Cell Division and Theory

• Cells Beget Cells describes the cell division

Rudolf Virchow (1821-1902)

• Proposed the third tenet of the cell theory that cells come from preexisting cells
• Was a trained German Physician

Cell Division

• A parent cell normally gives rise to two daughter cells
• A daughter cell ends up with a complete set of identical hereditary material from each parent cell and half of its cytoplasm.
• All cells (bacteria, plants, animals, and humans) undergo cell division

Unicellular Organisms

• Made of a single-cell such as bacteria, amoeba, paramecium, and yeast
• Binary Fission is a method of asexual reproduction commonly seen in prokaryotic organisms

Multicellular Organisms

• Composed of multiple cells that work together
• Cells are specialized and organized into tissues organs and systems

Cellular reproduction

• A way of production of new tissues or body parts during growth and development
• Replaces worn-out and damaged cells
• Produces new plants or animals in asexually reproducing organisms

Cell Differentiation

• Daughter cells may grow and divide again
• Cells may be specialized for specific functions, such as:
  • fighting infection (white blood cells)
  • contraction (muscle cells)
  • impulse conduction (nerve cells)
  • digestive enzyme secretion (stomach or intestine cells)

Cell Categories

• Permanently differentiated cells are differentiated cells that never divide again like cells in the heart and brain
• Stem cells have the ability to perform self-renewal and differentiate into a variety of cell types

Organelles Involved in Cell Division

• The nucleus contains the cell's genetic material (DNA, or Deoxyribonucleic Acid)
• Cell division starts in the nucleus
• Chromatin fibers are the genetic material of the cell that is found inside the nucleus
• The nucleus is responsible for regulating cell division by also controlling the processes of mitosis or meiosis
• Chromosomes are condensed structures of DNA and proteins that carry genetic information
  • Humans have an estimated 25,000 genes contained mostly in the nucleus of 46 chromosomes.
  • Chromosomes are essential for accurate distribution of genetic material to daughter cells
  • Chromosomes have a centromere, a region containing kinetochore
• Centrioles are paired organelles found in the cytoplasm near the nucleus and become visible only to take part in cell division
  • Found in animal cells and play a critical role in the formation of the mitotic spindle during cell division
  • During cell division duplicates itself and helps in organizing the movement of chromosomes
• Spindle Apparatus is composed of microtubules and other proteins that help separate chromosomes during cell division
  • Are formed from microtubules
  • Facilitates chromosome movement to opposite poles of the cell
• Nuclear membrane disintegrates during cell division
• Cytoplasm is the fluid-filled region where organelles are suspended.
  • Provides the necessary environment for cell division to occur.
• The plasma membrane plays a role in cytokinesis (the physical separation of the cytoplasm) during cell division

Summary of the Process:

• Chromosomes get copied and get ready to split
• Centrosomes organize the spindle fibers to pull the chromosomes apart
• The nucleus disappears to allow chromosome separation
• Cytoskeleton aids in physically separating the cell into two
• Either a cleavage furrow or cell plate forms to divide the cell's contents

Stages of Cellular Reproduction

• Cellular reproduction is the process by which cells divide to create new cells
  • Essential for growth, repair, and reproduction in living organisms and occurs in 2 stages:

1. Cellular growth and maturation
   • The cell undergoes processes to prepare for division including:
     • growth to normal size
     • assembly of organelles and cellular components
     • energy accumulation
     • replication of genetic material (DNA)
2. Cell division

• The genetic material (chromosomes) is divided equally into two daughter cells and organelles, and cytoplasmic components are also distributed

Cell Growth and Division in the Human Body

• Cells increase in number, not size as the body grows
• Adult human cells are similar in size to young human cells, but adults have more cells

Limits of Cell Growth

• Cells do not grow indefinitely due to two main factors:

1. As the cell is larger, it becomes more difficult for its DNA to handle the demands on the cell.
   • DNA stores the genetic information needed for growth (DNA stands for Deoxyribonucleic Acid)
   • A larger cell would demand more from the fixed amount of DNA
   • If cells grow too large, DNA cannot keep up with the needs of the cell
   • If a cell grows, it does not make extra copies of DNA
2. As the cell is larger, it becomes harder to move nutrients and waste across its membrane
   • The size of a cell affects how efficiently it can transport nutrients and remove waste
   • Smaller cells have a higher surface area-to-volume ratio, allowing for faster exchange of materials
   • Larger cells struggle to move substances efficiently, leading to slower cell functions.
   • Before the cell becomes too large, a growing cell divides, forming two identical daughter cells

Factors that Limit Cell Size

1. Surface Area-to-Volume Ratio
   • Surface area determines how much material can enter or leave the cell
   • Volume determines how much material the cell needs
   • As a cell grows, its volume increases faster than its surface area, making nutrient and waste exchange inefficient
2. Essential nutrients such as (oxygen, water, food) and waste move through the cell membrane
   • In larger cells, diffusion takes longer, making it difficult to transport materials effectively
3. DNA Limitations
   • DNA controls all cell activities, but the amount of DNA remains the same even if the cell grows.
   • A larger cell places higher demands on its DNA, which can slow down cell functions

Cell Division

• Cells divide instead of growing too large, to maintain an efficient surface area-to-volume ratio, to ensure they can transport nutrients, eliminate waste, and function properly

Molecular Biologists

• Study molecules present in the make up of living things plus their interactions and genetic information and how it affects structure and functions of the body

The Cell Cycle

• Stage 1: G1 (Gap 1) Phase
  • Longest phase in most cells
  • Cell is young at this stage and growing until it attains its normal size via forming organelles and producing proteins such as the enzymes needed in S phase
  • Cells that do not divide remains at this stage throughout their life cycle (muscle and nerve cells do not divide)
• Stage 2: S (Synthesis) Phase
  • DNA duplicates through a process called replication
  • At the end of this stage, each chromosome is made up of sister chromatids attached to the centromere
• Stage 3: G2 (Gap 2) Phase
  • The preparation stage for cell division
  • Proteins such as microtubules (help cause the chromosomes to move during cell division) will assemble

Stages of Mitosis

• Stage 4: M (Mitosis/Meiosis) Phase
  • Where the cell undergoes division
  • Division of the nucleus is called karyokinesis; division of the cytoplasm is called cytokinesis

Mitosis

• A type of cell division in which the nucleus divides into two nuclei with identical genetic material
  • The resulting two daughter cells have the same number of chromosomes similar to the parent cell
  • Only occurs in eukaryotes
  • Used by multicellular organisms for growth and development, for repair of tissues, and in asexual reproduction
• A diploid parent cell contains two sets of chromosomes (maternal and paternal chromosome sets)
  • In a normal human body cell containing 46 chromosomes, mitosis produces 2 genetically identical daughter cells with 46 chromosomes

Mitosis consists of 4 stages

1. Prophase
   • Chromosomes condense, the nuclear membrane breaks down, and spindle fibers begin to form
2. Metaphase
   • The chromosomes align at the center (metaphase plate) as spindle fibers attach to centromeres
3. Anaphase
   • Sister chromatids are pulled apart toward opposite poles of the cell
4. Telophase
   • Nuclear membranes start to reform, and chromosomes begin to decondense

Cytokinesis

• The cytoplasm divides, fully splitting the cell into 2 identical daughter cells
• Cleavage furrow formation forms during cytokinesis (the final step of cell division in animal cells)
• Cell plate formation occurs during cytokinesis in plant cells and other cells with cell walls; cytokinesis starts at the middle of the cell and moves outward.

Explore cell theory, division, and function. Test your knowledge of cellular processes, including mitosis, stem cells, and prokaryotic reproduction. Learn about the roles of the nucleus, chromosomes, and spindle apparatus in cell division.