What you need to know
- 3.14The cell cycle and genetically identical daughter cells for growth and asexual reproduction.
By the end, you should be able to…
- Describe G1, S, G2 and division.
- Track chromosomeA long DNA molecule wound around histone proteins. After replication it is two sister chromatids joined at a centromere. and DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. through one cycle.
- Explain why replication must precede division.
Build your understanding
A cycle of growth, copying and division
During interphaseThe stage of the cell cycle between divisions, when the cell grows and its DNA replicates. a cell grows, carries out metabolism and prepares for division. In G1 it synthesises cellular components; in S phase it replicates DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code.; in G2 it continues preparation, including synthesis of proteins used in division. Interphase is active, not a resting stage.
mitosisNuclear division that produces two genetically identical nuclei with the same number of chromosomes as the parent. divides the nucleusThe organelle that contains the cell’s chromosomes, surrounded by a nuclear envelope with pores.. cytokinesisDivision of the cytoplasm to form two cells, after nuclear division. divides the cytoplasmEverything inside the cell surface membrane except the nucleus: the fluid cytosol and the organelles. to form daughter cells. Together with DNA replication, these processes allow each daughter cell to inherit a complete copy of the genetic information. Not all specialised cells continue cycling.
Count chromosomes by centromeres
Before replication, a chromosomeA long DNA molecule wound around histone proteins. After replication it is two sister chromatids joined at a centromere. has one DNA molecule. After S phase it consists of two sister chromatidOne of the two identical copies of a chromosome after DNA replication, joined to its sister at the centromere. joined at a centromere; each chromatid contains a DNA molecule. Replication doubles DNA content but does not double chromosome number or change a diploidHaving two sets of chromosomes (2n), one from each parent. cell into a tetraploid cell.
Count chromosomes by centromeres (continued)
A diploid speciesA group of organisms that can interbreed to produce fertile offspring. with 2n = 6 has six chromosomes in G1. After S phase it still has six chromosomes, now with twelve chromatids. Each daughter nucleus produced by normal mitosis receives six chromosomes. Once sister chromatids separate, each is counted as a daughter chromosome.
DNA amount and ploidy describe different things
Ploidy is the number of sets of homologous chromosomesA pair of chromosomes, one from each parent, with the same genes at the same loci but possibly different alleles.. DNA amount measures the quantity of DNA. Let C represent the DNA in one unreplicated haploidHaving one set of chromosomes (n), as in gametes. set: a diploid G1 nucleus is 2n, 2C; after replication it is 2n, 4C; each daughter nucleus after mitosis is 2n, 2C.
When reading a graph, check whether the measurement refers to a whole dividing cell, one nucleus or a completed daughter cell. During anaphaseThe stage of mitosis when centromeres divide and sister chromatids are pulled to opposite poles., before cytokinesis, the whole cell contains both future daughter chromosome sets.
Why make identical copies?
Growth requires an increase in cell number. Mitosis also replaces lost cells and supports repair. In asexual reproduction a single parent produces offspring without fusion of gameteA haploid sex cell, such as a sperm or an egg cell.; mitosis preserves its genetic information, except for changes such as new mutationA change in the sequence of bases in DNA.. DNA replication alone does not separate the copies into daughter cells.
NOT: replication does not double the chromosome number. Interphase is not resting.
- Count chromosomes by centromeres
- After S, a chromosome is two sister chromatids
- Mitosis divides the nucleus; cytokinesis divides the cytoplasm
- Not every specialised cell keeps cycling
Two traps this diagram blocks
- Do not say the cell becomes tetraploid when DNA replicates.
- Do not call interphase a resting stage.
From the diagram alone
Question
Explain why a cell can have twice as much DNA after S phase but the same chromosome number. [3]
Show a model answer
Each chromosome is copied, so the DNA amount doubles, but the two sister chromatids stay joined at one centromere. Chromosomes are counted by centromeres, so the number remains 2n until the chromatids separate.
| Stage | Ploidy per nucleus | DNA per nucleus |
|---|---|---|
| G1 | 2n | 2C |
| After S phase / G2 | 2n | 4C |
| Each daughter nucleus | 2n | 2C |
Make your reasoning visible
State when DNA is copied
Replication occurs in S phase of interphaseThe stage of the cell cycle between divisions, when the cell grows and its DNA replicates., before mitosisNuclear division that produces two genetically identical nuclei with the same number of chromosomes as the parent.. Do not place it in prophase.
Name the measured quantity
A doubled DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. mass does not by itself show doubled chromosomeA long DNA molecule wound around histone proteins. After replication it is two sister chromatids joined at a centromere. number or ploidy.
Key explanations
- Chromosome number stays 2n through S. DNA amount becomes 4C.
Avoid this misconception
The claim: Interphase cells are doing nothing.
The correction: They carry out metabolism, growth and, during S phase, DNA replication.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
A diploid cell has 10 chromosomes in G1. How many chromosomes and chromatids does it have after S phase?
Show suggested answer
10 chromosomes and 20 chromatids. The chromosome number is unchanged because sister chromatids remain joined.
Practice question
Retrieval and explanation · suggested answer
A G1 nucleus contains 8 pg of DNA. Give the amount after replication and in each daughter nucleus after mitosis.
Show suggested answer
16 pg after replication; 8 pg in each daughter nucleus.
Practice question
Retrieval and explanation · suggested answer
Why is DNA replication alone insufficient for growth by cell division?
Show suggested answer
The replicated copies must be separated into nuclei and the cytoplasm divided, producing additional cells with complete genetic information.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A graph doubles from 2C to 4C and later returns to 2C. Does this prove the cell underwent mitosis?
Show a hint
Ask what is measured and whether alternative divisions produce the same DNA change.
Explore the reasoning
No. A diploid mitotic cycle can do this, but meiosis I also reduces DNA per nucleus from 4C to 2C while changing ploidy. You need chromosome behaviour, ploidy or information about subsequent division.
Recall this lesson
Lesson recall questions
Question 1 of 5: State what is meant by the cell cycle.
Mitosis and the cell cycle
Practice question
Retrieval and explanation · suggested answer
State what is meant by the cell cycle.
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Discovering the controls
3B · The Cell CycleThe 2001 Medicine Nobel Prize recognised discoveries of key regulators of the cell cycle. This connects basic cell research with understanding how division is coordinated. Cyclins and other…Specification 3.14Read & use the scienceClose article
Why this matters
The 2001 Medicine Nobel Prize recognised discoveries of key regulators of the cell cycle. This connects basic cell research with understanding how division is coordinated. Cyclins and other regulators help order the stages of the cycle. The Nobel work shows that a cell does not divide simply because time has passed. DNA must be copied and cell events coordinated before division can produce two genetically identical daughters. Cancer researchers examine what happens when control breaks down, but the existence of a regulator alone does not diagnose a tumour. For exam answers, name the stage or process affected and explain the consequence for chromosome inheritance rather than writing only that a checkpoint failed.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.14 understand the role of mitosis and the cell cycle in producing genetically identical daughter cells for growth and asexual reproduction
Use the science
Predict the consequence of starting division before DNA replication is complete.
Explore a possible response
Daughter cells may inherit incomplete genetic information; correct order is essential.
Explore the source
- Nobel Prize: the 2001 Physiology or Medicine awardNobel Prize Outreach
When cell-cycle control is lost
3B · The Cell CycleCancer researchers study how cells escape the usual controls on division. The National Cancer Institute describes cancer as uncontrolled growth caused by changes in genes that govern cell…Specification 3.14Read & use the scienceClose article
Why this matters
Cancer researchers study how cells escape the usual controls on division. The National Cancer Institute describes cancer as uncontrolled growth caused by changes in genes that govern cell behaviour. The familiar cycle of DNA replication and mitosis is therefore relevant well beyond a root tip diagram. A tumour is not simply a collection of cells caught in mitosis: researchers must investigate what changed and whether the resulting daughter cells survive and keep dividing. This article gives the cell-cycle lesson a medical context without implying that every fast-growing tissue is cancer. Growth in a healing wound is coordinated and normally limited.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.14 understand the role of mitosis and the cell cycle in producing genetically identical daughter cells for growth and asexual reproduction
Use the science
Why does rapid cell division in a healing wound not by itself establish that the cells are cancerous?
Explore a possible response
Normal repair can increase division under regulatory control. Cancer requires evidence of abnormal, persistent loss of control, not just a high division rate.
Explore the source
- What is cancer?National Cancer Institute