Cycles in biology 25 Marks - Paper 3 Practise Essay
All cells arise from other cells; this occurs by the process of the cell cycle, the process of
mitosis. However, not all cells within an organism retain the ability to divide. The first stage of
the cell cycle is called interphase, this occupies most of the cell cycle and involves replication of
genetic material, including membrane bound organelles; no cell division occurs in this stage.
The next stage involves cell division, either as meiosis where 4 genetically different daughter
cells of mitosis, where 2 genetically identical daughter cells are produced. The first stage of
mitosis is prophase; in this stage chromosomes condense and become shorter and thicker, the
nuclear membrane breaks down and spindle fibres extend from the centrioles which. The next
stage is metaphase where the chromosomes align themselves at the equator and the spindle
fibres attach to the centromere, joining the two sister chromatids together. After metaphase is
anaphase, where the centromere joining the sister chromatids split and the sister chromatids
are pulled to opposite poles, we now refer to them as chromosomes. This process requires
energy from mitochondria, which respire and produce ATP. once the chromosomes reach their
respective poles, they uncoil and become longer. A nuclear membrane then reforms around
each chromosome and from this the cytoplasm divides by a process known as cytokinesis; this
is cell division; completing the cycle. The process can continue as new cell growth and repair is
needed. However, this becomes unnecessary if uncontrolled cell division occurs, potentially
leading to the formation of a tumour. This can occur if a tumour suppressor gene is inactivated
and so a protein that codes for slower cell division is not produced and cell division increases.
Another cycle within biology includes the phosphorus cycle, life depends on this cycle as
phosphorus is an important element required for DNA, AP and many other biological molecules.
The phosphorus cycle does not start in the atmosphere, like the nitrogen cycle but rather
instead starts as phosphorus ions in sedimentary rock deposits, which originate in the sea but
are brought to surface by geological uplifting. When these rocks are weathered, the phosphate
ions become available for absorption into the plant and thus conversion into part of their
biomass. The phosphate ions are then passed through the ecosystem as consumers feed on
the plants and excess phosphate ions are excreted and may accumulate as guano. When
animals that have the phosphate ions as part of their biomass as phosphate is a central
component of DNA and ATP and therefore used within protein synthesis and mitosis, the
phosphate ions are released in decomposition. The cycle therefore restarts as plants take up
the phosphate ions from the soil or as the phosphate ions are compressed to form sedimentary
rock deposits again
The nitrogen cycle involves the transfer of nitrogen compounds and nitrate ions through an
ecosystem, involving both the abiotic and biotic factors that inhabit it. Nitrogen gas is first fixed
into ammonium ions in the soil by nitrogen fixing bacteria. Similarly, saprobiontic bacteria fix the
nitrogen compounds in the protein and amino acids of dead organisms and break them down to
provide a further source of ammonium in the soil. This ammonium is then fixed into nitrite ions
and then nitrate ions by nitrogen fixing bacteria in aerobic conditions, where there is plenty of
oxygen present. These nitrite ions are then available to be actively transported into the root of
plants, where nitrogen is used for the formation of DNA and thus is an important contributor to
