Preperation for cell division.
- Before cells divide they need an exact copy of their DNA by using a supply of organic bases, pentose sugar molecules and phosphates.
- The method by which DNA is copied is Semi-Conservative Replication.
- The DNA unwinds by the influence of the enzyme DNA Helicase.
- Hydrogen Bonds between the two chains break and the two DNA strands seperate.
- Each complementary strand the acts as a template to build its opposite strand from free nucleotides.
- The enzyme DNA Polymerase joins the nucleotides toegther.
- This results in the production of two identical copies of double stranded DNA.
Evidence for semi-conservative replication.
- Meselson and Stahl experiment.
- Bacteria were cultured with a heavy isotope of nitrogen located in the organic bases of their DNA.
- The bacteria were then supplied with bases containing the normal light nitrogen.
- They replicated their DNA using these bases.
- Their population increased.
- Each molecule of DNA of the next generation had one strand containing heavy nitrogen and one strand containing light nitrogen.
- The mass of DNA was therefore midway between the original heavy and the normal light DNA.
Types of cell division.
- Cells divide for growth, repair and reproduction.
- When the chromosomes are in pairs the cell is said to be Diploid.
- The pairs of chromosomes are Homologous pairs.
- Mitosis produces genetically identical copies of cells with the same number of chromosomes for growth and repair.
- Meiosis produces cells that have half the number of chromosomes, one from each pair.
- The cells are Haploid and are used as gametes.
- Meiosis introduces variation because the pairs can be split up in many different ways; Independent Assortment.
- Genetic material can also be exchanged between homologous chromosomes by crossing over.
The cell cycle.
- The length of time between a cell being formed and it dividing is called the cell cycle.
- G1 phase- the cell grows making new proteins and more organelles.
- S phase- the DNA of the chromosome is replicated by semi-conservative replication.
- G2 phase- more organelles are made and a spindle forms.
- M phase- this is mitosis involving the seperating of the genetic material into 2 nuclei.
- C phase- cytokinesis, where the cell divides in 2.
- Interphase is the period of time between cell divisions.
- It is made up of G1, S and G2 phases.
1. Prophase: Each chromosome forms 2 chromatids joined by a centromere. 2 centrioles begin to move forming a spindle.
2. Metaphase: The chromatids, still joined by a centromere move to the middle of the cell. Each of the 2 chromatids has identical DNA to the other.
3. Anaphase: The spindle fibres join to the centromeres. The spindle fibres shorten and the centromeres split. The seperated chromatids are now chromosomes.
4. Telophase: Identical chromosomes move to each pole. The nuclear membrane re-forms, the cell membrane narrows at the middle and two daughter cells are formed.