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TOPIC 8.docx

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University of Waterloo
HLTH 101
Glenn Ward

TOPIC 8: INNATE FACTORS IN THE HUMAN BODY AND HEALTH: GENETIC DISEASES BASIC PRINCIPLES OF GENETICS Nitrogenous Bases > Nucleic Acids > Genes > Proteins > Phenotype 1. NUCLEIC ACIDS  there are 2 nucleic acids: i. deoxyribonucleic acid (DNA) ii. ribonucleic acid (RNA)  each consists of polynucleotide chains of nitrogenous bases on a backbone of sugar- phosphate links  there are 2 classes of nitrogenous bases: i. purines — adenine & guanine (A & G) ii. pyrimidines — cytosine (C) & thymine (T) (found in DNA) OR cytosine (C) & uracil (U) (found in RNA) 2. GENES  genes are the basic entity of hereditary information  genetic code: sequences of the nitrogenous bases described above  3 adjacent bases form a codon o each codon codes for a specific amino acid o thus, there are 64 possible codons  however, there are 20 common amino acids  therefore, there is usually more than one codon for each amino acid  amino acids are the basic building blocks of polypeptides called proteins o a linear sequence of amino acids forms a specific protein  there can be any number of amino acids comprising a protein o a gene is a sequence of codons combining to produce a specific protein 3. PROTEINS  main types of proteins: o enzymes: catalyze chemical reactions (cell metabolism) o transport: means of transporting molecules o structural: material for cell and tissue structure o regulatory: regulate cellular processes (eg: hormones) o antibodies: immune substances which recognize foreign substances 4. CENTRAL DOGMA OF MOLECULAR BIOLOGY  genetic information flows in one direction: DNA —> DNA (intergenerational transmission) DNA —> RNA —> protein (phenotypic expression)  2 stages: i. transcription: information from a segment of DNA is copied onto mRNA o this segment provides the information to produce 1 gene ii. translation: amino acids are transferred to mRNA (by tRNA) to form a protein  the Central Dogma of Molecular Biology itself has been rejected to some degree because of such phenomena as regulator genes which respond to environmental events and thereby influence transcription and translation  the new field of epigenetics is the systematic study of how gene expression may be altered by such processes as methylation which responds to signals from the environment  furthermore, we now believe that at least some of the genome in the so-called higher organisms (such as mammals) may have been determined by contributions by retroviruses which can alter the genome of the cell  Note: all genetic activity occurs within the cell DNA —> RNA in nucleus RNA —> protein in cytoplasm o genetic information acts only within the cell o all subsequent events resulting from these intracellular events (at the level of tissues, organs, etc.) are at least partly dependent upon other factors above the level of the cell  Tissues > Organs > Organ Systems > Individual > Group/Society 5. CHROMOSOMES  genetic material is contained in a substance called chromatin o the chromatin forms discrete units (called chromosomes) prior to cell division  karyotypes consist of the chromosome complement and shape unique to a particular species o knowledge of karyotypes is essential in health i. to recognize chromosomal abnormalities ii. to be able to identify the location of individual genes (loci)  the genome consists of all chromosomal DNA contained within all chromosomes  there is also mitochondrial DNA transmitted in mitochondria of egg  no mitochondrial DNA from the sperm is transmitted to the egg, so each individual receives their mitochondrial DNA only from their mother 6. ALLELES  genes are inherited in pairs, one from each parent  each gene of a particular pair is called an allele of the other  if the alleles are identical, then we say that the individual is homozygous at that locus  otherwise, we say that they are heterozygous  a dominant trait is one in which the allele is expressed in both the homozygous and heterozygous situations* o if serious, usually late onset (reproduced and passed on through family) o due to “toxic” gene product  a recessive trait is one in which the allele is expressed only if the alleles are homozygous (it can be partially expressed if heterozygous)* o if serious, can be early onset (requires both copies of the allele from parents) o due to lack of gene product (non-functional)  the pattern of inheritance can be determined by use of the Punnett square  i.e., assume that there is a trait determined by allele A A = dominant allele a = recessive allele i. Both parents are carriers (heterozygous) for the trait. Parent 1 Aa Parent 2 Aa Offspring: 1/4 = AA 2/4 = Aa 1/4 = aa  ii. One parent is a carrier (heterozygous), and the other parent has the recessive trait (homozygous for the recessive allele). Parent 1 aa Parent 2 Aa Offspring: 2/4 = Aa 2/4 = aa  note that these rules do not strictly apply if the alleles are located on the sex chromosomes (X & Y)  genes on the sex chromosomes are distributed unevenly to males and females  i.e., Parent 1 X Y Parent 2 XX  the Y chromosome is passed by the father to his sons o it is not passed by the father to his daughters o this is termed holandric inheritance o holandric inheritance is not associated with any known syndromes  the X chromosome is passed from either parent to the daughters o it is passed by the mother to her sons only o each son will have one X chromosome, and any alleles on it will always be expressed o each daughter will inherit two X chromosomes, but one X chromosome in each cell will be inactivated  therefore, each allele will be present in, on average, 50% of her cells  effects of harmful alleles on the X chromosome may therefore be absent or mild, depending upon what cell types they are expressed in 7. SINGLE GENE MUTATIONS  abnormal or defective genes are referred to as mutations  eventually, if the mutation has spread throughout the population and occurs at a high frequency, it may simply be referred to as an allele o therefore, mutations usually refer to the appearance of a new form of the allele  there are 3 main types of mutations: i. substitutions: in this case, the base pair at a specific location is replaced by a different base pair so that the mutated copy of the allele codes for a different amino acid. ii. deletion: in this case, the complete sequence becomes altered because a specific base pair has been deleted from the DNA iii. insertion: in this case, the complete sequence becomes altered because a specific base pair has been inserted into the DNA  substitutions are often called point mutations while deletions and insertions are often called frame-shift mutations. If you think about how they change the DNA sequence, the reason they are called these names should be clear. 8. MENDELIAN GENETIC DISORDERS  in a Mendelian disorder, an existing mutation codes for an inappropriate amino acid sequence, which leads to the production of an abnormal protein, which leads to a disordered state*  the first three stages (the production of the abnormal protein) occur at the level of the cell  if this results in a disease state, then the disease state generally occurs above the level of cell  generally, there are two main categories of Mendelian genetic traits, each with characteristic features (note that these features were established in animal models in whic
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