Cell Growth + Division

• Limits to Cell Growth

  • The larger a cell gets, the more difficult it is for the cell to produce all the necessary proteins
  • When the cell gets bigger there is less membrane to exchange oxygen, water, food and carbon dioxide.
• Ratio of Surface Area to Volume

  • To increase the surface area/volume ratio of the cell membrane to cytoplasm, the cell divides. Cell division produces 2 cells called daughter cells which are identical to each other and to the "parent cell".

  
• Chromosomes - Part 1

  • Organisms have genetic information coded in a molecule called DNA
  • 2 forms of DNA:
  • In a non-dividing cell the DNA is called chromatin
  • In a dividing cell the DNA is tightly coiled into structures called chromosomes
  • Organisms have a set number of chromosomes

  
• Chromosomes - Part 2

  • When a cell divides DNA in each DNA molecule first replicates and then condenses to form a replicated, condensed chromosome.
  • To condense, the DNA coils around proteins called histones
  • Each replicated chromosome is now made of two identical strands called chromatids attached in the middle by a protein bundle called a centromere
  • When a cell divides, chromatids separate and each cell gets a copy of DNA.

  
• During the cell cycle, a cell grows and prepares for cell division, replicating DNA.
• The cell divides to form two daughter cells, each of which will begin the cell cycle again.
• The sequence of the cell cycle is the same in all cells but some spend more time in certain stages.
• Most cells in an adult multicellular organism are in G0, a state of rest.
• Cell division takes place in the tips of roots and stem in plants.
• Cell division takes place all over the body to produce many different kinds of cells in animals.
• The timing of cell division is critical to the development of tissues and organs.
• DNA must be replicated and equally divided or it can lead to birth defects, cancer or serious disease.
• Two phases of the cell cycle:

  1. Interphase
  2. Mitosis - nuclear division
• Steps of Interphase

  • G1 - 1st gap
  • S - DNA synthesis
  • G2 - 2nd gap
  • G0 - "resting"
• Steps of Mitosis

  • prophase
  • metaphase
  • anaphase
  • telophase
  • cytokinesis: cytoplasm divides
• Interphase

  • the period between cell divisions
  • the longest part of cell cycle - 75%
  • G1: cells grow & make new proteins + organelles
  • G0: some cells enter a prolonged period where they're just carrying out normal cell processes
  • S: DNA replicates (synthesize) - DNA is copied so that each cell will get a copy
  • once a cell enters the S phase, it usually completes the rest of the cell cycle
  • G2: organelles + molecules (enzymes) needed for mitosis are made
  • cell is ready for mitosis -> division
• Mitosis - Prophase

  • key word: preparation
  • Longest stage of mitosis - spindle forming
  • Chromatin condenses into chromsomes
  • Nuclear envelop + nucleolus disappear
  • Centrioles split + move to opposite ends of cells
  • Mitotic spindle (spindle fibers) forms from chromatids to centrioles in order to help organize the chromatids
• Mitosis - Metaphase

  • Key word - middle
  • Shortest phase of mitosis
  • Chromosomes line up along the equator of the cells
  • Microtubules connect centrioles to spindle
  • Ends when chromatids begin to separate
• Mitosis - Anaphase

  • key word: apart
  • centermeres splits
  • sister chromatids separate to poles of the cell
  • two groups of chromosomes are formed
  • ends when chromatids are completely separated
• Mitosis - Telophase

  • key word: two
  • chromosomes uncoil into chromatin
  • spindle fibers disappear
  • nuclear envelop & nucleolus starts to form
  • ends when cell membrane starts to pinch in
• Cytokinesis

  • membrane begins to pinch in until the cytoplasm is formed into two balls
  • in plant cells, a cell plate forms instead of pinching in