Cell Structure and Function: Organelles, Biomolecules and Division
Cell Structure and Function covers cell theory, prokaryotic and eukaryotic organisation, the structure and function of organelles, biomolecules, and the cell cycle including mitosis and meiosis.
Core concepts
- Cell theory: Schleiden and Schwann proposed that all organisms are made of cells; Virchow added omnis cellula e cellula.
- Prokaryotes lack a membrane-bound nucleus and organelles; they have 70S ribosomes, a single circular DNA and often a cell wall.
- Fluid mosaic model (Singer and Nicolson, 1972): a phospholipid bilayer with embedded proteins, giving selective permeability.
- Mitochondria and chloroplasts are semi-autonomous, with their own 70S ribosomes and circular DNA; eukaryotic cytoplasmic ribosomes are 80S.
- A nucleosome is DNA wound around a histone octamer.
Cell cycle and division
- Interphase: G1, S (DNA doubles) and G2. Mitosis keeps the chromosome number constant; meiosis halves it and generates variation by crossing over in pachytene of prophase I.
Worked example
A human somatic cell has 46 chromosomes at G1. After S phase its DNA content doubles to 4C but the chromosome number stays 46. After mitosis each daughter cell has 46 chromosomes and 2C DNA. After meiosis four cells form, each with 23 chromosomes and 1C DNA.
Traps
- Chromosome number does not double in S phase; only DNA content does.
- Centromeres split in anaphase, not prophase.
- Meiosis II resembles mitosis but starts from haploid cells.
- Plant cytokinesis uses a cell plate; animal cells form a cleavage furrow.
- Lysosomes arise from the Golgi apparatus.