Session 8: Organelles 1

Question 1

What is the function of lysosomes?

Answer 1

Degradation
- DNA delivered as gene therapy might get degraded before reaching the nucleus for transcription bc of lysosomes

Question 2

Explain the advantages of nuclear seperation from the cytoplasm.

Answer 2

1) protection - protects fragile DNA molecules from mechanical stress
2) function - allows concentration of RNA synthetic enzymes
- concentrated DNA would be more efficient at transcription and translation

Question 3

Disadvantages of nuclear seperation from the cytoplasm.

Answer 3

transportation - import of large proteins and export of large products is difficult because of double membrane and pores

Question 4

Describe the membrane structure of the nucleus.

Answer 4

• double membrane
• nuclear pores that connect the cytoplasm to the nucleoplasm

Question 5

Describe the internal structure of the nucleus (3).

Answer 5

chromatin - interphase DNA (histones and accessory proteins)

heterochromatin - condensed (no active transcription)
- little to no transcription can occur in dense areas

euchromatin - dispersed (transcriptionally active)

Question 6

Describe the nuclear matrix.

Answer 6

Chromatin and lamins (intermediate filaments)
- scaffold for TNA transcription and mRNA splicing
- site for DNA replication
- lamins promote heterochromatin formation directly under nuclear envelope by binding to transcription repressor proteins

Question 7

What molecules can transport through the nuclear membrane?

Answer 7

• small molecules <5000MW can passively diffuse
• most proteins, RNA, and ribosomes require active transport

Question 8

The nuclear membrane can only allow molecules with MW < 5000Da to passively diffuse across. Explain how this can be a problem? How can we solve it?

Answer 8

There is no way to passively export/import larger items (DNA/RNA)
- r-protein made in cytoplasm must be imported
- DNA/RNA polymerase is made in cytoplasm (needs to be imported)

Solution: active transport
- using Nuclear Localization Signals (NLS) rich in arginine and lysine

Question 9

Explain the nucleolus? What is it? Where is it found?

Answer 9

• Site of rRNA transcription and ribosome assembly
• rRNA is transcribed by pol 1
• NOT membrane bound
• breaks down before and reassembles after mitosis
• specialized scaffolding proteins provide structure to nucleolar surface in absence of membrane
• 70% of RNA production in the nucleolus (high metabolic activity)

Question 10

Describe what the endoplasmic reticulum is.

Answer 10

• tubules and flattened sacs of membrane throughout the cytoplasm (starting from the nuclear envelope)

Question 11

What reactions happen in the ER?

Answer 11

• protein synthesis (rough)
• lipid synthesis (smooth)

Question 12

Name the difference between the smooth and rough ER.

Answer 12

rough - ribosomes present
- secreted proteins on rER into lumen of rER

smooth - no ribosomes
- lipid/cholesterol synthesis occurs at sER

*the relative amount of rER and sER depends on the cell (do they want more proteins or lipids?)

Question 13

What are the two benefits from organization via membranes of the rough ER?

Answer 13

1) Facilitates protein modification (ex. glycosylation)
- only proteins in ER lumen will be modified (not cytoplasm)
- vesicles from rER are routed to golgi then fused into cell via exocytosis –> leaves the cell

2) facilitates protein secretion (ex. digestive enzyme)
- vesicles containing proteins in rER –> golgi –> fusion with cell membrane –> exocytosis

Question 14

Describe lipid synthesis in the smooth ER.

Answer 14

• HMG-CoA reductase: processing enzyme (target of statin drugs)
• enzymes combine precursors in stepwise fashion –> more lipid soluble at each step
• new lipids increase sER membrane surface