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Cell Biology and Biochemistry > 7 - The Nucleus > Flashcards

7 - The Nucleus Flashcards

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1
Q

what is a nucleosome

A

A nucleosome is when DNA is wrapped around a histone protein

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2
Q

what is chromatin

A

chromatin a series of repeated nucleosomes

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3
Q

where does transcription occur and translation occur

A

transcription occurs in the nucleus

translation in ribosomes of the cytoplasm

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4
Q

what are the 2 processes of modification that mRNA needs to undergo after transcription

A
  • adding a 5’ (prime) cap

- 3’ polyadenylation ( adding a poly-A tail)

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5
Q

what do post transcription modifications help with?

A
  • stabilising the mRNA
  • The binding of mRNA to ribosomes and in initiating translation
  • export from the nucleus
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6
Q

describe the structure of a nucleus

A
  • Double membrane nuclear envelope with peri nuclear space between
  • double membrane forms regions of ER
  • nuclear pore complexes
  • nuclear lamina which anchors chromatin to envelope
  • chromatin
  • Nucleolus
  • ribosomes on ER
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7
Q

Describe Nuclear pore complexes

A
  • go through both levels of the nuclear envelope
  • made from 30 different proteins called nucleoporins
  • each pore is made from 8 identical proteins
  • has cytosolic fibrils extending the cytoplasmic side
  • has a nuclear basket on the nucleus side
  • each NPC has 500-1000 proteins in so very large
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8
Q

Proteins larger than 60000 Daltons cannot diffuse through nuclear pore complexes so how are they transported into the nucleus

A
  • large proteins have nuclear localisation signals on their end terminus
  • this is a special signal sequence of amino acids that let the protein import into the nucleus
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9
Q

what is the mechanism behind how larger proteins enter the nucleus through a nuclear pore complex

A
  • Protein with nuclear localisation signal gets recognised by the nuclear import receptors (importins)
  • importins then bind to the nuclear localisation signal
  • the complex moves to the nuclear pore complex and binds to the cytosolic fibrils via FG repeats
  • the complex then moves through the NPC repeatedly unbinding and binding to the next FG complex which line the inner cavity of the NPC
  • The complex eventually reached the nucleus
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10
Q

What is a GTP binding protein?

A

A protein that binds to GTP and also when activated acts as a GTPase hydrolysing GTP into GDP and Pi

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11
Q

what is GAP?

A
  • GTPase Activating Protein

- This binds to GTP binding protein and induces GTP hydrolysis

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12
Q

What is GEF?

A
  • Guanine nucleotide Exchange factor

- Binds to GTP binding protein and causes it to release GDP in exchange for a new GTP

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13
Q

How does the nuclear import protein release the protein into the nucleus?

A

Ran-GTP binds to the nuclear import protein causing a conformational change in the importin releasing the protein into the nucleus

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14
Q

How is the importin/nuclear import protein recycled back into the cytoplasm and thus how is Ran-GTP cycled in this mechanism

A
  • Ran-GTP binds to the importin in the nucleus causing it to release its protein
  • The Ran-importin complex travels along FG repeats back into the cytoplasm
  • A Ran binding protein then seeks out a Ran anf binds to Ran in the Ran-importin complex, this causes the importin to dissociate from Ran-GTP being free for another use
  • A Ran GAP protein then hydrolyses the GTP into GDP and Pi causing the Ran binding protein to dissociate from Ran-GDP
  • Ran-GDP can freely move back into the nucleus through a Nuclear pore complex
  • Ran GEF regenerates Ran-GTP by exchanging the GDP for a GTP molecule
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15
Q

How does mRNA change change just before being exported to the nucleus

A

before export mRNA is complexed with numerous proteins being converted to mRNP

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16
Q

what is mRNP

A

mature mRNA with a 3’ Poly-A tail and 5’ Cap binding complex

17
Q

How is mRNP exported from the nucleus

A
  • Nuclear export receptors bind to the adaptor proteins, which bind to the 5 prime cap binding complex
  • This then travels along the FG repeats into the cytoplasm
18
Q

describe the nucleolus

A
  • not membrane bound
  • distinctive because its dark and packed with RNA’s and proteins
  • site of ribosome synthesis
  • constantly making ribosomes to keep up with protein demand
19
Q

Describe ribosomes

A
  • 1 Large subunit, 60s, 49 proteins and 3 rRNA’s
  • 1 Small subunit, 18s, 33 proteins and 1 rRNA’s
    Large subunit made from 5.8s, 28s, 5s rRNA
    Small subunit made from 18s rRNA
20
Q

there is extremely high demand for ribosomal synthesis, how does the body deal with this

A
  • 4 rRNA’s are encoded by more than 200 rRNA genes across many chromosomes
21
Q

where and how are the 4 rRNA’s transcribed in the cell?

A
  • 18s, 5.8s, 28s rRNA’s transcribed by RNA polymerase 1 in the nucleolus
  • 5s rRNA is transcribed by RNA Polymerase 3 in the nucleoplasm
22
Q

Describe how DNA becomes 90s-rRNA

A
  • the rRNA gene on DNA is transcribed into 45s pre-rRNA
  • Modification occurs where ribosomal proteins enter the nucleus from the cytoplasm through the NPC.
  • these proteins bind the the 45s pre-rRNA in the nucleolus forming 90s pre-rRNA
23
Q

How does 90s pre-rRNA become 3 rRNA’s

A
  • snoRNP’s are small nuclear RNA’s with proteins bound
  • snoRNP’s are complimentary to the target regions of 90s pre-rRNA
  • the proteins bound on snoRNP’s are enzymes which modify the 90s pre-rRNA in 3 ways
  • The ITS and ETS regions of 90s pre-rRNA get cleaved to end up with pure rRNA
24
Q

Which methods do SnoRNP’s use to modify 90s pre-rRNA

A

1) methylation of hydroxy groups on ribose
2) Conversion of uridine to pseudouridine
3) Cleavage of 45s pre-RNA

25
Q

explain how pre-RNA is cleaved into 3 rRNA’s and which rRNA are they

A
  • 18s, 5.8s, 28s
  • Exoribonucleases on the snoRNP’s cleave off the ETS (externally transcribed spaces) which are regions in the pre-RNA outside of the regions that are rRNA
  • Endoribonucleases on the snoRNP’s cleave off the ITS (internally transcribed spaces) which are regions that dont code for rRNA between rRNA coding regions
26
Q

where do the large and small subunits of the ribosomes finally combine to become a full ribosomes

A

In the cytoplasm after each subunit is separately transported out of the nucleus

27
Q

which type of rRNA is not transcribed in the nucleolus

A

5s rRNA, which is transcribed in the nucleolus

Cell Biology and Biochemistry (9 decks)

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