Replication of DNA.
Cell division occurs in 2 stages: nuclear division - nucleus divides (mitosis and meiosis), and cell division - follows nuclear and whole cell divides. Before nuclear DNA must replicate so all cells have genetic info.
- Requirements: 4 types of nucleotides, each with bases, must be present; both strands must act as template; enzyme DNA Polymerase needed to catalyse; and a source of chemical energy.
- 1. Enzyme DNA Helicase breaks hydrogen bonds linking bases.
- 2. Leads to double helix separating to 2 strands.
- 3. Each polynucleotide strand acts as template that complimentary nucleotides attach to.
- 4. Energy activates nucleotides.
- 5. Activated nucleotides joined by DNA Polymerase to form missing strand on each strand.
- 6. Hydrogen bonds form.
Division of nucleus leads to daughter cells having exact copy of DNA, expect in mutations. It is always preceded by a period when the cell is not dividing where DNA replicates.
- 1. Interphase - protein synthesis, DNA replicates, chromosomes invisible.
- 2. Prophase - chromosomes become visible, nuclear envelope disintegrates.
- 3. Metaphase - chromosomes line up on equator.
- 4. Anaphase - spindles contract, chromatids pulled to poles.
- 5. Telophase- chromatids reach poles & become distinct, nucleolus and nuclear envelope reform, spindle disintergrates.
Importance of Mitosis:
- 1. Growth - when haploid cells form diploid and diploid divides then all cells must be identical to this.
- 2. Differentiation - cells change to become specialised which then divide. Tissues can only function if all cells have same structure and function.
- 3. Repair - new cells must be the same as dead ones or tissue will not function.
The Cell Cycle.
Cells don't divide continuously but undergo a regular cycle separated by periods of cell growth (cell cycle):
- 1. Interphase - majority of cycle (resting phase).
- a. 1st Growth Phase (G1) - proteins that cell organelles are synthesised from are produced.
- b. Synthesis Phase (S) - DNA replicated.
- c. 2nd Growth Phase (G2) - organelles grow and divide and energy stores released.
- 2. Nuclear Division - nucleus divides into 2 (mitosis) or 4 (meiosis).
- 3. Cell Division - follows nuclear and whole cell divides into 2 or 4.
- Growth disorder of cells - damage to genes that regulate mitosis and cell cycle.
- Uncontrolled growth so group of abnormal cells develop and expands.
- Treatment blocks part of cell cycle so cell division ceases.
- Drugs prevent DNA replicating (cisplatin) or inhibit metaphase as interfere with spindle formation. They disrupt normal cells too but are more effective with rapidly dividing cells.