 protein)

Question 22

ribosomal subunits (2)

Answer 22

1. small subunit
2. large subunit

Question 23

ribosomal small subunit

Answer 23

platform where tRNAs matched to codons of mRNA

Question 24

ribosomal large subunit

Answer 24

catalyses formation of peptide bonds linking amino acids in polypeptide chain

Question 25

when do ribosomal subunits come together

Answer 25

come together on an mRNA for translation

Question 26

why are rRNA genes highly transcribed?

Answer 26

cells require lots of ribosomes

Question 27

how are rRNA coding genes organised in higher eukaryotes?

Answer 27

large tandem arrays

Question 28

ribosomal RNAs in higher eukaryotes (4)

Answer 28

1. 55 2. 5.8S 3. 18S 4. 28S

Question 29

processing of rRNAs transcription

Answer 29

5.8S, 18S and 28S rRNAs are transcribed as single 45S pre-rRNA the is subsequently cleaved to give rise to 3 mature rRNAs

Question 30

where does rRNA processing occur?

Answer 30

nucleuolus

Question 31

what happens as rRNAs mature?

Answer 31

additional ribosomal proteins are added to the pre-ribosomal particles

Question 32

what happens to mature ribosomes

Answer 32

exported from the nucleus via nuclear pores to yield active ribosomal subunits

Question 33

nuclear transport receptor structure

Answer 33

repeated a-helices that stack into large arches or snail-shaped coils