All cells arise from other cells
The cell cycle and mitosis
Not all cells in a multicellular organism can divide. Eukaryotic cells that do divide pass through a cell cycle: they grow and copy their DNA during interphase, then separate the copied chromosomes into two nuclei during mitosis. Division of the cytoplasm, called cytokinesis, usually follows.
Interphase: growth and DNA replication
During interphase, the cell grows and replicates its DNA. After replication, each chromosome consists of two genetically identical copies called sister chromatids, joined at a centromere.
The DNA is not highly condensed during interphase, so individual chromosomes are not distinct under a light microscope.
Mitosis: separation into two nuclei
During mitosis, the sister chromatids separate so that each new nucleus receives one copy of each chromosome. The stages occur in order:
- Prophase: The chromosomes condense and become visible. The nuclear envelope breaks down, and a structure of fibres called the spindle forms.
- Metaphase: The chromosomes align at the cell equator, midway between its opposite poles. Spindle fibres attach to their centromeres.
- Anaphase: The sister chromatids separate at their centromeres. Spindle fibres move them to opposite poles of the cell.
- Telophase: The chromosomes reach the poles and decondense. A nuclear envelope forms around each set, producing two nuclei.
Cytokinesis: formation of daughter cells
Cytokinesis usually divides the cytoplasm after mitosis, producing two daughter cells. Each daughter cell receives the same genetic information as the parent cell: the DNA was copied during interphase, and one of each pair of identical sister chromatids went to each new nucleus.
Uncontrolled cell division and cancer
Mitosis produces new cells. Normally, cell division is controlled so that cells divide when appropriate. If this control is lost, cells can divide excessively, producing a mass of cells called a tumour.
A tumour is not necessarily cancer. Cancer is a malignant growth: its cells can invade the surrounding tissues. As its cells continue to divide, the cancer can grow.
Many cancer treatments aim to control the rate of cell division. By slowing or limiting division, they can limit the growth of the cancer.
Binary fission
Binary fission is a process by which one prokaryotic cell divides to form two daughter cells. Before the cell divides, it copies its DNA so that each daughter cell can receive a copy.
A prokaryotic cell has a main circular DNA molecule. It may also contain plasmids: small, separate rings of DNA. The main circular DNA is replicated, and any plasmids are also copied.
As the cell divides, the copies of the main circular DNA are distributed so that each daughter cell receives one copy. The cytoplasm then divides, producing two daughter cells. Plasmid copies can be distributed unevenly, so the daughter cells may receive different numbers of copies of the plasmids.
Viral replication in host cells
Viruses are non-living and do not reproduce by cell division. Instead, a virus produces new virus particles by using the machinery of a host cell.
Viral nucleic acid, which carries the virus’s genetic information, enters the host cell. The host cell’s machinery uses this information to produce viral components, including copies of the viral nucleic acid and viral proteins.
The viral components are then assembled into new virus particles. Each new particle contains viral nucleic acid enclosed in a protein coat.
Root-tip squash microscopy and mitotic index
A root-tip squash produces a thin layer of plant cells that can be examined with an optical microscope. The root tip is useful because it contains cells that are dividing. Staining makes their chromosomes visible, allowing cells at different stages of the cell cycle to be identified.
Preparing and examining the squash
The root tip is softened and stained, then placed under a coverslip. Gentle pressure on the coverslip spreads the cells into a thin layer, making individual cells easier to see.
The slide is first examined at low magnification to locate a suitable area where cells are clearly spread. The magnification is then increased so that chromosome arrangements can be distinguished.
Identifying cell-cycle stages
The appearance and position of the chromosomes help identify the stage of each cell:
- Interphase: the nucleus is visible, but individual condensed chromosomes cannot usually be distinguished.
- Prophase: chromosomes condense and become visible.
- Metaphase: chromosomes are arranged across the middle of the cell.
- Anaphase: separated chromatids move towards opposite ends of the cell.
- Telophase: chromosomes are at opposite ends, and two nuclei begin to form.
Interphase is part of the cell cycle, but it is not a stage of mitosis.
Calculating the mitotic index
The mitotic index is the proportion of observed cells that are undergoing mitosis:
Count cells in any stage of mitosis in the numerator. Include all observed cells, including those in interphase, in the total. For example, if 10 of 50 observed cells are in mitosis, the mitotic index is $$\frac{10}{50} = 0.2$$.
A larger mitotic index means that a greater proportion of the observed cells are in mitosis.
Calculating actual size from magnification
A cell in an image usually appears larger than it really is. Image size is the apparent size measured on the image; actual size is the corresponding size of the real cell.
Magnification tells us how many times larger the image is than the actual cell. For example, at a magnification of $$\times 400$$, a measured length on the image is 400 times the corresponding actual length. To find the actual size, divide the image size by the magnification.
Measure the part of the image whose actual size you want to find, such as the cell’s width. The answer will initially be in the same unit as that measurement. Image size and actual size must be expressed in consistent units when comparing them; you can convert units before or after calculating.
Calculating a cell’s actual width
A cell image is $$20\ \text{mm}$$ wide at a magnification of $$\times 400$$. Its actual width is:
Since $$1\ \text{mm} = 1000\ \mu\text{m}$$, the actual width is also $$50\ \mu\text{m}$$. The magnification has no unit: it is a ratio between two sizes.