Cell

• A cell is the basic unit of all living things
• A cell is classified by having three different things:

  • A cell membrane (phospholipid bilayer)
  • A nucleus (houses genetic material in the form of chromatin)
  • Cytoplasm
• Eukaryotic cells have different organelles like ribosomes and mitochondria
• DNA replication occurs in the S phase of the cell cycle
• DNA replication involves opening the DNA to form a replication bubble and synthesizing new DNA based on nucleotides
• DNA replication is maintained by DNA polymerase enzymes (type 1 and type 3)
• DNA replication is very accurate, with few mistakes made
• Tumor suppressor genes and DNA repair genes ensure there are no errors in DNA replication
• G1 phase of the cell cycle:

  • Cells make more organelles
  • Cells synthesize proteins and enzymes
  • Cells repair thymidine dimers to prevent mistakes in DNA replication
• Labile cells (proliferative cells) constantly undergo cell cycle replication:

  • Epithelium of skin
  • GI tract
  • Urinary tract
  • Hematopoietic stem cells in red bone marrow
• Stable cells replicate when stimulated:

  • Liver (hepatocytes)
  • Kidney tubules
  • Alveolar cells of the lungs
• Permanent cells do not undergo mitosis:

  • Neurons
  • Skeletal muscle cells
  • Cardiac muscle cells
• During cell division, the total number of chromosomes in a cell doubles from 46 to 92
• The process of cell division involves two main phases: interphase and mitosis
• Interphase consists of three components: G1 phase, S phase, and G2 phase
• G1 phase is followed by the S phase, where DNA replication occurs
• Before entering the S phase, there is a G1/S checkpoint to ensure DNA quality
• G2 phase focuses on cell growth and ensuring the cell is ready to split into two equal cells
• Mitosis, also known as the M phase, consists of four main stages: prophase, metaphase, anaphase, and telophase
• Prophase involves condensing chromatin into visible chromosomes and dissolving the nuclear envelope
• Centrioles form the microtubule organization center to help separate chromosomes during prophase
• Metaphase is characterized by chromosomes aligning at the metaphase plate with polar microtubules connecting to kinetochores
• Anaphase sees sister chromatids separating and moving towards opposite poles of the cell
• Cytokinesis, a part of telophase, involves separating the cytoplasm equally to form two daughter cells
• Chromosomes are separated into sister chromatids
• There are a total of 92 chromosomes, with 46 needed to go to each end
• Motor proteins, such as dynein and kinesin, help move chromatids towards microtubules
• Cytokinesis involves the equal distribution of cytoplasm into two cells
• Chromatin becomes loose again in telophase
• Cells can enter a quiescent phase (G0) where they rest and do not replicate
• Telomeres shorten with age, leading to cell senescence
• Checkpoints in the cell cycle include G1/S, G2/M, and the metaphase checkpoint
• The nucleus is the brain of the cell and the center where all cell activities begin and revolve around
• The nucleus is composed of the nuclear envelope, which has an outer layer with ribosomes and an inner layer with lamins
• The nuclear envelope also contains nuclear pores for transport between the cytoplasm and nucleus
• The nucleolus is the site of rRNA synthesis, which combines with proteins to form ribosomes
• Chromatin, made up of DNA and histone proteins, is found within the nucleus and can be euchromatin (loose) or heterochromatin (tight)
• The nucleus functions in DNA replication, transcription, and the production of different types of RNA molecules (tRNA, mRNA, rRNA)
• The rough endoplasmic reticulum (ER) is a site of protein synthesis
• Ribosomes are found on the rough ER, which helps in protein synthesis