The Hungry Cell
Live to eat or eat to live? At Christmas, the former may be true, but in a day-to-day reality our cells need to eat constantly to obtain energy for a variety of tasks necessary for the cell to grow and replicate.
The main energy currency in the cell is adenosine triphosphate, commonly known as ATP. ATP is produced during the process of respiration, in organelles called mitochondria, where the chemical energy stored in glucose is converted to an easily utilisable form in the cell, i.e., ATP.
Task 1 – DNA replication
A single human cell contains approximately 6,000,000,000 base-pairs (sub-units), which when laid end-to-end have a total length of around 2 metres – enough to occupy around a 1000 double-sided A4 pages, at font size 12. Each time the cell divides, it must copy all this information – and without any mistakes! Different cells in the body divide at different rates, but on average most cells duplicate themselves once every 24 hours.
Have a look at the video below, which shows an animation of DNA replication. Synthesis of new DNA strands is carried out by mainly by an enzyme (a specialised protein molecule) called DNA polymerase III – this protein can string together 100 new base-pairs per second. In total, a human cell spends 8 hours replicating its DNA before the cell divides. Addition of each new base uses up a molecule of ATP.
Task 2 – Cell division
Each cell then splits into two in a process called mitosis. During mitosis, the contents of the ‘parent’ cell split into half, forming two new ‘daughter’ cells. In order to achieve this, the cell must first replicate it’s DNA, then duplicates all the accessory organelles within the cell (e.g. mitochondria, enzymes).
During cell division, the two DNA strands are arranged along the equator of the cell, connected to opposite poles by fibres called as ‘spindles‘. When these fibres contract, they pull the DNA towards opposite ends of the cell. The cell membrane then pinches off in the middle, creating two new cells. Synthesis of new organelles, new cell membranes and the actual process of separating the DNA consumes a large amount of energy in the cell.
This video shows how the spindle fibres pull DNA apart during cell division. This is followed by ‘pinching’ off at the membrane to form two new cells.
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