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NURS 500 Patho Ch 4

Patho 500 Ch 4 Genes & Genetic Disease

genetics the study of biologic heredity
gene basic unit of heredit
genomics field of genetics concerned with s/f of genome
genome DNA representing all genes for a given species
components of DNA Pentose sugar (deoxyribose) Phosphate molecule Four nitrogenous bases
pyrimidines-single ring cytosine and thymine
purines - double ring adenine and guanine
DNA has antiparallel structure with these nucleotide pairings Adenine with thymine 2 hydrogen bonds Cytosine with guanine 3 hydrogen bonds
human genome contains how many genes 20,000 - 25,000
proteins are composed of 20 amino acid polypeptides which are directed by . . . directed by sequence of bases = CODONS
DNA replication - untwisting and unzipping of DNA strand where single strand acts as template with these complementary base pairings adenine-thymine----------cytosine-guanine
What happens in transcription RNA is synthesized from DNA template bwo RNA POLYMERASE----results in formation of mRNA---mRNA moves OUT of nucleus into cytoplasm
gene splicing occurs during during transcription bwo introns and exons
translation a process by which RNA directs synthesis of polypeptide bwo tRNA at ribosome---
tRNA contains the sequence of nucleotides called called anticodon---which is complementary to the triad of nucleotides on the mRNA CODON
DNA organized on a xome composed of composed of 2 longitudinal sister chromatids
xomes found in somatic cells, gametes and in meiosis. What number where? somatic cells = 23 pairs/diploid----gametes 23 XOMES/haploid cells which have one member of each xome pair-----meiosis is the formation of haploid cells from diploid cells
autosomes The first 22 of the 23 pairs of chromosomes in males and females The two members are virtually identical and thus said to be homologous
sex xomes Remaining pair of chromosomes In females, it is a homologous pair (XX) In males, it is a nonhomologous pair (XY)
xome structure has centromere, which is attachment of sister chromatids occur with SHORT ARM 'p' and LONG ARM 'q'
telomere region with multiple base pairs that SHORTEN with each cell division
karyotype is an ordered display of xomes
mutation any inherited alteration of genetic material
mutations occur bwo chromosome aberrations one type is base pair substitution, another is silent substitution which DOES NOT result in an amino acid change--for example---RNA codons GUU, GUC, GUA, GUG all code for the amino acid valine
are mutations in SOMATIC cells transmitted to offspring? no--only mutations in gametes are transmitted to offspring
what is a point mutation a single nucleotide base-pair change in DNA
do spontaneous mutations occur? yes, varies by gene
examples of mutagens, which are agents known to increase frequency of mutations radiation---chemicals
what is a silent mutation again? A DNA sequence change that does not change the amino acid sequence of a gene
missense mutation A type of mutation that results in a single amino acid change in the translated gene product
and what is a nonsense mutation A type of mutation in which an mRNA stop codon is: Produced, resulting in premature termination of the protein sequence or Removed, resulting in an elongated protein sequence
in a nonsense mutation we have premature termination of the protein sequence OR an elongation of the protein sequence
frameshift mutations An alteration of DNA in which an addition or deletion of a base pair occurs Results in a change in the entire “reading frame”
consequences of mutations with respect to GAIN of function Associated with dominant disorders Production of new protein product Overexpression of a protein product Inappropriate expression of a protein product
consequences of mutations with respect to LOSS of function Associated with recessive disorders Loss of 50% of the protein product May or may not be adequate for normal function
chromosome abn - euploid cells Cells that have a multiple of the normal number of chromosomes Haploid and diploid cells are euploid forms
when a euploid cell has more than a diploid number, it is called a polyplid cell
triploidy a zygote having three copies of each chromosome (69)
tetraploidy four copies of each (92 total)
do triploid and tetraploid fetuses survive? no, they do not
disjunction normal separation of xomes during cell division
nondisjunction USUALLY THE CAUSE OF ANEUPLOIDY---failure of homologous xomes or sister chromatids to separate normally during meiosis or mitosis
xome aneuploidy a somatic cell that does not contain a multiple of 23 xomes
an aneuploidy cell that contains 3 copies of one xome is called trisomic or trisomy
monosome is the presence of only one copy of any xome
monosomy is often fatal, but infants infants can survivie with trisomy of certain xomes - is better to have extra than less!
xome disorders are the leading cause of mental retardation and miscarriage
Incidence of xomal abns fun facts 1/12 conceptions Approximately 95% of conceptions with chromosome disorders result in miscarriage 50% of first-trimester miscarriages associated with a major chromosomal abnormality 1/150 live births with a major diagnosable chromosomal abnormality
Best example of aneuploidy is Trisomy 21 also known as Down Syndrome---1:800 live births Mentally retarded, low nasal bridge, epicanthal folds, protruding tongue, poor muscle tone Risk increases with maternal age Increased risk of congenital heart disease, gastrointestinal disease, and leukemia
other autosomal aneuploidies Trisomy 13 and 18 More severe clinical manifestations than trisomy 21 Death in early infancy is common
autosomal aneuploidy - partial trisomy Only an extra portion of a chromosome is present in each cell
autosomal aneuploidy - chromosome mosaics trisomies occuring in only SOME cells of the body
sex xome aneuploidy - one of most common is Trisomy X where female has THREE X xomes Termed “metafemale” Symptoms are variable: sterility, menstrual irregularity, and/or mental retardation Symptoms worsen with each additional X
sex xome aneuploidy - Turner syndrome Females with only one X chromosome Characteristics Absence of ovaries (sterile) Short stature (~ 4'7") Webbing of the neck Edema Underdeveloped breasts; wide nipples High number of aborted fetuses X is usually inherited from mother
sex xome aneuploidy - Klinefelter syndrome---individuals with AT LEAST 2 X's and one Y---have these characteristics Male appearance Develop female-like breasts Small testes Sparse body hair Long limbs Some individuals can be XXXY and XXXXY. The abnormalities will increase with each X
alt in xome structure - deletion loss of a sequence of DNA from an xome
alt in xome structure - inversion xomal rearrangement in which a segment of xome is reversed END TO END
ALT IN XOME STRUCTURE - translocation transfer of one xome segment to another
alt in xome structure - ring xome Structurally abnormal chromosome in which the telomere of each chromosome arm has been deleted and the broken arms have joined
alt in xome structure - xome breakage - bwo ionizing radiation, chemicals and viruses If a chromosome break does occur, physiologic mechanisms usually repair the break, but the breaks often heal in a way that alters the structure of the chromosome
alt of xome structure - break or loss of DNA - Cri du Chat syndrom Deletion of short arm of chromosome 5 (5p-) Low birth weight, metal retardation, and microcephaly
alt in xome structure - duplication - rare repeated gene or gene sequence Less serious consequences because better to have more genetic material than less (deletion) Duplication in the same region as cri du chat causes mental retardation but no physical abnormalities
alt in xome structure - inversions Two breaks on a chromosome Reversal of the gene order Usually occurs from a breakage that gets reversed during reattachment ABCDEFG may become ABEDCFG
alt in xome structure - translocation The interchanging of material between nonhomologous chromosomes Translocation occurs when two chromosomes break and the segments are rejoined in an abnormal arrangement
alt in xome structure - fragile sites Fragile sites are areas on chromosomes that develop distinctive breaks or gaps when cells are cultured No apparent relationship to disease
alt in xome structure - fragile X syndrome Site on the long arm of the X chromosome Associated with mental retardation; second in occurrence to Down syndrome Higher incidence in males because they have only one X chromosome
genes are basic units of heredity, with sequences of xomal DNA coding for the production of a functional product Templates for mRNA that code for specific proteins All genes are contained in each cell of the body
genes - locus location occupied by a gene on a xome
genes - allele - is an alternate version of a gene at a locus Each individual possesses two alleles for each gene
genes - homozygous possessing identical alleles of a given gene
genes - heterozygous possessing two different alleles of a given gen
genes - polymorphism locus that has 2+ alleles that occur with appreciable frequency
example of homozygous loci Loci on a pair of chromosomes have identical alleles Example O blood type (OO)
example of heterozygous loci Loci on a pair of chromosomes have different alleles Example AB blood type (A and B alleles on pair of loci)
genotype - what they have the genetic makeup of an organism
phenotype - what they demonstrate the observable, detectable or outward appearance of genetics
example of phenotype A person with the A blood type could be AA or AO. A is the phenotype; AA or AO is the genotype.
genetics - if 2 alleles are found together, then the allele that is observable is dominant, and the one whose effects are hidden is recessive
genetics - the dominant allele is represented by a capital letter, and the recessive by a lowercase letter
true or false - alleles can be codominant true
genetics, what is a carrier and give one example A carrier is one that has a disease gene but is phenotypically normal For a person to demonstrate a recessive disease, the pair of recessive genes must be inherited Example Ss = sickle cell anemia carrier ss = demonstrates sickle cell disease
what is a genetic pedigree Used to study specific genetic disorders within families Begins with the proband Propositus (male) Proposita (female)
single gene disorder - autosomal dominant disorder Abnormal allele is dominant, normal allele is recessive, and the genes exist on a pair of autosomes
autosomal dominant disorders - characteristics Condition is expressed equally in males and females Approximately half of children of an affected heterozygous individual will express the condition Homozygous affected individuals are rare No generational skipping
autosomal dominant traits see slide 66 and 67
single-gene disorders - recurrence risk The probability that parents of a child with a genetic disease will have yet another child with the same disease Recurrence risk of an autosomal dominant trait When one parent is affected by an autosomal dominant disease and the other is normal, the
recurrence risk of autosomal dominant trait When one parent is affected by an autosomal dominant disease and the other is normal, the occurrence and recurrence risks for each child are one half
penetrance The percentage of individuals with a specific genotype who also express the expected phenotype
incomplete penetrance and example Individual who has the gene for a disease but does not express the disease Retinoblastoma (eye tumor in children) demonstrates incomplete penetrance (90%)
gene mapping see slide 70
expressivity definition and cause Expressivity is the variation in a phenotype associated with a particular genotype This can be caused by modifier genes
example of expressivity disorder - see slide 72 for image von Recklinghausen disease Autosomal dominant Long arm of chromosome 17 Disease varies from dark spots on the skin to malignant neurofibromas, scoliosis, gliomas, neuromas, etc
single gene disorders - autosomal RECESSIVE disorder Abnormal allele is recessive and a person must be homozygous for the abnormal trait to express the disease The trait usually appears in the children, not the parents, and it affects the genders equally because it is present on a pair of autosomes
autosomal recessive disorder recurrence risk When two parents are carriers of an autosomal recessive disease, the occurrence and recurrence risks for each child are 25%
characteristics of autosomal recessive disorder - characteristics Condition expressed equally in males and females Affected individuals most often the offspring of asymptomatic heterozygous carrier parents Approximately 1/4 of offspring will be affected; 1/2 will be asymptomatic carriers; and 1/4 will be unaffected
autosomal recessive disorder characteristics #2 Individuals must be homozygous for the condition to be expressed Generational skipping may be present Consanguinity may be present
autosomal recessive disorder - Cystic Fibrosis Associated with mutation of the CF gene located on the long arm of chromosome 7 Abnormal expression CF transmembrane conductance regulator protein on epithelial cells
how does CF cystic fibrosis manifest? Defect in chloride transport leading to salt imbalances, dehydrated mucus secretions plugging airways, digestive organ obstructions and malnutrition Lung disease or heart failure is usual cause of death
autosomal recessive disorder see slide 77 for image
Cosanguinity Mating of two related individuals Dramatically increases the recurrence risk of recessive disorders
sex-linked disorders - X linked usually expressed by males because females have another X chromosome to mask the abnormal allele Most are recessive
sex linked disorders - Y linked uncommon because Y chromosome contains relatively few genes Father-son transmission present No father-daughter transmission
Characteristics of X-linked recessive disorders Males most commonly affected Affected males cannot transmit the genes to sons, but they can to all daughters Unaffected carrier females Sons of female carriers have a 50% risk of being affected Pedigree analysis Generational skipping often present
in X-linked recessive disorders, is there generational skipping? what about father-to son transmission? Generational skipping often present No father-to-son transmission
Example #1 of X-linked Recessive Disorders Hemophilia Bleeding disorders resulting from a congenital deficiency of coagulation factors Hemophilia A: factor VIII deficiency 20.6/100,000 male births in U.S. Hemophilia B: factor IX deficiency 5.3/100,000 male births in U.S
In hemophilia, mutations associated with factor VIII deficiency Large deletions or insertions, frameshift and splice junction changes, and nonsense and missense mutations Mutations vary across families but tend to be similar within families
X-linked recessive disorders - Duchenne muscular dystrophy Characterized by progressive muscle degeneration Mutation of dystrophin gene resulting in down-regulation or absence of dystrophin Affects 1:3500 males, cardiac or respiratory failure before 20 years of age
manifestations of Duchenne muscular dystrophy Produces generalized weakness, progressive muscle wasting, lethal
X-linked recurrence risk - see slide 83 for graphic A. Outcomes for offspring of an unaffected father and a heterozygous unaffected carrier mother (most common scenario) B. Outcomes for offspring of an affected father and a homozygous unaffected mother C. (see next slide)
X-linked recurrence risk - part C Outcomes for offspring of an affected father and a heterozygous unaffected carrier mother
Created by: lorrelaws