Cell membranes, transport mechanisms, organelles, the cell cycle, and cell division.
During DNA replication each new strand must be built in the 5’→3′ direction. Because the two template strands run opposite ways, one can be copied continuously (the leading strand) while the other is copied in short sections (the lagging strand). This difference arises from the antiparallel nature of DNA and the requirement that DNA polymerases add nucleotides only to a free 3’‑OH.
Prokaryotic cells lack a true nucleus and membrane‑bound organelles, are typically 0.1–5 µm in diameter, and contain a single circular chromosome. Eukaryotic cells have a membrane‑surrounded nucleus, numerous organelles, and are usually 10–100 µm across. Both share a plasma membrane, cytoplasm, DNA, and ribosomes, but their structural organization underlies the major functional and evolutionary differences.
Mitosis and meiosis are the two fundamental ways eukaryotic cells divide. Mitosis copies a diploid genome once and splits it into two identical diploid daughters, while meiosis copies the genome once but divides twice to produce four genetically diverse haploid gametes. This guide explains their steps, key differences, and biological significance.
Substances cross the cell membrane by passive processes that follow concentration gradients or by active mechanisms that require energy. Understanding the structure of the membrane and the different transport proteins explains how cells control what enters and leaves.