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Molecular Biology And Molecular Processes > The Nucleus > Flashcards

The Nucleus Flashcards

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1
Q

What are the 2 main functions of the nucleus ?

A
  • replication
  • transcription
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2
Q

What is replication ?

A

The constant formation of genetic material for information transfer

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3
Q

What is transcription ?

A

Copying genetic material for expression of genetic instructions

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4
Q

Describe the structure of the nucleus in a neutrophil

A
  • it is bilobed and the 2 lobes are connected by a piece of chromatin
  • there is a greater amount of cytoplasm compared to nucleus
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5
Q

Describe the structure of the nucleus in a lymphocyte

A
  • it is spherical and the cytoplasm is crescent shaped
  • there is very little cytoplasm compared to nucleus
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6
Q

Describe the structure of the nucleus in a myeloma (blood tumour cell)

A
  • nucleus is quite round
  • there is an equal ratio of cytoplasm to nucleus
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7
Q

What is the evidence for the complete genome ?

A

1) an unfertilised frog egg was exposed to UV light causing the nucleus to be destroyed
2) the skin cells of another frog were scraped off its back
3) ex vivo skin cells were regrown in vitro
4) micro-dissection of random skin cells to isolate its nucleus
5) the nucleus was transplanted into the cytoplasmic shell of the egg cell
6) this egg is developed into an embryo
7) the embryo developed into a tadpole and matured into an adult frog

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8
Q

What did the frog experiment show ?

A
  • showed the relationship of the nucleus and the cytoplasm
  • cells have specialised functions and express genes required for their functions
  • the remaining genes in the nucleus are switched off (repressed)
  • repression is controlled by the cytoplasm
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9
Q

What is the role of the inner face of the nuclear envelope ?

A

Binds chromatin to regulate DNA replication and transcription

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10
Q

What is the role of the outer face of the nuclear envelope ?

A

Binds the cytoskeleton to position the nucleus

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11
Q

What is the diameter of the nucleus ?

A

7 - 10 micrometers

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12
Q

What is the role of the nuclear envelope ?

A

Acts as a barrier that keeps ions, solutes and macromolecules from passing between the nucleus and the cytoplasm

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13
Q

What are the nuclear components ?

A
  • lamina
  • pores
  • matrix
  • nucleolus
  • nucleolar organising region (NOR)
  • chromatin
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14
Q

Describe the structure of the nuclear lamina

A
  • made of intermediate filaments
  • 10 nm thick
  • embedded under the nuclear envelope
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15
Q

What is the function of the nuclear lamina ?

A
  • used for strength
  • acts as a anchor for chromatin
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16
Q

What are nuclear pores ?

A

They are little breaks in the nuclear envelope which act as gateways across the barrier

17
Q

What are the components that make up a nuclear pore channel ?

A

• cytoplasmic filaments
• central transporter
• proximal filaments
• spoke ring
• nuclear basket

18
Q

How many nuclear pores are there in a cell ?

A

~ 6000

19
Q

What is the role of the central transporter in the nuclear pore channel ?

A

It is the route through which proteins cross the nuclear envelope
It is 40 nm in diameter

20
Q

What is the role of the spoke channels in the nuclear pore channel ?

A

It is a 10 nm aqueous channel for small uncharged molecules

21
Q

Briefly summarise how import and export takes place in the nucleus

A
  • nuclear localisation signals are needed for import
  • nuclear export signals are needed for export
22
Q

How are molecules imported into the nucleus ?

A

They move through the nuclear pore channel in a linear array (one by one)

23
Q

What are the 2 types of import in the nucleus ?

A
  • passive import for small and water soluble molecules (this happens in the spoke channels)
  • active import for proteins used in replication and transcription
24
Q

Describe the process of import into the nucleus

A
  • a transport protein known as importin is used
  • the localisation signal binds to the importin protein
  • this complex binds to the nuclear pore channel
  • energy from GTP is used
25
Q

Describe the process of export in the nucleus

A
  • a transporter protein known as exportin is used
  • the signal binds to the exportin protein
  • energy from GTP is used
26
Q

What is the function of the matrix ?

A

It is an internal support structure that acts as a scaffold to support the nucleoplasm

27
Q

What is the structure of the nuclear matrix ?

A

The isolated nucleus is treated with detergent and salt to remove the lipids, histone and non histone proteins of chromatin to form the matrix

28
Q

What is the function of the nucleolus ?

A
  • makes ribosomal subunits
  • eukaryotic cells contain ribosomes containing rRNA and ribosomal proteins
29
Q

Which types of cells contain more than one nucleolus ?

A
  • cells which actively synthesise proteins have more than one nucleolus
  • more nucleoli = more protein synthesis
30
Q

What is the nucleolar organising region ?

A

A circular area surrounded by a rim of electron dense filaments

31
Q

Describe the structure of the nucleolar organising region

A
  • there are 3 distinct regions of morphology :
    1) fibrillar centre
    2) dense fibrillar component
    3) granular component
32
Q

Describe the function of the nucleolar organising region

A
  • fibrillar centre contains DNA that codes for rRNA
  • dense fibrillar component contains nascent pre-rRNA transcripts and associated proteins
  • the granular component contains the ribosomal subunits
  • pre-rRNA precursor transcription takes place at the border between the fibrillar centre and dense fibrillar component
33
Q

If an isolated nucleolus is stretched out what does it resemble ?

A

A Christmas tree

34
Q

Describe the Christmas tree theory

A
  • the top of the tree is short rRNA branches
  • the bottom of the tree is long rRNA branches
  • there are 100 Christmas trees per nucleolus
  • each tree is linked by DNA spacers
  • the baubles are RNA polymerase 1 (protein knobs)
35
Q

Describe the structure of chromatin

A
  • naked DNA molecules wrap around histones to form nucleosome
  • nucleosomes organise into 30nm fibres
  • 30nm fibres are organised into loop domains
  • loops become further compacted into mitotic chromosomes
36
Q

What are the 2 forms of chromatin ?

A
  • euchromatin (interphase) - diffuse
  • hetero-chromatin (mitosis) - condensed
37
Q

Describe the chromatin found in interphase

A

90% diffuse euchromatin
- the histone proteins are not touching and the DNA is accessible

38
Q

Describe the chromatin found in mitosis

A

10% condensed hetero-chromatin
- the histone proteins are stacked and so the DNA is not accessible

Molecular Biology And Molecular Processes (5 decks)

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