Cycles in Biology sample essay

this is a sample essay i wrote for my teacher - there may be a few mistakes and i cant find the edited versiom so please dont try to learn it word for word. its more useful to make notes on what can be used in an essays and what is meant by 'synoptic'

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  • Created on: 26-08-15 22:14
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Cycles in biology
Vegetative reproduction is a part of a plant's life cycle because it is a form of asexual reproduction,
which usually produces a stem from for a new plant to be produced from this occurs by producing
genetically identical eukaryotic cells allowing growth of the plant, via mitosis. The cell cycle
facilitates mitosis to produce two daughter cells with identical chromosomes. The first stage is
prophase: the chromosomes appear in the cell and its nuclear envelope disappears. In metaphase, the
chromosomes then line up at the equator of the cell where spindle fibres can attach to the chromatid
from the poles of the cell and during anaphase the fibres retract to pull the chromatids apart to
segregate the chromosomes to each pole of the cell. In telophase, the nuclear envelope reforms and
cytokinesis occurs in which two separate cells with identical genetic information is created for growth
and repair of an organism.
In the life cycles of animals, they must be able to sexually reproduce for the survival of their species.
Meiosis is a form of cell division essential for the parent organisms of offspring to produce gametes
with the haploid number of chromosomes, therefore the offspring will obtain the diploid number of
chromosomes during fertilisation and the zygote is produced during sexual reproduction. This ensures
the offspring in turn will also produce gametes with the correct haploid number of chromosomes to be
reproductively successful therefore meiosis is essential for the sexual life cycle by maintaining the
correct number of chromosomes in offspring from generation to generation and as a result a species'
Predator prey cycles are also influenced by the process of meiosis because it causes intraspecific
variation, genetic variation between individuals of the same species. This is because the process of
meiosis creates variation in alleles, before the cell division occurs the homologous pairs of
chromosomes line up at the equator of the cell randomly, which is independent segregation for when
they separate during anaphase one, and the chromatids wrap around each other causing crossing over
­ where sections of the chromosomes swap between the pairs. Therefore, the alleles passed onto the
next generation are random, causing genetic variation within a species. As a result, organisms such as
rabbits whom may have a selection pressures of biotic factors such as the presence of a predator, for
example foxes, may have an increased likelihood of survival if there is one individual who has the
advantageous gene that is better adapted to the environment due to variation, such as the rabbit being
able to run faster to get away from the foxes. Therefore, the prey that is slower will drop in numbers,
and the faster rabbit will be selected for and as a result reproductively successful. This means that
foxes in the predator prey cycle turn have a selection pressure to be faster, thus natural selection leads
to a drop in predators that are slower, and thus only predators who can outrun the rabbits will survive.
The passage of a nerve impulse is of a cyclic nature, it allows organisms effectors to produce the
effective action in response to a change in its environment ­ a stimulus, for example when a predator
needs to move away from prey, a nerve impulse is sent to the motor neurones and sent to the muscles
to run away. The nerve impulse stimulates the prey's muscles which is the effector as it carries out the
response to move away. The stimulus causes the sodium voltage gates on the axon membrane to
open, thus a sudden influx of sodium ions occurs, and the charge on the axon membrane is reversed ­
it is negative on the outside and positive on the inside. This region now has localised electrical circuits
and leading to the sodium channels to open further along the membrane and causing sodium ion to
influx in that region too, thus causing depolarisation. However, in the previous region the sodium
gates are closed, and the potassium ions leave the axon along their electrochemical gradient. The

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It is this process that stimulates the movement of muscles for the prey to run away
from its predator via the cross bridge cycle of muscle contraction.
The cross bridge cycle occurs when action potential travels down to the sarcoplasm of the muscle
(through the tubules that run to the cytoplasm in the muscle). This stimulates the release of calcium
ions from the endoplasmic reticulum, allowing them to diffuse into the muscle cytoplasm.…read more


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