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BIL 250 Chapter 15 review Part 2

Terms in this set (235)

Scientists rapidly accumulating information about how specific types of mutations contribute to
human disorders
polygenic
caused by variations in several genes
Although most human genetic diseases are polygenic - that is caused by variations in several genes - even a single base pair change
in the approximately 20000 human genes can cause a serious inherited disorder
monogenic diseases
can be caused by many different types of the single gene mutations
As of 2014, OMIM database has
cataloged over 4000 known phenotypes of the molecular basis is known
Geneticists estimate that apprixatley 30 percent of the mutations that cause human diseases are
single base pair changes that create nonsense mutations
These mutations not only code for
a prematurely terminated protein product, but also trigger rapid decay of the mRNA
Many more mutations are missense mutations that
alter the amino acid sequence of a protein
frameshift mutations
alter the protein sequence and crate internal nonsense codons
Other common disease associated mutations affect the sequences of
gene promoters, mRNA splicing signals, and other noncoding sequences that affect transcription, processing and stability of mRNA or protein
One recent study showed that about 15 percent of all point mutations that cause human genetic diseases result in
abnormal mRNA splicing
Approximately 85 percent of these pcsing mutations alter the
sequence of the 5' and 3' splice signals

The remainder create new splice sites within the gene
Splicing defects often result in
degradation of the abnormal mNRA or creation of abnormal protein products
Although some genetic diseases such as sickle cell anemia are caused by one specific base pair change within a single gene
most are caused by any of a large number of different mutations
B thalassemia is
an inherited autosomal recessive blood disorder resulting from a reduction or absence of hemoglobin

Is is the most common single gene diseases in the world, affecting people worldwide but especially populations in meditteranean, north africanm middle astern, central asian and southeast asnan countries
People with b thalassemia have
varying degrees of anemia - from mild to severe - with symptoms, including weakness, delayed development, jaundice, enlarged organs and often a need for frequent blood transfusions
Mutations in the b globin gene or HBB gene caused
by thalassemia
HBB gene
The HBB gene encodes the 146 amino acid B globin polypeptide

Two B globin polypeptides associated with two alpha globin polypeptides to form the adult hemoglobin tetramer

The HBB gene spans 1.6 ilobases

It is made up of three exons and two introns on chromosome 11
Scientists have discovered over
25 different mutations in the HBB gene that cause B thalassemia, although most cass worldwide are associated with about 20 of these mutations
Most mutations change
a single nucleotide within or surrounding the HBB gene or create small insertions and deletions
In addition each population affected by B thalassemia has
a unique mix of mutations
most prevalent mutation in a sardinian population
a mutation that accounts for more than 95 percent of cass - is a single base pair change at codon 39, creating a nonsense mutation and premature termination of the b globin polypeptide
A study of the b thalassemia mutations in a population from the former yugoslavia revealed
14 different mutations with only 3 accounting for over 75 percent of all cases
The types of mutations that cause B thalassemia not only affect
the B globin amino acid sequence, missense, nonsense and frameshift mutations, but also alter HBB transcription efficiency, mRNA splicing and stability, translation and protein stability
More than half of the mutations are
single base pair changes and the remainder are short insertions, deletions and duplications
Beginning in about 1990, molecular analysis of genes responsible for a number of inherited human disorders revealed
a remarkable set of observations
Researchers discovered that some mutant genes contain
expansion of trinucleotide repeat sequences - specific short DNA sequence repeated many times
Normal individuals have
a low number of repetitions of these sequences; however, individuals with over 20 different human disorders appear to have abnormally large numbers of repeat sequences - in some cases over 200 within and surrounding specific genes
Examples of diseases associated with these trinucleotide repeat expansions are
fragile x syndrome , myotonic dystrophy and huntington disease
When trinucleotide repeats such as CAG(n) occur within the coding region
they can be translated into long tracks of glutamine
what happens when trinucleotide repeats such as CAG(n) occur within the coding region?
they can be translated into long tracks of glutamine

The glutamine tracks may cause the proteins to aggregate and marley
When the repeats occur outside coding regions, but within the mRNA
it is thought that the mRNAs may act as "toxic" RNAs that bind to important regulatory proteins, sequestering them away from their normal functions in the cell
Another possible consequence of long include repeats is that the
DNA containing regions with the repts ay become abnormally methylated, leading to silencing of gene transcription
The mechanisms by which the repeated sequences expand from generation to generation are
of great interest
It is thought that expansion may result in
either errors during DNA replication or errors during DNA damage repair
Whatever the cause may be
the presence of these short and unstable repeat sequences seems to be prevalent in humans and in any other organisms
Living systems have evolved a
variety of elaborate repair systems that counteract both spontaneous and induced DNA damage
These DNA repair systems are
absolutely essential to the maintenance of the gnei integrity of organisms and as such to the survival of organisms on Earth
The balance between mutation and repair results
in the observed mutation rates of individual genes and organisms
The ability of these systems to counteract genetic damage that would otherwise result in
genetic diseases and cancer
Some of the most common types of mutations arise during DNA replication
when an incorrect nucleotide is inserted by DNA polymerase
The major DNA synthesizing enzyme in bacteria (DNA polymerase III) makes an error
approximately once every 100,000 insertion leading to an error rate of 10^-5

Fortunately, DNA polymerase proofreads each step, catching 99 percent of the errors
If an incorrect nucleotide is inserted during polymerization
the enzyme can reocnzniaethe error and "reverse" its direction

It then behaves as a 3' to 5' exonuclease, cutting out the incorrect nucleotide and replicaign it wht the correct one
This improves the efficiency of replication
100 fold, creating only 1 mismatch in every 10^7 insertion for a final error rate of 1 x 10^-7
To cope with such errors,such as bae base mismatches
small insertions and deletions that remain after proofreading, another mechanism, called mismatch repair, may be activated
During mismatch repair,
the mismatches are detected, the incorrect nucleotide is removed and the correct nucleotides inserted in its place
If the mismatch is recognized by no discriminaion occurs
the excision will be random and the strand bearing the correct base will be clipped about 50 percent of the time
process of strand discrimination is based on
DNA methylation
E coli
contain an enzyme, adenine methylase, which reoznies the sequence of the 5' GATC 3' and 3' CTAG 5' as a substrate, adding a methyl group to each of tehadeien resuidues during DNA repliation
process of strand discrimination in E coli
These bacteria contain an enzyme, adenine methylase, which reoznies the sequence of the 5' GATC 3' and 3' CTAG 5' as a substrate, adding a methyl group to each of tehadeien resuidues during DNA repliation

Following replication,
the newly synthesized
DNA strand remains
temporarily
unmethylated as the
adenine methylase
lags behind the DNA
polymerase

Prior to methylation,
the repair enzyme
recognizes the
mismatch and binds to
the unmethylated
(newly synthesized)
DNA strand

An endonuclease
enzyme creates a nick
in the backbone of the
unmethylated DNA
strand, either 5' or 3'
to the mismatch

An exonuclease
unwinds and degrades
the nicked DNA
strand, until the region
of the mismatch is
reached

Finally DNA polymerase fills the
gap created by the
exonuclease, using the correct DNA strand as
a template

DNA ligase seals the gap
A series of E coli genes
Mut H, MutL and MutS, as well as exonucleases, DNA polymerase III and ligase are involved in mismatch repair
Mutations in MUtH, MUtL and Muts genes result in
bacterial strains deficient in mismatch repair
While the preceding mechanism occurs in E coli
similar mechanisms involving homologous proteins exist in yeast and in mammals
In humans, mutations in genes that code for DNA mismatch repair proteins are
associated with hereditary nonpolyposis colon cancer
Mismatch repair defects are commonly found in
other cancers, such as leukemias, lymphoma and tumors of the ovary, prostate and endometrium
Cells from these cancers show
genome wide increase in the rate of spontaneous mutation
The link between defective mismatch repair cancer is supported by
experiments with mice

Mice that are engineered to have deficiency in mismatch repair genes accumulate larger numbers of mutations and are cancer prone
Mice that are engineered to have deficiency in mismatch repair genes accumulate
larger numbers of mutations and are cancer prone
Another DNA repair system, called postreplication repair, responds after
damaged DNA has escaped repair and has failed to be completely replicated
postreplication repair

process
When DNA bearing a lesion of some sort such as a pyrimidine dimer is being replicated, DNA polymerase may stall at the lesion and then skip over it, leaving an unreplicated gap on the newly synthesized strand

To correct the gap, the RecA proteins directs a recombinational exchange with the corresponding region on the undamaged parental strand of the same polarity

When the undamaged segment of the donor strand DNA replicates the gap design,gap is created on the donor strand

The gap can be filled by repair synthesis as replication proceeds

Because a recombinatorial event is involved in this type of DNA repair,it is considered to be a form of homologous recombination repair
E coli SOS repair system
responds to damaged DNA but in a different way
In the presence of a large number of unrepaired DNA mismatches and gaps
bacteria can induce the expression of about 20 genesis, involving lexA, recA whose products allow DNa replication to occur even in the presence of these pistons
E coli SOS repair
a last resort the minimize DNA damage, hence its name
During SOS repair, DNA synthesis becomes
error prone, inserting random and positively incorrect nuclide sin places that would normally stahl DNA replication

As a result, SOS repair itself becomes mutagenic - although it may allow the cell to survive DNA damage that would otherwise kill it
UV light is
mutagenic, as a result of the creation of pyrimidine dimers
UV induced damage to E coli DNA can be
partially reversed if, following irradiation, the cells are exposed briefly to light in the blue range of visible spectrum
photo reaction enzyme PRE
The enzyme's mode of action is to cleave het bonds between thymine dimers thus directly reverse ing the effect of UV radiation on DNA
Hot H the enzyme will associate with a thymine dimer in the dark
it must absorb a photon of light to cleave the dimer
In spite of its ability to reduce the number of UV induced mutations
photoreactivation repair is not absolutely essential in E coli, a mutation creatine ga null allele in the gene coding for PRE Is not lethal
PRE enzyme is detectable in
many organisms, including bacteria, fungi, plants and some vertebrates though not in humans
Humans and other organisms that lack photoreactive repair must rely on
other repair mechanisms to reverse the effects of UV radiation
A number of light independent DNA repair systems exist in
all prokaryotes and eukaryotes
The basic mechanism involved in these types of repair - collectively referred to as excision repair, or cut and paste mechanisms consist of the following three steps
The distortion or error present on one of the two strands of the DNA helix is recognized and enzymatically ripped out by an endonuclease

Exicisons in the
phosphodiester
backbone usually
include a number of
nucleotides adjacent
to the or as well,
leaving a gap on one
strand of the helix

A DNA polymerase
fills in the gap by
inserting nucleotides
complementary to
those on the intact
strand, which it uses as
a replicative template

The enzyme adds
nucleotides to the free
3' OH end of the
clipped DNA
In E coli this step is
performed by DNa
polymerase I

DNA ligase seals final
nick that remains at the 3' OH end of the last nucleotide inserted closing the gap
two types of excision repair
base excision repair

nucleotide excision repair
base excision repair
corrects DNA that contains a damaged AND base
base excision repair

process
The first step in the BER pathway in E coli nioves the recognition of the altere base by an enzyme called DNA glycosylase

There are a number of
DNA glycosylase each
of which recognizes a
specific base

The enzyme uracil
DNA glycosylase
recognizes the
presence of uracil in
DNA

DNA glycosylases first
cut the geodesic bond
between the base and
the sugar, creating a
apyrimidinic or
apurinic site

The sugar with the
missing base is then
recognized by an
enzyme called ATP
endonuclease

The AP endoculuae makes a cut in the
pshodpidiater
backbone at teh
aprymdiic or apurinic
site

Enconulaes then
remove the
deoxyribose sugar
and the gap is filled by
DNA polymerase and
DNA ligase
Although much has been learned about the mehcaimss of BER in E coli, BER systems have also been detected in
eukaryotes from yeast to humans
Experimental evidence shows that
both mouse and human cells that are defective in BER activity are hypersensitive to the killing effects of gamma rays an oxidising agents
Nucleotide excision repair NER pathways
repair bulky lesions in NDa that alter or distort the double helix
These lesions include
the UV induced pyrimidine dimers and the DNA adducts
The NER pathway was first discovered in E coli by Paul Howard Flanders who
isolated severa deepnet mutations that are sensitive to UV radiation
UV radiation mutations
One group of genes was designated uvr and include the uvrA, B and C mutations
process of the NER pathway
In the NER pathway, the uvr gene products are involved in recognizing and clipping out lesions in the DNA

Usually a specific number of nucleotides is clipped out around both sides of the lesion

In E coli, usually a total of 13 nucleotides is removed including the lesion

The repair is then completed by DNA polymerase I and DNA ligase

The undamaged strand opposite the lesion is used as a template for the repo clayton, resulting in repair
The mechanisms of NER in eukaryotes is much more complicated than that in
prokaryotes and involves many more proteins, encoded by about 30 genes
xeroderma pigmentosum
a rare recessive genetic disorder ath predisposes individual sot severe skin abnormalities, skin cancers and a wide range of other symptoms including developmental and neurological defects

Patients with XP are extremely sensitive to UV radiation in sunlight

They have a 2000 fold rate of cancer, particularly skin cancer, than the general population

The condition is severe and may be lethal, although early detection and protection from sunlight can arrest it
The repair of UV induced lesions in XP has been investigated
in vitro, using human fibroblast cell cultures derived from normal individuals and those with XP
Fibroblasts Are
undifferentiated connective tissue cells
The results of these studies suggest that
the XP phenotype is caused by defects in the NEW pathways and by mutations in more than one gene
In 1968, james cleaver showed that
cells from XP patients were deficient in DNA synthesis other than that occurring during chromosome replication - a phenomenon known as unscheduled DNA synthesis
Unscheduled DNA synthesis is elicited
in normal cells by UV radiation
Because this type of synthesis is thought to represent the activity of DNA polymerization during NER
the lack of unscheduled DNA synthesis in XP patients suggested that XP may be a deficiency in NER
The invivlemeth of mtilipe genes in NER nad XP was further investigated by studies using
somatic cell hybridization
somatic cell hybridization
Fusion of somatic cells of different types.
Fibroblast cells from any two unrelated XP patients when grown together in tissue culture, can
fuse together forming heterokaryons
A heterokaryon is
a single cell with two nuclei form different organisms but a common c palmas
NER in the heterokaryon can be measured by
the level of unscheduled DNA synthesis
If a mutation in each of the two XP cells occurs in the same gene, the heterokaryon that is focused to from it will
still be able to undergo NER

This is because there is no normal copy of the relevant gene present in the heterokaryotic
However if NER does occur in the heterokaryon
the mutations in the two XP cells must have been present in two different genes
the two mutants demonstrate
complementation
Complementation occurs because
the heterokaryon has at least one normal copy of each gene in the fused cell
By fusing XP cells from a large number of XP patients
researchers were able to determine how many genes continue to the XP phenotype
study of the defective genes in XP
XP patients were divided into seven competition groups, indicating that at least seven different genes are involved in nucleotide excision repair in humans

A gene representing
each of these
communication
groups, XPA to XPG
has now been
idenfiied and a
homologous gene for
each hs been
identified in yeast

Approixatley 20
percent of XP patients
do not fall into any of
the seven
complementation
groups

Cells form most of
these patients have
mutations in the gene
coding for DNa
polymerase H and are
defective in repair
DNA synthesis

Approximately 6
percent of XP patients
do not have mutations
or the polymerase H gene, suggesting that other genes or mutations outside of coding regions may be involved in XP
As a result of the study of defective genes in XP
a great deal is now known about how NER counteracts DNA damage in normal cells
how NER counteracts DNA damage in normal cells
The first step in humans is recognition of the damaged DNA by proteins encoded by the XPC, XPE and XPA gnees

These proteins then recruit the remainder of the repair proteins to the site of DNA damage

The XPB and XPD genes encode helicases and the XPF and XPG genes encode nculase

The excision repair complex containing these and other factors is responsible for the excision of an approximately 28 nucleotide long fragment from the DNA strand that contains the lesion
Two other rare autosomal recessive diseases are associated with defects in NER pathways
cyclone doraemon and trichothiodystrophy
CS
The symptoms of CS in blue developmental and neurological defects and sensitivity to sunlight but not an increase in cancers

Patients with CS age prematurely and usually die before the age of 20
TTD
Patients with TTD suffer from dwarfism, retardation, brittle hair and skin a fairchild deformities

Like CS, these patients are sensitive to sunlight but do not have higher than normal rates of cancer
TTD patients have a median life span of six years
Both CS and TTD arise from
mutations in some of the same genes involved in XP as well as other genes that encode proteisn involved in NER within transcribed regions of the genome
It si no known why such a variety of different symptoms result for mutations in
the same genes or DNA repair pathways however it may reflect that affect the products of many NER genes are also involved in other essential process
double strand break
These types of damage are extremely dangerous to cells, leading to chromosome rearrangements, cancer or cell death
Specialized forms of DNA repair, the DNA double strand break repair pathways are
activated and are responsible for reattached itwo broken DNA strands
Recently, interest in DSB repair has grown because
defects in these pathways are associated with X ray hypersensitivity and immune deficiency

Such defects may also underlie familial disposition to burst an ovarian cancer
Several human disease syndromes, such as fanconi anemia and ataxia telangiectasia result from
defects in DSB repair
process of homologous recombination repair
The first step in this process involves the activity of an enzyme that recognizes the double strand break and then digests back the 5' ends of the broken DNA helix, leaving overhanging 3' ends

One overhanging end
searches for a region
of sequence
complementarity on
the sister chromatid
and then invades the
homologous DNA
duplex, aligning the
complementary
sequences

Once aligned, DNA
synthesis proceeds
from the 3'
overhanging ends using the undamaged DNA strands as templates

The interaction of two
sister chromatids is
necessary because
when both strands of
one helix are broken
there is an undamaged
parental DNa strand
available to use as a
source of the commentary template
DNA sequence during
repair

After DNA repair
synthesis, the resulting
heteroduplex
molecule is resolved
and the two chromatids separate
DSB repair usually occurs during
the late S or early G2 phase of the cell cycle, after DNA replication a time when sister chromatids are available for repair templates
Because an undamaged template is used during repair synthesis
homologous recombination repair is an accurate process
process of non homologous end joining
the mechanism does not recruit a homologous region of DNA during repair

This system is
activated in G1, prior
to DNA replication

End joining involves a
complex of many
proteins, and may
include the DNA
dependent protein
kinase and the breast
cancer susceptibility
gene product BRCA1


These And other
proteins bind to the
free ends of the
broken DNA, trim the
ends and ligate them
back together

Because some
nucleotide sequences
are lost in techprocess
of end joining, it is an
error protein system

In addition,if more
than one chromosome suggest a souclb e strand break, the wrong ends could be joined together, leading to abnormal chromosome structures
There is great concern about
the possible mutagenic properties of any chemical that enters the human body, weather through the skin, the digestive system or the respiratory tract
Examples of synthetic chemicals that concern us are those
found in air and water pollution, food preservatives, artificial sweeteners,m herbicides, and pharhamecuetila products
Mutagenicity an be used in
various organisms, including fungi plants and cultured mammalian cells
The ames test uses a
number of different patterns of the bacterium salmonella typhinurium that have been selected for their ability to reveal the presence of specific types of mutations
Some strains are used to detect
base pairs usbustitions and other strains detect various frameshift mutations
Each strain contains a
mutation in one of the genes of the histidine operon
Ames test
The mutant strains are unable to synthesize histidine and therefore required histine for growth

The assay measures the frequency reverse mutations that occur within the mutant gene yielding wild type bacteria
These salmonella strains have also an
increased sensitivity to mutanes due itot presence of mutations in genes involved in both DNA damage repair and synthesis of the lipopolysaccharide barrier that coats bacteria and protects them from external substances
Many substances entering the human body are relatively innocuous until
activated metabolically usually the liver, to more chemically reactive products
Thus the Ames test includes a step in which
the compound is incubated in vitro in the presence of a mammalian liver extract
Test compounds may be injected into
mouse where they are modified by liver enzymes and then recovered for use in the ames test
In the initial use of AMes testings in the 1970s
a large number of known carcinogens or cancer causing agents, there examined a more than 80 percent of these were shown to be strong mutagens

This is not true as the transformation of cells to the malignant state occurs as a result of mutations
More than 60 compounds found in cigarette smoke test positive in the ames test and
cause cancer in animal tests
Although a positive response in the ames test doesn't prove that a compound is concarniogenic
the ames test is useful as a rpemiatiary screening device
Transposable elements also known as transposons or jumping genes, can
move or tnrpasoe within and between chromosomes, inserting themselves into various locations within the genome
Transposable elements are present in
the genomes of all organisms from bacteria to humans
Not only are they ubiquitous, but they also
comprise large person of some eukaryotic genomes
Almsot 50 percent of the human genome is derived from
transposable elements
Some organisms with unusually large genomes such a salamanders and barley contain
hundreds of thousands of pieces of various types of transposable elements
Although the function of these elements is unknown, data from human genome sequencing suggest that
some genes may be evolved from transposable elements and the presence of these elements may help modify and reshape the genome
Transposable elements are also valuable tools in
genetic research
transposable elements in genetic research
Geneticists harness transposons as mutagens, as cloning tags and as vehicles for indotrading foreign DNA into modeling aims
The movement of transposable elements from one place in the genome to another has
the capacity to disrupt genes and cause mutations as well as to create chromosomal damage such as double strand breaks
There are two types of transposable elements in bacteria
insertions sequences and bacterial transposons
insertion sequences can
move from one location to another and if they insert into a gene or gene regulatory region may cause motions
IS elements were
first identified during analysis of mutations in the gal operon of E coli
discovery of IS elements and subsequent research
Researchers discovered that certain mutations in this operon were due to the presence of several hundred base pairs of extra DNA inserted into the beginning of the operon

Surprisingly, the segment of mutagenic DNA could spontaneously excise from this location, restoring wild type function to the gal operon

Subsequent research revealed that several other DNA elements could behave in a similar fashion,m inserting into bacterial chromosomes and affecting gene function
IS elements are
relatively short, not exceeding 2000 bp
IS sequences
The first insertion sequence to be characterized in e coli, IS1, is about 800 bp long

Other IS elements such as IS2, 3, 4 and 5 are 1250 to 1400 bp long
IS elements are present in
multiple copies in bacterial genomes
All IS elements contain
two features that are essential for their movement
two features essential to IS
First, they contain a gene that encodes an enzyme called traspase

Second, the end of IS elements contain inverted terminal repeats or ITRs
transpase
This enzyme is responsible for making staggered cuts in chromosomal DNA, into which the IS element can insert
ITRs
shortcomings of DNA that have the same nucleotide sequence as each other, but are oriented in the opposite direction

They usually contain about 20 to 40nucletide pairs

ITRs are essential for transportation and act as reiconiton sies of the beginning of the transposase enzyme
Bacterial transposons or TN elements are larger than
IS elements and contain protein coding genes that are unrelated to their transpiration
Tn 10
composed of a drug resistance gene flanked by two IS elements present in opposite orientations
The IS elements encode the transposase enzyme that is necessary for
transposition of the Tn element
Other types of N elements such as Tn3 have
shorter inverted repeat sequences at their ends and encode their transposase enzyme from a transpose agnee located in the middle of the Tn element
like IS elements, Tn elements are
mobile in both bacterial chromosomes and plasmids and cause mutations if they insert into genes or gene regulatory regions
Tn elements are
currently of interest because they can introduce multiple drug resistance onto bacterial plasmids
R factors
may contain many Tn elements conferring simultaneous resistance to heavy metals, antibiotics and other drugs

These lentsn can move form plasmids into bacterial chromosomes and can spread multiple drug resistance between different strains of bacteria
About 20 years before the discovery of transposons in bacteria, McClintock discovered
mobile genetic elements in corn plants or maize
McClintock and discovery of mobile genetic elements
She did this by analyzing the generic behavior of mwo mautions, dissociation Ds ad activator As expressed in either the endosperm or aleurone layers

She then correlated her genetic observations with cytolical examines of the maize chromosomes
Initially, McClinktock determined that
Ds was located on chromosome 9
If Ac was also present in the genome, Ds induced
breakage at a point on the chromosome ajudante to its own location
If chromosome breakage occurred in somatic cells during their development
progeny cells often lost part of the broken chromosome, causing a variety of phenotypic effects
Subsequent analysis suggested to mcClintock that both Ds and Ac elements sometimes
moved to new chromosomal locations
While Ds moved only if Ac was also present
Ac was capable of autonomous movement
Where Ds came to reside denied its genetic effects
it might cause chromosome breakage or it might inhibit expression of certain gene
In cells in which Ds caused a gene mutation
Ds might move again restoring the gene mutation to wild type
In mcclintock's original observation
pigment synthesis was restored in cells in which the Ds element jumped out of chrome 9
McClintock concluded that
Ds and Ac genes ere mobile controlling elements

They are now transposable elements
Several Ac and Ds elements have now been
analyzed and the relationship between the two elements has been clarified
The first Ds studied Ds9 is nearly identical to
Ac except for a 194 bp deletion within the transposase gene
The deltion of part of th rnasposage gene in the Ds9 element explains
its dependence on the Acelent for transposon
Several other Ds elements have been sequence and each
contains an even larger deletion within the transposase gene
Although the significance of barbara Mcclinktokc's mobile controlling elements was not fully appreciated following her initial observations
molecular analysis has since verified her conclusions
There are more than 30 families of transposable elements in
drosophila, each of which is present in 20 to 50 copies in tahoe
Togher these families constitute about
5 percent of the drosophila genome and over half the middle repetitive DNA of this organism
On study suggest that 50 percent of tall visible mutations in drosophila are the
result of the insertion of transposons into otherwise wild type genes
In 1975, hogness and finnegan, rubin and young identified
a class of DNA elements in drosophila emlangogaser than they designated as copia

The elements are transcribed into "copious" amounts of RNA
Copia elements are present in
10 to 100copies in the genomes of drosophila cells
Mapping studies show that
they are transposable to different hcronsmal location and are dispersed throughout the genome
Each copia element consists of
approximately 5000 to 8000 bp of DNA, including a long direct terminal repeat DTR sequence of 267 bp at each end
Within each DTR is
an inverted terminal repeat of 17 bp
The short ITR sequences are characteristic of
copia elements
The DTR sequences are often
in other transposons in other organism but the are not universal
Insertion of copia is dependent on
the presence of the ITR sequences and seems to occur preferentially at specific target sites in the genome
The copia like lentes demonstrate
regulatory effects at the point of their insertion in the chromosome
certain mutations including those affecting eye color and segment formation are
due to copia insertions within genes
The eye color mutation white apricot sicard by
an ally of the whtie gene which contains a copia element within the gene

Transposition of the cpia element of the whtie apricot allele can restore on the alley to wild type
P elements
discovered while studying the phenomenon of hybrid dysgenesis
hybrid dysgenesis
a condition characterized by sterility elevated mutation rates a chromosome rearrangement in the offspring of crosses between certain strains of fruit flies
hybrid dysgenesis is caused by
a high rate of P element transposition in the germline in which tnaposiaton insert themselves into or nearer genes thereby causing mutations
P elements
range from 0.5 to 2.9 kb long
Full length P elements encode at last
two proteins one of which is the transpose enzyme that isrueired for transposition and another is a repressor protein that inhibit atnr position
The transposase gene is expressed only in
the general line, accounting for the tissue specificity of p element transition
Strains of lies that contain full length P elements inserted
into their genomes are sisitna to further transportation due to the presence of ht repressor protein encoded by the P elements
Mutations can arise from several kinds of insertional events
If a planet insets in a coding region of gene it can eliminate transcription of ge gene and destroy normal gene expression

If its assets in to the promoter region of a gene it can affect the level of expression for the gen

Insertions into introns can affect splicing or cause the premature termination of transcription
Geneticists have harnessed
P elements as tools for genetic analysis
One of the useful applications of P elements is as
vectors to introduce transgenes into dropshola - a technique known as germline transformation
Germline transformation
introducing trans genes into organisms
P elements are also used to generate
mautions and to clean mutants to large P elements insertions to precise single chromosomal sites, which should increase the precision of germ line transformation in the analysis of gene activity
The human genome is riddled with
DNA derived from transposons
Recent genomic sequencing data is compared of
transposable element DNA
The major families of human transposable elements are the
long interspersed elements or LINES and SINEs

The compose over 30 percent of the human genome
Other families of treasonable elements account for a
further 11 percent
As coding sequences comprise only about 1 percent of the human genome
there are about 40 to 50 times more transposable element NDA in the human genome than DNA in functional genes
Although most human transposable elements appear to be inactive
the potential mobility and mutagenic effects of these elements have far reaching implications for human genetics
male child with hemophilia

transposable elements
Once cause of hemophilia is a defect in blood clotting factor VIII an x linked gene
Kazakian found
LINES inserted at two points within the gene
Researchers were interested in
determining if one of the noters x heroes also contained his specific LINEIf so, the unaffected mother would be heterozygous and pas the LINE containing chromosome to the other son
The surprising finding was
that the LINE sequence as not present on either of her X chromosomes but was detected on chrome 22 of both parents
This suggests that
this mobile element mayyu have transposed from one chrome to another in the gamete forming cells of the other prior to being transmitted to the son
LINE insertions

Duchenne muscular dystrophy
In one cae, a LINE inserted into exon 48 and in another car, it inserted into exons 44, reading to frameshift mutations and premature termination of translation of the dystrophin protein
Tehri are also reports that LINes have inserted into the APC and c nyc genes leading to humans that may have
contributed to the development of some colon and breast cancers
In the latter, transposition has only occurred within
one or a few somatic cells
As of 2009, researchers have determined that
at last 11 human diseases are due to insertions of LINE elements
SINE insertions are also responsible for
more than 30 cases of human disease
In one caes, an Alu element integrated
into the BRCA2 gene, inactivating this tumor suppressor gene and ledin otra familia case of breast cancer
Other genes that have been muted by Alu integrations are the
factor IX gene leading to hemophilia B, the Che gene leading to acholesteromesima and the NF1 gene leading t neurofibromatosis
The insertion of a transposon into the condign region of gene may
disrupt the gene'normal translation reading frame or may induce premature termination of translation of the mNRA transcribed from the gene
Many transposable elements contain
their own promoters and ehanchersm as well as specie sites and polyadenylation signals
The presence of these regulatory sequences can
have effects on nearby genes
The insertion of a transposable elements containing poyadylation or transcription termination signals into a gene's intron may bring
about the termination of the gene's transcription within helens
it can cause
aberrant splicing of an RNA transcribed from the gene
Insertions of transposon into agene's transcription regulatory region may disrupt
the gene's normal regulation or may cause the gene to be expressed differently as a result of the presence of the transposon's own promoter or enhancer sequences
The presence of two or more identical teaspoons in a genome creates
the potential for recombination between transposons leading to duplications, deltions, inversion or chrome translation nations

Any of these rearrangements may bring about phenotypic changes or disease
It is thought the path about 0.2 percent of detectable human mutations may be
due to transposable element insertions
Other organisms appear to suffer
more damage due to transposition
About 10 percent of new mouse mutations and 50 percent of drosophila mutations are caused by
insertion of transposable elements in or near gees
Because of their ability to alter genes and chromosomes
transposons may contribute to the viability that underlies evolution
Tn elements
carry antibiotic resistance genes between groangasm conferring a survival advantage to the bacteria under certain incisions
Another example of a transposon's contribution to evolution is provided by drosophila telomeres
LINE like elements are present at the ends of drosophila chromosomes and the elkins act as tellers, maintaining the length of drosophila chromosomes over successive cell divisions
Other examples are the RAG1 and RAG2 genes in humans
These genes encode recombinant enzymes that are essential to the development of the immune system

These genes have evolved from transposons
Transposons may also affect the
evolution of genomes by altering gene expression patterns in a way that are subsequently retained by the host
Human amylase gene contains
an enhancer that cause the gene to be expressed in the parotid gland

This enhancer evolved or transposon sequences that were inserted into the gene regulatory region early in primate evolution
Other examples of gene expression patterns that were affected by the presence of transposon sequences are
T cell specific expression pattern that were affected by the presence of transposon sequences are T cell specific expression of the CD8 gene and placenta specific expression of the leptin and CYP19 genes
Sets found in the same folder