How do Cells divide?
Like a washing machine, the stages of a cells life must be completed before the next stage commences. In a cell this is known as the cell cycle; the cell grows, rests, copies its DNA, and finally divides into two new cells.
Different cells will take different amounts of time to complete a cell cycle. Human liver cells for example may take up to a whole year to complete a cell cycle.
Each cells contains DNA, the code for your cell set up as 46 bundles of chromosomes. Before the cell can divide it must unravel these chromosomes and make a copy of this unique DNA, thereby providing each new cell with a complete copy of this genetic blueprint.
Initially the cell gets bigger (Interphase) and makes a copy of its DNA. When a copy has been made a cell will usually divide into two new cells in a process known as mitosis. This process is divided up into phases:
Pro phase This is where the nuclear envelope starts to unfold. The chromosomes which are usually long strings in the nucleus start to condense. The 46 chromosomes coil up into compact bundles.The edge of the nucleus membrane starts to dissolve and spindle fibres a temporary protein scaffolding start to appear in the cell.
Metaphase The chromosomes use the spindle to line up along the equatorial line of the fibres, in the centre of the cell.
Anaphase The chromosomes split apart and each identical copy pulls away to opposite ends of the cell.
Telophase the nucleus membrane forms around each chromosome, the spindle fibres dissolve. A pinch point starts to appear in the cytoplasm; it narrows in the middle as the cell starts to divide.
Cytokineisis. Two identical daughter cells split to form complete new cells.
Each cell receives the complete copy of the DNA, bundles into 46 chromosomes. (egg and sperm cells are different as they each contain only half of the DNA from the original cell -with 23 chromosomes.)
Credit: Matthew Daniels, Wellcome Images.
The 46 chromosomes in a human cell consist of 23 pairs – one set inherited from each parent. When cells divide to make egg or sperm cells they receive only one chromosome from each of the pairs. The process of meiosis means the new cells usually end up with 23 chromosomes. Each set is known as a gamete. When an egg and sperm join together at fertilisation, the new cell they make will have the normal number of 46 chromosomes again – the complete set needed for a making a new person, this first union is called a zygote.