10. Nucleus and Cytoplasm

Question 1

Eukaryotic cells

Answer 1

Only cells with nucleus

eukaryon = true nucleus

Question 2

Nucleus

Answer 2

Prominent and distinguishing feature of eukaryotic cells.

Store of most of the genetic material within the cell.

Control centre for its expression

Allows segregation of events in cytoplasm and nucleoplasm

Provides selective barrier.

Allows more comple gene organisation, regulation and transcript processing

occupies 10 - 50 % of the cell

It is a membrane bound organelle with internal substructure

Question 3

Chromatin

Answer 3

DNA and associated proteins (80-90 % of nuclear mass).

Heterochromatin:

-> Inactive DNA
-> Electron dense
-> Associated to nuclear envelope
=> Constitutive:
repetitive DNA (centromeres, telomeres)
=> Facultative:
varies from tissue to tissue from time to time

Euchromatin:

-> Active DNA

Question 4

Chromosomes

Answer 4

Occupy distinct territories within the nucleus

• > Prevents intertwining
• > Possibly involved in regulating gene activity
• > Positions are not fixed

Question 5

Chromatin packing

Answer 5

1.) nucleosomes - basic unit of chromatin
At the simplest level - double staranded DNA

2. ) DNA completed with histones -> form nucleosome
3. ) 8 histone proteins in each nucleosome. DNA wraps around this 1.65 times.
4. ) Nucleosomes form beads on DNA strand.
5. ) nucleosome fold up to produce a 30 nm fibre.
6. ) Loops are forming from step 5 - 300 nm length
7. ) 300 nm - compressed and folded to produce 250 nm wide strand
8. ) tight coiling of 250 nm produces the chromatid of chromosome.

Question 6

Nuclear bodies

Answer 6

Small, highly dynamic non-membrane enclosed structures

1. ) Nucleolus
2. ) Cajal bodies and Germs
3. ) Speckles
4. ) PML bodies

Question 7

1.) Nucleolus

Answer 7

• > factory for transcription
• > processing of non-coding RNAs
• > assembly with proteins - rRNA (pre-ribosome assembly), tRNAs, telomerase, signal recognition particle.

Nucleolus organiser region:

stretch of DNA that carries multiple copies of rRNA genes - 5 per haploid set in humans

Granules:

ribosomal subunits being assembles

Fibrils:

DNA being transcribed into rRNA

Question 8

2.) Cajal bodies and Germs

Answer 8

(Gemini of Cajal bodies)

• > processing of snRNA and snoRNA
• > assembly of snRNPs - molecules involved in RNA-splicing

Question 9

3.) Speckles

Answer 9

(interchromatin granule clusters)

-> storage of mature snRNPs and other rRNA processing components

Question 10

4.) PML bodies

Answer 10

(pro-myelocytic leukemia)

• > storage, assembly, modification of protein complexes involved in DNA repair
• > Sites of SUMOylation

Question 11

Assembly of ribosomal subunits

Answer 11

1. ) ribosomal proteins made in cytoplasm
2. ) ribosomal RNA (rRNA) remains in nucleolus after transcription
3. ) ribosomal proteins imported into nucleus - targeted to nucleolus
4. ) ribosomal proteins and rRNA form intact ribosomes in cytoplasm - once mRNA bound

Question 12

Nuclear envelope

Answer 12

Double lipid bilayer continuous with endoplasmatic reticulum

Question 13

Nuclear Pore

Answer 13

Allows selective nuclear import and export

mammalian cell -> 3000 - 4000 nuclear pores

Question 14

Nuclear pore complex (NPC)

Answer 14

consists of ~30 proteins (nucleoporins)

nuclear mass of ~70 MDa

probably largest protein complex of cell

structural details revealed by electron microscope

Question 15

Transport in and out of nucleus

Answer 15

• > takes place through nuclear pores
• > small molecules such as nucleotides, metabolites diffuse freely
• > large proteins and complexes selectively enter and exit the nucleus - sequence information

Question 16

Transport of macromolecules in and out of the nucleus

Answer 16

1. ) Transcription of RNA processing
2. ) Chromosome replication
3. ) Protein synthesis

Question 17

1.) Transcription of RNA processing

Answer 17

Proteins are needed for transcription

DNA -> rRNA

     - > mRNA
     - > tRNA

Protein component of other ribonucleoproteins, including ribosomal proteins are being delivered to the ribosomal subunits (snRNPs, telomerase)

Question 18

2.) Chromosome replication

Answer 18

Lamins and other structural proteins

Proteins involved in DNA repair and modification

Proteins needed for chromosome replication (enzymes, histones, transcription factors)

Question 19

3.) Protein synthesis

Answer 19

tRNA + growing polypeptide - Ribosome + mRNA -> completed protein

Question 20

Nuclear localisation

Answer 20

a) Nucleoplasmin is transported into the nucleus
b) Where is the nuclear localization signal located
c) Pinpointing the nuclear localization signal

Question 21

a) Nucleoplasmin is transported into the nucleus

Answer 21

-

Question 22

b) Where is the nuclear localization signal located

Answer 22

-

Question 23

c) Pinpointing the nuclear nlocalization signal

Answer 23

-

Question 24

Nuclear import is regulated via specific signal sequences on proteins

Answer 24

Nuclear localization signal (NLS)

a) Localization of T-antigen containing its normal nuclear import signal
b) Localization of T-antigen containing a mutated nuclear import signal

Question 25

Proteins with a nuclear destination bind to import receptors

Answer 25

Several proteins are involved in nuclear import

1. ) The GTPase Ran
2. ) The NLS receptor Importin (Karyopherin family)
3. ) the guanine nucleotide-exchange factor
4. ) the GTPase-activating protein GAP
5. ) Nucleoporins (part of the pore complex, contain FG repeats)