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UNIT 4
DNA Replication
Objectives Discuss experimental evidence supporting semi-
conservative mechanism of DNA replication Explain DNA replication in Prokaryotes and Eukaryotes Define mutation Identify different types of mutations and their effects on
the protein products produced
DNA runs 5’ to 3’ and the two strands run anti-parallel to each other
This thus suggests a method by which DNA replication might occur
RECAP
1. Conservative replication
Following replication, one daughter molecule contains both of the parental strands.
The other daughter molecule contains two newly synthesized DNA strands
However three theories were put forward as to how DNA replication may occur
2. Dispersive replication
Both strands of each daughter molecule contain nucleotides derived from the parental molecule.
However three theories were put forward as to how DNA replication may occur
3. Semi- conservative replication
molecules are produced with both old and new DNA each molecule would be composed of one old strand and one new one.
However three theories were put forward as to how DNA replication may occur
Which is correct?
Each new double helix retains or conserves one of the two strands of the original double helix
This proven by experiments conducted by two scientist.
DNA Replication is semi-conservative
(1957) Mathew Meselson and Franklin Stahl conducted an experiment to prove that DNA replication was semi- conservative.
1. They used two N (nitrogen)
isotopes (Isotopes are variants of a particular chemical element)
2. 15N heavy isotope and 14N light isotope
3. They made 2 growth media one with each isotope and grew E. coli on each
DNA Replication is semi-conservativeExperimental Proof
5. They allowed the bacteria to grow for several generations ( E.coli reproduces every 20 mins)
6. This allowed the 15N isotope and the 14N to be incorporated into the E. coli DNA and label it.
7. Thus the DNA contained either light or heavy DNA but not both
DNA Replication is semi-conservativeExperimental Proof
8. They took samples from each bacterial culture
9. The two cultures were then mixed together.
10. Cesium chloride (CsCl) was then added to the mixture
DNA Replication is semi-conservativeExperimental Proof
11. They sample was placed on a centrifuge and spun at high speed for 20 hrs
12. After centrifugation a consentration gradient is formed in the tube with the densest area being at the bottom
13. The lighter DNA containing the 14N floats to the top and the heavier DNA containing 15N is found close to the bottom
DNA Replication is semi-conservativeExperimental Proof
Since Meselson and Sathl now knew how they could differentiate the DNA based on density then they could test the semi-conservative replication using this newly found basis
DNA Replication is semi-conservativeExperimental Proof
1. They made medium with the 15N heavy isotope and added E. coli they allowed the E. coli to grow for 14 generations to make sure that all the DNA would be contain the isotope
2. They then isolated a sample extracted the DNA and added CsCl
DNA Replication is semi-conservativeExperimental Proof
3. They made medium with the 14N light isotope and transferred some of the bacteria from the 15N heavy isotope media to it.
4. They were left to grow on the 14N for a few generations
5. Thy called the first sample from the 15N medium generation 0
6. They took a sample every 20 mins from the 14N with mixed with bacteria from the 15N
7. They called the 1st sample generation 1 and the other after 20 mins generation 2 and so on
DNA Replication is semi-conservativeExperimental Proof
8. They added CsCl to every sample and centrifuged for 20 hrs
9. Generation 0 -all the DNA was found to be heavy DNA (as expected)
10. Generation 1- all DNA were in the neither heavy or light but found in between the two (intermediate)
11. Generation 2- 50% heavy and 50% intermediate
Why did this occur if only heavy and light isotopes were used?
DNA Replication is semi-conservativeExperimental Proof
Why did this occur if only heavy and light isotopes were used?
The answer must be that the two combined therefore DNA replication must be SEMI -CONSERVATIVE
DNA Replication is semi-conservativeExperimental Proof
In that one strand that contained the 15N isotope acted as a template
so that during replication the new DNA strand being synthesized had to incorporate the light 14N isotope the two separate strands then recombined so that the resulting DNA would have both light 14N and heavy 15N isotpoes incorporated
DNA Replication is semi-conservativeExperimental Proof
This finding supported the semi-conservative model - each double helix would contain one previously synthesized strand and a newly synthesized strand.
http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter14/animations.html
Self help Tutorial http://www.sumanasinc.com/webcontent/ani
mations/content/meselson.html
Animation of DNA Replication Experimental Proof
In general, DNA is replicated by:◦ uncoiling of the helix◦ strand separation by breaking of the
hydrogen bonds between the complementary strands
◦ synthesis of two new strands by complementary base pairing
DNA Replication
• Recap- bacterial DNA is circular
• Replication begins at a specific site in the DNA
called the origin of replication
(ori)
DNA Replication in Bacteria
DNA replication is bidirectional from the origin of replication
DNA replication occurs in both directions from the origin of replication in the circular DNA found in most bacteria.
DNA Replication in Bacteria
To begin DNA replication, unwinding enzymes called DNA helicases bind to the DNA molecule
This causes the two parent DNA
strands to unzip and unwind thus separating from each other separate from one another at the origin of replication to form two "Y"-shaped replication forks.
These replication forks are the actual site of DNA copying
Two replication forks are actually formed in bacteria that is why DNA replication is bidirectional
DNA Replication in BacteriaDNA Helicase
Replication Fork
http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter14/animations.html#
Animation of Replication Fork
Helix destabilizing proteins bind to the single-stranded regions so the two strands do not rejoin
Enzymes called topoisimerases produce breaks in the DNA and then rejoin them in order to relieve the stress in the helical molecule during replication.
DNA Replication in Bacteria
As the strands continue to unwind in both directions around the entire DNA molecule, new complementary strands are produced by the hydrogen bonding of free DNA nucleotides with those on each parent strand
As the new nucleotides line up opposite each parent strand by hydrogen bonding, enzymes called DNA polymerases join the nucleotides by way of phosphodiester bonds.
DNA Replication in Bacteria
The nucleotides lining up by complementary base pairing are deoxynucleoside triphosphates. (sugar and base together are called nucleosides)
As the phosphodiester bond forms between the 5' phosphate group of the new nucleotide and the 3' OH of the last nucleotide in the DNA strand, two of the phosphates are removed providing energy for bonding
DNA Replication in Bacteria
http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter14/animations.html#
Animation of How Nucleotides are added
DNA replication in a 5' to 3' direction
DNA replication is more complicated than this because of the nature of the DNA polymerases.
DNA polymerase enzymes are only able to join the phosphate group at the 5' carbon of a new nucleotide to the hydroxyl (OH) group of the 3' carbon of a nucleotide already in the chain.
As a result, DNA can only be synthesized in a 5' to 3' direction while copying a parent strand running in a 3' to 5' direction.
DNA Replication in Bacteria
Since DNA runs from 5’to 3’ this poses a problem at the replication fork
Recap the double helix has a unique form of polarity determined by the way each sugar is linked to the next and the two strands run in opposite directions
DNA Replication in Bacteria
This therefore means that at the replication fork one new strand of DNA is being made on a template that runs in one direction 3’ to 5’
Where as the other strand is made on a template running 5’ to 3’
DNA Replication in Bacteria
The replication fork is therefore asymmetrical at first glance the both new strands seem to be running in the same direction that is the direction in which the replication fork is moving
It then suggest that one strand is being synthesized 3’ to 5’ and the other 5’ to 3’
DNA Replication in Bacteria
DNA polymerase can however only catalyze the growth of DNA chain in one direction
It can only add new subunits only to the 3’ end
As a result a new DNA strand can only be synthesized in the 5’ to 3’ direction
This can thus account for synthesis on one strand called the leading strand
But what about the other strand that runs in the opposite direction? Lagging strand)This means another DNA polymerase would have to add to the 5’ end
NO SUCH DNA POLYMERASE EXISTS
DNA Replication in Bacteria
The two strands are antiparallel
• one parent strand - the one running 3' to 5' is called the leading strand can be copied directly down its entire length Continously
• the other parent strand - the one running 5' to 3' is called the lagging strand
• but there is a problem how can this be copied?
DNA Replication in Bacteria
So how can this problem be fixed? This is done by “ Backstiching”
Thus one strand must grow discontinuously (while the other is continuously being synthesized) in separate small pieces with DNA polymerase moving backward to the movement of the replication fork as each new piece is made in the 5’ to 3’ direction
DNA Replication in Bacteria
These resulting small pieces of DNA are called Okazaki fragments
These fragments later join together to make one continuous strands
How is this done?
DNA Replication in Bacteria
DNA polymerase enzymes cannot begin a new DNA chain from scratch.
Recap It can only attach new nucleotides onto 3' OH group of a nucleotide in a pre-existing strand.
DNA Replication in Bacteria
Therefore a new enzyme is needed one that can begin a new polynucleotide chain by simply joining two nucleotides together
Without the need for a base paired end
This enzyme cannot synthesize DNA
One such enzyme is called RNA polymerase
Which can make short single stranded RNA pieces approx 10 nucleotides long which can be base paired to the 3’ end as a starting point for DNA polymerase
DNA Replication in Bacteria
◦ To start the synthesis of the leading strand and each DNA fragment of the lagging strand, an RNA polymerase complex called a primosome or primase is required
◦ The primase is capable of
joining RNA nucleotides without requiring a preexisting strand of nucleic acid - forms what is called an RNA primer
DNA Replication in Bacteria
After a few nucleotides are added, primase is replaced by DNA polymerase.
DNA polymerase can now add nucleotides to the 3' end of the short RNA primer.
The primer is later degraded and filled in with DNA by DNA Ligase
DNA Replication in Bacteria
Bacteria have 5 known DNA polymerases: Pol I:
◦ DNA repair◦ has 5'→3' (Polymerase) activity ◦ both 3' → 5' and 5' → 3' exonuclease activity (in
removing RNA primers).
DNA polymerase I is not the replicative polymerase.The enzyme is too slow!
adds dNTPs at a rate of 20 nt/sec. So it would require 460,000 sec (= 7667 min = 128 hr = 5.3 days) to replicate the E. coli chromosome! Too slow for an organism which can divide every 20 mins.
The enzyme is too abundantThere are 400 molecules per E. coli cell. This is excessive given that there are generally only 2 replication forks per cell.
The enzyme is not processive enoughDNA polymerase I dissociates after catalysing the incorporation of 20-50 nucleotides.
DNA Replication in Bacteria
Pol II: ◦ involved in repair of damaged DNA◦ has 3' → 5' exonuclease activity.
Strains lacking the gene show no defect in growth or replication.
Synthesis of Pol II is induced during the stationary phase of cell growth - a phase in which little growth and DNA synthesis occurs. But DNA can accumulate damage such as short gaps
Pol II has a low error rate but it is much too slow to be of any use in normal DNA synthesis.
DNA Replication in Bacteria
Pol III: ◦ the main polymerase in bacteria (elongates in DNA
replication)◦ has 3' → 5' exonuclease proofreading ability.
is the principal replicative enzyme is highly processive catalyses polymerization at a high rate.
There are two forms of the enzyme.Core enzyme - consists of only those subunits that are required for the basic underlying enzymatic activity: alpha (a), epsilon (e) and theta (q).
Holoenzyme- the fully functional form of an enzyme, complete with all of its necessary accessory subunits. The DNA polymerase III holoenzyme consists of the core enzyme, the b sliding clamp and the clamp-loading complex.
DNA Replication in Bacteria
Pol IV: a Y-family DNA polymerase. Pol V: a Y-family DNA polymerase;
participates in bypassing DNA damage
DNA Replication in Bacteria
http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter11/animations.html#
Animation of bidirectional replication of DNA
Unusual features of DNA polymerase functionFigure 11.9
Problem is overcome by the RNA primers
synthesized by primase
DNA polymerases cannot initiate DNA synthesis
DNA polymerases can attach nucleotides only in
the 5’ to 3’ direction
Problem is overcome by synthesizing the 3’ to 5’
strands in small fragments
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
The two new daughter strands are synthesized in different ways Leading strand
One RNA primer is made at the origin DNA pol III attaches nucleotides in a 5’ to 3’ direction
as it slides toward the opening of the replication fork
Lagging strand Synthesis is also in the 5’ to 3’ direction
However it occurs away from the replication fork Many RNA primers are required DNA pol III uses the RNA primers to synthesize small
DNA fragments (1000 to 2000 nucleotides each) These are termed Okazaki fragments after their discoverers
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
DNA pol I removes the RNA primers and fills the resulting gap with DNA It uses its 5’ to 3’ exonuclease activity to digest the RNA
and its 5’ to 3’ polymerase activity to replace it with DNA
After the gap is filled a covalent bond is still missing
DNA ligase catalyzes a phosphodiester bond Thereby connecting the DNA fragments
Breaks the hydrogen bonds between the
two strands
Alleviates supercoiling
Keep the parental strands apart
Synthesizes an RNA primer
Synthesizes daughter DNA strands
III
Covalently links DNA fragments together
Eukaryotic DNA replication is not as well understood as bacterial replication
◦ The two processes do have extensive similarities,
◦ Nevertheless, DNA replication in eukaryotes is more complex Large linear chromosomes Tight packaging within nucleosomes More complicated cell cycle regulation
11.3 EUKARYOTIC DNA REPLICATION
Eukaryotes have long linear chromosomes They therefore require multiple
origins of replication To ensure that the DNA can
be replicated in a reasonable time
In 1968, Huberman and Riggs provided evidence for the multiple origins of replication
DNA replication proceeds bidirectionally from many origins of replication.
Multiple Origins of Replication
multiple origins of replication in eukaryotes ◦ human genome about 30,000 origins
each origin produces two replication forks◦ moving in opposite direction
DNA Replication in Eukaryotes
Origin recognition complex (ORC) A six-subunit complex that acts as the initiator of
eukaryotic DNA replication It appears to be found in all eukaryotes
Multiple Origins of Replication
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Mammalian cells contain well over a 15 different DNA polymerases
Four: alpha (a), delta (d), epsilon (e) and gamma (g) have the primary function of replicating DNA a, d and e Nuclear DNA g Mitochondrial DNA
Eukaryotes Contain Several Different DNA Polymerases
11-68
DNA Replication in Eukaryotes
Pol α : act as a primase (synthesizing an RNA primer), elongates the primer
Pol β : repairs DNA, (excision repair and gap-filling).
Pol γ: Replicates and repairs mitochondrial DNA and has proofreading 3' → 5' exonuclease activity.
Pol δ: Highly processive and has proofreading 3' → 5' exonuclease activity.
Pol ε: Highly processive and has proofreading 3' → 5' exonuclease activity.
η, ι, κ, Rev1 and Pol ζ are involved in the bypass of DNA damage.
θ, λ, φ, σ, and μ are not as well characterized: There are also others, but the nomenclature has become quite jumbled.
Eukaryotes have at least 15 DNA Polymerases:
the polymerases that deal with the elongation arePol α, Pol ε,Polδ.
Pol α : forms a complex to act as a primase (synthesizing an RNA primer), and then elongates that primer with DNA nucleotides. After around 20 nucleotides elongation is taken over by Pol ε (on the leading strand) and δ (on the lagging strand).
DNA Replication in Eukaryotes
DNA Replication in Eukaryotes
DNA replication occurs until a series of termination sequences are identified and then the enzymes are removed
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
DNA replication exhibits a high degree of fidelity Mistakes during the process are extremely rare
DNA pol III makes only one mistake per 108 bases made
There are several reasons why fidelity is high
1. Instability of mismatched pairs 2. Configuration of the DNA polymerase active site 3. Proofreading function of DNA polymerase
Proofreading Mechanisms
11-42
1. Instability of mismatched pairs Complementary base pairs have much higher stability
than mismatched pairs This feature only accounts for part of the fidelity
It has an error rate of 1 per 1,000 nucleotides
2. Configuration of the DNA polymerase active site DNA polymerase is unlikely to catalyze bond formation
between mismatched pairs This induced-fit phenomenon decreases the error rate to
a range of 1 in 100,000 to 1 million
Proofreading Mechanisms
3. Proofreading function of DNA polymerase
DNA polymerases can identify a mismatched nucleotide and remove it from the daughter strand
The enzyme uses its 3’ to 5’ exonuclease activity to remove the incorrect nucleotide
It then changes direction and resumes DNA synthesis in the 5’ to 3’ direction
Proofreading Mechanisms
A schematic drawing of proofreading
Site where DNA backbone is cut
DNA polymerases also play a role in DNA repair DNA pol b is not involved in DNA replication It plays a role in base-excision repair
Removal of incorrect bases from damaged DNA
Recently, more DNA polymerases have been identified Lesion-replicating polymerases
Involved in the replication of damaged DNA They can synthesize a complementary strand over the
abnormal region
DNA damage bypass All organisms need to deal with the problems that arise when
a moving replication fork encounters damage in the template strand.
The best way to deal with this situation is to repair the damage by excision mechanisms.
In some cases, however, the damage may not be repairable, or the advancing replication fork may already have unwound the parental strands, thus preventing excision mechanisms from using the complementary strand as template for repair, or excision repair may not yet have had an opportunity to repair the damage.
DNA damage bypass
It is important for the cell to be able to move replication forks past unrepaired damage:◦ Long-term blockage of replication forks leads
to cell death. ◦Replication of damaged DNA provides a sister
chromatid that can be used as template for subsequent repair by homologous recombination.
Replication fork bypass mechanisms cannot, strictly speaking, be considered examples of DNA repair, because the damage is left in the DNA, at least temporarily.
Rate of Replication
In prokaryotes replication proceeds at about 1000 nucleotides per second, and thus is done in no more than 40 minutes.
In Eukaryotes replication takes proceeds at 50 nucleotides per second, and is completed in 60 minutes.
Mutations changes in the nucleotide sequence of the
DNA.
organisms have special systems of enzymes that can repair certain kinds of alterations in the DNA.
once the DNA sequence has been changed, DNA replication copies the altered sequence just as it would copy a normal sequence.
provide the variation necessary for evolution to happen in a given species.
Types of MutationsSomatic mutations Occurs in cells not dedicated to sexual
reproduction
The mutant genes disappear when the cell in which it occurred dies and can only be passed on through asexual reproduction.
Germline mutations found in every cell descended from the
zygote to which that mutant gamete contributed.
If an adult is successfully produced, every one of its cells will contain the mutation.
often caused by chemicals or malfunction of DNA replication, exchange a single nucleotide for another.
These changes are classified as transitions or transversions.
Most common is the transition that exchanges a purine for a purine (A ↔ G) or a pyrimidine for a pyrimidine, (C ↔ T).
Less common is a transversion, which exchanges a purine for a pyrimidine or a pyrimidine for a purine (C/T ↔ A/G). An example of a transversion is adenine (A) being converted into a cytosine (C).
Types of Mutations- Point mutations
In short a point mutation is a Single-base Substitution
exchanges one base for another◦ If one purine [A or G] or pyrimidine [C or T] is
replaced by the other, the substitution is called a
transition. ◦ If a purine is replaced by a pyrimidine or vice-versa,
the substitution is called a transversion
Original: The fat cat ate the wee ratPoint Mutation: The fat hat ate the wee rat
Types of Mutations- Point mutations
Point mutations
Point mutations causes changes in the amino acids the bases code for
Three bases make a codon one condon codes for an amino acid example CTG codes for valine GAG codes for glutamic acid
Types of Point mutations
1. Missense mutation. A change in a codon to one that encodes a different amino acid and cause a small change in the protein produced.
Example sickle-cell disease
A → T at the 17th nucleotide of the gene for the beta chain of hemoglobin changes the codon GAG (glutamic acid) to GTG (valine)
Therefore: 6th amino acid glutamic acid → valine
Missense mutation
Types of Point mutations
2. Silent mutations change a codon to one that encodes the same
amino acid and causes no change in the protein produced
Cys
CysT G T
T G C
No detrimental effect
Silent mutation
nonsense mutationchange an amino-acid-coding codon to a single "stop" codon → an incomplete protein
◦ can have serious effects since the incomplete protein probably won't function.
◦ protein wont function because protein synthesis is stopped prematurely
Types of Point mutations
Nonsense Mutation
Examples of Diseases caused by point mutations
Color blindness- is the inability or decreased ability to see colour, or perceive colour differences, under normal lighting conditions. Eg. red/ green
Cystic fibrosis- affects the lungs, and also the pancreas, liver, and intestine. It is characterized by abnormal transport of chloride and sodium across an epithelium , leading to thick, viscous secretions.
Haemophilia- impair the body's ability to control blood clotting or coagulation, which is used to stop bleeding when a blood vessel is broken.
Phenylketonuria - Phenylketonuria (PKU) is a rare condition in which a baby is born without the ability to properly break down an amino acid called phenylalanine.
Tay Sachs- Tay-Sachs disease is a deadly disease of the nervous system passed down through families Tay-Sachs disease occurs when the body lacks hexosaminidase A, a protein that helps break down a chemical found in nerve tissue called gangliosides. Without this protein, gangliosides, particularly ganglioside GM2, build up in cells, especially nerve cells in the brain
Types of Mutations- InsertionInsertion
extra base pairs are inserted into a new place in the DNA.
Original: The fat cat ate the wee rat.Insertion: The fat cat xlw ate the wee rat.
Insertion mutation
Types of Mutations
An example of a human disorder caused by insertion is Huntington’s disease.
In this disorder, the repeated trinucleotide is CAG, which adds a string of glutamines (Gln) to the encoded protein (called huntingtin).
The abnormal protein increases the level of the p53 protein in brain cells causing their death by apoptosis.
Huntington’s
Types of Mutations- Deletion
Deletion
a section of DNA is lost, or deleted.Original: The fat cat ate the wee rat.Deletion: The fat ate the wee rat.
Deletion Mutation
Examples of Diseases caused by deletions
Cri du chat syndrome,-is a rare genetic disorder due to a missing part of chromosome 5. Its name is a French term (cat-cry or call of the cat) referring to the characteristic cat-like cry of affected children
De Grouchy syndrome- is a genetic condition caused by a deletion of genetic material within one of the two copies of chromosome 18 De Grouchy syndrome manifests clinically as mental retardation.
Shprintzen syndrome- is a syndrome caused by the deletion of a small piece of chromosome 22 A congenital anomaly with cleft palate, heart defect, abnormal face, learning defects, short stature, microcephaly, mental retardation, ear anomalies, slender hands/digits, inguinal hernia
Wolf-Hirschhorn syndrome- It is a characteristic phenotype resulting from a partial deletion of chromosomal material of the short arm of chromosome 4 growth and mental retardation, muscle hypotonia, seizures, and congenital heart defects
Duchenne muscular dystrophy- is an inherited disorder that involves muscle weakness, which quickly gets worse. The disorder is caused by a mutation in the dystrophingene, located on the human X chromosome, which codes for the protein dystrophin, an important structural component within muscle tissue that provides structural stability to the dystroglycan complex (DGC) of the cell membrane. While both sexes can carry the mutation, females rarely exhibit signs of the disease.
Types of Mutations
Insertion and deletions involving one or two base pairs (or multiples ) ◦ can have devastating consequences to the gene
because translation of the gene is "frameshifted"
◦ DNA reads in sequences of three bases therefore the addition or removal of one or more bases alters the sequence that follows as the bases all shifted.
◦ The entire meaning of the sequence has changed.
Frameshifts often create new STOP codons → nonsense mutationsOriginal: The fat cat ate the wee rat.Frame Shift: The fat caa tet hew eer at.
Frame shift mutation
Types of Mutations-Duplications
Duplications
Duplications are a doubling of a section of the genome.
During meiosis, crossing over between sister chromatids that are out of alignment can produce one chromatid with an duplicated gene and the other having two genes with deletions.◦ Example of disease :DM1 (Myotonic dystrophy)
Types of Mutations-TranslocationsTranslocations
Translocations are the transfer of a piece of one chromosome to a nonhomologous chromosome.
Translocations are often reciprocal; that is, the two nonhomologues swap segments.
Types of Mutations
Translocations can alter the phenotype in several ways:
the break may occur within a gene ◦ destroying its function ◦ creating a hybrid gene.
translocated genes may come under the influence of different promoters and enhancers so that their expression is altered.
Types of Mutations- InversionsInversion
an entire section of DNA is reversed.
A small inversion may involve only a few bases within a gene, while longer inversions involve large regions of a chromosome containing several genes.
Original: The fat cat ate the wee rat.Insertion: The fat tar eew eht eta tac.
Inversion
Types of Mutations-SuppressorSuppressor mutation
partially or completely masks phenotypic expression of a mutation but occurs at a different site from it (i.e., causes suppression)
may be intragenic or intergenic.
It is used particularly to describe a secondary mutation that suppresses a nonsense codon created by a primary mutation.
Naming genes given an official name and symbol by a formal committee
The HUGO Gene Nomenclature Committee (HGNC) – US and UK designates an official name and symbol (an abbreviation of the name) for each known human gene.
Some official gene names include additional information in
parentheses, such as related genetic conditions, subtypes of a condition, or inheritance pattern.
The Committee has named more than 13,000 of the estimated 20,000 to 25,000 genes in the human genome.
a unique name and symbol are assigned to each human gene, which allows effective organization of genes in large databanks, aiding the advancement of research.
How are genetic conditions named?
Disorder names are often derived from one or a combination of sources:
The basic genetic or biochemical defect that causes the condition (alpha-1 antitrypsin deficiency)
One or more major signs or symptoms of the disorder (sickle cell anemia)
The parts of the body affected by the condition (retinoblastoma)
The name of a physician or researcher, often the first person to describe the disorder (Marfan syndrome - Dr. Antoine Marfan)
A geographic area (familial Mediterranean fever)
The name of a patient or family with the condition (Lou Gehrig disease)
Disorders named after a specific person or place are called eponyms.
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http://www.google.com.jm/url?sa=t&rct=j&q=&esrc=s&source=web&cd=10&ved=0CE8QFjAJ&url=http%3A%2F%2Fwww.csus.edu%2Findiv%2Fd%2Fdulaik%2FGeneticsLectures07%2Fchapt11_lecture.ppt&ei=-WhoULPgJYHu9ATTjYC4DA&usg=AFQjCNESac2iCvfwRMIo7DqnpdJs_C7HTQ&sig2=17FVyTgpt7Ga-ChOgmRBEA&cad=rja
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