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Cell Biology > Nucleus > Flashcards

Nucleus Flashcards

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1
Q

What are the general functions of the nucleus?

A

> storage of DNA
transcription of RNA from DNA
mechanical element
- stiffer than rest of cell so limiting factor in how squashed a cell can get

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2
Q

What are the benefits of compartmentalisation of genetic material in eukaryotes?

A

> protect DNA from reactions in cell - enzymes
increase SA for membrane-localised reactions
increase efficiency of transcription & synthesis
more ways to regulate gene expression

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3
Q

How many nuclei do cells have?

A

Most have 1
Red blood cells = 0
Skeletal muscle = many

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4
Q

Describe the nuclear membrane

A

Double membrane

- contiguous w/ lumen of ER

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5
Q

What is the ER the site of?

A

Protein production by ribosomes, protein folding & glycosylation

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6
Q

What is the theory for the evolution of the nucleus?

A

DNA in ancient prokaryotic cell was tethered to membrane until it became bound to membrane bound ribosomes

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7
Q

What are nuclear lamina?

A

> structure composed of nuclear lamins

> dense fibrillar network inside the nucleus

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8
Q

What are nuclear lamins?

A

> related to intermediate filaments
structural: form meshwork on inner nuclear membrane
functional: associate w/ DNA
2 types: A/C & B

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9
Q

What can mutations in lamins lead to?

A

‘Laminopathies’

–> muscular dystrophy & progeria (premature ageing)

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10
Q

What is the LINC complex?

A

Protein complex that links nuclear lamina w/ cytoskeleton

Inner membrane: SUN/KASH proteins

Outer membrane: nesprins
(connect to diff cytoskeleton elements)
(4 types w/ structural & functional roles)

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11
Q

What is the nuclear matrix?

A

Network of fibres found throughout the inside of a nucleus

- analogous to a cytoskeleton (‘nuceloskeleton’)

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12
Q

What is the nuclear matrix composed of?

A 
> NuMa
> Titin
> Spectrin
> PLF
> Nuclear actin 
> Motor proteins
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13
Q

In what 2 forms is DNA arranged within the nucleus?

A

Chromosomes
= DNA-protein complexes specially packaged for cell division

Chromatin
= complex of DNA, RNA & proteins

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14
Q

What are the 2 types of chromatin?

A

> Euchromatin

> Heterochromatin

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15
Q

Describe euchromatin

A

= ‘true’ chromatin

  • less densely packed
  • majority of genome
  • localised in central part of nucleus
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16
Q

Describe heterochromatin

A

= ‘different’ chromatin

  • dark under microscope
  • tightly packed condensed DNA
  • localised around lamina & nucleoli
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17
Q

How is DNA organised within the nucleus?

A

Chromosomes exist in discrete territories within nucleus

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18
Q

What does localisation of DNA within the nucleus affect?

A

Whether gees are expressed (=transcribed into RNA)

Less RNA production in LADs (heterochromatin) than away from lamina (euchromatin)

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19
Q

What are LADs?

A

Lamina-associated domains

20
Q

How does info get out of the nucleus?

A

Export of RNA & ribosomes

- mRN, rRNA, tRNA & assembled ribosomal units

21
Q

Which proteins need importing into the nucleus?

A

RNA polymerase
Ribosomal proteins
Transcription factors
All other structural proteins (lamins, histones etc)

22
Q

Through which structure do proteins move in & out of the nucleus?

A

Nuclear Pore Complex (NPC)

23
Q

What are the features of NPCs?

A

> ring structure w/ 8-fold symmetry
30 diff types of nuclear pore proteins (Nups)
central channel is filled w/ FG-Nups that form a selective barrier to transport

24
Q

What are FG-Nups?

A

Phenylatanine-glycine motifs - Nuclear pore proteins

25
Q

What do NPCs have common ancestry with?

A

COP I, COP II & clathrin

26
Q

What are the 2 types of movement through the NPC?

What types of molecules are transported these ways?

A

Passive transport
- small molecules & proteins

Facilitated transport
- RNAs, larger proteins, macromolecular complexes

27
Q

Where does the energy for facilitated transport come from?

What else does facilitated transport require?

A

> ATP hydrolysis for mRNA
GTP hydrolysis for proteins, tRNA & ribosomes

Nuclear transport receptors = karyopherins
(= karyo + phero(‘bring’))

28
Q

What is mRNP?

What does it include?

A

mRNA-protein complex

Proteins that process, cap & splice
Export factors

29
Q

How is mRNP exported & imported?

How does ATP hydrolysis affect mRNP?

A

Via the NPC

Release of Dbp5 in cytoplasm & remodelling on mRNP

30
Q

How are proteins imported into the nucleus?

A
  1. Karyopherins = AKA nuclear transport receptors = AKA importins
    - ->bind to cargo proteins in cytoplasm
  2. interact w/ FG-Nups
  3. release cargos in the nucleus
31
Q

How do importins release their cargo in the nucleus?

A

Ran-GTP binds to importin-beta
–> conformational change
= complex breaks apart

32
Q

How are proteins exported from the nucleus?

A
  1. Exportins bind cargo proteins in nucleus
  2. interact w/ FG-Nups
  3. release cargo in cytoplasm
33
Q

How do exporting load their cargo in the nucleus?

A

Ran-GTP binds to exportin
–> conformation change
= complex assembly

Ran-GTP then hydrolysed to Ran-GDP in cytoplasm

34
Q

What are GTPases?

What happens when they bind to GTP?

A

Molecular switches

Hydrolyse a phosphate group
–> forms GDP

35
Q

What is GTP hydrolysis into GDP assisted by?

What is GTP loading after hydrolysis assisted by?

A

GAP proteins in cytosol
= GTPase activating proteins

GEFs in nucleus
= guanine nucleotide exchange factors

36
Q

What are nuclear localisation signals (NLS)?

A

> Protein motifs approx 8 amino acids long
Can occur anywhere in protein
Many basic residues e.g. lysine, arginine, proline
Binds to importins

37
Q

What are nuclear export signals (NES)?

A

> Sequence w/ 4 hydrophobic amino acids
e.g. LxxxLxxLxL (L= leucine, x = any other aa)
Binds to exportins

38
Q

How can NLS & NES be modified?

A

Phosphorylation

39
Q

Describe nucleocytoplasmic shuttling

A
  1. NLS of the transcription factor NF-kB is masked when bound to IkB in cytoplasm
  2. signal like cytokine (TNFa) triggers degradation of IkB
  3. NF-kB can be imported to nucleus
  4. NF-kB relocalises to cytoplasm via exportins binding to its NES
40
Q

Which non-membrane-bound organelles are contained in the nucleus?

A

> Nucleoli
- cells can have 1 large nucleolus or many small nucleoli

> Cajal bodies
PML bodies
Speckles

41
Q

What are the major functions of the nucleolus?

What are their other functions?

A

> Transcription of rRNAs
Assembly of ribosomal subunits

> sensing & responding to stress
cell cycle regulation

42
Q

What is the nucleolus composed of?

A

> segments of 10 chromosomes encoding rRNA genes
proteins for processing rRNAs
ribosomal subunit proteins
snoRNA oligonucleotides that help process rRNAs

43
Q

What are the functions of Cajal bodies?

What are they made of?

A

RNA processing
Genome organisation

Dense foci of coilin protein

44
Q

What are the functions of PML bodies?

A

DNA repair
Cell proliferation
Programmed cell death

45
Q

What are the functions of speckles?

A

Gene transcription

mRNA processing

46
Q

What is the role of the Ran-GTP/GDP cycle?

A

Maintain gradient of import/export

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