Leaving Cert Biology

Cell Continuity:

• Cells that arise from cells of the same type
  • Living organisms maintain cell continuity by mitosis and cell division

Chromatin and Chromosomes

• Chromatin is elongated DNA
• Chromosomes are composed of DNA and protein
  • Chromosomes are arranged into homologous pairs – pairs of chromosomes that contain genes that control the same characteristics

Haploid and diploid

• Haploid means one set of chromosomes
• Diploid means two sets of chromosomes
  • In humans the haploid number is 23
  • The diploid number is 46 – arranged into 23 pairs

The Cell cycle

• The cell cycle occurs under three distinct stages:
  • Interphase
  • Mitosis
  • Cell division

​1. Interphase

• Interphase is the stage where a cell is not dividing and a cell spends the vast majority of its time in this stage​
• Chromosomes are elongated and the cell goes about its daily functions

• Towards the end of interphase the cell organelles and DNA are replicated

End of interphase

• The DNA is replicated and condenses into duplicated chromosomes which are held together by centromeres

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2. Mitosis

• Mitosis is nuclear division that leads to the formation of two identical daughter cells

Stages:

• Stage 1: Prophase
• Stage 2: Metaphase
• Stage 3: Anaphase
• Stage 4: Telophase

Stage 1: Prophase

• The nuclear membrane begins to disappear and spindle fibres begin to appear from the centrioles:

• The replicated chromosomes line up along the equator of the cell held in place by spindle fibres:

• The spindle fibres begin to contract pulling one chromosome from each pair to each pole:

• The chromosomes at each pole begin to unravel and lengthen; spindle fibres disappear; and the nuclear membranes reform at each pole:

• Cell division occurs immediately after mitosis and involves formation of cleavage furrow (in animals cells) or cell plate (in plant cells) prior to cell splitting

• Meiosis is nuclear division whereby four new daughter cells are produced each with half the number of chromosomes as the parent cell

• Cancer is a disease whereby a cell loses its ability to control both the rate of mitosis and cell division
• Mitosis is normally carefully controlled but when it is out of control it can result in cancer
• Cancer can be either benign or malignant
  • Benign cancer involves cells that divide out of control for a limited period of time and do not spread
  • Malignant cancers involve cells that divide rapidly uncontrollably and spread and invade other tissues – destroying that tissue in the process
• Anything that has the potential to cause cancer is called a carcinogen – which damage DNA
• Two carcinogens:
  • UV light – damages DNA of skin cells
  • Cigarette smoke – damages the DNA of cells lining the mouth, airways and lungs