Lecture 4-Nuclear structure and function

Question 1

Functions of the nucleus

Answer 1

• Protect chromosomes
• Repair chromosomes when damaged
• Facilitate expression of genetic programs through the transcription of gene sets
• Construct ribosomes

Question 2

Nuclear envelope

Answer 2

Double membrane
Barrier between nucleus and cytoplasm
Outer membrane continuous with ER
Attached to and supported by the nuclear lamina

Question 3

Nuclear lamina

Answer 3

Meshwork of intermediate filaments from three intermediate filament proteins (Lamins A, B and C)
Attaches to nuclear envelope and to chromatin
Supports nuclear envelope
Organizes chromosomes
Regulate nuclear breakdown at mitosis (phosphorylation induces breakdown)

Question 4

Nuclear pores

Answer 4

Semiselective gates
8 fold symmetry, about 100 proteins, 125 kDa
Solutes and small proteins (44 kDa) can diffuse through

Question 5

Mechanism of nucleocytoplasmic transport

Answer 5

Importin binds cargo
Docks onto fibrils
Transcends the pore
GTP bound Ran directs dissociation of carrier from cargo and release of the NLS containing protein into the nucleus (nuclear localization signal)

Export works the opposite way

Question 6

Nucleolus

Answer 6

Ribosome biogenesis

Formed on rRNA repeats that are located on five different chromosomes, ten total chromosomes per diploid cell.

Question 7

Ribosome biogenesis

Answer 7

• Begins in nucleolus with the transcription of the genes for the 46s rRNA large precursor
• Ribosomal proteins and processing factors assemble onto the transcripts in addition to the 5s rRNA that what transcribed somewhere else by RNA pol III.
• 45s rRNA precursor is processed within the particle by nucleases to create the 18s rRNA of the small ribosomal subunit and the 5.8S r RNA and 28s rRNA of the large subunit (plus the 5s rRNA)

Question 8

Components of the histone proteins

Answer 8

H2A, H2B, H3, H4 subunits (2 of each)