2.2 All cells arise from other cells

What is Stage 1 of the cell cycle called, and what three key events occur?

• Stage 1: Interphase.

• DNA replicates semi-conservatively in the S phase.

• The number of organelles increases.

• The volume of cytoplasm increases (due to protein synthesis in G1/G2).

What are Stage 2 and Stage 3 of the cell cycle, and what is their outcome?

• Stage 2: Mitosis – The nucleus divides.

• Stage 3: Cytokinesis – The cytoplasm and cell membrane divide.

• Outcome: Two new, genetically identical daughter cells.

What is the specific result of DNA replication in interphase?

• Each chromosome consists of two identical sister chromatids.

• These chromatids are joined at a centromere.

How do chromosomes change at the start of prophase?

• They condense, becoming shorter and thicker so they become visible.

• Each is seen as two sister chromatids joined by a centromere.

What happens to the nucleus and spindle apparatus in prophase?

• The nuclear envelope breaks down.

• Centrioles move to opposite poles of the cell.

• A spindle network (made of microtubules) begins to form.

What is the defining event of metaphase?

• Chromosomes align along the equator (middle) of the cell.

• Spindle fibres are attached to each chromosome at its centromere.

What causes the chromatids to separate in anaphase?

• The spindle fibres shorten / contract.

• This pulls the sister chromatids apart after the centromere divides.

• Chromatids are pulled to opposite poles.

What events 'reverse' the changes of prophase during telophase?

• Chromosomes uncoil, becoming longer and thinner.

• Nuclear envelopes reform around each set of chromosomes, forming two nuclei.

• The spindle fibres break down.

Summarise the key roles of spindle fibres in mitosis.

• Attach to chromosomes at the centromere (metaphase).

• Shorten / contract to pull chromatids apart to opposite poles (anaphase).

• Break down after separation is complete (telophase).

Why do some eukaryotic cells (e.g., neurons) not undergo the cell cycle?

• In multicellular organisms, not all cells retain the ability to divide.

• Only undifferentiated cells (e.g., stem cells) or those specified for growth/repair go through the cycle.

How does mitosis contribute to the growth of an organism?

• It increases the total cell number in a multicellular organism.

How does mitosis contribute to tissue repair?

• It produces new cells to replace damaged or dead cells.

How does mitosis facilitate asexual reproduction?

• It produces genetically identical offspring from a single parent organism.

What is the fundamental genetic cause of uncontrolled cell division?

• Mutations in the genes that control mitosis (e.g., proto-oncogenes or tumour suppressor genes).

What is a tumour?

• A mass of abnormal cells formed due to uncontrolled cell division.

What is the difference between a benign and a malignant tumour?

• Benign: Non-cancerous, does not invade other tissues or spread.

• Malignant: Cancerous, can metastasise (spread) to other parts of the body.

How do some drugs that disrupt spindle fibres prevent mitosis?

• They prevent chromosomes from attaching to the spindle by their centromere.

• This stops anaphase, as chromatids cannot be separated and pulled to opposite poles.

How do some drugs that prevent DNA replication slow mitosis?

• They stop DNA replication during the S phase of interphase.

• Without replication, chromosomes cannot form two sister chromatids, so mitosis cannot proceed.

Why are these treatments more effective against cancer cells, and what is a major side effect?

• They target rapidly dividing cells. Cancer cells divide uncontrollably.

• A side effect is that they also disrupt the cell cycle in rapidly dividing healthy cells (e.g., in hair follicles, bone marrow).

What is the first step in the replication of a prokaryotic cell by binary fission?

• Replication of the circular DNA molecule.

What other genetic material is replicated independently?

• Replication of plasmids (small, circular DNA).

What is the final step and the genetic outcome for the two daughter cells?

• Division of the cytoplasm (cytokinesis).

• Each daughter cell receives: one copy of the circular DNA and a variable number of plasmid copies.

Do viruses undergo cell division? Why or why not?

• No. Viruses are non-living and cannot replicate independently.

• They must use the machinery of a host cell.

What is the first, specific step in viral replication?

• Attachment proteins on the virus bind to complementary receptor proteins on the surface of a host cell.

How does the viral genetic material enter the host cell?

• The virus injects its nucleic acid (DNA or RNA) into the host cell's cytoplasm.

• The capsid (protein coat) often remains outside.

What is the first thing the infected host cell does with the viral genetic material?

• The host cell's machinery is hijacked to replicate the viral nucleic acid (making many copies).

What other viral components does the host cell produce?

• The host cell's ribosomes are used to synthesise viral proteins.

• This includes proteins for the capsid, enzymes, and attachment proteins.

What are the final two steps inside the host cell?

• Assembly: The new viral nucleic acid and proteins are assembled into complete virus particles.

• Release: The new virus particles are released from the host cell (often lysing/destroying it).