Chapter 10 Cell division

• humans have increased in size due to 


  increase of number of cells
• new cells are constantly needed for 


  growth and to replace cells that have died or been damaged
• more cells need to be reproduced 


  so organisms can grow as it is made up of more cells

  replace old, dead or damaged cells
• the more wear or tear on a cell 


  the shorter the life span

  cells in intestine live for >2 days

  nerve cells in brain last a lifetime
• cell cycle 


  events that take place from one cell division to the next

  called a cycle because events keep repeating as the cell divides again and again
• events that occur in cell cycle

  G1 phase (first growth phase)

  S phase (synthesis)

  G2 phase (second growth phase)

  M phase (mitotic phase)
• G1 phase 


  cell produces new proteins, grows and carries out its normal tasks for the body

  this phase ends when the cells DNA begin to duplicate
• S phase 


  the DNA molecules in the cell nucleus form exact copies of themselves
• G2 phase 


  relatively short phase involves preparation for cell division
• M phase 


  cell divides into two daughter cells
• after division cells may continue 


  the cycle and re-enter the G1 phase
• G0 phase 


  some cells leave the cycle and stop dividing for days, years or even for the rest of the persons life

  these cells are in G0 phase
• cytokinesis

  division of cytoplasm
• while the events of telophase is occurring 


  the cytoplasm usually begins to divide
• furrow develops in the 


  cytoplasm between two nuclei

  gradually depends until it cuts the cytoplasm into two parts, each with its own nucleus
• mitosis and cytoplasmic division result in


  formation of two daughter cells, which are now in interphase
• because each chromosome was duplicated prior to 


  mitosis and a copy went into each daughter cell, each daughter cell has exactly the same number and type of chromosomes as the parent cell
• genetic information is therefore passed from 


  parent cell to daughter cells and without change
• DNA controls 


  the structure and actions of cells and hence the body
• vital that when a cell reproduces 


  each new cell gets exactly the same DNA as the parent cell

  to contain same genetic information as parent cell
• containing same genetic information as parent cell 


  achieved by division of the nucleus, known as mitosis
• mitosis ensures that 


  each body cell receives the exact same hereditary material (DNA) as that possessed by its parent cell
• stages of mitosis

  prophase

  metaphase

  anaphase

  telophase

  process is continuous, does not occur in steps
• interphase 


  period between nuclear divisions

  cell undergo through the G1, S and G2 phases

  in S phase, DNA molecules in nucleus forms exact copies of themselves

  in period between one cell division and the next, the quantity of DNA in the nucleus doubles
• prophase 


  first phase of mitosis

  two pairs of centrioles become visible early

  move to opposite ends (poles) of cell and microtubules begin to radiate from them

  nucleolus disappears and the nuclear membrane breaks down

  chromatin threads become tightly coiled and can be seen as chromosomes
• coiling the long delicate DNA molecules makes it 


  easier to distribute the DNA to the daughter cells
• each chromosomes consist of

  two chromatids, which are joined at a point celled the centromere
• two chromatids are 


  identical, tightly coiled DNA molecules produced from DNA replication interphase
• end of prophase 


  centrioles have reached opposite poles of the cell and

  the microtubules radiating from them join to form a framework of fibres called a spindle

  nuclear membrane has now completely disappeared

  chromatids pairs migrate towards the centre of the cell
• metaphase 


  chromatin pairs line up in the equator of the spindle

  the centromere of each pair is attached to a spindle fibre
• anaphase 


  each pair of chromatics separates at the centromere

  as the chromatids have become independent of each other, they are now each called chromosomes

  new chromosomes are then pulled away from one another towards opposite poles of the cell

  centromeres are still attached to the spindle fibres, seems that the spindle fibres pull the chromosomes apart in some way
• telophase 


  two sets of chromosomes form tight groups at each pole of the cell

  a nuclear membrane forms around each group, and a nucleolus appears in each new nucleus

  spindle fibres disappear, and the chromosomes gradually uncoil to become chromatin threads once more
• cells can be classified as either

  stem cells or specialised cells
• specialised cells are 


  unable to divide

  must be replaced by other means
• when stem cells undergo mitosis 


  the daughter cells may be new stem cells (stem proliferation) or cells that differentiate to form specialised cells
• cells are specialised so that they 


  can carry out particular tasks
• differentiation 


  process by which cells become specialised
• as the cells undergo division by mitosis


  different genes become activated

  makes the cells differentiate into specialised cells that can perform particular functions

  e.g. stomach cells secrete enzymes, red blood cells can carry oxygen
• cells that can undergo differentiation are 


  called stem cells

  very different from other cells because they are bot specialie=sed and are capable of repeated division
• in the right conditions stem cells can 


  differentiate into specialised cells

  because stem cells have the potential to develop into any cell type, they could possibly provide an unlimited source of cells for repair of tissues such as bone, skin, muscle, liver or blood