CELL DIVISON

Reasons for Cell Division

• Cell division is required for:
  • a) growth
  • b) repair & replacement of damaged parts
  • c) reproduction of the species

In Cell Division Copies of the DNA Must Be Sent to Both New Cells

• Since the instructions for making cell parts are encoded in the DNA, each new cell must get a complete set of the DNA molecules
• This requires that the DNA be copied (replicated, duplicated) before cell division

Genetic Blueprints for Cells Are Organized Into Chromosomes

• The plans for making cells are coded in DNA
• DNA is organized into giant molecules called chromosomes
  • Each chromosome is a single DNA molecule containing many genes
  • Each gene gives the directions for making 1 protein
  • In humans each chromosome has approximately 2000 genes
• Chromosomes have distinct parts
  • Centromeres:
    • Hold duplicated chromosomes together before they are separated in mitosis
    • Kinetochore proteins bind to centromere and attach chromosome to spindle in mitosis
  • Telomeres: ends of chromosomes: important in cell aging
• DNA in chromosomes is associated with proteins
  • Proteins strengthen DNA fiber
  • Package chromosomes when they condense
  • Control activity of genes
• Humans body cells have 23 pairs of chromosomes (46 total)
  • Diploid = pair of each chromosome = 46 total
  • The members of a chromosome pair are called homologues
  • One of each pair came from mother, the other from father
• Human reproductive cells (sperms & eggs) have 23 single chromosomes
  • Haploid = single copy of each chromosome = 23 total
• One of each pair came from the father and the other came from the mother

Genetic Instructions are Organized Into Genes

• A section of DNA which codes for a protein is called a gene
  • “One gene, one enzyme”
• We have approximately 50,000 genes (approx. 2000 per chromosome)
• Most of DNA in chromosome (~95%) is “junk” DNA- function not known

Before a Cell Can Divide it Must Duplicate its Chromosomes

• To make a new cell the old cell must duplicate all its parts
• Duplication takes place in interphase
  • DNA (chromosomes) duplicated in the S subphase
    • Entire chromosome is duplicated at the same time
    • The duplicated chromosome remains attached to the original chromosome by its centromere
    • The original chromosome and its duplicated partner are called sister chromatids
      
• In duplication the DNA strands separate (“unzip”)
  • DNA is a double helix (spiral) with the 2 strands held together by hydrogen bonds
  • In replication the 2 strands come apart and each acts as a template (pattern) to form a new strand
  • The coming apart (“unzipping”) is made possible because the strands are held together by hydrogen bonds

Chromosomes Must Be Tightly Packaged for Division

• DNA must be tightly packaged for division- otherwise it would tangle
  • DNA is wound up on histones and other proteins
  • Strands become 10,000 times shorter and much thicker (called condensation)
  • They become visible in microscope
• Condensation occurs in prophase

Mitosis is Used for Growth and Repair

• Object of mitosis is to produce 2 identical cells (same number of chromosomes)
• DNA duplicates and there is a single division, giving each cell 23 pairs of chromosomes
• Some tissues must be repaired often: lining of gut, white blood cells, skin- cell lifespan is only a few days
• Other cells do not divide at all after birth: nerve and muscle
• Red blood cells intermediate- lifespan is ~ 120 days
• General scheme of mitosis:
  • DNA duplicates -> 2 sister chromatids
  • Chromosomes attach to spindle and separate
  • Used for growth, repair and reproduction (in single-cell organisms)
  • Makes 2 identical cells (each has the original number of chromosomes)
  • 2D = DNA content of diploid cell; 4D = amount after duplication

Meiosis is Used for Sexual Reproduction

• Object of meiosis is to reduce the number of chromosomes to single copy of each (23 total)
• Used for making gametes: sperm and eggs (haploid)
  • When a sperm fertilizes an egg to form a zygote the diploid number of chromosomes is restored (23 + 23 = 46)
• In meiosis cells divide twice after a single DNA duplication
• General scheme of meiosis:
  • DNA duplicates -> 2 sister chromatids
  • Chromosomes attach to spindle & separate
  • Two divisions
    • First separates homologues
    • Second separates sister chromatids
  • Used for sexual reproduction (makes sperm & eggs)
  • Makes 4 haploid cells (each has half the number of chromosomes)
    
  • 2D = amount of DNA in diploid cell; 4D = amount after duplication; 1D = amount of DNA in haploid cells (sperm & eggs)

In Mitosis (and Meiosis) Chromosomes are Separated by the Spindle Apparatus

• Spindle is formed of microtubule fibers between the 2 centrosomes (see below)
• Chromosomes attach to spindle at centromeres
• Separation of chromosomes requires energy (ATP)

The Cell Division Cycle Has Five Stages:

• Interphase:
  • Longest phase: essentially the cell must duplicate all its parts
  • DNA replicates
  • Proteins synthesized
  • Centrioles duplicated
  • Replicated chromosomes (sister chromatids) remain attached by centromeres until anaphase
  • Longest phase
• Prophase:
  • Chromosomes condense, become visible
  • Centrosomes move to opposite ends of cell
  • Spindle forms
  • Nuclear membrane dissolves
• Metaphase:
  • Chromosomes attach to spindle fibers at their centromeres
  • Line up in center of spindle apparatus
• Anaphase:
  • Centromeres split, freeing the sister chromatids
  • Chromosomes move toward centrosomes to opposite ends of cell
• Telophase:
  • Cell cleaves to form 2 cells (cytokinesis)
  • Nuclear membrane reforms
• The last 4 phases are called mitosis
  • Mitosis = Prophase + Metaphase + Anaphase + Telophase
  • (PMAT)

Cancer is Uncontrolled Mitosis

• Mitosis must be controlled, otherwise growth will occur without limit (cancer)
• Control is by special proteins produced by oncogenes
• Mutations in control proteins can cause cancer

Summary of Mitosis:

Interphase: Duplication of DNA, organelles, proteins. Nucleolus present in nucleus. Chromosomes not visible.	Early Prophase: Chromosomes condense, become visible. Centrosome divides. Spindle starts to form between the centrosomes.
Late Prophase: Spindle formed, with centro- somes at opposite poles. Nuclear membrane dissolves. Chromosomes start to attach to spindle at their centromeres.	Metaphase: Chromosomes line up on spindle in center of cell.
Anaphase: Centromeres split apart. Then chromosomes move to towards centrosomes at opposite poles of cell.	Telophase/Cytokinesis: Nuclear membrane reforms. Cell pinches into 2 cells in animals. In plants a cell plate separates the 2 new cells.

More Information

San Diego State University has a good animated tutorial of the cell cycle and mitosis. You can control the speed of the movie by increasing or decreasing the frames per second.

The University of Arizona has good tutorials on both mitosis and meiosis. Test your knowledge of mitosis by identifying dividing onion root tip cells.