Cell Division

The Cell Cycle
The cell cycle is the process that all body cells in multicellular organisms use to grow and divide.

1. The cell cycle starts when a cell has been produced by cell division and ends with the cell dividing to produce two identical cells
2. The cell cycle consists of a period called Cell Growth and DNA Replication which is collectively called Interphase. Next is Cell Division, which is known as M Phase. M Phase involves Mitosis and Cytokinesis.
3. Interphase is divided into 3 separate Growth Stages, G1, S, and G2
4. The cell cycle is regulated by checkpoints, these checkpoints let the cell know it is okay to continue.

Mitosis

Mitosis is needed for Growth of multicellular organisms and to Repair Damaged Tissues. It is also a method of asexual reproduction in some animals, plants and fungi. Mitosis is really just one continuous process but is described as a series of stages, Prophase, Metaphase, Anaphase and Telophase (PMAT)

INTERPHASE

Interphase actually comes Before Mitosis and is essentially the Preparation Phase, where cells Grow and Replicate their DNA ready for division. All while carrying out its Normal Functions.

1.  INTERPHASE: The cells DNA is Unravelled and Replicated, to double its genetic content. Organelles are also Replicated so it has spare ones and its ATP content is Increased (ATP is the energy needed for the cell to divide.
2. PROPHASE: The CchromosomesCcondense, getting Shorter and Fatter. Tiny bundles of protein called Ccentrioles start moving to Opposite Ends of the cell, forming a network of protein fibres across called a Spindle. The Nuclear Envelope Breaks Down and the Chromosomes lie Free in the Cytoplasm. 
3. METAPHASE: The chromosomes, each with two chromatids line up along the middle of the cell and become attached to the spindle by their centromere. At the metaphase checkpoint, the cell checks that all chromosomes are attached to the spindle before mitosis can continue.
4. ANAPHASE: The centromeres divide, separating each pair of sister chromatids to opposite ends of the cell, centromere first.
5. TELOPHASE: The Chromatids reach Opposite Poles on the spindle. They Uncoil and become long and thin again. They're now called chromosomes again. A Nuclear Envelope Forms around each group of chromosomes, so there are now Two Nuclei.
6. CYTOKINESIS: The Cytoplasm Divides. In animal cells, a Cleavage Furrow Forms to divide the cell membrane. There are now two daughter cells genetically identical to the original cell and to each other. Cytokinesis usually starts in anaphase and ends in telophase. It is a Separate Process to mitosis.

Sexual Reproduction and Meiosis

In sexual reproduction, two Gametes (sex cells) join together at fertilisation to form a Zygote. The zygote then develops into a new organism. Meiosis is the process that takes place in the reproductive organs to produce gametes. Meiosis involves a Reduction Division. Cells have a full number of chromosomes to start with, but after meiosis, they only have half. The product cells are called Haploid Cells. Cells formed  by meiosis are all Genetically Different because each new cell has a different set of chromosomes.

• Meiosis involves two separate parts, Meiosis I, and Meiosis II.
• Meiosis I is the Reduction Division, where the number of chromosomes are Halved.
• Both Meiosis parts are each split into Prophase, Metaphase, Anaphase and Telophase (PMAT) as with Mitosis.
• The whole of meiosis begins with Interphase, where the DNA Unravels and Replicates itself to produce double armed chromosomes called sister chromatids.

MEIOSIS I

• Prophase I - The Chromosomes Condense, getting shorter and fatter and they arrange themselves into Homologous Pairs (Chromosomes from Mum and Dad) and crossing over occurs. Centrioles move to Opposite Ends forming Spindle Fibres and the Nuclear Envelope Breaks Down.
• Metaphase I - The homologous pairs Line Up across the Centre  of the cell and Attach to the Spindle Fibres by their Centromere.
• Anaphase I - The Spindles Contract separating the homologous pairs, one chromosome to each end of the cell
• Telophase I - A Nuclear Envelope Forms around each group of chromosomes.
• Cytokinesis - The Division of the Cytoplasm and two haploid daughter cells are produced.

MEIOSIS II

In Meiosis II, the two daughter cells go through the same process, Prophase II, Metaphase II, Anaphase II and Telophase II. In anaphase II, each of the sister chromatids are separated, so each daughter cell has one chromatid from each chromosome, so four genetically different haploid daughter cells are produced. These are the Gametes.