有丝分裂细胞周期

A chromosome 染色体 is one very long molecule of DNA wound tightly around special proteins 蛋白质 called histones 组蛋白. Winding the DNA like this lets a huge length fit inside the nucleus and keeps it tidy.

Before a cell divides, its DNA is copied (this copying is called replication 复制). After copying, each chromosome is made of two identical copies joined together. These two copies are the sister chromatids 姐妹染色单体, and they are held together at a point called the centromere 着丝粒. The tips of each chromosome are capped by telomeres 端粒, which protect the ends.

Mitosis 有丝分裂 is a type of nuclear division that makes two daughter cells 子细胞 that are genetically identical — they carry exactly the same genes 基因 as the parent cell and as each other.

This matters for:

• growth of multicellular 多细胞 organisms 生物体 (making more cells).
• replacement of damaged or dead cells.
• repair of tissues 组织 by making new cells.
• asexual reproduction 无性生殖 (one parent makes identical offspring).

The cell cycle 细胞周期 is the full life of a cell from one division to the next. It has three parts:

1. interphase 间期 — the longest part. The cell grows in the G₁ phase, copies its DNA in the S phase (replication), and grows again and prepares to divide in the G₂ phase.
2. mitosis — the nucleus divides into two identical nuclei.
3. cytokinesis 胞质分裂 — the rest of the cell splits, giving two separate daughter cells.

The mitotic index

The mitotic index is the fraction of cells in a sample that are in mitosis. You find it by counting cells under a microscope:

Worked example. In a root-tip sample, $8$ of the $50$ cells seen are in mitosis. Find the mitotic index.

A high mitotic index means many cells are dividing quickly — normal in a growing root tip, but in adult animal tissue it can be a warning sign of uncontrolled growth.

When DNA is replicated, the copying cannot reach the very end of the molecule, so a little is lost each time. Telomeres are short, repeated lengths of DNA at the ends that carry no genes. Because the telomeres are shortened instead, no important genes are lost during replication.

A stem cell 干细胞 is an unspecialised cell that can keep dividing by mitosis and can differentiate 分化 (change) into different specialised cell types. Stem cells are the source of new cells for replacing lost cells and repairing tissues.

The cell cycle is normally tightly controlled, so cells divide only when needed. If this control is lost, a cell may divide again and again without stopping. This uncontrolled division produces a lump of cells called a tumour 肿瘤.

Mitosis runs through four stages. You should be able to recognise them in photomicrographs and slides.

Cytokinesis then follows. In an animal cell the cell surface membrane pinches inwards to split the cell; in a plant cell a new wall forms across the middle. The result is two genetically identical daughter cells.

Common mistakes.

• Saying DNA is copied in prophase. It is copied earlier, in the S phase of interphase; by prophase each chromosome already has two chromatids.
• Calling interphase a "resting" phase. The cell is very active: it grows, makes proteins and organelles, and replicates its DNA.
• Saying the chromosome number doubles in the S phase. The amount of DNA doubles, but the number of chromosomes stays the same: each chromosome now has two chromatids joined at a centromere.
• Putting the splitting of the centromeres in metaphase. In metaphase the chromosomes only line up; the centromeres split at the start of anaphase.
• Counting a cell as "in mitosis" for the mitotic index just because its nucleus is visible. Count only cells whose chromosomes are visible as threads (prophase to telophase).

• Learn the stages in order (prophase, metaphase, anaphase, telophase) with one key event each; interphase (G1, S, G2) is not part of mitosis.
• Mitosis gives two genetically identical diploid cells — for growth, repair and asexual reproduction.
• Mitotic index $=$ cells in mitosis $\div$ total cells; a high value means rapid division.
• Link uncontrolled mitosis to tumours and telomere shortening to the limit on the number of divisions.