Chapter 5: The Mitotic Cell Cycle

Chromosomes

Chromosomes are thread-like structures present in the nucleus, which carries genetic information from one generation to another. They play a vital role in cell division, heredity, variation, mutation, repair and regeneration.

  • Chromosomes are made of one very long, condensed DNA molecule associated with proteins (in eukaryotic cells)
    • The main proteins present are the large positively charged globular proteins called histones, their role is to organize and condense the DNA tightly so that it fits into the nucleus
    • The other proteins are enzymes used in copying and repairing the DNA
  • The tightly coiled combination of DNA and proteins is called chromatin – this is what chromatids, and therefore chromosomes, are made of from four subunits, each containing a pair of histone subtypes.
  • Structurally chromosomes are differentiated into three parts:
    • Pellicle
    • Matrix
    • Chromonemata

Interphase

  • Interphase consists of three phases:
    • G1 phase
    • S phase
    • G2 phase
  • During interphase, the cell grows (G1), replicates its DNA (S) and prepares for mitosis (G2). G1(Gap 1), in which the cell grows and functions normally. During this time, a high amount of protein synthesis occurs and the cell grows (to about double its original size) – more organelles are produced and the volume of the cytoplasm If the cell is not to divide again, it will enter G0.
  • Synthesis(S), in which the cell synthesizes its DNA and the amount of DNA is doubled but the number of chromosomes remains constant (via semiconservative replication).
  • G2(Gap 2), in which the cell resumes its growth in preparation for division. The mitochondria divide and the cell continues to grow until mitosis begins. In plants, chloroplasts also divide during G2.

Interphase = G1 + S + G2

Nuclear Division (Mitosis)

  • Follows Interphase
  • It is referred to as M phase. During the mitotic (M) phase, the cell divides its copied DNA and cytoplasm to make two new cells. M phase involves two distinct division-related processes: mitosis and cytokinesis. … Mitosis takes place in four stages: prophase (sometimes divided into early prophase and prometaphase), metaphase, anaphase, and telophase.

Cytokinesis

Cytokinesis is the physical process of cell division, which divides the cytoplasm of a parental cell into two daughter cells. In animal cells, cytokinesis involves constriction of the cytoplasm between the two nuclei and in plant cells a new cell wall is formed.

Significance of Mitosis

Mitosis is important for three main reasons: development and growth cell replacement and asexual reproduction.

  1. Development and Growth: It is responsible for growth and development of multicellular organisms. They help in repairing the damaged cells or regrowth of cells in cuts or wounds
  2. Cell Replacement: This occurs when the original cell is damaged or wounded. New cells are created to replace those that were damaged
  3. Asexual Reproduction: Single-celled organisms and certain multicellular organisms use mitosis for asexual reproduction. This includes reproduction by fragmentation, as in the case of planaria, and reproduction by budding, as in the case of sea anemones. Many plants reproduce using mitosis.

Significance of Telomeres

  • The ends of chromosomes are capped with special regions of DNA known as telomeres. Telomeres are needed to prevent the loss of genes from the ends of chromosomes during replication of DNA.
  • Telomeres therefore act as a ‘buffer’ regionof non-essential DNA and ensure that no important coding sections near the ends of the DNA molecules are left out of the replication process

Stem Cells

Cells produced by mitosis are undifferentiated which are called stem cells. A stem cell is a cell with the unique ability to develop into specialized cell types in the body. They serve as a repair system for the body.. They are essential for the growth of Zygote to adult and for cell replacement and tissue repair in an adult.

Cancers

Cancers are tumors resulting from repeated and uncontrolled mitosis. They are thought to start as the result of mutation.