All cells arise from other cells

• Viruses replicate inside living cells, making it difficult to kill them without killing host cells.
• The cell cycle is the cycle of division with intermediate growth periods, including interphase, mitosis or meiosis (nuclear division), and cytokinesis (cytoplasmic division).
• After differentiation, some types of cell in multicellular organisms (e.g. neurons) no longer have the ability to divide.
• The cell cycle includes growth period between divisions; mitosis is only 10% of the cycle & refers only to nuclear division.
• During interphase, the cell synthesises proteins for replication, such as tubulin for spindle fibres, and its size doubles.
• During G1, the cell synthesises proteins for replication, such as tubulin for spindle fibres, and its size doubles.
• During S, DNA replicates, forming chromosomes consisting of 2 sister chromatids joined at a centromere.
• During G2, organelles divide.
• The purpose of mitosis is to produce 2 genetically identical daughter cells for growth, cell replacement/ tissue repair, and asexual reproduction.
• The stages of mitosis are prophase, metaphase, anaphase, and telophase.
• During prophase, chromosomes condense, becoming visible as X-shaped structures with 2 sister chromatids joined at a centromere.
• Mutation to tumour suppressor genes and proto-oncogenes can cause cancer.
• Tumour suppressor genes are those that do not produce a protein needed to slow the cell cycle.
• Proto-oncogenes are genes that form permanently-activated oncogenes.
• Disruption to the cell cycle can lead to uncontrolled cell division and tumour formation.
• Cancer treatments aim to control the rate of cell division by disrupting the cell cycle.
• Prokaryotic cells replicate through a process called binary fission, where one DNA loop replicates and both copies stay attached to the cell membrane, and plasmids replicate in the cytoplasm.
• Bacteria can undergo exponential growth within 8 hours if binary fission occurs once every 20 minutes and there is 1 bacterium at the start.
• Viruses are classified as non-living because they are acellular, meaning they have no cytoplasm, no metabolism and cannot self-replicate.
• Viruses replicate by attaching to receptors on host cell membrane, fusing with cell membrane or moving in via endocytosis and releasing DNA/RNA into cytoplasm, and the host cell uses viral genetic information to synthesise new viral proteins/nucleic acid.
• Components of new viral particle assemble.
• New viral particles leave the host cell by budding off and using the cell membrane to form an envelope, or causing the lysis of the host cell.
• Centrioles move to opposite poles of the cell (animal cells) and mitotic spindle fibres form during prophase.
• During metaphase, sister chromatids line up at the cell equator, attached to the mitotic spindle by their centromeres.
• Telophase requires energy from ATP hydrolysis.
• During telophase, spindle fibres contract, centromeres divide, and sister chromatids separate into two distinct chromosomes and are pulled to opposite poles of the cell, forming 'V' shapes facing each other.
• After telophase, chromosomes decondense, becoming invisible again, and new nuclear envelopes form around each set of chromosomes, resulting in two new nuclei, each with one copy of each chromosome.
• A root tip squash experiment involves preparing a temporary mount of root tissue, focusing an optical microscope on the slide, counting the total number of cells in the field of view and the number of cells in a stage of mitosis, calculating the mitotic index (proportion of cells undergoing mitosis), preparing a temporary mount of root tissue, placing the root in hydrochloric acid to halt cell division and hydrolyse the middle lamella, staining the root tip with a dye that binds to chromosomes, macerating the tissue in water using a mounted needle, using the mounted needle at 45° to press dow
• Two dyes that bind to chromosomes are toluidine blue (blue) and acetic orcein (purple-red).
• The root tip is used when calculating a mitotic index because meristematic cells at the root tip are actively undergoing mitosis, while cells further from the root tip are elongating rather than dividing.
• Tumour suppressor genes are genes that code for proteins to trigger apoptosis (programmed death of damaged cells)/ slow cell cycle (e.g. p53 acts between G1 & S in interphase so damaged DNA cannot replicate).
• Proto-oncogenes are genes that code for proteins to stimulate cell cycle to progress from one stage to the next.
• During anaphase, the chromosomes separate and move towards the poles of the cell.
• Cytokinesis is the cytoplasmic division that follows mitosis.