why DNA/RNAs are called nucleic acids if they are made up of bases also?
Asked by: Mahua Datta
Latest Reply:
Hello Mahua,
You’re right: DNA is built of both acidic and basic components. The acidic component of DNA is its phosphate group, and the basic component of DNA is its nitrogenous base. So, why is DNA called a nucleic “acid” when it also has a basic component? As a reminder, an acid is traditionally a hydrogen donor (often with a negative charge), and a base is traditionally a hydrogen acceptor (often with a positive charge).
Now, let’s review some key aspects of DNA structure. As you already know, the letters in DNA stand for "Deoxyribonucleic Acid." DNA is a polymer made up of monomers called nucleic acids, which are linked together in long chains. Each nucleic acid monomer is made up of a sugar (deoxyribose in DNA), a nitrogenous base, and a phosphate group. The nitrogenous bases are called adenine (A), guanine (G), thymine (T), and cytosine (C). A nitrogenous base linked to a sugar is called a nucleoside; a nitrogenous base linked to a sugar and one or more phosphate groups is called a nucleotide; and polymers built of multiple linked nucleotides are called nucleic acids.
Notably, most DNA is found in a double-stranded form with complementary base pairing between the bases of the two DNA strands: A pairs with T, and C pairs with G. Base pairing occurs via the formation of hydrogen bonds. Two hydrogen bonds form between A–T base pairs, and three hydrogen bonds form between G–C base pairs.
The formation of phosphodiester bonds between adjacent nucleotides forms alternating sugar and phosphate groups, called the “sugar-phosphate backbone” of a DNA molecule. In DNA, the phosphodiester bond is the linkage between the 3' carbon atom of one sugar molecule and the 5' carbon atom of another; importantly these asymmetric bonds mean that each DNA strand has a “direction.” Notably, the phosphate groups in the phosphodiester bond are negatively charged at physiological pH.
Finally, DNA forms a double helix. In a nutshell, the structure of DNA can be thought of as a twisted ladder with its complementary base pairs making up the rungs of the ladder and the sugar-phosphate backbone of each strand making up each side of the ladder. And the exposed phosphate groups have a net negative charge.
Now, let’s revisit your original question. Why is DNA called a nucleic “acid” when it also has a basic component (nitrogenous bases)? The most straightforward answer is that the phosphate group — with its negative charge and exposure to the outside environment — plays a leading role in DNA structure. Although the nitrogenous bases play key roles in base pairing, their basic properties are not as prominent as the acidic properties of the negatively charged phosphate groups that make up the phosphate backbone. We have provided you with a collection of links to helpful websites to help you learn more about the fascinating structure of DNA. We hope the information we’ve provided helps clarify your understanding of DNA!
For more information about DNA and its nucleotide building blocks, check out these links:
http://www.nature.com/scitable/topicpage/dna-is-a-structure-that-encodes-biological-6493050
http://www.nature.com/scitable/topicpage/discovery-of-the-function-of-dna-resulted-6494318
http://www.nature.com/scitable/topicpage/Discovery-of-DNA-Structure-and-Function-Watson-397
http://nobelprize.org/educational_games/medicine/dna_double_helix/readmore.html
http://www.nature.com/scitable/topicpage/Discovery-of-DNA-as-the-Hereditary-Material-340
http://www.nature.com/scitable/topicpage/Isolating-Hereditary-Material-Frederick-Griffith-Oswald-Avery-336
http://www.nature.com/scitable/content/DNA-is-a-double-helix-24263
http://www.nature.com/scitable/content/DNA-is-Packed-into-a-Mitotic-Chromosome-3497
http://www.nature.com/scitable/content/Under-the-electron-microscope-DNA-molecules-undergoing-29586
Submit
You’re right: DNA is built of both acidic and basic components. The acidic component of DNA is its phosphate group, and the basic component of DNA is its nitrogenous base. So, why is DNA called a nucleic “acid” when it also has a basic component? As a reminder, an acid is traditionally a hydrogen donor (often with a negative charge), and a base is traditionally a hydrogen acceptor (often with a positive charge).
Now, let’s review some key aspects of DNA structure. As you already know, the letters in DNA stand for "Deoxyribonucleic Acid." DNA is a polymer made up of monomers called nucleic acids, which are linked together in long chains. Each nucleic acid monomer is made up of a sugar (deoxyribose in DNA), a nitrogenous base, and a phosphate group. The nitrogenous bases are called adenine (A), guanine (G), thymine (T), and cytosine (C). A nitrogenous base linked to a sugar is called a nucleoside; a nitrogenous base linked to a sugar and one or more phosphate groups is called a nucleotide; and polymers built of multiple linked nucleotides are called nucleic acids.
Notably, most DNA is found in a double-stranded form with complementary base pairing between the bases of the two DNA strands: A pairs with T, and C pairs with G. Base pairing occurs via the formation of hydrogen bonds. Two hydrogen bonds form between A–T base pairs, and three hydrogen bonds form between G–C base pairs.
The formation of phosphodiester bonds between adjacent nucleotides forms alternating sugar and phosphate groups, called the “sugar-phosphate backbone” of a DNA molecule. In DNA, the phosphodiester bond is the linkage between the 3' carbon atom of one sugar molecule and the 5' carbon atom of another; importantly these asymmetric bonds mean that each DNA strand has a “direction.” Notably, the phosphate groups in the phosphodiester bond are negatively charged at physiological pH.
Finally, DNA forms a double helix. In a nutshell, the structure of DNA can be thought of as a twisted ladder with its complementary base pairs making up the rungs of the ladder and the sugar-phosphate backbone of each strand making up each side of the ladder. And the exposed phosphate groups have a net negative charge.
Now, let’s revisit your original question. Why is DNA called a nucleic “acid” when it also has a basic component (nitrogenous bases)? The most straightforward answer is that the phosphate group — with its negative charge and exposure to the outside environment — plays a leading role in DNA structure. Although the nitrogenous bases play key roles in base pairing, their basic properties are not as prominent as the acidic properties of the negatively charged phosphate groups that make up the phosphate backbone. We have provided you with a collection of links to helpful websites to help you learn more about the fascinating structure of DNA. We hope the information we’ve provided helps clarify your understanding of DNA!
For more information about DNA and its nucleotide building blocks, check out these links:
http://www.nature.com/scitable/topicpage/dna-is-a-structure-that-encodes-biological-6493050
http://www.nature.com/scitable/topicpage/discovery-of-the-function-of-dna-resulted-6494318
http://www.nature.com/scitable/topicpage/Discovery-of-DNA-Structure-and-Function-Watson-397
http://nobelprize.org/educational_games/medicine/dna_double_helix/readmore.html
http://www.nature.com/scitable/topicpage/Discovery-of-DNA-as-the-Hereditary-Material-340
http://www.nature.com/scitable/topicpage/Isolating-Hereditary-Material-Frederick-Griffith-Oswald-Avery-336
http://www.nature.com/scitable/content/DNA-is-a-double-helix-24263
http://www.nature.com/scitable/content/DNA-is-Packed-into-a-Mitotic-Chromosome-3497
http://www.nature.com/scitable/content/Under-the-electron-microscope-DNA-molecules-undergoing-29586
Submit
Reply From:
Nature Education
Jun 08, 2012 09:07AM
I will surely switch to XL-1 blue cells and will let you know the results, as i have been doing my transformation in DH5 alpha cells and has faced trouble of genomic DNA contamination as I mentioned in earlier post. However, another doubt that comes to my mind is when I treat my vector pUC19 with EcoRI/HindIII in order to do cohesive end ligation, so is it neceassry to give SAP treatment to vector even its being double digested, as ligation seems to be a be big big problem..
Asked by: Anjali Saxena
Latest Reply:
Welcome back Anjali!
We hope the XL-1 blue cells help improve your miniprep DNA purity and restriction enzyme digestion results! And thanks for checking in about shrimp alkaline phosphatase (SAP) treatment of your vector. As you likely know, SAP treatment will remove the 5’ phosphate groups at both ends of the vector and prevent it from re-ligating with itself, since ligase enzyme requires at least one free 5’ phosphate group to do its job. This is important when a vector is digested with a single enzyme or with two enzymes that have compatible ends, since the vector would be much more likely to ligate back with itself than to ligate with the DNA fragment (insert) you would like to clone into it.
The quick answer to your question is that it is not necessary to treat your vector with SAP if you are digesting it with two different enzymes with incompatible ends. In your case, the overhangs produced following your digestion with EcoRI and HindIII will not be capable of ligating with each other. As long as both of the enzymes have optimal activity in the reaction buffer you are using with your restriction enzyme digest, you should not need to include the SAP step. In some cases, one buffer is not optimal for both enzymes; in this case, you can digest your vector with the first enzyme using its optimal buffer, and then do an ethanol precipitation step (as described in our previous answer) before digesting your vector with the second enzyme in its optimal buffer.
As you may know, SAP treatment of any vector will decrease the efficiency of the ligation reaction. Ligase enzyme is most efficient when both the vector and the insert have 5’ phosphate groups attached to their ends. Also, even though you heat-inactivate the SAP, some residual activity often remains; this may lead to dephosphorylation of your insert and loss of ligation activity. Therefore, you will likely have many more transformants after you skip the SAP treatment step. And you’ll be able to rely on blue-white screening to identify vectors with inserts. We hope this information has been helpful! And best of luck to you as you continue your cloning experiments!
To learn more about SAP and other types of phosphatase treatments, follow these links:
http://www.neb.com/nebecomm/products/faqproductM0290.asp
http://www.neb.com/nebecomm/products/productm0289.asp
http://www.neb.com/nebecomm/tech_reference/modifying_enzymes/ligation_tips.asp#.T7bMdu3KyME
http://www.vivo.colostate.edu/hbooks/genetics/biotech/enzymes/phosphatase.html
phosphatase treatment and ligation-google results
The quick answer to your question is that it is not necessary to treat your vector with SAP if you are digesting it with two different enzymes with incompatible ends. In your case, the overhangs produced following your digestion with EcoRI and HindIII will not be capable of ligating with each other. As long as both of the enzymes have optimal activity in the reaction buffer you are using with your restriction enzyme digest, you should not need to include the SAP step. In some cases, one buffer is not optimal for both enzymes; in this case, you can digest your vector with the first enzyme using its optimal buffer, and then do an ethanol precipitation step (as described in our previous answer) before digesting your vector with the second enzyme in its optimal buffer.
As you may know, SAP treatment of any vector will decrease the efficiency of the ligation reaction. Ligase enzyme is most efficient when both the vector and the insert have 5’ phosphate groups attached to their ends. Also, even though you heat-inactivate the SAP, some residual activity often remains; this may lead to dephosphorylation of your insert and loss of ligation activity. Therefore, you will likely have many more transformants after you skip the SAP treatment step. And you’ll be able to rely on blue-white screening to identify vectors with inserts. We hope this information has been helpful! And best of luck to you as you continue your cloning experiments!
To learn more about SAP and other types of phosphatase treatments, follow these links:
http://www.neb.com/nebecomm/products/faqproductM0290.asp
http://www.neb.com/nebecomm/products/productm0289.asp
http://www.neb.com/nebecomm/tech_reference/modifying_enzymes/ligation_tips.asp#.T7bMdu3KyME
http://www.vivo.colostate.edu/hbooks/genetics/biotech/enzymes/phosphatase.html
phosphatase treatment and ligation-google results
Reply From:
Nature Education
May 21, 2012 08:26AM
Dear sir, thanx for your valuable inputs and suggestions. Now, what should i do to concentrate my ligation mix?? Another problem, a bit weired, i am encountreing, i got blue white transformants once on my plate, ofcourse white colonies will be my interest, i checked them using PCR with my specific primers as well as M13 primers, and give me correct amplification, but i dont know somehow, the clones are unstable as after 2-3 days if i grow them on lb amp plate, they show growth, but when i isolate plasmid (using promega kit) i gt only genomic DNA on gel and also smear pattern on gel after digestion. I am really stuck here as firstly i hardly get any clones on plate after ligation and even if i gt some clones, they show me very weird pattern but at the same time they show correct amplification.
Asked by: Anjali Saxena
Latest Reply:
Welcome back Anjali,
Congratulations on your successful acquisition of white colonies — that is fantastic! And better yet, your PCR reactions using gene-specific primers or M13 primers yield the expected amplification products.
Despite your successes, you are now facing a new type of technical problem: your clones appear unstable, since your miniprepped plasmid DNA does not show up on an agarose gel either before or after you digest it with restriction enzymes. Prior to digestion, your DNA is difficult to visualize. And after digesting the DNA, you see a smeared pattern on an agarose gel. How can you overcome these technical hurdles?
We’re happy to let you know that you are not alone! Indeed, we have encountered a similar problem in the lab ourselves. When this happened to us, we discovered that some of the E. coli strains (e.g., HB101, JM series) that are often used for blue-white screening contain extra endonuclease activity (endA+). These endonucleases directly impact plasmid yield and stability when you carry out the DNA miniprep procedure. We used two different approaches to overcome this issue. First, we used miniprep kits that include an extra wash step to inactivate endonucleases from the E. coli host cells. Second, we used a commercially available E. coli strain (XL1-Blue) that is optimized for blue-white screening and is endonuclease-deficient (endA-) for the transformation step. You could simply take one of your clones that tested positive in your PCR reactions and use the miniprep DNA you isolated it to transform an endA- E. coli strain. You could even use DH5alpha E. coli cells, which are endA- but cannot be used for blue-white screening.
Before we end, we’ll also answer your question about how to concentrate your ligation mixture (although this is likely not necessary based on your ability to obtain white clones during your blue-white screening). To accomplish this, we carry out a standard ethanol precipitation step and resuspend the resulting dried DNA pellet in a smaller volume of water or Tris-EDTA (TE) buffer.
We’ve provided a set of links below to help you learn more about the technical tips we’ve provided. We hope they help you successfully isolate and digest your clones of interest. Best of luck to you!
Check out the links below to learn more about alternative miniprep kits, E. coli strains, and ethanol precipitation protocols:
http://www.genomics.agilent.com/files/Manual/200249.pdf
http://public.wsu.edu/~kahn_sci/Flow/E2-QIAprep_Miniprep_Handbook.pdf
http://userpages.umbc.edu/~jwolf/m5.htm
http://kitto.cm.utexas.edu/research/Kittolabpage/Protocols/Microbiology/ethanolPpt.html
http://bitesizebio.com/articles/the-basics-how-ethanol-precipitation-of-dna-and-rna-works/
Congratulations on your successful acquisition of white colonies — that is fantastic! And better yet, your PCR reactions using gene-specific primers or M13 primers yield the expected amplification products.
Despite your successes, you are now facing a new type of technical problem: your clones appear unstable, since your miniprepped plasmid DNA does not show up on an agarose gel either before or after you digest it with restriction enzymes. Prior to digestion, your DNA is difficult to visualize. And after digesting the DNA, you see a smeared pattern on an agarose gel. How can you overcome these technical hurdles?
We’re happy to let you know that you are not alone! Indeed, we have encountered a similar problem in the lab ourselves. When this happened to us, we discovered that some of the E. coli strains (e.g., HB101, JM series) that are often used for blue-white screening contain extra endonuclease activity (endA+). These endonucleases directly impact plasmid yield and stability when you carry out the DNA miniprep procedure. We used two different approaches to overcome this issue. First, we used miniprep kits that include an extra wash step to inactivate endonucleases from the E. coli host cells. Second, we used a commercially available E. coli strain (XL1-Blue) that is optimized for blue-white screening and is endonuclease-deficient (endA-) for the transformation step. You could simply take one of your clones that tested positive in your PCR reactions and use the miniprep DNA you isolated it to transform an endA- E. coli strain. You could even use DH5alpha E. coli cells, which are endA- but cannot be used for blue-white screening.
Before we end, we’ll also answer your question about how to concentrate your ligation mixture (although this is likely not necessary based on your ability to obtain white clones during your blue-white screening). To accomplish this, we carry out a standard ethanol precipitation step and resuspend the resulting dried DNA pellet in a smaller volume of water or Tris-EDTA (TE) buffer.
We’ve provided a set of links below to help you learn more about the technical tips we’ve provided. We hope they help you successfully isolate and digest your clones of interest. Best of luck to you!
Check out the links below to learn more about alternative miniprep kits, E. coli strains, and ethanol precipitation protocols:
http://www.genomics.agilent.com/files/Manual/200249.pdf
http://public.wsu.edu/~kahn_sci/Flow/E2-QIAprep_Miniprep_Handbook.pdf
http://userpages.umbc.edu/~jwolf/m5.htm
http://kitto.cm.utexas.edu/research/Kittolabpage/Protocols/Microbiology/ethanolPpt.html
http://bitesizebio.com/articles/the-basics-how-ethanol-precipitation-of-dna-and-rna-works/
Reply From:
Nature Education
May 11, 2012 04:14PM
Are viruses harmful to the respiratory system especially the airborne ones!
Asked by: eva ROSAS
Latest Reply:
Hello Eva,
Although there are many different types of viruses, your question deals primarily with respiratory viruses. As with other types of viruses, people of all ages can be infected with respiratory viruses; however, young children and elderly individuals are most at risk of experiencing severe symptoms and associated complications. Respiratory viruses include influenza (flu) viruses, human parainfluenza viruses (HPIVs), respiratory syncytial virus (RSV), severe acute respiratory syndrome (SARS), and adenoviruses.
Now, let’s focus on your specific question about the impact of viral infections on the respiratory system. Although the viral infections we’ve mentioned are not often associated with long-term effects on the respiratory system, they can sometimes lead to bronchitis or pneumonia if they are left untreated or if they spread to the lower respiratory system.
What can you do to prevent or reduce your risk of being infected by these viruses? One answer is to wash your hands frequently and thoroughly in order to prevent them from spreading. Vaccines are another preventative measure. However, although vaccines are available for some of these viruses (e.g., influenza, RSV), vaccines for most of them are currently under development. We’ve provided you with a collection of links to informative websites below to help you learn more about these different types of viruses. We hope you find them helpful!
Follow these links to some introductory websites focused on viruses:
http://faculty.ccbcmd.edu/courses/bio141/lecguide/unit3/viruses/virinf.html
http://www.nature.com/scitable/topicpage/the-origins-of-viruses-14398218
To learn about respiratory viruses, check out these links:
http://www.hpa.org.uk/Topics/InfectiousDiseases/InfectionsAZ/RespiratoryViruses/
http://www.nlm.nih.gov/medlineplus/flu.html
http://www.flu.gov/
http://www.chp.edu/CHP/P02522
http://www.cdc.gov/ncidod/dvrd/revb/respiratory/hpivfeat.htm
http://www.nlm.nih.gov/medlineplus/respiratorysyncytialvirusinfections.html
http://www.niaid.nih.gov/topics/rsv/understanding/Pages/quickFacts.aspx
http://www.nlm.nih.gov/medlineplus/severeacuterespiratorysyndrome.html
http://www.cdc.gov/sars/about/fs-SARS.html
http://www.cdc.gov/adenovirus/hcp/clinical-overview.html
http://kidshealth.org/parent/infections/lung/adenovirus.html
Although there are many different types of viruses, your question deals primarily with respiratory viruses. As with other types of viruses, people of all ages can be infected with respiratory viruses; however, young children and elderly individuals are most at risk of experiencing severe symptoms and associated complications. Respiratory viruses include influenza (flu) viruses, human parainfluenza viruses (HPIVs), respiratory syncytial virus (RSV), severe acute respiratory syndrome (SARS), and adenoviruses.
Now, let’s focus on your specific question about the impact of viral infections on the respiratory system. Although the viral infections we’ve mentioned are not often associated with long-term effects on the respiratory system, they can sometimes lead to bronchitis or pneumonia if they are left untreated or if they spread to the lower respiratory system.
What can you do to prevent or reduce your risk of being infected by these viruses? One answer is to wash your hands frequently and thoroughly in order to prevent them from spreading. Vaccines are another preventative measure. However, although vaccines are available for some of these viruses (e.g., influenza, RSV), vaccines for most of them are currently under development. We’ve provided you with a collection of links to informative websites below to help you learn more about these different types of viruses. We hope you find them helpful!
Follow these links to some introductory websites focused on viruses:
http://faculty.ccbcmd.edu/courses/bio141/lecguide/unit3/viruses/virinf.html
http://www.nature.com/scitable/topicpage/the-origins-of-viruses-14398218
To learn about respiratory viruses, check out these links:
http://www.hpa.org.uk/Topics/InfectiousDiseases/InfectionsAZ/RespiratoryViruses/
http://www.nlm.nih.gov/medlineplus/flu.html
http://www.flu.gov/
http://www.chp.edu/CHP/P02522
http://www.cdc.gov/ncidod/dvrd/revb/respiratory/hpivfeat.htm
http://www.nlm.nih.gov/medlineplus/respiratorysyncytialvirusinfections.html
http://www.niaid.nih.gov/topics/rsv/understanding/Pages/quickFacts.aspx
http://www.nlm.nih.gov/medlineplus/severeacuterespiratorysyndrome.html
http://www.cdc.gov/sars/about/fs-SARS.html
http://www.cdc.gov/adenovirus/hcp/clinical-overview.html
http://kidshealth.org/parent/infections/lung/adenovirus.html
Reply From:
Nature Education
May 04, 2012 01:47PM
IN WOMEN ONE X CHROMOSOME IS RANDOMLY INACTIVATED.(LYONIZATION).. IN TURNER SYNDROME 45 X0 ONE X CHROMOSOME IS ABSENT.. ONLY ONE X CHROMOSOME IS FUNCTIONAL.... NORMALLY ALSO ONE X CHROMOME IS INACTIVATED.. THEN HOW THE SYNDROME DEVELOPS...?
Asked by: muthu chidambaram
Latest Reply:
Hello Muthu,
Let's begin answering by going over some background on X chromosomes.
As you already know from your research, Turner syndrome is caused by the absence of a sex chromosome. Human cells normally have 23 pairs of chromosomes and 46 chromosomes in total. One of these pairs is the sex chromosomes, which determine whether a human is male or female. Normally, males have an X and a Y chromosome (they are XY), whereas females have two X chromosomes (they are XX). Turner syndrome is caused in females when one X chromosome is present but the other is missing entirely (or missing in some cells). Since individuals with Turner syndrome have 45 total chromosomes in each cell and only one X chromosome, Turner syndrome is also called "45,X."
Turner syndrome does not involve a carrier parent as do many other genetic disorders, and it is generally not inherited in families across multiple generations. It is more commonly caused by a random mistake when the eggs or sperm were formed and one X chromosome was lost.
How is Turner syndrome identified at the molecular level? It is often diagnosed via two widely used tests: fluorescent in situ hybridization (called "FISH") and karyotyping. These tests can be used to identify, examine, and count the chromosomes in the cell. When used together, the tests can confirm one another's results. Additionally, a number of fetal abnormalities may suggest that a baby has Turner syndrome, including cystic hygroma. Turner syndrome can be diagnosed using these tests at any stage of a baby’s development since they show that one X chromosome is missing.
As you mentioned, X chromosome inactivation occurs in female mammals, resulting in chromosome mosaicism at the level of gene expression. Most genes on the inactivated X chromosome will not be expressed. This phenomenon is very important for maintaining X chromosome gene dosage at an acceptable level in females. Interestingly, while one X chromosome might be inactivated in one set of cells, the other might be inactivated in other cells.
An example of where X chromosome inactivation produces an observable phenotype is in tortoiseshell cats, which have a mottled black and orange coat. In cats, the primary gene for coat color has two alleles: the b allele produces an orange pigment and the B allele produces a black pigment. In heterozygous female cats, cells where the X chromosome carrying the b allele has been inactivated will produce the black pigment; cells in which the X chromosome carrying the B allele has been inactivated will express the orange pigment. The result is a female cat with black and orange patches of fur in its coat. Can you figure out why it is extremely rare to find a male tortoiseshell cat?
Now, let’s address your original question. If individuals with Turner syndrome have only one X chromosome but only one X chromosome is normally transcriptionally active in females, then why does Turner syndrome occur? The quick answer to your question is that around 15% of the genes that reside on the inactivated X chromosome in humans escape X inactivation. Notably, these genes are often referred to as “escape genes.” As a result, individuals with Turner syndrome have a deficit in the expression levels of the escape genes on their X chromosome. Presumably, this decrease in X chromosome escape gene expression is largely responsible for the symptoms that accompany Turner syndrome. Intriguingly, many of these genes are normally expressed in specific tissues. To help you learn more about these fascinating topics, we’ve provided you with a set of links to helpful below. Happy reading!
If you are interested in reading more about Turner syndrome, here are some links that might be helpful:
http://www.nlm.nih.gov/medlineplus/turnersyndrome.html
http://ghr.nlm.nih.gov/condition=turnersyndrome
http://www.turnersyndrome.org/
http://www.nature.com/scitable/topicpage/Chromosomal-Abnormalities-Aneuploidies-290
http://www.nature.com/scitable/topicpage/Somatic-Mosaicism-and-Chromosomal-Disorders-867
http://www.nature.com/scitable/topicpage/Mitosis-Meiosis-and-Inheritance-476
Here are some links about how Turner syndrome and other genetic diseases are diagnosed and the reliability of these tests:
http://www.turnersyndrome.org/index.php?option=com_content&view=article&id=26:how-is-turner-syndrome-diagnosed&catid=55:faqs-general-questions&Itemid=300053
http://www.nature.com/scitable/topicpage/Cytogenetic-Methods-and-Disease-Flow-Cytometry-CGH-772
http://www.nature.com/scitable/topicpage/Prenatal-Screen-Detects-Fetal-Abnormalities-306
http://www.turnersyndrome.org/index.php?option=com_content&view=article&id=103:how-reliable-are-the-fish-and-the-kayotype-tests&catid=55:faqs-general-questions&Itemid=300053
Check out these websites to learn more about X chromosome inactivation:
http://www.nature.com/scitable/topicpage/x-chromosome-x-inactivation-323
http://www.vivo.colostate.edu/hbooks/genetics/medgen/chromo/mosaics.html
http://www.bio.miami.edu/dana/dox/calico.html
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/SexChromosomes.html#x-inactivation
http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter13/x_inactivation.html
Follow these links to primary research and review articles focused on Turner syndrome and X chromosome inactivation:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2837040/?tool=pubmed
http://www.nature.com/nature/journal/v434/n7031/full/nature03479.html
http://genesdev.cshlp.org/content/20/14/1848.full
Let's begin answering by going over some background on X chromosomes.
As you already know from your research, Turner syndrome is caused by the absence of a sex chromosome. Human cells normally have 23 pairs of chromosomes and 46 chromosomes in total. One of these pairs is the sex chromosomes, which determine whether a human is male or female. Normally, males have an X and a Y chromosome (they are XY), whereas females have two X chromosomes (they are XX). Turner syndrome is caused in females when one X chromosome is present but the other is missing entirely (or missing in some cells). Since individuals with Turner syndrome have 45 total chromosomes in each cell and only one X chromosome, Turner syndrome is also called "45,X."
Turner syndrome does not involve a carrier parent as do many other genetic disorders, and it is generally not inherited in families across multiple generations. It is more commonly caused by a random mistake when the eggs or sperm were formed and one X chromosome was lost.
How is Turner syndrome identified at the molecular level? It is often diagnosed via two widely used tests: fluorescent in situ hybridization (called "FISH") and karyotyping. These tests can be used to identify, examine, and count the chromosomes in the cell. When used together, the tests can confirm one another's results. Additionally, a number of fetal abnormalities may suggest that a baby has Turner syndrome, including cystic hygroma. Turner syndrome can be diagnosed using these tests at any stage of a baby’s development since they show that one X chromosome is missing.
As you mentioned, X chromosome inactivation occurs in female mammals, resulting in chromosome mosaicism at the level of gene expression. Most genes on the inactivated X chromosome will not be expressed. This phenomenon is very important for maintaining X chromosome gene dosage at an acceptable level in females. Interestingly, while one X chromosome might be inactivated in one set of cells, the other might be inactivated in other cells.
An example of where X chromosome inactivation produces an observable phenotype is in tortoiseshell cats, which have a mottled black and orange coat. In cats, the primary gene for coat color has two alleles: the b allele produces an orange pigment and the B allele produces a black pigment. In heterozygous female cats, cells where the X chromosome carrying the b allele has been inactivated will produce the black pigment; cells in which the X chromosome carrying the B allele has been inactivated will express the orange pigment. The result is a female cat with black and orange patches of fur in its coat. Can you figure out why it is extremely rare to find a male tortoiseshell cat?
Now, let’s address your original question. If individuals with Turner syndrome have only one X chromosome but only one X chromosome is normally transcriptionally active in females, then why does Turner syndrome occur? The quick answer to your question is that around 15% of the genes that reside on the inactivated X chromosome in humans escape X inactivation. Notably, these genes are often referred to as “escape genes.” As a result, individuals with Turner syndrome have a deficit in the expression levels of the escape genes on their X chromosome. Presumably, this decrease in X chromosome escape gene expression is largely responsible for the symptoms that accompany Turner syndrome. Intriguingly, many of these genes are normally expressed in specific tissues. To help you learn more about these fascinating topics, we’ve provided you with a set of links to helpful below. Happy reading!
If you are interested in reading more about Turner syndrome, here are some links that might be helpful:
http://www.nlm.nih.gov/medlineplus/turnersyndrome.html
http://ghr.nlm.nih.gov/condition=turnersyndrome
http://www.turnersyndrome.org/
http://www.nature.com/scitable/topicpage/Chromosomal-Abnormalities-Aneuploidies-290
http://www.nature.com/scitable/topicpage/Somatic-Mosaicism-and-Chromosomal-Disorders-867
http://www.nature.com/scitable/topicpage/Mitosis-Meiosis-and-Inheritance-476
Here are some links about how Turner syndrome and other genetic diseases are diagnosed and the reliability of these tests:
http://www.turnersyndrome.org/index.php?option=com_content&view=article&id=26:how-is-turner-syndrome-diagnosed&catid=55:faqs-general-questions&Itemid=300053
http://www.nature.com/scitable/topicpage/Cytogenetic-Methods-and-Disease-Flow-Cytometry-CGH-772
http://www.nature.com/scitable/topicpage/Prenatal-Screen-Detects-Fetal-Abnormalities-306
http://www.turnersyndrome.org/index.php?option=com_content&view=article&id=103:how-reliable-are-the-fish-and-the-kayotype-tests&catid=55:faqs-general-questions&Itemid=300053
Check out these websites to learn more about X chromosome inactivation:
http://www.nature.com/scitable/topicpage/x-chromosome-x-inactivation-323
http://www.vivo.colostate.edu/hbooks/genetics/medgen/chromo/mosaics.html
http://www.bio.miami.edu/dana/dox/calico.html
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/SexChromosomes.html#x-inactivation
http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter13/x_inactivation.html
Follow these links to primary research and review articles focused on Turner syndrome and X chromosome inactivation:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2837040/?tool=pubmed
http://www.nature.com/nature/journal/v434/n7031/full/nature03479.html
http://genesdev.cshlp.org/content/20/14/1848.full
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Nature Education
May 04, 2012 01:45PM
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