Comprehensive Study Notes on Genes and Genetic Diseases

Deoxyribonucleic Acid (DNA) and Clinical Genetics

Deoxyribonucleic Acid (DNA) Structure: * Nucleotides: DNA is composed of subunits called nucleotides. Each nucleotide contains three components: 1. One pentose sugar (specifically deoxyribose). 2. One phosphate group. 3. One nitrogenous base. * Nitrogenous Bases: There are four types of nitrogenous bases in DNA: * Cytosine (C) * Thymine (T) * Adenine (A) * Guanine (G) * Double Helix: DNA molecules are structured as a double-stranded helix. * Genes: Chromosomes contain genes, which are considered the basic unit of inheritance. Genes are composed of DNA sequences.

DNA as the Genetic Code and Replication

The Genetic Code: * Protein Synthesis: DNA provides the template/code for all proteins in the body. * Polypeptides: Proteins are constructed from one or more polypeptides. Polypeptides, in turn, are made of amino acids. * Amino Acids: There are $20$ distinct amino acids used in protein construction. * Codons: The production of amino acids is directed by sequences of three bases known as codons. * Stop Signals: Termination and nonsense codons serve to stop the production of a protein sequence.
The Process of DNA Replication: * Unzipping: The DNA strand is untwisted and unzipped to expose a single strand that acts as a template. * DNA Polymerase: This enzyme pairs complementary bases to the template strand: * Adenine pairs with Thymine ( $A-T$ ). * Cytosine pairs with Guanine ( $C-G$ ). * Proofreading: DNA polymerase adds new nucleotides and "proofs" the new protein. If an incorrect nucleotide is identified, it is excised and replaced.

Mutations and Mutagens

Mutation Definitions: A mutation is any inherited alteration of genetic material. Key types include: * Chromosome Aberrations: Changes in the number or structure of chromosomes. * Base Pair Substitution (Missense Mutation): One base pair is replaced by another. This may change the amino acid sequence and may or may not result in disease. * Frameshift Mutation: Involves the insertion or deletion of one or more base pairs in the DNA molecule, which shifts the reading frame.
Mutagens: These are agents that increase the frequency of mutations. Examples include: * Ionizing radiation * Chemicals

From Genes to Proteins: Transcription and Translation

Overview: DNA is formed in the nucleus, but protein synthesis occurs in the cytoplasm. This requires genetic transport mediated by RNA.
Ribonucleic Acid (RNA): * RNA is a single-stranded molecule. * It contains the base Uracil (U) instead of Thymine (T). The other three bases (Adenine, Cytosine, Guanine) are identical to DNA.
Transcription: * RNA is synthesized from a DNA template. * RNA Polymerase binds to a promoter site on the DNA to initiate synthesis. * The DNA specifies the sequence of messenger RNA (mRNA). * The process continues until a termination sequence is reached. * mRNA moves from the nucleus into the cytoplasm. * Gene Splicing: Involves the removal of introns (non-coding regions) and the joining of exons (coding regions).
Translation: * RNA directs polypeptide synthesis through interaction with transfer RNA (tRNA). * Anticodon: A sequence of nucleotides on tRNA that is complementary to the codon on the mRNA strand. * Ribosome: The physical site of protein synthesis. It facilitates the interaction between mRNA and tRNA to form polypeptides. * Termination: Synthesis ceases when the ribosome reaches a termination signal on the mRNA.

Human Chromosomes and Cell Division

Somatic Cells: * Contain $46$ chromosomes arranged in $23$ pairs. * They are diploid cells, meaning one member of each pair comes from the mother and one from the father.
Gametes: * Sperm and egg cells. * Contain $23$ chromosomes and are haploid cells.
Meiosis: The specific process of cell division that forms haploid cells from diploid cells.
Classification of Chromosomes: * Autosomes: The first $22$ of the $23$ pairs. They are virtually identical in males and females and are termed homologous. * Sex Chromosomes: The remaining pair ( $23^{rd}$ ). * Females: Homologous pair ( $XX$ ). * Males: Nonhomologous pair ( $XY$ ).
Karyotype: An ordered display of chromosomes based on length and centromere location.

Chromosomal Aberrations: Numerical Abnormalities

Euploid Cells: Cells with a multiple of the normal number of chromosomes ( $23$ ). Both haploid and diploid are euploid.
Polyploidy: A condition where a euploid cell has more than the diploid number ( $46$ ). * Triploidy: A zygote with three copies of each chromosome ( $69$ total). * Tetraploidy: A zygote with four copies of each chromosome ( $92$ total). * Clinical Significance: Triploid and tetraploid fetuses do not survive; they are typically stillborn or spontaneously aborted.
Aneuploidy: A somatic cell that does not contain a multiple of $23$ chromosomes. * Trisomy (Trisomic): A cell containing three copies of one specific chromosome. Survival is possible for certain chromosomes. * Monosomy: Presence of only one copy of any chromosome; this is often fatal.
Nondisjunction: The failure of homologous chromosomes or sister chromatids to separate normally during meiosis or mitosis. This is the primary cause of aneuploidy.
Sex Chromosome Aneuploidy: * Generally less serious than autosomal aneuploidy. * The $Y$ chromosome contains little genetic material. * Extra $X$ chromosomes are largely silenced through inactivation.

Clinical Examples of Aneuploidy

Autosomal Aneuploidy: * Trisomy: Only trisomies of chromosomes $13$ , $18$ , and $21$ are typically compatible with survival beyond birth. * Partial Trisomy: An extra portion of a chromosome is present; symptoms are less severe than total trisomy. * Chromosomal Mosaics: Trisomies occurring in only some cells of the body, resulting in different cell lines with different karyotypes. * Down Syndrome (Trisomy 21): * Occurs in $1\,in\,800$ live births. * Manifestations: Mental challenges, low nasal bridge, epicanthal folds, protruding tongue, flat low-set ears, and poor muscle tone. * Risks: Risk increases with maternal age. Higher incidence of congenital heart disease, respiratory infections, and leukemia.
Sex Chromosome Aneuploidy Examples: * Trisomy X: Females with three X chromosomes ( $1\,in\,1000$ female births). Symptoms vary: sterility, menstrual irregularity, or cognitive deficits. Symptoms worsen with additional X chromosomes. * Turner Syndrome: Females with only one X chromosome (Karyotype $45,X$ ). * Occurs $1\,in\,2500$ female births. * Characteristics: Sterile (absence of ovaries), short stature, webbed neck, widely spaced nipples. * The single X is usually inherited from the mother. High rate of spontaneous abortion. * Klinefelter Syndrome: Individuals with at least one Y and two or more X chromosomes ( $XXY, XXXY, XXXXY$ ). * Occurs $1\,in\,1000$ male births. * Characteristics: Male appearance, gynecomastia (femalelike breasts), small testes, sparse body hair. * Risk increases with maternal age; abnormalities increase with more X chromosomes.

Abnormalities of Chromosomal Structure

Chromosome Breakage: Usually repaired without damage, but can heal in ways that alter structures. Caused by ionizing radiation, chemicals, and viruses.
Deletion: Loss of DNA/breakage. * Cri du chat Syndrome ("cry of the cat"): Deletion of the short arm of chromosome $5$ ( $5p$ deletion). Features include low birth weight, mental challenges, and microcephaly.
Duplication: Excess genetic material; usually results in less serious consequences than deletions.
Inversion: Chromosomal rearrangement where a segment is reversed (e.g., $ABCDEFG \rightarrow ABEDCFG$ ). Primarily affects offspring.
Translocation: Interchange of genetic material between nonhomologous chromosomes. * Robertsonian Translocation: Fusion of the long arms of two nonhomologous chromosomes at the centromere; common in Down syndrome. * Reciprocal Translocation: Breaks in two different chromosomes with an exchange of material.
Fragile Sites: Breaks and gaps that develop when cells are cultured in folate-deficient medium. * Fragile X Syndrome: Located on the long arm of the X chromosome. It involves an elevated number of repeated DNA sequences. It is the second most common cause of being mentally challenged after Down syndrome.

Elements of Formal Genetics and Inheritance

Terminology: * Locus: The specific location of a gene on a chromosome. * Allele: Different forms of a gene at a specific locus. One from mother, one from father. * Homozygous: Identical alleles at a locus. * Heterozygous: Different alleles at a locus. * Polymorphism: A locus with two or more alleles occurring with appreciable frequency. * Genotype: The genetic composition at a locus. * Phenotype: The outward appearance resulting from genotype and environment. * Example: Phenylketonuria (PKU). Untreated, the infant has cognitive impairment (phenotype). If treated, the infant maintains the PKU genotype but develops a normal phenotype. * Dominance: The allele whose effects are observable ( $capital\,letter$ ). * Recessiveness: The allele whose effects are hidden ( $lowercase\,letter$ ). * Co-dominant: Both alleles are expressed. * Carrier: Individual with a disease allele who is phenotypically normal.

Principles and Modes of Inheritance

Mendel’s Laws: 1. Principle of Segregation: Homologous genes separate; each cell carries only one. 2. Principle of Independent Assortment: Transmission of one gene does not affect another.
Pedigrees: Tools to study family genetic disorders. * Proband: The first person diagnosed (Propositus = male; Proposita = female).
Autosomal Dominant Inheritance: * Rare; occurs in fewer than $1\,in\,500$ people. * Affected heterozygous parent + normal parent = $50\%$ chance of affected offspring for each birth. * No generational skipping; males and females affected equally. * Germline Mosaicism: Two or more offspring affected when parents show no symptoms (parent carries mutation in germline only). * Penetrance: Percentage of individuals with a genotype who express the phenotype. * Incomplete Penetrance: Gene is present but not expressed (e.g., $Retinoblastoma$ at $90\%$ ). * Age-dependent Penetrance: Symptoms delay until a certain age (e.g., Huntington disease). * Expressivity: Variation in phenotype for the same genotype. * von Recklinghausen disease: Autosomal dominant; varies from brown skin spots to malignant tumors.
Autosomal Recessive Inheritance: * Must be homozygous to express (abnormal allele is recessive). * Parents are usually heterozygous carriers. Risk for each child is $25\%$ . * Consanguinity: Inbreeding (mating of related individuals). Dramatically increases risk for recessive disorders. * Cystic Fibrosis: Defective sodium channels and salt imbalance; causes thick dehydrated mucus; survival rarely past $40$ years of age.

X-Linked Inheritance

Mechanisms: * X-Inactivation: One X chromosome in female somatic cells is permanently inactivated (forms a Barr body). * Number of Barr bodies = $Number\,of\,X\,chromosomes - 1$ . * SRY Gene: Sex-determining region on the Y chromosome. Initiates male gonadal development at the $6^{th}$ week of gestation.
X-Linked Recessive Inheritance: * Commonly affects males more because they are hemizygous (only one X). * Affected fathers pass the gene to all daughters (who become carriers) but none of their sons. * Generational skipping often occurs as it passes through female carriers. * Duchenne Muscular Dystrophy (DMD): $1\,in\,3500$ males; deletion of DMD gene leads to non-functional dystrophin and muscle cell death.
Other Traits: * Sex-limited trait: Occurs in only one sex. * Sex-influenced trait: Occurs significantly more often in one sex than the other.

Linkage and Gene Identification

Linkage: Loci that are close together do not follow independent assortment.
Crossing Over: Creates new alleles during recombination.
Genetic Testing: Used to confirm diagnosis, identify carriers, and perform presymptomatic identification for diseases with delayed onset.

Questions & Discussion

Question 1: Which information is correct regarding DNA polymerase? DNA polymerase functions to: * Answer: Add the correct nucleotides to a DNA strand.
Question 2: At what site does protein synthesis occur? * Answer: The ribosome.
Question 3: A female has one X chromosome. Which diagnosis will the nurse observe documented on the chart? * Answer: Turner syndrome.
Question 4: Which information indicates that the nurse has a good understanding of X-linked recessive inheritance? * Answer: The gene is passed from an affected father to all of his daughters.