Cells from Cells: Why Cells Divide and the Cell Cycle

1An unbroken line of cells

A human life begins as a single cell. A few years later that life is a child made of an enormous number of cells: skin, bone, nerve, muscle, blood. None of those cells appeared out of thin air. Every one of them descends from that first cell through a long chain of divisions.

This observation is one of the Big Ideas in the BC Science 9 curriculum: cells are derived from cells. A new cell never forms from non-living material in an ordinary body or an ordinary pond. It always comes from a pre-existing cell that divides. The same is true across the whole history of life: every living cell alive today is the latest link in a chain that reaches back through countless earlier cells. (For a refresher on cell parts, see What is a cell?)

This first lesson sets the stage. It explains why cells must divide, what has to be copied before they do, and how a cell organizes its life into an orderly repeating pattern called the cell cycle. The next lesson follows the dividing step itself (mitosis) in detail.

2Why cells divide instead of just growing bigger

If a cell can grow, why not simply let a single cell grow until it is as large as a whole animal? Size is a trap. A cell takes in nutrients and oxygen and gets rid of wastes across its outer surface, the cell membrane. But the cell must serve everything inside it, which is a matter of volume. As a cell grows, its volume increases faster than its surface area. The surface can no longer keep up with the needs of the interior.

OpenStax Biology describes exactly this problem: as a cell grows larger, it becomes less efficient because its surface-to-volume ratio drops. Dividing into two smaller cells restores a more favourable ratio.

The four jobs of cell division

• Growth. A baby becomes an adult mainly by adding cells.
• Repair. After a scrape, nearby skin cells divide to fill the gap.
• Replacement. Many cells wear out and need to be replaced regularly, so division continues throughout life.
• Reproduction. In many single-celled organisms, one cell dividing into two is reproduction (covered in the lesson on asexual reproduction).

3What must be copied: genomes, genes and chromosomes

A dividing cell cannot simply be cut in half. Each new cell needs a complete set of instructions. These instructions are stored in DNA (see What is DNA?). A cell's complete set of DNA is its genome (see What is a genome?).

DNA is organized into units called chromosomes. A gene is a segment of a chromosome that codes for a specific protein, and genes are the functional units that influence characteristics such as hair colour. The different forms a characteristic can take (blonde, brown or black hair) are called traits.

Chromosomes in different organisms

Plasmids are not essential for normal growth, but bacteria can pass them to one another. This is one way a trait such as antibiotic resistance can spread through a bacterial population.

Diploid and haploid cells

Human body cells (also called somatic cells) contain 46 chromosomes. These are arranged as 23 homologous pairs. Homologous chromosomes have the same length and the same genes in the same places. One chromosome in each pair came from the mother and one from the father. A cell with two sets of chromosomes is diploid, written 2n.

Human sex cells (gametes: sperm and eggs) contain only 23 chromosomes, one from each pair. A cell with a single set is haploid, written n (or 1n). At fertilization, a sperm and an egg each contribute one set, restoring the diploid number in the new individual.

Two chromosomes in a homologous pair carry the same genes, but they may carry different versions of those genes. Different versions of a gene are called alleles. ABO blood type is an example: it is determined by the combination of two alleles, such as AA, AO or BO (see What are blood types?).

4Packing a very long molecule into a tiny space

A human cell's DNA, if stretched out, would be about two metres long, yet only about 2 nanometres wide. It must fit inside a microscopic nucleus, and it still must remain accessible so that genes can be read. The solution is packing in levels.

1. Wrapping. DNA winds around cores of proteins called histones, forming beads called nucleosomes connected by short stretches of linker DNA. The result is called chromatin, and this step shortens the DNA about sevenfold.
2. Coiling. The beaded string coils into a thicker fibre, shortening the DNA about 50-fold compared with its extended form.
3. Looping and condensing. Additional proteins pack the fibre further. In cells that are not dividing, each chromosome occupies its own region of the nucleus. During division, packing is at its most extreme, which gives the short, thick, rod-shaped chromosomes seen under a light microscope.

Copied chromosomes: sister chromatids

Before a cell divides, every chromosome is copied in a process called DNA replication. After copying, each chromosome consists of two identical halves called sister chromatids. They are held together by proteins called cohesins, and the grip is tightest at a pinched region called the centromere. The joined pair is still counted as one chromosome. Later, when the cell divides, the sister chromatids are pulled apart and each becomes a separate chromosome of one of the two new cells.

5The cell cycle: an orderly routine

The cell cycle is the ordered series of events from the formation of a cell until it divides into two genetically identical daughter cells. It has two major parts: interphase (preparation) and the mitotic phase (division).

Interphase: the long preparation

Interphase is not a resting period. It is the busiest part of a cell's life, and for most cells it takes up the majority of the cycle. It has three stages.

• G1 (first gap). Little changes visibly, but the cell is biochemically active. It builds the materials needed to copy DNA and stores energy for the job. Cells that are not going to divide again may leave the cycle here.
• S (synthesis). DNA is copied. Every chromosome becomes a pair of sister chromatids. The centrosome, the structure that organizes the spindle (the fibres that move chromosomes), is also duplicated. Animal cells have a pair of centrioles inside the centrosome; plants and most fungi lack centrioles.
• G2 (second gap). The cell restores its energy reserves and makes proteins needed for moving chromosomes. Some organelles are duplicated, and the cell may grow a little more.

The mitotic phase

The mitotic phase (M phase) has two parts. Mitosis divides the nucleus so that each new nucleus receives one complete set of chromosomes. Cytokinesis divides the cytoplasm so that two separate cells result. These are the subject of the next lesson.

G0: stepping out of the cycle

Some cells leave the cycle and enter an inactive state called G0. Cells may enter G0 temporarily, for example when nutrients are scarce or growth signals are missing, and return to G1 later. Others, such as mature heart muscle cells and nerve cells, remain in G0 permanently.

6Checkpoints: quality control for division

Copying and splitting DNA is high-stakes work. A cell that divides with damaged or incomplete DNA would pass the problem on to all of its descendants. To prevent this, the cell cycle has checkpoints, points where the cell pauses until certain conditions are met.

If a problem is found, the cell can pause and repair the damage, or at the G1 checkpoint exit to G0. The cell also responds to outside signals. Growth signals such as hormones can encourage division, and crowding by neighbouring cells can discourage it.

Inside the cell, regulatory proteins called cyclins rise and fall in amount at set points in the cycle, and they work with enzymes called Cdks to push the cell forward. A protein called p53 acts as a guardian at G1: it responds to DNA damage by halting the cycle and calling in repair enzymes, or by triggering the cell to self-destruct if repair fails. The details of these proteins go beyond Science 9, but the principle matters: division is controlled, not automatic. The consequences when control is lost are explored in the mitosis lesson.

Investigating the cell cycle: a data-handling example

How can anyone know how long a cell spends in each stage when cells under a microscope are just frozen snapshots? One classic approach used in OpenStax lab activities counts cells instead of timing them. A prepared slide of a rapidly dividing tissue (such as whitefish blastula) shows many cells at different stages. The percentage of cells found in each stage estimates the percentage of the cycle that stage occupies.

This kind of task connects to the curricular competencies in BC Science 9: gathering and analyzing data, noticing patterns, and evaluating how reliable a method is. Questions that can drive an investigation include: Do cells in a growing root tip divide more often than cells in older parts of the root? Do cells divide at the same rate in all parts of an organism?

Connecting the pieces

This lesson established the foundation for the rest of the biology strand.

• Cells come only from cells, so new cells are produced by division of existing cells.
• Division is needed because large cells are inefficient (surface area to volume), and for growth, repair, replacement and reproduction.
• DNA is packaged into chromosomes, and chromosomes must be copied (S phase) before division, producing sister chromatids.
• The cell cycle runs from G1 through S and G2 (interphase) to mitosis and cytokinesis, with checkpoints that guard against errors.
• Body cells are diploid (2n); sex cells are haploid (n). Producing haploid cells needs a different kind of division, meiosis, covered later.

?Quick check

Try each question first, then reveal the answer.