Cell division and differentiation

Copying doubles the DNA but not the chromosome count. Each of the 46 chromosomes is now two sister chromatids joined at the centromere.

• Correct: 46 chromosomes and 92 chromatids:
• 92 chromosomes and 92 chromatids:
• 23 chromosomes and 46 chromatids:

Transcription factors bind specific DNA sequences near a gene and make RNA polymerase more or less likely to start copying it. The mix a cell holds decides which genes it expresses.

• It joins amino acids into a chain on the ribosome:
• Correct: It binds DNA near a gene and raises or lowers that gene's transcription:
• It removes introns from the first RNA copy:

Microtubules are hollow tubes of tubulin that grow from the centrosome and can lengthen or shorten quickly. Actin filaments are thinner, and intermediate filaments are the stable, ropelike ones.

• Intermediate filaments:
• Actin filaments:
• Correct: Microtubules:

S phase doubles the DNA, so G2 holds 12 pg. Mitosis and cytokinesis split it evenly, so each daughter returns to 6 pg, the same as the parent in G1.

• 6 pg in G2; 3 pg in each daughter: This skips S phase. The DNA doubles before division, so the daughters are not left with half.
• 12 pg in G2; 12 pg in each daughter: Division splits the doubled DNA between two cells. Each daughter receives half of the 12 pg.
• Correct: 12 pg in G2; 6 pg in each daughter: Correct. Doubled in S, then halved at division.
• 6 pg in G2; 6 pg in each daughter: The DNA is replicated in S phase, so G2 holds twice the G1 amount.

In anaphase the 46 pairs of sister chromatids separate, and each separated chromatid counts as a chromosome. The cell briefly holds 92 chromosomes, 46 moving to each end.

• 23: 23 is the count in an egg or sperm cell. Mitosis never halves the chromosome number.
• 46: 46 is the count through metaphase, when sister chromatids are still joined. Once they separate, each is its own chromosome.
• Correct: 92: Correct. 46 chromatid pairs become 92 separate chromosomes.
• 184: This doubles the count twice. The DNA is replicated only once per cycle.

The mitotic spindle is built from microtubules that must grow and shrink to capture and pull chromosomes. Frozen microtubules cannot attach and pull correctly, so the spindle checkpoint keeps sister chromatids together and holds the cell in mitosis, where many cells die.

• In G1, at the G1 checkpoint: The G1 checkpoint checks size, nutrients, signals and DNA damage. Microtubules are not needed to pass it.
• In S phase, partway through replicating its DNA molecules: DNA replication uses DNA polymerase and helicase, not microtubules.
• In G0, as resting cells: The drug acts on dividing cells. It does not send them into G0, and cells already in G0 are largely unaffected.
• Correct: In mitosis, before sister chromatids separate: Correct. The spindle cannot work, so the spindle checkpoint stops the cell in mitosis.

p53 senses DNA damage and responds by halting the cycle or, if the damage is too great, triggering apoptosis. Without p53, neither response happens, so the cell divides with its damaged DNA and its descendants inherit the resulting mutations. That is why TP53 is a tumor suppressor gene.

• It halts at the G1 checkpoint until repair: Halting at the G1 checkpoint in response to DNA damage depends on p53, which this cell lacks.
• It triggers apoptosis at once: Damage-triggered apoptosis also depends on p53. Without it, the cell does not self-destruct.
• Correct: It keeps dividing and passes the damage to its descendants: Correct. With no p53, damaged cells keep dividing, and mutations pile up.
• It enters G0 permanently: Leaving the cycle is not the default response to DNA damage. Without p53, the cell has no signal to stop.

In metaphase the chromosomes line up on the metaphase plate with sister chromatids still joined. They separate in anaphase, and the spindle checkpoint prevents that until every chromosome is attached.

• Prophase: chromatin condenses and the spindle forms from the centrosomes.: This step is right. Prophase condenses the chromosomes and builds the spindle.
• Correct: Metaphase: sister chromatids separate and move to opposite ends of the cell.: This is the error. Metaphase is when the chromosomes line up; separation is anaphase.
• Anaphase: kinetochore microtubules shorten, pulling chromatids apart.: This step is right. Shortening kinetochore microtubules pull the chromatids apart in anaphase.
• Telophase: nuclear envelopes re-form around each set, and the chromosomes loosen back into chromatin.: This step is right. Telophase rebuilds the two nuclei.

Cytokinesis depends on a ring of actin and myosin tightening to form the cleavage furrow. Mitosis itself uses spindle microtubules, so it finishes, leaving two nuclei in one undivided cell.

• Two normal daughter cells: Without the contractile ring, no cleavage furrow forms, so the cell does not split.
• Correct: One cell with two nuclei: Correct. Mitosis completes, but cytokinesis fails.
• One cell whose chromosomes never separated: Chromosomes are separated by spindle microtubules, which the drug does not affect.
• Two cells, each without a nucleus: The nuclei form normally in telophase. The problem is that the cytoplasm is not divided.

Differentiation works through gene expression. Each cell holds a different mix of transcription factors, which switch on different genes. The proteins made from those genes build microvilli in one cell and digestive enzymes in the other. The genome is the same.

• Each lost the genes the other one uses: Differentiated cells keep their whole genome; unused genes are switched off, not removed. Induced pluripotent stem cells show that the genes are still there.
• They carry different numbers of chromosomes: Both are somatic cells made by mitosis, so both carry 46 chromosomes.
• One took up extra genes from a neighboring cell: Cells do not pick up genes from their neighbors. Both carry the genome they inherited from the stem cell.
• Correct: They hold different transcription factors, so they express different genes: Correct. Same genome, different genes switched on.