Mutations defined as changes in the sequence of the bases, can occur by a number of means: for example, single base substitutions that might result from errors during DNA replication (copying of the DNA prior to cell division; see below), or deletion of larger pieces of a chromosome or translocation events (swapping pieces from one chromosome to another).
It is also important to note that mutations can be grouped into two general categories, germline and somatic.
Germline mutations are those that we inherit from our parents and will pass on to our children, while somatic mutations are those that occur during our lifetime and are not passed on to our descendants. Most mutations have no effect on our health and lifespan because they result in changes to DNA base sequences that do not alter the coded genetic information.
However, those mutations that make changes to the genetic information such that cells no longer can control their growth are the hallmark of cancer. The discovery of the structure of DNA in 1953 by James Watson and Francis Crick initiated a rapid increase in the study of how the information encoded in the double-helical fibers of DNA was copied and passed from generation to generation. It has been estimated that the human genome carries approximately 35,000 sequences whose codes are read to produce protein molecules with specific metabolic functions.
In cells that are growing and dividing, and even in those cells in a resting phase, mutations occur at an alarmingly high rate, both from normal cellular processes (e.g., copying of the DNA strands prior to cell division) and from exposure to environmental or chemical carcinogens [e.g., the ultraviolet rays in sunlight or especially cigarette smoke (at least 40 different carcinogens are found in a typical cigarette)]. However, all cells use remarkably effective strategies to either avoid mutations or to repair them when they occur.
Showing posts with label MUTATIONS. Show all posts
Showing posts with label MUTATIONS. Show all posts
Monday, July 27, 2009
The Base pairs in a DNA Strand - A : Tand G : C
The bases in each of the two DNA strands form what arereferred to as base pairs, and each strand is polymerized in an orientation oppositerelative to its partner. The chemical composition of the bases dictates that A always pairs with T on the opposite strand, and G with C. Hence, normal DNA contains only A : Tand G : C base pairs.
Mutations defined as changes in the sequence of the bases, can occur by a number of means: for example, single base substitutions that might result from errors during DNA replication (copying of the DNA prior to cell division; see below), or deletion of larger pieces of a chromosome or translocation events (swapping pieces from one chromosome to another).
Mutations defined as changes in the sequence of the bases, can occur by a number of means: for example, single base substitutions that might result from errors during DNA replication (copying of the DNA prior to cell division; see below), or deletion of larger pieces of a chromosome or translocation events (swapping pieces from one chromosome to another).
MUTATIONS AND CELL DEFENSES
INTRODUCTION
The information required for the development and growth of an organism is encoded in its deoxyribonucleic acid (DNA), the genetic material. In human cells, the entire DNA content, or genome, is packaged in 46 chromosomes that reside in the cell’s nucleus. Because this information is critically important for all biological processes occurring during the lifetime of an organism, evolution has resulted in many and various means by which a cell can either avoid damage to this information, or repair damage once it has occurred.
DNA is a double-strandedhelical ribbon, with each strand consisting of a continuous sequence of bases. Bases can be considered as the letters of words that are strung together to make up the information code. Remarkably, only four bases—adenine, guanine, cytosine, and thymine (A, G, C, and T)—are required to generate the amazingly diverse information encoded in the human genome, and it is the varying, ordered sequence of these bases that creates all the different bits of information along the DNA strand. For the purposes of our discussion in this chapter, remember that genetic information is decoded by the following general path: DNA codes for ribonucleic (RNA), acid which in turn codes for protein molecules, whose amino acid sequence derives from the DNA sequence. Although this is an oversimplification of the complex information contained within DNA, it suffices for our appreciation of how changes in the DNA sequence (mutations) may adversely effect cellular function.
The information required for the development and growth of an organism is encoded in its deoxyribonucleic acid (DNA), the genetic material. In human cells, the entire DNA content, or genome, is packaged in 46 chromosomes that reside in the cell’s nucleus. Because this information is critically important for all biological processes occurring during the lifetime of an organism, evolution has resulted in many and various means by which a cell can either avoid damage to this information, or repair damage once it has occurred.
DNA is a double-strandedhelical ribbon, with each strand consisting of a continuous sequence of bases. Bases can be considered as the letters of words that are strung together to make up the information code. Remarkably, only four bases—adenine, guanine, cytosine, and thymine (A, G, C, and T)—are required to generate the amazingly diverse information encoded in the human genome, and it is the varying, ordered sequence of these bases that creates all the different bits of information along the DNA strand. For the purposes of our discussion in this chapter, remember that genetic information is decoded by the following general path: DNA codes for ribonucleic (RNA), acid which in turn codes for protein molecules, whose amino acid sequence derives from the DNA sequence. Although this is an oversimplification of the complex information contained within DNA, it suffices for our appreciation of how changes in the DNA sequence (mutations) may adversely effect cellular function.
Sunday, July 26, 2009
Cell signaling
Cells communicate and respond to the extracellular environment through a process designated signal transduction. Signal molecules bind to transmembrane receptors that span the cell membrane. The interaction of signal molecules with components of receptors located outside the cell modifies the intracellular components of the receptors. An environmental signal is thereby transduced into a cascade of regulatory steps that control genes which control cell proliferation and specialized properties of cells.
In some signaling pathways, scaffold proteins assemble signaling molecules into complexes for the initial passage of information from the transmembrane receptor to relay and adaptor proteins. Subsequent steps in the signaling process amplify and integrate signals. A chain of intracellular signaling proteins processes regulatory information through the cytoplasm and into the cell nucleus to activate or suppress genes. In other signaling pathways the regulatory cascades are abbreviated. The transduction of regulatory information from the intracellular component of the transmembrane receptor is more direct, circumventing intermediary steps in information transfer. At an early stage in the signaling process a signaling protein enters the nucleus and interacts directly with genes to modify expression.
Many cancer cells exhibit defects in one or more steps of signaling cascades that alter control of
cell growth, specialized cell properties, cell–cell communication, cell motility, and cell adhesion. The components of signaling pathways that are modified in tumor cells are targets for treatments that are effective and specific.
In some signaling pathways, scaffold proteins assemble signaling molecules into complexes for the initial passage of information from the transmembrane receptor to relay and adaptor proteins. Subsequent steps in the signaling process amplify and integrate signals. A chain of intracellular signaling proteins processes regulatory information through the cytoplasm and into the cell nucleus to activate or suppress genes. In other signaling pathways the regulatory cascades are abbreviated. The transduction of regulatory information from the intracellular component of the transmembrane receptor is more direct, circumventing intermediary steps in information transfer. At an early stage in the signaling process a signaling protein enters the nucleus and interacts directly with genes to modify expression.
Many cancer cells exhibit defects in one or more steps of signaling cascades that alter control of
cell growth, specialized cell properties, cell–cell communication, cell motility, and cell adhesion. The components of signaling pathways that are modified in tumor cells are targets for treatments that are effective and specific.
GENES, MUTATIONS, AND CANCER
These connections with environmental factors suggest that some cancers could originate from agents that change a cell’s genetic material (mutation). Each of the more than 100 trillion cells in a human body carries its genetic information in deoxyribonucleic acid (DNA), composed of long double-helical strands made of sequences of four building blocks (bases) linked in pairs. It is packaged in 23 pairs of chromosomes which can be seen with a microscope. The DNA in each cell carries information equal to the letters in 600 encyclopedia volumes. Genes are sequences of DNA that code for individual proteins.
Mutations are errors in DNA structure that alter this genetic information. Most mutations arise spontaneously, possibly from mistakes that arise while DNA duplicates during cell growth. Experiments have shown that foods contain many chemicals that cause carcinogenic damage to DNA. Errors can also be produced by damage from toxic chemicals (carcinogens) or radiation. Cell growth is stopped when molecular mechanisms termed checkpoints sense the damage, recruit the molecules to rectify the problem, and give time for corrections to be made.
Then enzymes for repair are activated, and the cell may recover if the damage was not too severe. Genes designated BRCA1 and BRCA2 are involved in DNA repair and are mutated in some breast and ovarian cancers. The inability to repair damaged DNA may result in cancer.
Mutations are errors in DNA structure that alter this genetic information. Most mutations arise spontaneously, possibly from mistakes that arise while DNA duplicates during cell growth. Experiments have shown that foods contain many chemicals that cause carcinogenic damage to DNA. Errors can also be produced by damage from toxic chemicals (carcinogens) or radiation. Cell growth is stopped when molecular mechanisms termed checkpoints sense the damage, recruit the molecules to rectify the problem, and give time for corrections to be made.
Then enzymes for repair are activated, and the cell may recover if the damage was not too severe. Genes designated BRCA1 and BRCA2 are involved in DNA repair and are mutated in some breast and ovarian cancers. The inability to repair damaged DNA may result in cancer.
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