How does meiosis create genetic variation, and how do stem cells build organised tissues?
2.1.6 Cell division, diversity and organisation: how meiosis produces haploid gametes and generates genetic variation through crossing over and independent assortment; the meaning and potential of stem cells (totipotent, pluripotent and multipotent); cell specialisation and the organisation of cells into tissues, organs and organ systems.
A focused answer to the OCR H420 2.1.6 dot point on meiosis, stem cells and cell organisation. Covers meiosis and how it generates variation, the potency of stem cells, their uses and ethics, and the organisation of cells into tissues, organs and systems.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
OCR wants you to describe how meiosis produces haploid gametes and generates variation, distinguish the types of stem cell by potency and source, outline their uses and the ethical issues, and explain how specialised cells are organised into tissues, organs and organ systems.
The answer
Meiosis and genetic variation
Meiosis is the reduction division that produces haploid gametes from a diploid cell. One DNA replication is followed by two divisions (meiosis I and meiosis II), so four haploid cells are formed, each with half the chromosome number. Restoring the diploid number at fertilisation depends on this halving.
Meiosis generates genetic variation in three ways:
- Crossing over (prophase I): homologous chromosomes pair as bivalents and exchange sections of chromatid at chiasmata, producing new allele combinations on a chromosome.
- Independent assortment (metaphase I and II): homologous pairs (then chromatids) line up randomly on the equator, so maternal and paternal chromosomes are distributed independently to the gametes.
- Random fertilisation then combines any two gametes, multiplying the variation further.
Contrast meiosis with mitosis: mitosis gives two genetically identical diploid cells for growth and repair; meiosis gives four genetically varied haploid cells for sexual reproduction.
Stem cells and potency
A stem cell is an unspecialised cell that can keep dividing and can differentiate into specialised cells. Potency describes how many cell types it can become:
- Totipotent: any cell type, including extra-embryonic (placental) tissue; the zygote and very early embryo.
- Pluripotent: any body cell type but not extra-embryonic tissue; embryonic stem cells of the blastocyst.
- Multipotent: a limited range; adult stem cells such as bone-marrow cells that form blood cells.
- Unipotent: only one cell type, for example cardiomyocytes from heart-tissue stem cells.
Stem cells are used (and researched) to treat conditions such as leukaemia (bone-marrow transplants), type 1 diabetes, Parkinson's disease and burns. Sources include adult tissue, umbilical cord blood, embryos and induced pluripotent stem cells (iPS cells) reprogrammed from adult cells. The ethical debate centres on the use of embryos, which are destroyed to harvest embryonic stem cells; iPS cells offer a way to obtain pluripotent cells without embryos.
Cell specialisation and organisation
Cells differentiate by expressing only part of their genome, so different genes are switched on in different cells, giving each a structure suited to its function (for example red blood cells lose their nucleus to carry more haemoglobin; root hair cells have a large surface area for water uptake). Specialised cells are then organised hierarchically:
- Tissue: a group of similar cells working together (for example squamous epithelium, xylem).
- Organ: different tissues working together for a function (for example the heart, a leaf).
- Organ system: organs working together (for example the circulatory system).
Examples in context
Example 1. Bone-marrow transplants. Multipotent stem cells in donated bone marrow can replace a patient's blood-forming cells after their own are destroyed (for example in leukaemia treatment), a real clinical use of adult stem cells.
Example 2. Down syndrome and non-disjunction. If homologous chromosomes fail to separate in meiosis (non-disjunction), a gamete gains an extra chromosome, which can produce trisomy 21, linking meiotic errors to inherited conditions.
Try this
Q1. State two ways meiosis increases genetic variation. [2 marks]
- Cue. Crossing over in prophase I and independent assortment in metaphase I (random fertilisation also acceptable).
Q2. Explain the difference between a pluripotent and a multipotent stem cell. [2 marks]
- Cue. Pluripotent cells can become any body cell type; multipotent cells can become only a limited range of cell types.
Q3. Put these in order of increasing organisation: organ, cell, organ system, tissue. [1 mark]
- Cue. Cell, tissue, organ, organ system.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H420/02 20194 marksExplain how meiosis produces genetically different gametes.Show worked answer →
Two named sources of variation, each with a mechanism (about 2 marks each).
- Crossing over (in prophase I): homologous chromosomes pair up and exchange sections of chromatid at points called chiasmata, producing new combinations of alleles on a chromosome.
- Independent assortment (in metaphase I): homologous pairs line up randomly on the equator, so the way maternal and paternal chromosomes are distributed to gametes varies from cell to cell. (Independent assortment of chromatids also occurs in metaphase II.)
Both create new allele combinations. Together with random fertilisation, this is why offspring differ.
Markers reward naming crossing over and independent assortment and correctly describing each mechanism.
OCR H420/02 20223 marksDistinguish between totipotent, pluripotent and multipotent stem cells, giving an example of where each is found.Show worked answer →
One mark per type, each with potency and a source.
- Totipotent
- can differentiate into any cell type, including extra-embryonic tissue (placenta); found in the very early embryo (zygote and first few divisions).
- Pluripotent
- can differentiate into any body cell type but not extra-embryonic tissue; found in the embryonic stem cells of the blastocyst.
- Multipotent (and unipotent)
- can differentiate into a limited range of cell types; found in adult tissues, for example bone marrow stem cells that form blood cells.
Markers reward the correct range of potency and a valid example for each.
Related dot points
- 2.1.6 Cell division: the cell cycle and its regulation by checkpoints; the main stages of mitosis (prophase, metaphase, anaphase and telophase) and cytokinesis; the significance of mitosis in growth, repair and asexual reproduction; the calculation and use of the mitotic index.
A focused answer to the OCR H420 2.1.6 dot point on the cell cycle and mitosis. Covers interphase and checkpoints, the four stages of mitosis and cytokinesis, the significance of mitosis, the link to cancer, and the mitotic-index calculation.
- 6.1.2 Patterns of inheritance: monohybrid and dihybrid crosses; the inheritance of codominant and multiple alleles, sex linkage and epistasis; the use of genetic diagrams to predict phenotypic ratios; and the chi-squared test to compare observed and expected results.
A focused answer to the OCR H420 6.1.2 dot point on patterns of inheritance. Covers monohybrid and dihybrid crosses, codominance and multiple alleles, sex linkage and epistasis, genetic diagrams and phenotypic ratios, and the chi-squared test.
- 2.1.1 Cell structure: the ultrastructure of eukaryotic and prokaryotic cells, the function of organelles including the role of the rough endoplasmic reticulum and Golgi apparatus in producing and secreting proteins; the use, calibration and resolution of light and electron microscopes.
A focused answer to the OCR H420 2.1.1 dot point on cell structure and microscopy. Covers every required eukaryotic and prokaryotic organelle, the protein secretory pathway, the three microscopes, eyepiece-graticule calibration and the magnification equation.
- 2.1.3 Nucleotides and nucleic acids: the semi-conservative replication of DNA and the roles of DNA helicase, DNA polymerase and the complementary base pairing rule; the nature of the genetic code as a triplet code that is degenerate and non-overlapping; the roles of mRNA and tRNA in protein synthesis.
A focused answer to the OCR H420 2.1.3 dot point on DNA replication and the genetic code. Covers semi-conservative replication, the roles of DNA helicase and DNA polymerase, the Meselson-Stahl evidence, and the triplet, degenerate, non-overlapping code with transcription and translation.
- 6.1.4 Cloning and biotechnology: natural and artificial cloning of plants (including micropropagation and tissue culture) and animals; the use of microorganisms in biotechnology and the conditions in an industrial fermenter; the principles and advantages of using immobilised enzymes; and the asepsis and growth curve of a microbial culture.
A focused answer to the OCR H420 6.1.4 dot point on cloning and biotechnology. Covers natural and artificial cloning of plants and animals, micropropagation and tissue culture, the use of microorganisms and the conditions in an industrial fermenter, immobilised enzymes, and the microbial growth curve.
Sources & how we know this
- OCR A Level Biology A (H420) Specification — OCR (2023)