cell cycle

• Cell Reproduction


  A process by which cells duplicate their contents and then divide to yield multiple cells with similar, if not duplicate, contents
• When cells become damaged or die the body makes new cells to replace them
• Chromosomes


  Threadlike structures made of protein and a single molecule of DNA that serve to carry the genomic information from cell to cell

  
• In plants and animals (including humans), chromosomes reside in the nucleus of cells
• All living things contain a self-replicating genetic material that directs the activities and functions of the cells
• Deoxyribonucleic acid (DNA)


  The genetic material located inside a chromosome in the nucleus of the cell

  
• The DNA from the parents is transmitted to the offspring to ensure the continuity of life
• Chromatids


  Two identical halves of a replicated chromosome after the Synthesis phase of the cell cycle
• Centromere


  The attachment points of the two chromatids of a chromosome. It is also described as the constriction point which divides the chromosome into two sections, or "arms"

  
• Short arm (p arm)


  Upper arms of the chromosome which is usually shorter
• Q arm


  Lower arms of the chromosome which is usually longer
• Cell Cycle


  A series of growth and developmental steps a cell undergoes between its "birth" and reproduction

  
• The chromosomes of a cell change their form as they undergo cell transitions from one stage to another in a typical cell cycle

  
• The cell cycle may be divided into two stages: the interphase and cell division (mitosis/meiosis)

  
• Interphase


  The interval between two cell divisions where the cell is not dividing, it obtains nutrients and metabolizes, grows, replicates its DNA in preparation for mitosis

  
• Interphase Subphases


  • G1 Phase (First Gap Period)
  • S Phase (Synthesis stage)
  • G2 Phase (Second Gap Period)
• G1 Phase


  Cell grows initially, synthesis of protein and ribonucleic acid or RNA occurs, mitochondria increase in number

  
• S Phase


  DNA Replication, DNA are synthesized thus replicating the chromosomes in preparation for the next cell division

  
• G2 Phase


  Cell grows rapidly, cell prepares for the actual cell division
• Cell Division


  Actual division of the cell, occurs every after interphase
• Types of cell division in eukaryotic cells

  • Mitosis
  • Meiosis

  
• Mitosis


  A cellular process wherein two nuclei and two cells are produced due to the division of the original nucleus, each of which contains the same chromosome number as the parent cell

  
• Stages of Mitosis


  • Prophase
  • Prometaphase
  • Metaphase
  • Anaphase
  • Telophase

  
• Prophase


  • The chromatin fibers become more tightly coiled
  • The nuclear membrane breaks down
  • Each duplicated chromosomes appears as 2 identical sister chromatids joined together at their centromeres & all along their arms by cohesions
  • The mitotic spindle fibers begins to form
  • The centrioles move away from each other
• Prometaphase


  • The nuclear envelope fragments
  • The microtubules can invade the nuclear area
  • The chromosomes become even more condensed
  • Each of the 2 chromatids of each chromosome now has a kinetochore
  • Some of the microtubules attach to the kinetochores, becoming "kinetochore microtubules," these jerk the chromosomes back & forth

  
• Metaphase


  • The centrioles are now at opposite poles of the cell
  • The chromosomes align on the "metaphase plate/ equatorial plane"
  • For each chromosome, the kinetochores of the sister chromatids are attached to kinetochore microtubules coming from opposite poles

  
• Anaphase


  • Spindle fibers begin to contract and become shorter. Continued contraction causes the separation of the genetically identical sister chromatids
  • Centromeres divide
  • The 2 liberated daughter chromosomes begin moving toward the opposite ends of the cell
• Telophase


  • The chromosomes are now at the opposing poles of the spindle
  • The microtubules disappear
  • Two sets of chromosomes are surrounded by new nuclear membranes, completing the nuclear division process known as karyokinesis
  • Cytoplasmic division called cytokinesis occurs concurrently, splitting the cell into two

  
• Meiosis


  Formation of Gametes (Eggs & Sperm)
• Facts About Meiosis


  • Preceded by interphase which includes chromosome replication
  • Two meiotic divisions --- Meiosis I and Meiosis II
  • Meiosis I is called Reduction division
  • Meiosis II is called Equational division
  • Original cell is diploid (2n)
  • Daughter cells contain half the number of chromosomes as the original cell
  • Produces gametes (eggs & sperm)
  • Occurs in the testes in males (Spermatogenesis)
  • Occurs in the ovaries in females (Oogenesis)

  
• Replication of Chromosomes


  Replication is the process of duplicating a chromosome, occurs prior to division, replicated copies are called sister chromatids, held together at centromere

  
• Meiosis: Two Part Cell Division


  1. Homologs separate

  2. Sister chromatids separate
• Start with 46 double stranded chromosomes (2n), after 1 division - 23 double stranded chromosomes (n), after 2nd division - 23 single stranded chromosomes (n), occurs in our germ cells that produce gametes
• Meiosis


  A type of cell division in sexually reproducing organisms that reduces the number of chromosomes in gametes (the sex cells, or egg and sperm)
• Both members of a single homologous pair of chromosomes in a diploid cells are replicated & that the copies are then sorted into 4 haploid daughter cells

  
• Why Do we Need Meiosis?

  It is the fundamental basis of sexual reproduction, two haploid (1n) gametes are brought together through fertilization to form a diploid (2n) zygote
• Meiosis I: Reduction Division


  Separates homologous chromosomes

  
• Meiosis II: Equational Division

  Separates sister chromatids
• Meiosis I: Prophase I


  • Homologs pair, Crossing over occurs
• Meiosis I: Late Prophase I


  • Chromosomes condense, Spindle forms, Nuclear envelope fragments