Cell Cycle and DNA Replication

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Summary of Cell Cycle

The cell cycle is the process of cell growth and division. There is a period of division then a period of growth

Interphase

  • The majority of the cell cycle is interphase, in which no division occurs. It consists of:
    • First growth (G1) - proteins for organelles are produced
    • Synthesis (S) - DNA is replicated
    • Second growth (G2) - organelles grow and divide + energy stores increase

Nuclear Division

  • Nucleus divides into 2 (mitosis) or 4 (meiosis)

Cell Division

  • Whole cell divides into 2 (mitosis) or 4 (meiosis)

The length of the cell cycle varies between organisms

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Mitosis

Mitosis is a form of nuclear division which produces 2 daughter cells which have an exact copy of the DNA of the parent cell. Used for growth and repair

Stages:

Always followed by interphase in which the DNA + organelles are replicated and ATP is increased

1) Prophase - Chromosomes become visible and condense + nuclear envelope disappears
2) Metaphase - Chromosomes arrange in a line in the middle and spindle fibres attach to the centromere
3) Anaphase - Spindle fibres pull apart the chromatids of each chromosome to opposite poles
4) Telophase - Chromosomes elongate and nuclear envelope reforms. Cell divides in two

Important for:

  • Growth - Ensures all cells contain the same genetic info
  • Differentiation - Cells differentiate to make specialise cells, mitosis forms specialised tissue
  • Repair - Replaces damaged or dead cells so new ones have identical structure + function
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DNA Replication - Semi-Conservative

DNA replicates before nucleous divides so each daughter cell has genetic info for producing proteins + enzymes

Requirements:

  • Four types of nucleotide, each with a different base, must be present
  • Both strands of DNA must act as a template for attachments of nucleotides
  • The enzyme DNA polymerase is needed
  • A source of chemical energy is needed

Process:

  • DNA helicase breaks the hydrogen bonds between the bases so that the double helix separates
  • Activated nucleotides are attracted to their complementary base on each strand which is acting as a template via specific base pairing
  • DNA polymerase joins the base pairs and hydrogen bonds form between each base pair so each DNA molecule contains an original strand and a complementary new strand. Each DNA strand is identical
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Cancer

  • Cancer is the result of uncontrolled cell division caused by a mutation in the gene which controls cell division
  • These cells which have been produced needlessly form a tumour
  • Cancer is when the tumour invades surrounding tissue
  • As cell division is part of the cell cycle, treatments target the cell cycle

Treatments

These treatments can't distinguish normal cells from cancer cells so kill both. However cancer cells divide more frequently so cancer cells are more likely to die

  • Some chemical drugs (chemotherapy) prevent the synthesis of enzymes in G1 phase needed for DNA replication. Unable to enter the synthesis stage the cell kills itself
  • Some radiation (radiotherapy) and drugs damage the DNA. In S phase the cell will detect damaged DNA and kill itself before it's replicated

Precautions are made to reduce the impact on normal cells:

  • Chunk of tumour removed first via surgery. Increases access of nutrients to cancer cells so are more succeptible to treatment. There are breaks between for recovery
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Cancer

  • Cancer is the result of uncontrolled cell division caused by a mutation in the gene which controls cell division
  • These cells which have been produced needlessly form a tumour
  • Cancer is when the tumour invades surrounding tissue
  • As cell division is part of the cell cycle, treatments target the cell cycle

Treatments

These treatments can't distinguish normal cells from cancer cells so kill both. However cancer cells divide more frequently so cancer cells are more likely to die

  • Some chemical drugs (chemotherapy) prevent the synthesis of enzymes in G1 phase needed for DNA replication. Unable to enter the synthesis stage the cell kills itself
  • Some radiation (radiotherapy) and drugs damage the DNA. In S phase the cell will detect damaged DNA and kill itself before it's replicated

Precautions are made to reduce the impact on normal cells:

  • Chunk of tumour removed first via surgery. Increases access of nutrients to cancer cells so are more succeptible to treatment. There are breaks between for recovery
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