Cell Division

• Cell Cycle - G1

  • cell growth
  • new organelles and proteins are made
  • G1 checkpoint - DNA damage? chemicals required are present? size?
• Cell Cycle - S

  • synthesis - DNA replication
• Cell Cycle - G2

  • cell keeps growing
  • proteins required for division are made
  • G2 checkpoint - DNA damage?
• Cell Cycle - M

  • mitosis and cytokinisis
  • metaphase checkpoint
• Cell Cycle - G0

  • cell cycle arrest
  • irreparable damage
  • e.g. cancerous, old
• Cell Cycle

  Process of cell growth and division
• Cell Cycle


  • consists of a period of cell growth and DNA replication -> interphase
  • period of cell division -> M phase, involves mitosis and cytokinesis
  • regulated by checkpoints
• INTERPHASE - G1, S and G2

  • cell's normal functions will still occur
  • DNA is unravelled and replicated - genetic content is DOUBLED
  • organelles are replicated
• Mitosis


  • needed for growth - repairing damaged tissues
  • asexual reproduction
  • a continuous process
• Stages of Mitosis

  1. prophase
  2. metaphase
  3. anaphase
  4. telophase
• MITOSIS - PROPHASE


  • chromatin condenses - chromosomes become visible
  • chromosomes contain 2 sister chromatids, joined by a centromere
  • nuclear membrane breaks down
  • centrioles move to opposite ends of the cell
  • microtubules start to assemble around them, forming the spindle
• MITOSIS - METAPHASE


  • centromeres attach to chromosomes
  • chromosomes line up at spindle equator of cell
  • metaphase checkpoint - checks all chromosomes are attached to spindle
• MITOSIS - ANAPHASE


  • centromeres divide
  • sister chromatids are pulled to opposite ends of the cell by spindle fibres
• MITOSIS - TELOPHASE


  • 'chromatids' reach poles of the spindle
  • 'chromosomes' unravel, become indistinct (chromatin)
  • nuclear membranes form
• CYTOKINESIS - ANIMALS


  • cytoplasm divides
  • separate process to mitosis
  • begins in anaphase and ends in telophase
  • cleavage furrow forms - ACTIN and MYOSIN microfilaments form a ring around the plasma membrane and pulls it in - myosin slides past actin, similarly to how muscles contract
• CYTOKINESIS - PLANTS


  • vesicles carrying cell wall and cell membrane components are delivered to place of division
  • vesicles fuse
  • cell plate grows from the centre outwards
  • cell wall forms alongside
• YEAST AND OTHER FUNGI

  • BUDDING
  • asexual form of reproduction
  • bud emerges from one side of cell, before DNA replicates
  • spindle forms between bud and parent
  • NOT MITOSIS
  • no clear S and M stages
  • clear G1 stage
• BACTERIA, MITOCHONDRIA AND CHLOROPLASTS


  • BINARY FISSION
  • prokaryotes - no nucleus or centromeres
  • NOT MITOSIS and asexual
  • produces genetically identical daughter cells
• BINARY FISSION 


  • no genetic variation in offspring - weaknesses are inherited, effected by environmental changes
  • rapid division - can take advantage of resources
  • same genetic material - easy to survive in current environment
  • can occur if sexual reproduction is not possible or fails
• mitotic index


  no. cells undergoing mitosis/total no. cells x100
• microscopy and cell division

  • can stain chromosomes to see them under a microscope
  • only see individual chromosomes DURING MITOSIS
  • during interphase they are spread out and not condensed (chromatin)
• Process of meiosis


  • two divisions - meiosis one and two
  • similar to mitosis - split into same stages
  • interphase is the same
• GeneS


  
  

  • A section of DNA that codes for a protein
  • every gene has a locus - its original position on a chromosome
  • every gene comes in different forms (alleles)
  • pairs of chromosomes are called homologous pairs
• Meiosis


  • used in sexual reproduction
  • two gametes fuse together at fertilisation to form a zygote
  • zygote divides and develops into a new organism
  • MEIOSIS OCCURS IN REPRODUCTIVE ORGANS TO PRODUCE GAMETES
  • involves reduction division - initially have full no. chromosomes and cells produced have half
  • cells produced are all genetically different due to combinations of chromosomes
• DIPLOID


  • full number of chromosomes
  • 46 in humans
• HAPLOID


  • half the full number of chromosomes
  • 23 in humans
• MEIOSIS 1 - prophase 1


  • chromosomes condense
  • homologous pairs form
  • nuclear membrane breaks down
  • centrioles move to opposite ends of the cell, microtubules start to assemble around them, forming the spindle
  • crossing over occurs
• MEIOSIS 1 - metaphase 1

  • bivalents (pairs of chromosomes) line up at cell equator
  • centromeres attach to chromosomes
  • metaphase checkpoint - checks all chromosomes are attached to spindle
• MEIOSIS 1 - anaphase 1

  • spindles contract
  • centromere divides
  • pairs of chromatids are pulled to opposite ends of celll
• MEIOSIS 1 - telophase 1


  • chromatids reach poles of spindle
  • chromosomes unravel
  • nuclear membrane forms (skipped in plants)
  • cytokinesis occurs
• MEIOSIS 2 - prophase 2

  • chromosomes condense again
  • nuclear membrane breaks down
  • centrioles move to opposite ends of cell
  • microtubules start to assemble, forming the spindle
• MEIOSIS - metaphase 2

  • chromosomes line up at the the equator of the cell
  • centromeres attach to chromosomes
• MEIOSIS - anaphase 2


  • individual chromatids are pulled apart
  • centromeres divide
  • chromatids are pulled to opposite ends of the cell by spindle fibres that contract
• MEIOSIS - telophase 2


  • chromatids reach poles of spindle - now called chromosomes
  • chromosomes unravel
  • nuclear membranes form
  • cytokinesis occurs
  • four haploid daughter cells are produced
• Sources of genetic variation

  • crossing over/recombination
  • independent assortment of chromosomes. (metaphase 1 and 2)
  • random fusion of gametes at fertilisation
  • mutation
• Crossing over/recombination


  • prophase 1
  • DNA exchange between homologous pairs - bivalents
  • shuffles alleles as chromatids twist around each other at chaismata
• independent assortment


  • metaphase 1 and 2
  • it is completely random which pole the chromosome/chromatid will go to in metaphase
  • four daughter cells produced have completely different combinations
• Random fertilisation


  • two gametes fuse to form a zygote
  • each gamete has a unique combination of genes
  • any of the numerous male gametes could fuse with the egg
• Gene mutation


  • although DNA is very stable, bases can change in DNA replication
  • mutations in gametes will be present in all cells of the offspring