TELOMERES /DISEASE CAUSING MUTATION- REPLICATION TRANSCRIPTION AND
TRANSLATION
Ex for 10/03/04
Topics in Nanobiotech- 2004
MVDuarte
http://www.swmed.edu/home_pages/cellbio/shay-wright/intro/facts/sw_facts.htmlWith Animation!!
What are telomeres and telomerase?
To better understand telomeres and telomerase, let's first review some basic principles of biology and
genetics. The human body is an organism formed by adding many organ systems together. Those organ
systems are made of individual organs. Each organ contains tissues designed for specific functions like
absorption and secretion. Tissues are made of cells that have joined together to perform those special
functions. Each cell is then made of smaller components called organelles, one of which is called the
nucleus. The nucleus contains structures called chromosomes that are actually "packages" of all the
genetic information that is passed from parents to their children. The genetic information, or "genes" are
really just a series of base pairs called Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). These
base pairs make up our cellular alphabet and create the sequences, or instructions needed to form our
bodies. In order to grow and age, our bodies must duplicate their cells. This process is called mitosis.
Mitosis is a process that allows one "parent" cell to divide into two new "daughter" cells. During mitosis,
cells make copies of their genetic material. Half of the genetic material goes to each new daughter cell.
To make sure that information is successfully passed from one generation to the next, each chromosome
has a special protective cap called a telomere located at the end of it's "arms". Telomeres are controlled by
the presence of the enzyme telomerase. Now that we have covered some basics, let's explore telomeres,
telomerase, and their importance to you!
A telomere is a repeating DNA sequence (TTAGGG) at the end of the body's chromosomes. The
telomere can reach a length of 15,000 base pairs. Telomeres function by preventing chromosomes from
losing base pair sequences at their ends. They also stop chromosomes from fusing to each other.
However, each time a cell divides, some of the telomere is lost (usually 25-200 base pairs per division).
When the telomere becomes too short, the chromosome reaches a "critical length" and can no longer
replicate. This means that a cell becomes "old" and dies by a process called apoptosis. Telomere activity
is controlled by two mechanisms: erosion and addition. Erosion, as mentioned, occurs each time a cell
divides. Addition is determined by the activity of telomerase.
Telomerase, also called telomere terminal transferase, is an enzyme made of protein and RNA subunits
that elongates chromosomes by adding TTAGGG sequences to the end of existing chromosomes.
Telomerase is found in fetal tissues, adult germ cells, and also tumor cells. Telomerase activity is
regulated during development and has a very low, almost undetectable activity in somatic (body) cells.
Because these somatic cells do not regularly use telomerase, they age. The result of aging cells is an aging
body. If telomerase is activated in a cell, the cell will continue to grow and divide. This "immortal cell"
theory is important in two areas of research: aging and cancer.
Cellular aging, or senescence, is the process by which a cell becomes old and dies. It is due to the
shortening of chromosomal telomeres to the point that the chromosome reaches a critical length. Cellular
aging is analogous to a wind up clock. If the clock stays wound, a cell becomes immortal and constantly
produces new cells. If the clock winds down, the cell stops producing new cells and dies. Our cells are
TELOMERES /DISEASE CAUSING MUTATION- REPLICATION TRANSCRIPTION AND
TRANSLATION
Ex for 10/03/04
Topics in Nanobiotech- 2004
MVDuarte
constantly aging. Being able to make the body's cells live forever certainly creates some exciting
possibilities. Telomerase research could therefore yield important discoveries related to the aging process.
Cancer cells are a type of malignant cell. The malignant cells multiply until they form a tumor that
grows uncontrollably. Telomerase has been detected in human cancer cells and is found to be 10-20 times
more active than in normal body cells. This provides a selective growth advantage to many types of
tumors. If telomerase activity was to be turned off, then telomeres in cancer cells would shorten, just like
they do in normal body cells. This would prevent the cancer cells from dividing uncontrollably in their
early stages of development. In the event that a tumor has already thoroughly developed, it may be
removed and anti-telomerase therapy could be administered to prevent relapse. In essence, preventing
telomerase from performing its function would change cancer cells from "immortal" to "mortal”
Knowing what we have just learned about telomeres and telomerase, it can be said that scientists are
on the verge of discovering many of telomerase's secrets. In the future, their research in the area of
telomerase could uncover valuable information to combat aging, fight cancer, and even improve the
quality of medical treatment in other areas such as skin grafts for burn victims bone marrow transplants,
and heart disease. Who knows how far this could go?
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/T/Telomeres.html
Each eukaryotic chromosome consists of a single molecule of DNA associated with a variety of proteins.
The DNA molecules in eukaryotic chromosomes are linear; i.e., have two ends. (This is in contrast to
such bacterial chromosomes as that in E. coli that is a closed circle, i.e. has no ends.)
The DNA molecule of a typical chromosome contains


a linear array of genes (encoding proteins and RNAs) interspersed with
much noncoding DNA.
Included in the noncoding DNA are


long stretches that make up the centromere and
long stretches at the ends of the chromosome, the telomeres.
.
TELOMERES /DISEASE CAUSING MUTATION- REPLICATION TRANSCRIPTION AND
TRANSLATION
Ex for 10/03/04
Topics in Nanobiotech- 2004
MVDuarte
Telomerase
Telomerase is an enzyme that adds telomere repeat sequences to the 3' end of DNA strands. By
lengthening this strand DNA polymerase is able to complete the synthesis of the "incomplete ends" of the
opposite strand.
Telomerase:




is a ribonucleoprotein;
Its single RNA molecule provides an AAUCCC (in mammals) template to guide the insertion of
TTAGGG.
Its protein component — called hTERT in humans ("human TElomere Reverse Transcriptase")
— provides the catalytic action.
Thus telomerase is a reverse transcriptase; synthesizing DNA from an RNA template.
Telomerase is generally found only in



the cells of the germline, including embryonic stem (ES) cells;
unicellular eukaryotes like Tetrahymena thermophila;
cancer cells.
However, some human cells have been shown to briefly express telomerase during S phase of their cell
cycle (a time when new chromosomes are being synthesized). Perhaps all our cells do.
When normal somatic cells are transformed in the laboratory with DNA expressing high levels of
telomerase, they continue to divide by mitosis long after their normal life span is over. And they do so
without any further shortening of their telomeres. This remarkable demonstration (reported by Bodnar et.
al. in the 16 January 1998 issue of Science) provides the most compelling evidence yet that telomerase
and maintenance of telomere length are the key to cell immortality.
http://www.nobel.se/medicine/educational/dna/index.html
DNA-RNA-Protein
DNA carries the genetic information of a cell and consists of thousands of genes. Each gene serves as a
recipe on how to build a protein molecule. Proteins perform important tasks for the cell functions or serve
as building blocks. The flow of information from the genes determines the protein composition and
thereby the functions of the cell.
The DNA is situated in the nucleus, organized into chromosomes. Every cell must contain the genetic
information and the DNA is therefore duplicated before a cell divides (replication). When proteins are
needed, the corresponding genes are transcribed into RNA (transcription). The RNA is first processed so
that non-coding parts are removed (processing) and is then transported out of the nucleus (transport).
Outside the nucleus, the proteins are built based upon the code in the RNA (translation).
TELOMERES /DISEASE CAUSING MUTATION- REPLICATION TRANSCRIPTION AND
TRANSLATION
Ex for 10/03/04
Topics in Nanobiotech- 2004
MVDuarte
The document has two levels, basic and advanced. This page is an introduction to both levels. You start at
the basic level, then you can advance if you want more and deeper information.
DNA Replication
Every cell has to contain the precious book of life, the DNA. To make this possible, a complex
machinery, like a copying machine, copies the cells DNA before it divides. That way each daughter cell
receives an exact copy of the dividing cell's DNA. This doubling of DNA is called replication.
In addition to this "copying machine" cells have evolved mechanisms to correct mistakes that sometimes
occur during DNA replication, a DNA repair system. Abnormalities in these processes, copying and
repairing, result in a failure of accurate replication and maintenance of DNA - a failure that can have
disastrous consequences, such as the development of cancer.
Thus replication constitutes the fundamental condition for biological growth (cell division) and
transmission of genetic information, DNA, from one generation to another (reproduction).
RNA-Transcription
We earlier imagined DNA as an instruction book. Let's even make it a reference book. When you need
information about something you make a copy of the pages (genes) you're interested in, returning the
book to the library. This way you don't have to risk losing or destroying the book.
In all eucaryotic cells DNA never leaves the nucleus, instead the genetic code (the genes) is copied into
RNA which then in turn is decoded (translated) into proteins in the cytoplasm. Why? Wouldn't it be
smarter if DNA itself was translated into proteins in the cytoplasm instead of using a RNA intermediate?
The answer, for many reasons, is no. One important reason is security. The cytoplasm is a dangerous
environment for the DNA and the daily transcription of genes to proteins would be very harmful to the
DNA, which has to stay intact in order to maintain life. Therefore, RNA works as a sort of throw-away
version of DNA (like the copies from the reference book) - good for limited work but not for long-term
storage. Another reason is to regulate the rate of protein synthesis. This will be further discussed in the
section about protein-translation.
TELOMERES /DISEASE CAUSING MUTATION- REPLICATION TRANSCRIPTION AND
TRANSLATION
Ex for 10/03/04
Topics in Nanobiotech- 2004
MVDuarte
Transport
Since the DNA is transcribed into mRNA in the nucleus, and protein synthesis takes place in the
cytoplasm, the mRNA has to exit the nucleus to the cytoplasm. The environment in the nucleus differs in
many ways from that of the cytoplasm. To separate these two environments from each other the nucleus
is enclosed by a double membrane, and the only connection to the surrounding cytoplasm is through
channels
called
the
nuclear
pore
complex
(NPC).
When it is time for the mRNA to leave the nucleus, the mRNA is believed to be "tagged" by proteins
which serve as export signals, directing the mRNA to the nuclear pore complex that the mRNA is to
leave. The mRNA and it's bound export proteins then attaches to export receptors and the whole complex
(RNA, export signal proteins and export receptors) is translocated through the nuclear pore complex. The
mRNA is released into the cytoplasm and is immediately ready for the next step: translation>
Protein / Translation
Translation process
The translation process is divided into three steps:
Initiation: When a small subunit of a ribosome charged with a tRNA+the amino
acid methionine encounters an mRNA, it attaches and starts to scan for a start
signal. When it finds the start sequence AUG, the codon (triplet) for the amino
acid methionine, the large subunit joins the small one to form a complete
ribosome and the protein synthesis is initiated.
TELOMERES /DISEASE CAUSING MUTATION- REPLICATION TRANSCRIPTION AND
TRANSLATION
Ex for 10/03/04
Topics in Nanobiotech- 2004
MVDuarte
Elongation: A new tRNA+amino acid enters the ribosome, at the next codon
downstream of the AUG codon. If its anticodon matches the mRNA codon it
basepairs and the ribosome can link the two aminoacids together.(If a tRNA with
the wrong anticodon and therefore the wrong amino acid enters the ribosome, it
can not basepair with the mRNA and is rejected.) The ribosome then moves one
triplet forward and a new tRNA+amino acid can enter the ribosome and the
procedure is repeated.
Termination: When the ribosome reaches one of three stop codons, for example
UGA, there are no corresponding tRNAs to that sequence. Instead termination
proteins bind to the ribosome and stimulate the release of the polypeptide chain
(the protein), and the ribosome dissociates from the mRNA. When the ribosome is
released from the mRNA, its large and small subunit dissociate. The small subunit
can now be loaded with a new tRNA+methionine and start translation once again.
Some cells need large quantities of a particular protein. To meet this requirement
they make many mRNA copies of the corresponding gene and have many
ribosomes working on each mRNA. After translation the protein will usually
undergo some further modifications before it becomes fully active.
http://www.ncbi.nlm.nih.gov/books/bv.fcgi?call=bv.View..ShowSection&rid=gnd.section.269
Wilson's
disease
Wilson's Disease is a rare autosomal recessive disorder of copper transport, resulting in copper
accumulation and toxicity to the liver and brain. Liver disease is the most common symptom in children;
neurological disease is most common in young adults. The cornea of the eye can also be affected: the
'Kayser-Fleischer ring' is a deep copper-colored ring at the periphery of the cornea, and is thought to
represent copper deposits.
The gene for Wilson's disease (ATP7B) was mapped to chromosome 13. The sequence of the gene was
found to be similar to sections of the gene defective in Menkes disease, another disease caused by defects
in copper transport. The similar sequences code for copper-binding regions, which are part of a
transmembrane pump called a P-type ATPase that is very similar to the Menkes disease protein.
A homolog to the human ATP7B gene has been mapped to mouse chromosome 8, and an authentic model
of the human disease in rat is also available (called the Long-Evans Cinnamon [LEC][ rat). These systems
will be useful for studying copper transport and liver pathophysiology, and should help in the
development of a therapy for Wilson disease.
Porphyria
TELOMERES /DISEASE CAUSING MUTATION- REPLICATION TRANSCRIPTION AND
TRANSLATION
Ex for 10/03/04
Topics in Nanobiotech- 2004
MVDuarte
Porphyria is a diverse group of diseases in which production of heme is disrupted. Porphyria is derived
from the Greek word "porphyra", which means purple. When heme production is faulty, porphyrins are
overproduced and lend a reddish-purple color to urine.
Heme is composed of porphyrin, a large circular molecule made from four rings linked together with an
iron atom at its center. Heme is the oxygen-binding part of hemoglobin, giving red blood cells their color.
It is also a component of several vital enzymes in the liver including the group known as cytochrome
P450. This enzyme family is important in converting potentially harmful substances such as drugs to
inactive products destined for excretion.
Heme synthesis takes place in several steps, each of which requires a specific enzyme of which there are
8 in total. The genes that encode these enzymes are located on different chromosomes, and mutations of
these genes can be inherited in either an autosomal dominant or autosomal recessive fashion, depending
on the gene concerned. Affected individuals are unable to complete heme synthesis, and intermediate
products, porphyrin or its precursors, accumulate.
Environmental triggers are important in many attacks of porphyria. Example triggers include certain
medications, fasting, or hormonal changes. Genetic carriers who avoid a triggering exposure may never
experience symptoms.
The cutaneous porphyrias cause sun sensitivity, with blistering typically on the face, back of the hands,
and other sun-exposed areas. The most common of these is porphyria cutanea tarda (PCT). Triggering
factors are alcohol use, estrogen, iron, and liver disease, particularly hepatitis C.
The acute porphyrias typically cause abdominal pain and nausea. Some patients have personality changes
and seizures at the outset. With time the illness can involve weakness in many different muscles.
The cutaneous and acute forms are treated differently. Cure of these genetic diseases awaits the results of
ongoing research on the safest and most effective means of gene transfer or correction