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Cell Division

Part of 4.1 Cell Biology.

Cell division explains how organisms grow, repair damage and replace worn-out cells. The central idea is that genetic information must be copied accurately before one cell becomes two.

Learning Objectives

ID Official specification wording Main teaching sections
4.1.2-lo-1 4.1.2.1 The nucleus of a cell contains chromosomes made of DNA molecules.
4.1.2.1 Each chromosome carries a large number of genes.
4.1.2.1 In body cells the chromosomes are normally found in pairs.		 Chromosomes
4.1.2-lo-2 4.1.2.2 Cells divide in a series of stages called the cell cycle. Students should be able to describe the stages of the cell cycle, including mitosis.
4.1.2.2 During the cell cycle the genetic material is doubled and then divided into two identical cells.
4.1.2.2 Before a cell can divide it needs to grow and increase the number of sub-cellular structures such as ribosomes and mitochondria. The DNA replicates to form two copies of each chromosome.
4.1.2.2 In mitosis one set of chromosomes is pulled to each end of the cell and the nucleus divides.
4.1.2.2 Finally the cytoplasm and cell membranes divide to form two identical cells.
4.1.2.2 Students need to understand the three overall stages of the cell cycle but do not need to know the different phases of the mitosis stage.
4.1.2.2 Cell division by mitosis is important in the growth and development of multicellular organisms.
4.1.2.2 Students should be able to recognise and describe situations in given contexts where mitosis is occurring.		 The Cell Cycle, Interphase, Mitosis and Cytokinesis
4.1.2-lo-3 4.1.2.3 A stem cell is an undifferentiated cell of an organism which is capable giving rise to many more cells of the same type, and from which certain other cells can arise from differentiation.
4.1.2.3 Students should be able to describe the function of stem cells in embryos, in adult animals and in the meristems in plants.
4.1.2.3 Stem cells from human embryos can be cloned and made to differentiate into most different types of human cells.
4.1.2.3 Stem cells from adult bone marrow can form many types of cells including blood cells.
4.1.2.3 Meristem tissue in plants can differentiate into any type of plant cell, throughout the life of the plant.
4.1.2.3 Knowledge and understanding of stem cell techniques are not required.
4.1.2.3 Treatment with stem cells may be able to help conditions such as diabetes and paralysis.
4.1.2.3 In therapeutic cloning an embryo is produced with the same genes as the patient. Stem cells from the embryo are not rejected by the patient’s body so they may be used for medical treatment.
4.1.2.3 The use of stem cells has potential risks such as transfer of viral infection, and some people have ethical or religious objections.
4.1.2.3 Stem cells from meristems in plants can be used to produce clones of plants quickly and economically.
4.1.2.3 • Rare species can be cloned to protect from extinction.
4.1.2.3 • Crop plants with special features such as disease resistance can be cloned to produce large numbers of identical plants for farmers.		 Stem Cells

Chromosomes

  • Chromosomes are long DNA molecules found in the nucleus. Genes are short sections of DNA carried on chromosomes.
  • Human body cells contain chromosomes in pairs, with one chromosome of each pair inherited from each parent.
  • Before a cell divides, the DNA must be copied so each new cell can receive a full set of genetic instructions.

The Cell Cycle

  • The cell cycle is a continuous process consisting of three main stages: interphase, mitosis and cytokinesis.
  • The length of the cell cycle varies between organisms and cell types. Cells that need frequent replacement, such as hair follicles, blood cells, skin cells and cells lining the digestive system, have very quick cell cycles.

Interphase

  • Interphase is the longest stage of the cell cycle during which the cell grows, replicates its DNA and produces more organelles.
  • DNA replication ensures that when the cell divides, each daughter cell receives a complete copy of the genetic instructions.
  • Organelle production is essential: mitochondria are made (needed for energy during growth), and ribosomes are made (needed for protein synthesis to support cell growth).
  • The cell membrane extends outward and the cytoplasm grows. The cell becomes larger and doubles its DNA content, preparing for division.

Mitosis and Cytokinesis

  • Mitosis is the process by which a cell divides its replicated DNA and produces two genetically identical daughter cells. It is divided into four stages: prophase, metaphase, anaphase and telophase.
  • Prophase: DNA condenses into visible chromosomes and the nuclear envelope breaks down.
  • Metaphase: chromosomes line up at the cell's equator (the middle).
  • Anaphase: sister chromatids separate and are pulled to opposite poles of the cell.
  • Telophase: the nuclear envelope reforms around each set of chromosomes at both poles, and two nuclei form.
  • Cytokinesis is the final stage where the cytoplasm and cell membrane divide. In animal cells a contractile ring pinches the cell in two. In plant cells a cell plate forms.
  • Mitosis produces two genetically identical daughter cells. This is important for growth in children and for repair and replacement in adults.
  • If the DNA is not copied accurately, new cells may not function properly, which is why control of the cell cycle matters.

Stem Cells

  • Stem cells are unspecialised cells that can divide by mitosis to form more stem cells, and then differentiate into specialised cell types.
  • For organisms to grow and repair damage, cells must divide (through mitosis) and differentiate. In plants, elongation also occurs at the root and shoot tips.
  • Embryonic stem cells are found in early embryos and are completely undifferentiated. They can become virtually any cell type and are pluripotent (can form many types).
  • Adult stem cells are found in limited locations such as bone marrow and teeth. They can only differentiate into a limited range of cell types and are multipotent (e.g., forming different blood cells).
  • Plant stem cells are found in meristems (growing regions) and can keep dividing and differentiating throughout the plant's life.
  • Stem cells offer possible medical benefits such as replacing damaged tissues and treating diseases, but their use raises ethical questions (especially regarding embryonic stem cells) and biological challenges (controlling their differentiation, ensuring safety).

Common Confusions

  • Mitosis vs. division: Mitosis is nuclear division; cytokinesis is cytoplasmic division. Mitosis must happen before the cell can physically split into two.
  • Prophase vs. metaphase: In prophase chromosomes condense and the nuclear envelope breaks down. In metaphase they line up at the cell's equator. These happen in sequence, not interchangeably.
  • Embryonic vs. adult stem cells: Embryonic stem cells are pluripotent (can become almost any cell); adult stem cells are multipotent (limited to certain cell types). This difference limits their medical use.
  • Sister chromatids separating: At anaphase, sister chromatids separate and move to opposite poles. Once separated, each is now called a chromosome.

Key Terms

  • Chromosome: a structure made of DNA that carries genes.
  • Gene: a short section of DNA that codes for a particular characteristic or protein.
  • Mitosis: cell division that produces two genetically identical daughter cells.
  • Cell cycle: the sequence of growth, DNA replication and division in a cell.
  • Interphase: the stage of the cell cycle during which the cell grows, replicates DNA and produces organelles.
  • Prophase: the stage of mitosis where chromosomes condense and the nuclear envelope breaks down.
  • Metaphase: the stage of mitosis where chromosomes line up at the cell's equator.
  • Anaphase: the stage of mitosis where sister chromatids separate and move to opposite poles.
  • Telophase: the stage of mitosis where nuclear envelopes reform around each set of chromosomes.
  • Cytokinesis: the final stage where the cytoplasm and cell membrane divide to form two separate cells.
  • Stem cell: an unspecialised cell that can divide and become specialised.
  • Pluripotent: able to differentiate into almost any cell type (describing embryonic stem cells).
  • Multipotent: able to differentiate into a limited range of cell types (describing adult stem cells).
  • Meristem: plant tissue containing cells that can keep dividing and differentiating.

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