Basic Biology

The One Thing

Chromosomes organise DNA so that genetic information can be copied and distributed accurately. The cell cycle coordinates growth, DNA replication and cell division, with mitosis maintaining chromosome number and meiosis reducing it to produce genetically varied reproductive cells.

Reactions, Processes and Experiments

Key Terms

• Chromosome: A thread-like structure in the nucleus that contains DNA and genes.
• DNA: The genetic material that stores instructions for the structure and functioning of an organism.
• Gene: A specific segment of DNA that controls an inherited characteristic or helps produce a particular protein.
• Chromatin: A fine network of DNA and proteins present in the nucleus when the cell is not dividing.
• Histone: A protein around which DNA is coiled, helping DNA become compact and organised.
• Sister chromatids: Two identical copies of a chromosome formed after DNA replication and joined together.
• Centromere: The constricted region joining sister chromatids and helping spindle fibres attach.
• Telomere: The protective end region of a chromosome that helps prevent loss or joining of genetic material.
• Homologous chromosomes: A matching pair of chromosomes, one inherited from each parent, carrying genes for the same traits.
• Diploid number: The full chromosome number in a body cell, represented as 2n.
• Haploid number: Half the chromosome number, represented as n, found in gametes.
• Cell cycle: The sequence of events through which a cell grows, duplicates its genetic material and divides.
• Interphase: The preparation stage of the cell cycle, including growth, DNA replication and preparation for division.
• G1 phase: The first growth phase, during which the cell increases in size and carries out normal activities.
• S phase: The synthesis phase, during which the cell copies its DNA.
• G2 phase: The second growth phase, during which the cell prepares proteins and structures needed for division.
• Mitosis: A type of nuclear division producing two genetically identical nuclei with the same chromosome number as the parent cell.
• Cytokinesis: The division of the cytoplasm to form separate daughter cells.
• Prophase: The stage of mitosis in which chromosomes become visible, the nuclear membrane breaks down and spindle fibres form.
• Metaphase: The stage in which chromosomes arrange themselves at the equator of the cell.
• Anaphase: The stage in which sister chromatids separate and move to opposite poles.
• Telophase: The stage in which chromosomes reach the poles, become less distinct and new nuclear membranes form.
• Meiosis: A specialised type of cell division that produces four genetically different haploid cells from one diploid cell.
• Crossing over: The exchange of genetic material between homologous chromosomes during meiosis, creating variation.
• Spindle fibres: Thread-like structures that attach to chromosomes and move them during cell division.
• Chromosome number: The characteristic number of chromosomes found in the cells of a species.

Easily Confused

• Chromatin and chromosomes: Chromatin is the uncondensed DNA–protein network present when a cell is not dividing; chromosomes are condensed, visible structures formed during cell division.
• Sister chromatids and homologous chromosomes: Sister chromatids are identical copies of one chromosome joined at the centromere; homologous chromosomes are a matching pair, with one inherited from each parent.
• Mitosis and meiosis: Mitosis produces two genetically identical diploid daughter cells, whereas meiosis produces four genetically different haploid daughter cells.
• Mitosis and cytokinesis: Mitosis is nuclear division; cytokinesis is division of the cytoplasm.
• Meiosis I and meiosis II: Homologous chromosomes separate during meiosis I; sister chromatids separate during meiosis II.
• Diploid and haploid numbers: Diploid cells have the full chromosome number, represented as 2n; haploid cells have half the chromosome number, represented as n.
• Interphase and a resting stage: Interphase is an active preparation period involving growth, normal cell functions and DNA replication; it is not a resting stage.
• Chromosome number and DNA amount: DNA replication produces sister chromatids but does not itself alter the chromosome number; chromosome number is maintained in mitosis and reduced by half in meiosis.

What Gets Asked

• Describe chromosome structure: Questions may require the relationship between DNA, histones, chromatin, chromosomes, sister chromatids and the centromere. A common mark-losing error is failing to state that a duplicated chromosome consists of two sister chromatids joined at the centromere.
• Sequence the cell cycle: Questions may ask for G1, S, G2 and M phases and their functions. The specific error is treating interphase as a resting stage or placing DNA replication outside the S phase.
• Compare mitosis and meiosis: Answers must distinguish the equations, products and chromosome numbers: mitosis produces 2 genetically identical diploid cells, whereas meiosis produces 4 genetically different haploid cells.
• Identify events in division stages: Questions may describe chromosome behaviour and ask for the stage. The key distinctions are chromosome alignment at the equator in metaphase, sister-chromatid separation in mitotic anaphase, and homologous-chromosome separation in meiosis I.
• Explain the significance of chromosome number: Human body cells contain 46 chromosomes, or 23 pairs, whereas human gametes contain 23 chromosomes. A frequent error is stating that meiosis maintains 2n rather than reducing 2n to n.
• Explain variation and abnormal division: Questions may ask how crossing over and independent assortment produce variation, or how incorrect separation and uncontrolled cell division have consequences. The specific omissions are failing to link meiosis with genetic variation or failing to connect uncontrolled division with tumours and cancer.