Cell division

• Interphase is part of the cell cycle and consists of three phases: G phase, S phase, and another G phase
• During Interphase, the cell increases in mass and size, carries out normal cellular functions like synthesizing proteins, and replicates its DNA in preparation for mitosis
• In the G phase of Interphase, the cell grows and receives a signal to divide
• During the S phase of Interphase, the DNA in the nucleus replicates, resulting in each chromosome consisting of two identical sister chromatids
• The G phase between the S phase and the next cell division event involves further cell growth, error checking of newly synthesized DNA, and preparations for cell division
• The cell cycle consists of interphase, nuclear division (mitosis), and cell division (cytokinesis)
• Mitosis is the process of nuclear division by which two genetically identical daughter nuclei are produced, essential for growth, cell replacement, repair of tissues, and asexual reproduction
• Mitosis can be divided into four main stages: Prophase, Metaphase, Anaphase, and Telophase
• In Prophase of mitosis, chromosomes condense, centrosomes move towards opposite poles, spindle fibers emerge, and the nuclear envelope breaks down
• In Metaphase of mitosis, chromosomes line up at the equator of the spindle and spindle fibers attach to the centromeres of the chromosomes
• In Anaphase of mitosis, sister chromatids separate at the centromere and are pulled to opposite poles by the spindle fibers
• During interphase (S phase), the 46 DNA molecules in the parent cell replicate to form sister chromatids, resulting in 92 molecules, ensuring the two daughter cells will be diploid (with 46 chromosomes each) when mitosis occurs
• Cytokinesis is the final step in the cell cycle where the cytoplasm divides to form two genetically identical cells, occurring after the telophase stage of mitosis
• In animal cells, cytokinesis forms a 'cleavage furrow' to separate the daughter cells, while in plant cells, a 'cell plate' forms at the site of the metaphase plate and expands towards the cell wall of the parent cell, separating the daughter cells
• Recognizing the stages of mitosis from images:

  • Prophase: chromosomes are visible, nuclear envelope is breaking down
  • Metaphase: chromosomes are lined up along the middle of the cell
  • Anaphase: chromosomes move away from the middle towards opposite poles
  • Telophase: chromosomes arrive at opposite poles, begin to decondense, nuclear envelope reforms
• The mitotic index is the proportion of cells undergoing mitosis, calculated by dividing the number of cells with visible chromosomes by the total number of cells
• Calculating the size of root tip cells involves studying meristems, using the squash technique to prepare slides, and measuring individual cells undergoing mitosis
• Micrograph showing a stained root tip can be used to calculate the actual size of root tip cells using the magnification equation: actual size = size of image ÷ magnification
• The size of cells is typically measured using the micrometre (μm) scale, with cellular structures measured in either micrometers (μm) or nanometers (nm)
• When doing calculations, all measurements must be in the same units, and it is best to use the smallest unit of measurement shown in the question
• To convert units, multiply or divide depending on whether the units are increasing or decreasing
• There are 1000 nanometers (nm) in a micrometer (µm), 1000 micrometers (µm) in a millimeter (mm), and 1000 millimeters (mm) in a meter (m)
• Cancers arise due to uncontrolled mitosis, with cancerous cells dividing repeatedly and uncontrollably, forming a tumor
• Changes in genes controlling cell division lead to cancer, with mutations that cause cancer referred to as oncogenes
• Carcinogens, such as UV light and tar in tobacco smoke, may cause cancer, and tumors can be benign (not spreading) or malignant (spreading)
• Malignant tumors can interfere with the normal functioning of organs and tissues, and they can spread through the blood and lymphatic system, a process known as metastasis
• Viruses are non-cellular infectious particles that replicate by infecting living cells, using attachment proteins to bind to host cells and injecting their DNA or RNA for replication
• Viral particles are released from host cells either by the cell bursting open or through budding, where they take a section of the host cell membrane with them