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Summary Genetics Chapter 25
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Cancer = a disease characterized by uncontrolled cell division. It is a genetic disease at thecellular level. More than 100 kinds of human cancers are known, these are
classified according to the type of cell that has become cancerous.
Most cancers originate from a single cell: A cancerous growth can be
considered to be clonal
At the cellular & genetic level, cancer is usually a multistep process; It
begins with a precancerous genetic change (i.e. benign growth =
‘goedaardig’); Following additional genetic changes, it progresses to
cancerous cell growth
Once a cellular growth has become malignant, the cells are invasive (i.e.,
they can invade healthy tissues); They are also metastatic (i.e., they can
migrate to other parts of the body)
An environmental agent that causes cancer is termed a carcinogen.
During the 1970s, RSV research led to the discovery of oncogenes (genes that
promote cancer): Mutant RSV strains did not transform chicken fibroblast cells
These RSV strains contained a defective viral gene designated src: For sarcoma, the
type of cancer it causes: The src gene is also designated v–src (for viral src): It is the
first example of a viral oncogene
The v–src gene is not important for viral replication
So researchers wondered why should the virus carry it?
Harold Varmus and Michael Bishop discovered that viral oncogenes had a cellular
origin!
A normal copy of the src gene is found in the host cell’s chromosome: It is
designated c–src (for cellular src): Once incorporated into the viral genome, c–src
can now cause cancer when a cell is infected by such a virus
There are two possible explanations:
1. Viral replication leads to overexpression of the src gene
2. The v–src gene may accumulate additional mutations that has converted it into
an oncogene.
Cancer cells often contain: aneuploidies, gene amplifications, translocations.
1. Oncogenes = generate tumors: a mutant gene that promotes cancer when it is
overexpressed:
Oncogenes have gain-of-function Oncogenes promote abnormal cell growth. Proto-oncogenes are normal cellular
mutations that affect cell division: a genes that can be mutated into an oncogene -> Expression becomes abnormally
proto-oncogene is a normal cellular active => This is a gain-of-function mutation
gene that through a gain-of-function This can occur in three ways:
mutation becomes an oncogene 1. The amount of the encoded protein is greatly increased.
2. A change occurs in the structure of the encoded protein that causes
it to be overly active.
3. The encoded protein is expressed in a cell type where it is not
normally expressed.
, Proto-oncogenes can mutate into
oncogenes.
Examples:
*The genes described in this table are
found in humans as well as other
vertebrate species. Many of these
genes were initially identified in
retroviruses. Most of the genes have
been given three-letter names that are
abbreviations for the type of cancer
the oncogene causes or the type of
virus in which the gene was first
identified.
Growth factors In eukaryotes, the cell cycle is regulated in part by polypeptide hormones (signaling
molecules) known as growth factors (because they promote cell division).
Cell cycle is regulated in part by growth factors that bind to cell surface receptors
and initiate a cascade of cellular events leading ultimately to cell division.
E.g. Epidermal growth factor (EGF) The activation of a cell-signaling pathway by a growth factor. In this example,
epidermal growth factor (EGF) binds to two EGF receptors, causing them to
dimerize and phosphorylate each other. An intracellular protein called GRB2 is
attracted to the phosphorylated EGF receptor, and it is subsequently bound by
another protein called Sos. The binding of Sos to GRB2 enables Sos to activate a
protein called Ras. This activation involves the release of GDP and the binding of
GTP. The activated Ras/GTP complex then activates Raf-1, which is a protein kinase.
Raf-1 phosphorylates MEK, and then MEK phosphorylates MAPK. More than one
MAPK may be involved. Finally, the phosphorylated form of MAPK activates
transcription factors, such as Myc, Jun, and Fos. This leads to the transcription of
genes, which encode proteins that promote cell division.
An oncogene may promote cancer by keeping the cell growth signaling pathway
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