Cell Cycle: Study Guide (Biology 4.5)

Why cells need a cycle

You started life as one cell. Now your body has trillions of them. Your skin and gut lining replace cells every day, and a cut heals by making new ones. Every one of these new cells needs a complete copy of your genome. If a cell divided without first copying its DNA, or split its chromosomes unevenly, the daughter cells would be missing genes.

The cell cycle is the ordered series of events by which a eukaryotic cell grows, copies its DNA and divides into two. The events always happen in the same order, and the cell checks its progress along the way (you will see how in Topic 4.6). The cycle has three main parts:

1. Interphase: growth and preparation, made of three phases called G1, S and G2.
2. Mitosis: division of the nucleus, in four steps.
3. Cytokinesis: division of the cytoplasm into two daughter cells.

Interphase: G1, S and G2

Interphase is not a resting stage. The cell is busy making molecules and, in S phase, copying all of its DNA. In a typical dividing cell, interphase takes up most of the cycle.

G1 (first gap). The cell is metabolically active. It grows, makes proteins and copies its organelles (such as mitochondria and ribosomes) and other parts of the cytoplasm. At this point each chromosome is a single DNA molecule, spread out as loose chromatin (DNA wrapped around proteins).

S (synthesis). The DNA, still in the form of chromatin, is replicated. Each chromosome is now two identical DNA molecules called sister chromatids, held together at a region called the centromere. The amount of DNA in the cell doubles, but the number of chromosomes does not change: a chromosome made of two sister chromatids still counts as one chromosome.

G2 (second gap). The cell makes the proteins it will need for division, such as the tubulin that builds spindle fibres. It produces ATP in large quantities to pay for the work ahead. The centrosome, the structure that organises the spindle, is duplicated: copying begins around S phase and is complete before mitosis, so the cell enters mitosis with two centrosomes.

G0: leaving the cycle

Not every cell keeps dividing. A cell can leave the cycle from G1 and enter a non-dividing state called G0. Most mature nerve cells stay in G0. A cell in G0 is alive and working, often very actively; it simply is not preparing to divide. Many G0 cells can re-enter the cycle when they receive the right cues. For example, most liver cells normally sit in G0, but if part of the liver is damaged or removed, many of them return to the cycle and divide until the lost tissue is replaced. Other non-dividing cells are held at a particular stage of the cycle until conditions change.

Mitosis: sharing the copies out

Mitosis divides the nucleus so that each new nucleus receives one copy of every chromosome. It gives each of the two daughter cells a full copy of the parent cell’s genome, so the daughters are genetically identical. Mitosis runs as a continuous process, but it is described in four steps.

Step	What you would see	Why it matters
Prophase	Chromatin coils up (condenses) into short, thick chromosomes, each made of two sister chromatids. The mitotic spindle, made of microtubules, starts to form. The two centrosomes move towards opposite poles. The nuclear envelope breaks down (some books call this last part prometaphase).	Compact chromosomes can be moved without tangling or breaking.
Metaphase	Spindle fibres attached at each centromere line the chromosomes up along the equator (middle) of the cell.	Every chromosome is attached to fibres from both poles before anything is pulled apart.
Anaphase	The sister chromatids of each chromosome separate. Spindle fibres shorten and pull the chromatids towards opposite poles, centromere first.	Each pole receives one copy of every chromosome.
Telophase	The spindle breaks down. A new nuclear envelope forms around each set of chromosomes, which start to uncoil. Cytokinesis usually begins.	Two nuclei with identical chromosome sets.

Cytokinesis: two new cells

Cytokinesis divides the cytoplasm and its organelles between the two new cells.

• In animal cells, a ring of protein filaments just under the plasma membrane contracts, pinching the cell in two. The groove you see is the cleavage furrow.
• In plant cells, the rigid cell wall cannot be pinched in. Instead, vesicles from the Golgi apparatus gather at the middle of the cell and fuse to form a cell plate. The plate grows outward until it joins the existing wall, making a new wall between the two daughter cells.

What mitosis is for

Because the daughter cells are genetically identical to the parent, mitosis is used whenever an organism needs more of the same cells:

• Growth: a seedling’s root tip or a child’s bones add cells.
• Tissue repair: new skin cells close a cut.
• Asexual reproduction: a strawberry plant’s runners or a yeast cell’s bud produce offspring that are clones of the parent.

Mitosis does not create variety, and it does not halve the chromosome number. Making gametes with half the chromosomes needs a different division, meiosis (Topic 5.1).

Worked example 1: mitotic index and stage timing

Question. A student stains a squash of a bean root tip and counts 500 cells in the region just behind the tip. The (fictional) results are below. The whole cell cycle in this region is known to take 20 hours.

Stage	Interphase	Prophase	Metaphase	Anaphase	Telophase
Number of cells	412	48	20	12	8

(a) Calculate the mitotic index (the percentage of cells in mitosis). (b) Estimate how long a cell spends in metaphase. (c) Describe what the data show about the length of interphase compared with mitosis.

(a) Mitotic index.

1. Cells in mitosis = 48 + 20 + 12 + 8 = 88.
2. Total cells = 412 + 88 = 500.
3. Mitotic index = (88 ÷ 500) × 100 = 17.6%.

(b) Time in metaphase. If cells are spread at random through the cycle, the fraction of cells in a stage matches the fraction of the cycle spent in that stage.

1. Fraction in metaphase = 20 ÷ 500 = 0.040.
2. Time = 0.040 × 20 h = 0.80 h.
3. Convert: 0.80 h × 60 min h⁻¹ = 48 minutes.

Check. Do the same for every stage: interphase 16.48 h, prophase 1.92 h, metaphase 0.80 h, anaphase 0.48 h, telophase 0.32 h. These add up to 20.00 h, the whole cycle.

(c) Describe the data. 412 of 500 cells (82.4%) are in interphase, so interphase takes about 16.5 h of the 20 h cycle, roughly 4.7 times as long as all of mitosis (3.52 h). Within mitosis, prophase is the longest step (48 cells) and telophase the shortest (8 cells).

Assumptions to state. The cells are dividing independently, not in step with each other; the counts are large enough to be representative; and stages were identified correctly (early prophase is easy to confuse with interphase). The method gives an estimate, not an exact time.

Worked example 2: chromosomes, chromatids and DNA content

Question. A body cell of a fictional animal has 8 chromosomes. In G1 the cell contains 6.4 pg of DNA (1 pg = 10⁻¹² g). Figure 3 shows the DNA content of one cell as it goes through the cycle.

(a) Complete a table of chromosomes, chromatids and DNA mass for G1, G2, anaphase and each daughter cell. (b) Calculate the mean rate of DNA synthesis during S phase. (c) Explain why the DNA content halves at 20 h.

(a) Counting. Count chromosomes by counting centromeres. A chromosome may be one chromatid (before S, or after separation in anaphase) or two sister chromatids joined at one centromere (after S).

Stage	Chromosomes	Chromatids (DNA molecules)	DNA per cell / pg
G1	8	8	6.4
G2 (and prophase, metaphase)	8	16	12.8
Anaphase (whole cell)	16	16	12.8
Each daughter cell after cytokinesis	8	8	6.4

In anaphase the sister chromatids have separated, so each one now counts as a chromosome. The cell briefly has 16 chromosomes, 8 heading to each pole.

(b) Rate. DNA rises from 6.4 pg to 12.8 pg during S phase, which lasts from 8 h to 14 h.

1. Change in DNA = 12.8 − 6.4 = 6.4 pg.
2. Time = 14 − 8 = 6 h.
3. Rate = 6.4 pg ÷ 6 h = 1.07 pg h⁻¹ (about 1.1 pg h⁻¹).

(c) Explanation. The DNA was copied in S phase and the two copies were shared out in mitosis, one complete set to each pole. At cytokinesis the cell splits, so each daughter cell gets one set: half of the parent’s 12.8 pg, which is 6.4 pg, the same as the parent had in G1. Nothing is lost; the copies are divided between two cells. The vertical drop is a simplification, because cytokinesis takes a short time.

Common misconceptions

• “Interphase is a resting phase.” The cell is working hard: growing, making proteins and ATP, copying organelles and replicating its DNA.
• “The chromosome number doubles in S phase.” The DNA doubles; the chromosome number does not. Each chromosome now has two sister chromatids joined at one centromere.
• “DNA is copied during prophase.” Replication happens in S phase. In prophase, already-copied chromosomes condense.
• “Chromatin and chromosomes are different substances.” Both are DNA with proteins. Chromatin is the loose form in interphase; chromosomes are the condensed form seen in mitosis.
• “Mitosis halves the chromosome number.” Each daughter cell has the same number of chromosomes as the parent. Halving happens in meiosis.
• “Plant cells divide with a cleavage furrow.” The cell wall prevents pinching. Plant cells build a cell plate from Golgi vesicles.
• “A cell in G0 is dead or will never divide again.” G0 cells are alive and functioning, and many can re-enter the cycle in response to the right cues.
• “A high mitotic index means mitosis is fast.” It means a large fraction of cells are in mitosis at that moment, which usually signals rapid division in that tissue. It does not tell you the speed of mitosis by itself.

Where this leads

The cell cycle does not run on its own: checkpoints and proteins called cyclins and cyclin-dependent kinases decide whether a cell moves on to the next stage, and failures can lead to cancer or programmed cell death. That is Topic 4.6, Regulation of Cell Cycle. In Topic 5.1 you will compare mitosis with meiosis. Test yourself with the practice questions, then use the revision notes and the checklist to consolidate.