Mitosis, meiosis and cell ageing

The cell cycle and mitosis

A dividing cell passes through the cell cycle, which has a long growth phase called interphase followed by division. During interphase the cell grows, copies its DNA and prepares the structures needed for division. Mitosis then separates the copied chromosomes so that each new cell receives a complete set, and cytokinesis divides the cytoplasm to produce two separate cells. Mitosis has four recognisable stages: prophase, when chromosomes condense and the spindle forms; metaphase, when chromosomes line up at the centre; anaphase, when the copies are pulled to opposite ends; and telophase, when two nuclei form. The result is two genetically identical diploid cells, meaning each has the full number of chromosomes. This is how a cut heals and how a growing child adds tissue.

For example: When you cut your skin, surviving cells around the wound divide by mitosis until the gap is closed, producing cells identical to those lost.

Meiosis and the formation of gametes

Meiosis is a special division that produces gametes. It involves two successive divisions after a single copying of DNA, so one starting cell produces four cells, each with half the usual number of chromosomes. These cells are haploid. The purpose is reproduction: if gametes carried the full set, fertilisation would double the chromosome number in every generation. Meiosis also creates genetic variation. During the first division, matching chromosomes can exchange pieces in a process called crossing over, and the pairs separate independently of one another, so each gamete receives a different combination. A gamete is a mature reproductive cell — a sperm or an egg. Meiosis occurs only in the testes and ovaries, whereas mitosis occurs throughout the body.

For example: Two children of the same parents differ because crossing over and independent assortment give each sperm and egg a unique combination of chromosomes.

Common misconception: Meiosis is not a faster mitosis. It has two divisions, halves the chromosome number and produces four genetically different cells; mitosis makes two identical cells.

Comparing mitosis and meiosis

The two processes can be compared along four lines. Purpose: mitosis maintains and repairs the body, while meiosis produces gametes for reproduction. Location: mitosis occurs in almost all body tissues, meiosis only in the reproductive organs. Number of divisions: mitosis has one division, meiosis has two. Result: mitosis gives two diploid cells that are genetically identical to the parent, while meiosis gives four haploid cells that are genetically different from one another. A simple way to remember is that mitosis copies, and meiosis halves. Both processes begin by copying DNA and both use a spindle to move chromosomes, which is why they are often studied together even though their purposes are opposite in effect.

For example: A skin cell dividing to close a wound is mitosis; a cell in the ovary producing an egg is meiosis. Same machinery, different purpose and outcome.

How cells age

Cells change with age in several ways. Each time most cells divide, the protective ends of their chromosomes, called telomeres, become slightly shorter; when they become very short, the cell stops dividing. Cells also accumulate damage to DNA, proteins and membranes from ordinary metabolism and from environmental exposures, and their repair systems become less efficient. Mitochondria produce less ATP and more damaging by-products, and the cell’s ability to respond to stress falls. Some cells enter a state in which they no longer divide but remain alive and release substances that affect nearby cells. Others die by apoptosis, a controlled self-destruction that removes damaged or unwanted cells without inflammation, in contrast to necrosis, which is unplanned death from injury and does cause inflammation.

For example: Skin and gut lining cells divide often and are replaced quickly, while most nerve and heart muscle cells divide rarely or not at all, so damage there is more permanent.

Common misconception: Ageing is not a disease, and it does not happen at the same rate in everyone or in every tissue. It reduces reserve and slows repair rather than following a fixed timetable.

From cell ageing to tissue ageing

The changes in individual cells combine to alter whole tissues. A tissue loses cells as division slows and death continues, and the cells that remain may function less efficiently. Connective tissue becomes less elastic and loses water, so skin sags and joints stiffen. Muscle tissue loses some fibres, reducing strength, and nervous tissue conducts and repairs more slowly. Because tissues depend on one another, the effects appear across systems: slower repair lengthens healing time, reduced reserve makes illness harder to tolerate, and altered connective tissue changes the shape and support of organs. Understanding cellular ageing is therefore the first step to understanding the tissue and system changes that appear later in this course and in clinical practice.

For example: A wound that heals in a week in a young adult may take considerably longer in an older adult, because division, repair and blood supply are all slower.