The cell cycle and its phases
A baby grows from one cell into trillions by repeated cell division. Before each division the cell has to grow and copy its DNA, like photocopying a book before splitting it into two.
The cell cycle is the series of events by which a cell grows, duplicates its DNA and divides into two. It has two parts: interphase (the long preparation phase) and the M phase (mitosis, the division). A human cell in culture takes about 24 hours; a yeast cell about 90 minutes.
In the figure : Phases of the cell cycle in order.
Rule 1: Interphase: G1, S and G2 — In G1 the cell grows and does normal metabolism but does not copy its DNA. In the S phase DNA replicates: the amount of DNA doubles (2C to 4C), but the number of chromosomes stays the same. In G2 proteins are made for mitosis and the cell grows further. Interphase takes up the longest time, in a human cell more than 95 per cent of the cycle.
- Easy : A cell with 2C DNA in G1 has 4C DNA after S phase.
- Medium : Chromosome number is 2n before and after S phase, but the DNA content doubles.
- Exam-type : Q: In which phase does DNA replicate? A: S phase.
Rule 2: M phase and G0 — The M phase is the visible division, which starts with karyokinesis (division of the nucleus) and ends with cytokinesis (division of the cytoplasm). Some cells leave the cycle after G1 and enter a quiescent stage, G0, where they are metabolically active but do not divide, and may divide again when needed. Heart cells are one example.
- Easy : A mature nerve cell does not divide and stays in G0.
- Medium : An adult liver cell in G0 can start dividing again after an injury.
- Exam-type : Q: What is the G0 phase? A: A resting stage in which the cell is active but does not divide.
Interphase = G1 + S + G2. DNA doubles in S. Chromosome number is unchanged until anaphase.
Calling interphase a resting phase. The cell is very active in making DNA and proteins during interphase.
Memory clue: G1-S-G2-M — Grow, Synthesise, Grow again, Mitosis.
Spot the difference
| Phase | Main event | DNA content |
|---|---|---|
| G1 | Cell growth | 2C |
| S | DNA replication | 2C to 4C |
| G2 | Preparation for division | 4C |
| M | Division | 4C to 2C per cell |
Practice questions
- 1What are the two main parts of the cell cycle?
- 2During which phase does DNA replicate?
- 3What is karyokinesis?
- 4What is the G0 phase?
- 5How long does a human cell in culture take for one cycle?
- 6What is the DNA content of a cell in G2 compared with G1?
- 7A cell in G1 has 2C DNA and 2n chromosomes. What are these values at the end of S phase?
- 8Why is interphase not a resting phase?
Show answers (try them yourself first)
- 1Interphase and the M phase.
- 2The S phase.
- 3Division of the nucleus.
- 4A quiescent stage in which a cell does not divide but remains metabolically active.
- 5About 24 hours.
- 6Double.
- 74C DNA and still 2n chromosomes.
- 8Cell growth, DNA replication and protein synthesis occur during it.
Mitosis: prophase and metaphase
Before two friends can share a packet of sweets equally, they sort the sweets into pairs. During mitosis, a cell does the same with its chromosomes, first condensing them, then lining them up.
Mitosis is an equational division in which a cell produces two daughter cells with the same chromosome number as the parent. Its four stages are prophase, metaphase, anaphase and telophase.
In the figure : The first two stages of mitosis.
Rule 1: Prophase — Chromosome material condenses into chromosomes, each with two chromatids joined at the centromere. The centrosomes move to opposite poles and the spindle starts to form from microtubules. The Golgi complex, ER, nucleolus and nuclear envelope disappear by the end of prophase.
- Easy : Under the microscope, a prophase cell shows thread-like chromosomes.
- Medium : A cell in late prophase has no nucleolus.
- Exam-type : Q: What happens to the nuclear envelope in prophase? A: It breaks down by the end of prophase.
Rule 2: Metaphase — Chromosomes are fully condensed and most clearly visible. The spindle is complete, and the kinetochores of each chromosome are attached to spindle fibres. The chromosomes line up at the equator, called the metaphase plate. The plane of alignment is the equatorial plane. This is the best stage to count chromosomes and to study their shape.
- Easy : A karyotype is prepared from cells in metaphase.
- Medium : Each chromosome at metaphase is attached to fibres from both poles.
- Exam-type : Q: Where do chromosomes lie at metaphase? A: At the equator (metaphase plate).
Prophase: condense, Metaphase: align. Nuclear envelope and nucleolus vanish in prophase.
Saying that chromosomes separate in metaphase. They separate in anaphase.
Memory clue: P-M-A-T — Pack, Middle, Apart, Two nuclei.
Spot the difference
| Point | Prophase | Metaphase |
|---|---|---|
| Chromosomes | Condensing | Fully condensed at the equator |
| Nuclear envelope | Disappears | Absent |
| Spindle | Forming | Complete |
Practice questions
- 1What is meant by equational division?
- 2Name the four stages of mitosis.
- 3What disappears at the end of prophase?
- 4What is the metaphase plate?
- 5At which stage are chromosomes best seen and counted?
- 6What attaches the chromosomes to the spindle?
- 7How are the centrosomes involved in prophase?
- 8Why is metaphase useful in the study of chromosomes?
Show answers (try them yourself first)
- 1A division in which the chromosome number of the daughter cells is the same as in the parent cell.
- 2Prophase, metaphase, anaphase and telophase.
- 3The nucleolus and the nuclear envelope.
- 4The plane at the equator where chromosomes line up.
- 5Metaphase.
- 6The kinetochores on the centromeres.
- 7They move to opposite poles and organise the formation of the spindle.
- 8They are fully condensed and lie at one plane, so they are easy to see, count and photograph.
Mitosis: anaphase, telophase and cytokinesis
A rope tied to two sides of a tent pulls the pegs apart when you release it. In anaphase, the spindle fibres pull the two halves of each chromosome toward opposite poles.
In anaphase the centromeres split and sister chromatids move to opposite poles. In telophase two nuclei re-form. Cytokinesis divides the cytoplasm into two cells.
In the figure : Final stages of mitosis.
Rule 1: Anaphase and telophase — At the start of anaphase, the centromere of each chromosome splits, and each chromatid, now a chromosome, moves to its pole as the spindle fibres shorten. In telophase the chromosomes reach the poles and decondense, the nuclear envelope forms around each group, the nucleolus, Golgi and ER reappear.
- Easy : A cell in anaphase shows two groups moving apart, often V-shaped.
- Medium : In telophase a new nuclear envelope forms around each chromosome group.
- Exam-type : Q: What happens to the centromere at anaphase? A: It splits so that sister chromatids separate.
Rule 2: Cytokinesis — In an animal cell a furrow in the plasma membrane deepens and divides the cell into two. In a plant cell, the wall makes it hard to form a furrow, so a cell plate forms at the centre and grows outward, joining the existing wall; it later becomes the middle lamella. Sometimes karyokinesis occurs without cytokinesis and results in a multinucleate syncytium, as in the liquid endosperm of coconut.
- Easy : A pinched animal cell is dividing by furrowing.
- Medium : Coconut water has many free nuclei without cell walls.
- Exam-type : Q: How does a plant cell divide its cytoplasm? A: By forming a cell plate from the centre outward.
Animal cytokinesis: furrow from outside in. Plant cytokinesis: cell plate from inside out.
Believing plants also form a furrow. Plant cells make a cell plate.
Memory clue: A-Apart, T-Two — Anaphase = Apart; Telophase = Two nuclei.
Spot the difference
| Point | Animal cell | Plant cell |
|---|---|---|
| Cytokinesis by | Furrow | Cell plate |
| Direction | Outside in | Centre outward |
| Centrioles | Present | Absent |
Practice questions
- 1What happens to chromatids in anaphase?
- 2What reappears in telophase?
- 3What is cytokinesis?
- 4What is a cell plate?
- 5What is a syncytium?
- 6Name the stage in which chromosomes decondense.
- 7Why can't plant cells divide by furrowing?
- 8Give an example of karyokinesis without cytokinesis.
Show answers (try them yourself first)
- 1They separate and move to opposite poles.
- 2The nuclear envelope and nucleolus (also the Golgi complex and ER).
- 3Division of the cytoplasm after nuclear division.
- 4A structure that forms in the centre of a dividing plant cell and grows outward to make the new wall.
- 5A multinucleate cell formed when nuclear division is not followed by cytokinesis.
- 6Telophase.
- 7The rigid cell wall prevents the membrane from pinching inward.
- 8The liquid endosperm of coconut, which has many free nuclei.
Significance of mitosis
A cut on your finger heals in a few days because skin cells near the wound divide again and again. This repair work is done by mitosis.
Mitosis produces genetically identical diploid daughter cells. It is the basis of growth, repair and maintenance of chromosome number, and of asexual reproduction in many organisms.
In the figure : Main roles of mitosis.
Rule 1: Why cells divide by mitosis — Cell growth increases the nucleus-to-cytoplasm ratio; division restores the proper ratio, so cells stay small enough to work efficiently. Mitosis also replaces dead or damaged cells, such as the cells of skin, gut lining and blood.
- Easy : The lining of the gut is replaced regularly by new cells.
- Medium : A child grows because the number of cells increases by mitosis.
- Exam-type : Q: Name one use of mitosis in the human body. A: Growth and repair.
Rule 2: Mitosis in plants — Plants have meristematic cells at the apices of roots and shoots which divide by mitosis throughout life, allowing continuous growth. The daughter cells are identical to the parent, so the number of chromosomes stays the same in all cells.
- Easy : A root tip keeps growing because it has dividing cells.
- Medium : Onion root tip is used to see mitosis in the laboratory.
- Exam-type : Q: Why are cells of an onion root tip used for mitosis? A: They are meristematic and divide actively.
Mitosis: 1 parent cell → 2 identical daughter cells, same chromosome number (2n → 2n).
Saying that mitosis makes gametes. Gametes are made by meiosis.
Memory clue: G-R-M — Growth, Repair, Maintain chromosome number.
Spot the difference
| Use | Example | Cells |
|---|---|---|
| Growth | Baby to adult | All somatic cells |
| Repair | Wound healing | Skin cells |
| Plant growth | Root tip, shoot tip | Meristem |
Practice questions
- 1Name two functions of mitosis.
- 2What is the chromosome number of daughter cells formed by mitosis?
- 3Where does mitosis occur in plants continuously?
- 4Which cell division produces gametes?
- 5What happens to the nucleo-cytoplasmic ratio as a cell grows?
- 6Give one example of cells replaced by mitosis.
- 7Why are the daughter cells of mitosis genetically identical?
- 8A cell of a plant has 24 chromosomes. How many chromosomes will each daughter cell have after mitosis?
Show answers (try them yourself first)
- 1Growth and repair.
- 2The same as the parent cell.
- 3In meristems at the root and shoot tips.
- 4Meiosis.
- 5It falls, and mitosis restores it.
- 6Skin cells, gut lining cells or blood cells.
- 7DNA is copied exactly in the S phase and the sister chromatids are shared equally.
- 824.
Meiosis I: reduction division
A deck of cards is split into two halves to deal each player a hand. In meiosis, a cell with two full sets of chromosomes divides so that each sex cell gets only one set.
Meiosis is a division in which a diploid cell (2n) makes four haploid cells (n) in two successive divisions, with only one round of DNA replication. In meiosis I the homologous chromosomes separate (reductional division).
In the figure : Five sub-stages of prophase I.
Rule 1: Prophase I — It is long and has five sub-stages. Leptotene: chromosomes start to condense. Zygotene: homologous chromosomes pair (synapsis) forming bivalents with the synaptonemal complex. Pachytene: bivalents are visible as tetrads, and crossing over (exchange of segments between non-sister chromatids) happens at recombination nodules. Diplotene: the synaptonemal complex dissolves, and homologues separate except at the X-shaped chiasmata. Diakinesis: the chiasmata move to the ends (terminalisation), chromosomes are fully condensed, and the nuclear envelope and nucleolus disappear.
- Easy : A pair of homologous chromosomes pairs up in zygotene.
- Medium : Crossing over takes place in pachytene and chiasmata are visible in diplotene.
- Exam-type : Q: In which stage does crossing over occur? A: Pachytene.
Rule 2: Metaphase I to telophase I — In metaphase I the bivalents arrange on the equatorial plate, with each homologue facing one pole. In anaphase I the homologous chromosomes separate, while the sister chromatids remain together. In telophase I, the nuclear envelope and nucleolus reappear, and cytokinesis follows to form two cells; this is the dyad of cells. A short pause called interkinesis follows with no DNA replication.
- Easy : In anaphase I, whole chromosomes (each with two chromatids) move apart.
- Medium : After meiosis I, each daughter cell has n chromosomes, each still having two chromatids.
- Exam-type : Q: What separates in anaphase I? A: The homologous chromosomes.
Meiosis I separates homologues (2n → n); meiosis II separates sister chromatids.
Saying that sister chromatids separate in anaphase I. Homologues separate; chromatids separate in anaphase II.
Memory clue: L-Z-P-D-D — Lazy Zebras Pass Dull Days: Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis.
Spot the difference
| Sub-stage | Event | Visible feature |
|---|---|---|
| Zygotene | Pairing of homologues | Bivalent, synaptonemal complex |
| Pachytene | Crossing over | Tetrad, recombination nodule |
| Diplotene | Homologues start to separate | Chiasmata |
| Diakinesis | Terminalisation | Envelope breaks |
Practice questions
- 1What is meiosis?
- 2What is synapsis?
- 3What is a bivalent?
- 4In which stage does crossing over occur?
- 5What is a chiasma?
- 6What is interkinesis?
- 7Why is meiosis I called reductional division?
- 8A cell with 20 chromosomes enters meiosis. How many chromosomes and how many chromatids per cell are present after meiosis I?
Show answers (try them yourself first)
- 1A division that gives four haploid cells from a diploid cell.
- 2The pairing of homologous chromosomes during zygotene.
- 3A pair of synapsed homologous chromosomes.
- 4Pachytene of prophase I.
- 5The X-shaped point where non-sister chromatids have crossed over.
- 6The short stage between meiosis I and meiosis II, without DNA replication.
- 7Homologous chromosomes separate, so the chromosome number is halved from 2n to n.
- 810 chromosomes, each with two chromatids (20 chromatids).
Meiosis II and the significance of meiosis
The two halves of a pizza can be cut again to give four slices. Meiosis II cuts each of the two haploid cells again, producing four cells in all, each different from the others.
Meiosis II is like mitosis: the sister chromatids separate and four haploid cells form. Meiosis keeps the chromosome number constant over generations and creates variation by crossing over and random assortment.
In the figure : One diploid cell gives four haploid cells through meiosis.
Rule 1: Meiosis II — Prophase II has a short stage; chromosomes are not duplicated again. In metaphase II the chromosomes align at the equator of each cell. In anaphase II the centromere splits and chromatids move to the poles. In telophase II nuclear envelopes form and cytokinesis gives four haploid cells (a tetrad of cells).
- Easy : Meiosis II looks similar to mitosis under the microscope.
- Medium : After meiosis II, each of the four cells has n chromosomes, each with a single chromatid.
- Exam-type : Q: What separates in anaphase II? A: The sister chromatids.
Rule 2: Significance of meiosis — Meiosis gives gametes with n chromosomes. Fertilisation brings back 2n, so the chromosome number stays constant from generation to generation. Crossing over and random assortment of chromosomes give new combinations of genes, which is the raw material for evolution.
- Easy : A human sperm has 23 chromosomes and an egg has 23; the zygote has 46.
- Medium : Brothers and sisters differ because meiosis makes unique combinations of genes.
- Exam-type : Q: How does meiosis keep the chromosome number constant? A: It halves the number in gametes, and fertilisation restores it.
Meiosis: 1 cell → 4 cells; 2n → n; variation from crossing over. Mitosis: 1 cell → 2 cells; 2n → 2n.
Thinking that meiosis II halves the chromosome number again. The halving is in meiosis I only.
Memory clue: MI-reduces, MII-divides — Meiosis I reduces the number; Meiosis II separates chromatids.
Spot the difference
| Point | Mitosis | Meiosis |
|---|---|---|
| Divisions | One | Two |
| Daughter cells | 2, diploid | 4, haploid |
| Crossing over | No | Yes |
| Role | Growth, repair | Gametes, variation |
Practice questions
- 1How many cells are formed at the end of meiosis?
- 2What separates in anaphase II?
- 3What is the chromosome number in a human gamete?
- 4Name two sources of variation in meiosis.
- 5Which division is like mitosis?
- 6Why is meiosis necessary in sexually reproducing organisms?
- 7List three differences between mitosis and meiosis.
- 8A cell of an organism with 2n = 14 undergoes meiosis. What is the chromosome number of each resulting cell?
Show answers (try them yourself first)
- 1Four haploid cells.
- 2The sister chromatids.
- 323.
- 4Crossing over and random assortment of chromosomes.
- 5Meiosis II.
- 6It halves the chromosome number in gametes, so fertilisation restores the diploid number.
- 7Mitosis has one division and gives two diploid cells with no crossing over; meiosis has two divisions, gives four haploid cells and has crossing over.
- 87.