A-Level Biology OCR Notes

Mitosis

Within multicellular organisms, not all cells retain the ability to divide
Mitosis produces genetically identical daughter cells, which is useful for growth, repair and asexual reproduction

The eukaryotic cell cycle has three main stages:
Interphase consists of two growth phases (G1& G2) and a DNA synthesis stage (S). The cell may exit the cell cycle at G0
Mitosis is the nuclear division
Cytokinesis is when the cell splits in two, forming two identical daughter cells.
- In animal cells, the plasma membrane folds inwards until the two dents meet to separate the two daughter cells
- In plant cells, cellulose starts to build up at the equator (the end plate). Plasma membrane forms in the middle of the end plate resulting in two fully separated plant cells

Stage	Description
Prophase	DNA condenses & coils, nuclear envelope breaks down, centrioles move to opposite poles
Metaphase	Spindle fibres attach to centromeres & chromosomes line at the equator
Anaphase	Centromeres divides, chromatids move to opposite poles
Telophase	Chromosomes uncoil, nuclear envelope reforms

Cell cycle checkpoints ensure the cell only divide at the appropriate time when ready and helps detect DNA damage.

	Checkpoint Location	Function
Restriction Point or G1/S Checkpoint	Between G1 and S	Decides whether cell proceeds to S-Phase..
G1 Checkpoint	End of G1	Checks cell is ready for S-Phase
G2/M Checkpoint	Between G2 and mitosis	Checks and repairs all of the DNA before the cell enters mitosis.
Mitosis Checkpoint	In the middle of mitosis	Checks cell is ready to proceed in mitosis. Ensures that the cell doesn't proceed to the next stage before it's ready.

Meiosis

Meiosis produces 4 haploid daughter cells (gametes) that are genetically different from each other.

In meiosis 1, homologous chromosomes are separated from each other, with one chromosome from each pair going into one of the two daughter cells. In the second meiotic division, the sister chromatids from each chromosome are separated

Stage	Description
Prophase 1	Chromatin condenses & coils, nuclear envelope breaks down, centrioles move to opposite poles. Crossing over occurs between homologous chromosomes
Metaphase 1	Spindle fibres attach to centromeres & chromosomes line at the equator
Anaphase 1	Homologous chromosomes move to opposite poles
Telophase 1 and cytokinesis	Chromosomes uncoil, nuclear envelope reforms. Animal cells divide by cytokinesis
Prophase 2	Chromatin condenses & coils, nuclear envelope breaks down, centrioles move to opposite poles.
Metaphase 2	Spindle fibres attach to the centromeres & chromosomes lines up at the equator
Anaphase 2	The centromeres divide and the chromatids are pulled to opposite poles
Telophase 2 and cytokinesis	Chromosomes uncoil, nuclear envelope reforms. Cells divide to form 4 haploid cells.