Genes and Genetic Diseases
Genes and Genetic Diseases Nursing 240
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Date Created: 04/05/16
Chapter 2: Genes and Genetic Diseases Definitions Genes are composed of DNA which has three basic components: 5 carbon deoxyribose, 1 phosphate molecule, & 4 types of nitrogenous bases Bases: cytosine & thymine are single carbons called pyrimidines & the other two bases are double carbons called adenine & guanine; represented as A, C, T, &G Watson and Crick showed how these molecules are assembled as DNA on a double helix model which looks like a twisted ladder Nitrogenous bases form the rungs of the ladder: A&T pairs & G&C pairs Each DNA subunit consists of 1 deoxyribose, 1 phosphate, &1 base molecules and is called a nucleotide DNA Composition Pentose sugar (deoxyribose 1) Phosphate molecule (1) Four nitrogenous bases: o Pyrimidines: cytosine and thymine o Purines: adenine and guanine Double helix model Nucleotide is DNA subunit One or more polypeptides Composed of amino acids o Twenty amino acids o Directed by sequence of bases (codons) DNA as the Genetic Code Directs the synthesis of all body proteins which are composed of one or more polypeptides (intermediate protein compounds) which in turn consist of sequences of amino acids (20 types in body) To code for 20 different amino acids with only 4 bases means that different combinations of bases, occurring in groups of 3, are used & these triplets of bases are called codons DNA Replication Untwisting and unzipping of the DNA strand breaks the weak hydrogen bonds between the bases Single strand of DNA then acts as a template for more strands Consistent pairing of A&T and G&C is known as complementary pairing and is the key to accurate replication and produces a new doublestranded molecule identical to the original Other proteins are involved in DNA replication and the most important is an enzymes known as DNA polymerase that travels along the single DNA strand adding the correct nucleotides to the free end of the new strand and checking to make sure that its base is the right complementary base; “proofreads” the sequence DNA Mutation An inherited alteration of genetic material o Chromosomeaberrations (abnormalities) Spontaneous mutation—mutation that occurs in absence of exposure to known mutagens (things that are known to derail the DNA replication) Mutational hot spots—areas of the chromosomes that have high mutation rates Base pair substitution—one base pair is substituted for another; may result in change of amino acid sequence have no consequence (due to redundancy) or may cause disease Frame shift mutation—insertion or deletion of one or more base pairs o Causes a change in the entire “reading frame” o Tend to cause severe disease consequences Mutagen Agent known to increase the frequency of mutations o Radiation o Chemicals—nitrogen mustard, vinyl chloride, alkylating agents, formaldehyde, sodium nitrite Mutations are rare events, but some DNA sequences have particularly high mutation rates and are called mutational hot spots Transcription RNA is synthesized from the DNA template that results in the formation of messenger RNA (mRNA) RNA polymerase binds to promoter site which is a sequence of DNA that specifies the beginning of a gene Transcription continues until termination sequence is reached mRNA moves out of the nucleus and into the cytoplasm and then activates all other structures “directs traffic” Translation Process by which RNA directs the synthesis of a polypeptide (long chain of amino acids) via interaction with transfer RNA (tRNA) Site of protein synthesis is the ribosome (molecule responsible for production in living cells) The ribosome moves along the mRNA sequence to translate the amino acid sequence & is eventually released from the nucleus into cell cytoplasm (material inside the cell that surrounds the nucleus) to perform its required functions Chromosomes Gametes—sperm and egg cells; have only 1 member of each chromosome pair for a total of 23 chromosomes; called haploid cells which are formed from diploid cells through meiosis Somatic cells—all cells other than gametes; have 46 chromosomes in 23 pairs; called diploid cells because the individual’s parents each donate one chromosome per pair; formed through mitosis Autosomes o The first 22 of 23 pairs of chromosomes in males and females o The two members are virtually identical and thus said to be homologous (identical) Sex chromosomes o Remaining pair of chromosomes In females, it is a homologous pair (XX) In males, it is a nonhomologous pair (XY) Karyotype (karyogram) : an ordered display of chromosomes Chromosome Abberations—Leading Cause of Mental Retardation and Miscarriages Euploid cell with a multiple of the normal number of chromosomes Aneuploid somatic cell that does not contain a multiple of 23 chromosomes Trisomic/trisomy—cell containing three copies of one chromosome Monosomy—presence of only one copy of any chromosome and is lethal Infants can survive with trisomy of certain chromosomes “It is better to have extra than less” in some cells Loss of chromosome material has more serious consequences than duplication of chromosome material Disjunction—normal separation of chromosomes during cell division Nondisjunction—abnormal separation of chromosomes during cell division Usually the cause of aneuploidy—absence of the right number of chromosomes Failure of homologous chromosomes or sister chromatids to separate normally during meiosis or mitosis Autosomal Aneuploidy—Down Syndrome Best known example of aneuploidy (abnormal chromosomal separation)/ Trisomy 21 Occurs in 1:800 births Mentally retarded, low nasal bridge, epicanthal folds, protruding tongue, poor muscle tone Risk increases with maternal age >35 Sex Chromosome Aneuploidy One of the most common is trisomy X (a female with 3 X chromosomes) o Symptoms are variable: sterility, menstrual irregularity, and/or mental retardation o Symptoms worsen with each additional X Turner Syndrome o Females with only one X chromosome o Characteristics: Underdeveloped ovaries (sterile) Short stature (~4’7”) Webbing of the neck Edema Underdeveloped breasts; wide nipples High number of aborted fetuses Klinefelter syndrome o Individuals with atleast two Xs and one Y chromosome o Some individuals can be XXY and XXXY o The abnormalities increase with each X o Characteristics Male appearance Develop femalelike breasts Small testes Sparse body hair Long limbs More About SexLinked Disorders The Y chromosomes contains only a few dozen genes, so most sexlinked traits are located on the X chromosome and are said to be Xlinked; males only have one X so it is more likely to inherit the abnormal gene Xlinked disorders are usually expressed (shown) by males because females have two X chromosomes to mask the abnormal gene In general, men are affected & women are carriers Abnormalities in Chromosome Structure Chromosome breakage—physiologic mechanisms will usually repair the break, but the break often will heal in a way that alters the chromosome structure Clastogens (synonyms for mutagen) increase the risk for chromosome breakage; include ionizing radiation, chemicals, viruses Deletions—breakage or actual loss of chromosomes Cri du Chat Syndrome/ Cry of the Cat Caused by DNA (deletion) on short arm of chromosome 5 Low birth weight, mental retardation, & microcephaly Alteration in Chromosome Structure Inversions two breaks on a chromosome that results in reversal of the gene order o Usually occurs from a breakage that gets reversed during reattachment: ABCDEFG may become ABEDCFG Position effect: genes may influence neighboring genes which can cause physical effects Unlike deletions and duplications, no loss or gain of genetic material occurs so inversions are “balanced” alterations and often do not cause any physical effects Translocations—interchanging of material between nonhomologous chromosomes Reciprocal translocation occurs when two chromosomes break and the segments are rejoined in an abnormal arrangement; usually do not produce disease/disorders Robertsonian translocation occurs when fusion at centromere, forming a single chromosome; carriers may be normal but offspring may have increased risk of down syndrome Fragile sites: areas on chromosomes that develop distinctive breaks or gaps where cells are cultured Fragile X syndrome o Gap on the long arm of the X chromosome o Mental retardation; autism o Higher incidence in males because that have only one X chromosome Genetics Locus—position of a gene along a chromosome Allele—different or variant form of a particular gene which is located at the same position (genetic locus) on a chromosome Homozygous—loci on a pair of chromosomes that have identical genes Heterozygous—loci on a pair of chromosomes that have different genes Genotype—the composition of a gene at a given locus; genetic makeup of an individual, is not observable/inward Phenotype—outward appearance of an individual which is result of genotype and environment, is observable/outward appearance The allele that is observable is dominant, and the one whose effects are hidden is recessive – brown eyed (dominant) father & blue eyed (recessive) mother – offspring will usually brown eyes If both parents have blue eyes (recessive), the offspring will have blue eyes Alleles can be codominant – AB blood type In genetics, the dominant allele is represented by a capital letter, and the recessive by a lowercase letter Carrier—one who has a disease gene but is phenotypically normal o For a person to demonstrate a recessive disease, the pair of recessive genes must be inheirited Autosomal Dominant & Recessive Patterns Autosomal dominance is a pattern of inheritance characteristic of some genetic diseases "Autosomal" means that the gene in question is located on one of the numbered, or nonsex, chromosomes. "Dominant" means that a single copy of the diseaseassociated mutation is enough to cause the disease. Autosomal recessive is one of several ways that a trait, disorder, or disease can be passed down through families. An autosomal recessive disorder means two copies of an abnormal gene must be present in order for the disease or trait to develop. Recurrence Risks of a Dominant Trait Probability that an individual will develop a genetic disease such as when parents of a child with a genetic disease will have yet another child with the same disease When one parent has an autosomal dominant disease (Dd) & the other is normal (dd), the occurrence & recurrence risk for each child is onehalf or 50% Examples: Huntington’s Disease, Marfan’s Syndrome, Von Willebrand’s Disease Huntington’s Disease has a delayed onset at about 40 years of age; neuro disorder whose main features are progressive dementia & increasingly uncontrollable limb movements Autosomal Recessive Disorder Abnormal allele is recessive and homozygous for the abnormal trait to express the disease (bb) Two copies of the gene (bb) must be mutated for a person to have ARD Affected person usually has two unaffected parents who each carry one copy of the mutated gene; affects the genders equally because it is present on a pair of autosomes Risk of occurrence for each child is 25%; 50% risk that a child will have one abnormal gene & be a carrier; 25% chance that child will not receive abnormal genes & will be neither a carrier or have the disease Examples: cystic fibrosis, sickle cell anemia, albinism