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Lecture 6

BIOLOGY 1A03 Lecture Notes - Lecture 6: Ionizing Radiation, Dna Replication, Oncogene

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Lovaye Kajiura

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2.5 Cell Division
2.5.1 - Outline the stages in the cell cycle, including interphase (G₁, S, G₂), mitosis and
Interphase is when the DNA replicates.
The cell will also replicate its
centrosome, which is important for
movement of chromosomes. This is split
into three stages:
G₁ is the first stage, and stands for
Gap 1. During this time, the
cytoplasm is still active, and the cell
continues with its normal
functions, such as protein synthesis, mitochondria replication or chloroplast
replication. There is all the activity of a growing cell.
S is the synthesis phase when the DNA is replicated. The mass of the DNA in the cell
doubles. All the chromosomes are copied and form chromatids. These remain
attached until they divide in mitosis.
G₂ is the third stage, standing for Gap 2, when there is more growth of the cell, then
preparation takes place for cell division.
Mitosis then happens. This consists of four stages: prophase, metaphase, anaphase and
telophase. The chromosomes are separated and distributed.
Cytokinesis is the division of the cytoplasm to form two daughter cells. The cell cycle is then
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2.5.2 - State that tumours (cancers) are the result of uncontrolled cell division and that
these can occur in any organic tissue
Tumours, or cancers, are cell mass formed as a result of uncontrolled cell division. They can
occur in any tissue.
In a tumour, the normal repressed state of mitosis is disrupted by mutation to the proto-
oncogene. As a result, the cells begin to divide uncontrollably. The proto-oncogene mutates
into the oncogene, resulting in the loss of control of cell division.
The cells form an irregular mass of cells; the tumour. Some cells may break away and form a
secondary tumour elsewhere. Eventually they take over the surrounding, healthy cells,
which leads to malfunction and death.
It is caused by damage to DNA chromosomes. The accumulation of mistakes in DNA causes
cancer, which is why it is more common in older people. Another cause is damage to the
gene that codes for p53, the protein which stops the copying of damaged DNA.
The damage to the DNA can result from ionising radiation (X-rays, gamma rays...), some
chemicals (tar in tobacco smoke) as well as virus infections. Some factors are also inherited.
The development of cancer requires at least two mutations; one of the proto-oncogene;
two of the tumour suppressor.
Cancer exerts its deleterious effect on the body by destroying the adjacent tissues (such as
compressing nerves, eroding blood vessels), replacing normal functioning cells (such as
replacing blood forming cells in the bone marrow or the heart muscles so that the heart
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