Quick questions on The cell cycle, mitosis and binary fission: VCE Biology Unit 1

Prokaryotes have a single circular chromosome and no nucleus, so they reproduce by a simpler process called binary fission.

The cell cycle has two main parts: interphase (preparing for division) and the mitotic phase (dividing).

A mnemonic: Prophase, Metaphase, Anaphase, Telophase.

In animal cells. A ring of actin and myosin proteins beneath the plasma membrane contracts, forming a cleavage furrow that deepens around the cell's equator until the cell pinches into two daughter cells.

Two daughter cells, each genetically identical to the parent, each with the full diploid number of chromosomes (2n in humans, 46). Mitosis underpins growth, tissue repair, asexual reproduction in some organisms, and the maintenance of multicellular bodies.

The cell cycle is tightly regulated by checkpoints that pause the cycle to verify conditions before proceeding. The three major checkpoints:

Chromosomes condense and become visible under the microscope as pairs of sister chromatids. The nuclear envelope breaks down. The mitotic spindle starts to form from microtubules organised by centrioles (in animal cells) at opposite poles.

Spindle fibres attach to the centromere of each chromosome. Chromosomes line up along the cell's equator at the metaphase plate. This is the diagnostic image of metaphase.

Centromeres divide. The two sister chromatids of each chromosome are pulled apart and dragged to opposite poles by shortening spindle fibres. The cell now has two identical sets of chromosomes, one at each pole.

Chromosomes arrive at the poles and decondense back into chromatin. A new nuclear envelope re-forms around each set, and nucleoli reappear. The spindle dissolves.

A ring of actin and myosin proteins beneath the plasma membrane contracts, forming a cleavage furrow that deepens around the cell's equator until the cell pinches into two daughter cells.

The rigid cellulose cell wall prevents pinching. Instead, vesicles full of cell-wall material bud off the Golgi and gather at the cell's equator. They fuse to form a cell plate that grows outwards until it meets the existing cell wall, dividing the cell into two and laying down a new wall between them.

Before S phase: each chromosome is one chromatid. After S phase and through G2 and most of mitosis: each chromosome is two sister chromatids joined at the centromere. After anaphase: each chromatid is now its own chromosome.

Mitosis is only 10% of the cell cycle. Most of a cell's life is spent in interphase.

Binary fission is simpler: no spindle, no chromosomes condensing visibly, no PMAT phases. It is the prokaryotic equivalent.