Difficulty: Introductory | Prerequisites: Basic understanding of cell structure (nucleus, membrane, cytoplasm). If those terms are unfamiliar, review your cell structure notes first.
The cell cycle is the sequence of events a cell goes through from one division to the next. It sits at the heart of all biology: every organism you will study, from bacteria to humans, depends on cells dividing reliably. This topic bridges your earlier work on cell structure (what a cell is made of) with later material on genetics and cancer (what happens when division goes wrong). You should already be comfortable with the difference between prokaryotes and eukaryotes, and know roughly what DNA is, before diving in.
Cells reproduce by dividing. Prokaryotes use binary fission (simple splitting), while eukaryotes use the cell cycle, which includes a long growth-and-preparation phase called interphase (G1, S, G2) followed by mitosis and cytokinesis. The whole point is to copy the DNA faithfully and distribute one complete set to each daughter cell.
Cell division
The process by which a parent cell splits into two or more daughter cells. In simple terms, this is how organisms grow, repair damage, and reproduce.
Binary fission
The method prokaryotes use to reproduce: the single circular chromosome replicates, and the cell pinches in two. Think of it as the simplest possible way to copy a cell.
Genome
The complete set of genetic information in an organism. In prokaryotes this is typically one DNA molecule; in eukaryotes it spans multiple chromosomes.
Chromosome
A DNA molecule packaged with proteins. In simple terms, chromosomes are the organised bundles your DNA gets wound into so it can be moved around during division.
Sister chromatids
The two identical copies of a single chromosome, joined at the centromere after DNA replication. Think of them as twin copies held together until the cell is ready to split them apart.
Centromere
The region of a duplicated chromosome where the two sister chromatids are most tightly attached. In simple terms, it is the pinch-point that holds the twin copies together.
Cohesin
Proteins that hold sister chromatids together along their length from S phase until anaphase.
Interphase
The portion of the cell cycle between one division and the next, comprising G1, S, and G2 phases. This is when the cell does most of its growing, working, and DNA copying.
G1 phase (first gap)
The phase of interphase in which the cell grows and carries out normal functions. Think of it as the cell's main working period before it commits to dividing.
S phase (synthesis)
The phase of interphase during which all the DNA is replicated. Each of the 46 chromosomes (in humans) is duplicated.
G2 phase (second gap)
The phase of interphase after DNA replication, during which the cell checks the copied DNA for errors and makes repairs before entering mitosis.
Somatic cells
Body cells (muscle, nerve, skin, etc.) that contain the diploid (2n) chromosome number and are produced through mitosis.
Gametes
Sex cells (sperm and egg) that contain the haploid (n) chromosome number and are produced through meiosis.
Diploid (2n)
Having two complete sets of chromosomes, one from each parent. All human somatic cells are diploid (2n = 46).
Haploid (n)
Having a single set of chromosomes. Human gametes are haploid (n = 23).
Prokaryotes have a single circular chromosome. Replication begins at the origin of replication and proceeds in both directions.
As the chromosome copies, the two daughter chromosomes move apart toward opposite ends of the cell.
The plasma membrane then pinches inward, dividing the cell into two identical daughter cells.
Binary fission is asexual reproduction: the offspring are exact genetic copies of the parent.
Key fact: prokaryotes evolved before eukaryotes. Mitosis probably evolved from binary fission, and some protists still have forms of division that sit between the two.
Eukaryotes depend on cell division for three things: reproduction (developing from a fertilised egg), growth, and repair (you lose roughly 50 million skin cells a day).
Because eukaryotes have multiple chromosomes, they need a more elaborate mechanism to ensure every daughter cell gets a complete set. That mechanism is mitosis.
Only cells that are actively dividing replicate their DNA. In multicellular organisms, these are mainly stem cells (in animals) and meristem cells (in plants).
Most mitotic divisions produce daughter cells with identical DNA to the parent cell.
Meiosis is a separate type of division that halves the chromosome number: daughter cells are haploid (n), whereas the parent cell was diploid (2n). Meiosis produces gametes.
The genome is the full set of an organism's DNA. In prokaryotes, this is usually a single circular DNA molecule. In eukaryotes, it is divided among several linear chromosomes.
DNA molecules are packaged into chromosomes with the help of histone proteins.
In sexually reproducing organisms, cells fall into two categories:
Somatic cells (body cells): contain the diploid chromosome number (2n), are produced by mitosis. Examples: muscle cells, nerve cells, stomach cells.
Gametes (sex cells): contain the haploid chromosome number (n), are produced by meiosis. Examples: sperm cells, egg cells.
The cell cycle has two broad parts: interphase (the long preparation period) and the mitotic phase (division itself). Interphase takes up roughly 90% of the cycle.
G1 phase (first gap)
The cell grows and carries out its normal functions.
Cellular contents, including organelles, are duplicated.
The cell is metabolically active and increasing in size.
This is when the cell decides whether to commit to division or exit the cycle into G0 (a resting state).
S phase (synthesis)
DNA replication occurs. Each of the 46 chromosomes (in humans) is duplicated.
By the end of S phase, every chromosome consists of two sister chromatids joined at the centromere.
Cohesin proteins begin holding the sister chromatids together.
G2 phase (second gap)
The cell checks the duplicated chromosomes for errors and carries out any needed repairs.
Additional growth and protein synthesis occur, preparing the cell for mitosis.
Organelles needed for division (e.g. the centrosome, which has already replicated) finish maturing.
Requirements for cell division (summary)
Receipt of a signal directing the cell to divide
DNA replication (S phase)
DNA distribution to daughter cells (mitosis)
Cytoplasmic division (cytokinesis)
Your body replaces about 50 million skin cells every day through mitotic cell division. Wound healing, bone marrow producing new blood cells, and the growth of an embryo from a single fertilised egg all depend on the cell cycle running correctly. When the cycle's checkpoints fail, uncontrolled division can lead to cancer, which is why so much medical research targets cell cycle regulation.
Students often think "mitosis" and "the cell cycle" are the same thing. They are not. Mitosis is only the division stage; interphase (G1, S, G2) makes up most of the cycle.
Students sometimes believe all cells are constantly dividing. Most mature cells in your body are in G0, a non-dividing resting state. Only specific populations (stem cells, gut lining, bone marrow) divide regularly.
"Sister chromatids" and "chromosomes" are frequently confused. After replication, a chromosome consists of two sister chromatids joined at the centromere. Once they separate in anaphase, each chromatid is called a chromosome again.
Binary fission is sometimes described as a type of mitosis. It is not. Binary fission is a simpler, prokaryote-specific process. Mitosis is exclusive to eukaryotes.
Be able to list and describe each phase of interphase (G1, S, G2) and state what happens in each.
Know the four requirements for cell division (signal, replication, distribution, cytokinesis).
Distinguish binary fission from mitosis: prokaryotes vs. eukaryotes, single chromosome vs. multiple.
Know the difference between somatic cells (diploid, produced by mitosis) and gametes (haploid, produced by meiosis).
Understand what sister chromatids are, when they form (S phase), what holds them together (cohesin at the centromere), and when they separate (anaphase).
True or false: Binary fission produces genetically unique offspring. (False, offspring are exact copies.)
Fill in the blank: DNA replication occurs during the ___ phase of interphase. (S phase)
True or false: Most cells in an adult human body are actively dividing at any given time. (False, most are in G0.)
Fill in the blank: After DNA replication, each chromosome consists of two ___ joined at the ___. (sister chromatids; centromere)
True or false: Gametes are diploid. (False, gametes are haploid.)
Q: What are the three functions of cell division in eukaryotes?
A: Reproduction (development from a fertilised egg), growth, and repair of damaged tissues.
Q: Describe binary fission in two to three sentences.
A: The prokaryote's single circular chromosome replicates starting at the origin of replication. The two copies move to opposite ends of the cell, and the plasma membrane pinches inward to divide the cell into two genetically identical daughter cells.
Q: What is the difference between a chromosome and a chromatid?
A: A chromosome is a single DNA molecule packaged with proteins. After replication (S phase), each chromosome consists of two sister chromatids joined at the centromere. Once the chromatids separate during anaphase, each is again called a chromosome.
Q: List the phases of interphase in order and state the main event of each.
A: G1 (cell growth and normal function), S (DNA synthesis, chromosome duplication), G2 (error-checking of duplicated DNA and final preparation for mitosis).
Q: A human somatic cell has 46 chromosomes. How many sister chromatids does it have at the end of S phase?
A: 92 sister chromatids (46 chromosomes, each made up of 2 sister chromatids).
This material connects directly to the upcoming unit on mitosis and cytokinesis (Part 2 of these notes), where you will learn what happens after interphase ends. It also lays the groundwork for understanding cancer biology, because cancer is fundamentally a failure of cell cycle control. Later in the course, meiosis will build on the concepts of chromosome duplication and sister chromatids introduced here, extending them to explain genetic variation.
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