Cell Division

The Cell Cycle

• The cell cycle has two main stages: interphase and the m phase (mitosis). Interphase is where a cell spends most of its time - growing and carrying out its functions.
• Interphase
• Interphase has 3 stages: G1, S, and G2. After each phase is a checkpoint - these make sure the DNA and cell are correct before division.
• G1: This is the first growth phase of the cell. Here new proteins and organelles are made.
• G1 checkpoint: Size of the cell is checked, as well as the chemicals needed for replication. DNA integrity is also checked.
• S: Here the cell replicates its DNA for cell division.
• G2: The cell continues to grow and further proteins needed for cell division are made.
• G2 checkpoint: The cell is checked for its size and DNA. If the DNA is faulty, the cell leaves the cell cycle at G0. Here it either leaves temporarily: for DNA replication to be completed or repaired, or permanently: via apoptosis (programmed cell death).

Mitosis

• 
• Prophase: This is the first stage of cell division. Here the chromosomes condense - meaning they shorten and fatten. Then the centrioles move to the poles and form spindle fibres. The nuclear envelope begins to break down and the chromosomes are free in the cytoplasm.
• Metaphase: The chromosomes line up along the metaphase plate. Here the spindles attach to the centromeres (centre of the chromosomes.
• Anaphase: Now the centromeres divide which separates the chromosome into sister chromatids.
• Telophase: The chromatids reach the poles and uncoil forming chromatin. The nuclear envelope reforms.

Meiosis

• Prophase 1: The chromosomes condense, coil and become visible. The nuclear envelope breaks down. Homologous chromosomes pair up. These are called bivalents. Chromatids entangle, this is called crossing over
• Metaphase 1: Bivalents line up down the metaphase plate. They become attaches to the spindle fibres. Independent assortment occurs.
• Anaphase 1: The Homologous chromosomes are pulled to opposite poles due to contraction of the spindles. Bivalents are separated. Sections of DNA which became entangled break off and re-join. The point at where this occurs is called the chiasmata. This forms recombinant chromatids
• Telophase 1: The nuclear envelope forms around each group of chromosomes. Cytokinesis occurs.
• Prophase 2: The chromosomes condense and the nuclear envelope breaks down. The centrioles move to the poles and the spindle fibres form.
• Metaphase 2: The chromosomes line up on the plate and independent assortment occurs.
• Anaphase 2: The chromosomes are pulled apart and the centromeres split.
• Telophase 2: The chromatids assemble at poles where they uncoil forming chromatin. The nuclear envelope reforms.

Plant Cell Division

• 
• Due to the presence of the cellulose cell walls, therefore it isn't possible for cleavage furrows to form.
• The Golgi apparatus produce vesicles which assemble on the equator - where the metaphase plate would have formed.
• The vesicles fuse with each other and the cell surface membrane. This divides the cell into two.
• New sections of cell wall then form along the new sections of cell membrane.

Cell Division - Pictures (Onion cell)

Comparing human and plant cell division

• Human
• Cleavage furrow forms.
• Induced by a number hormones.
• Plant
• Cleavage furrow doesn't form due to the presence of the cell wall.
• Induced by a hormone called cytokinin.