What is the primary function of nuclear pores in the nuclear envelope?
ATo provide structural rigidity to the nuclear envelope
BTo actively regulate the passage of molecules between nucleus and cytoplasm
CTo anchor the nucleus to the cell membrane
DTo replicate DNA during cell division
Nuclear pores are not simple openings — they are elaborate protein complexes (nuclear pore complexes) that selectively regulate molecular traffic. Small molecules diffuse freely, but larger molecules like proteins and RNA require active, facilitated transport. This selectivity is essential for controlling which proteins enter the nucleus and which RNA transcripts exit.
Question 2 True / False
The nucleolus is a membrane-bound organelle located inside the nucleus that produces ribosomal RNA.
TTrue
FFalse
Answer: False
The nucleolus lacks a surrounding membrane, so it is not technically an organelle by the standard definition. It is a functional region that forms around clusters of ribosomal RNA genes and dissolves during cell division. Because it has no membrane, it assembles and disassembles dynamically rather than persisting as a stable enclosed compartment.
Question 3 Short Answer
Why does chromatin condense into tightly packed chromosomes during mitosis, even though condensed chromatin cannot be transcribed?
Think about your answer, then reveal below.
Model answer: During mitosis, gene expression is temporarily halted so the cell can accurately segregate chromosomes. Tight condensation makes each chromosome compact enough to be moved by spindle fibers without tangling, breaking, or misdistributing genetic material.
This question targets the apparent paradox that the cell silences its own genes during mitosis. The mechanical benefit — orderly chromosome segregation — outweighs the cost of pausing transcription. The relationship between chromatin compaction and transcriptional activity (loose euchromatin = active; condensed heterochromatin = silent) is a core theme running from the nucleus into the cell cycle.