genetics

Overview of Genetics

• The genetic system comprises a network of molecular components for storing, expressing, and transmitting genetic information.

  • DNA (Deoxyribonucleic acid) and RNA (Ribonucleic acid) are primary carriers of genetic instructions.

  • Genes serve as blueprints for protein synthesis.

  • Chromosomes organize and ensure the inheritance of genetic material.

  • These components influence traits and guide biological processes across generations.

DNA Structure and Function

• Definition: DNA is a nucleic acid responsible for storing and transmitting genetic information.

  • Contains sugar deoxyribose.

  • Mainly located in the nucleus, also in mitochondria and cytoplasm.

• Key Functions:

  • Determines an organism's traits and functions.

• Structural Composition:

  • Consists of four nitrogenous bases:

  • Purines: Adenine (A) and Guanine (G)

  • Pyrimidines: Cytosine (C) and Thymine (T)

  • Base Pairing Rules:

  • Adenine (A) pairs with Thymine (T)

  • Cytosine (C) pairs with Guanine (G)

  • This pairing facilitates accurate replication and transmission of genetic information.

DNA Molecule Structure

• DNA is structured as a double-stranded molecule coiled into a double helix.

  • The strands are held by hydrogen bonds between complementary base pairs.

• The sequential order of bases forms genes and chromosomes, crucial for development, growth, and cellular function.

  • Genes: units of heredity, segments of DNA located on chromosomes that encode instructions for protein synthesis.

  • Vary in size from hundreds to millions of base pairs.

  • Typically occur in pairs in somatic cells but singly in gametes (sperm and eggs).

Role of RNA in Protein Synthesis

• Function of RNA: Similar to DNA but contains Uracil (U) instead of Thymine (T).

  • Plays a critical role in carrying genetic information from DNA to the sites of protein synthesis in the cytoplasm.

• Types of RNA:

  1. Messenger RNA (mRNA):

  • Template for protein synthesis, carries genetic information from DNA to ribosomes.

  1. Transfer RNA (tRNA):

  • Transports specific amino acids to ribosomes during protein assembly.

  1. Ribosomal RNA (rRNA):

  • Forms ribosomes, the sites of protein synthesis.

Transcription and Translation Process

• Transcription:

  • mRNA synthesizes using a section of DNA, similar to copying information from a book.

  • Non-coding segments (introns) are removed; coding segments (exons) are retained.

  • Alternative splicing allows a single gene to produce different mRNA variants.

• Translation:

  • The process whereby ribosomes assemble proteins using the mRNA sequence.

  • mRNA codons (3-letter sequences) specify amino acids; tRNA brings appropriate amino acids to ribosomes, forming polypeptide chains.

  • Ribosomes link amino acids into a growing protein chain, leading to complete protein formation.

Chromosomes and Cell Division

• Chromosome Structure:

  • Mostly located in the nucleus; DNA wraps around histone proteins to form compact structures.

  • Humans have 46 chromosomes in somatic cells, organized into 23 pairs:

  • 44 are autosomes (non-sex chromosomes).

  • 2 are sex chromosomes (X and Y), which determine biological sex.

  • Chromosomes have telomeres at their ends, providing stability and protecting genetic information.

• Cell Division:

  1. Mitosis:

  • Process through which somatic cells divide, ensuring equal chromosome distribution.

  • Stages: Prophase, Metaphase, Anaphase, Telophase.

  1. Meiosis:

  • Division for gamete (egg and sperm) formation, halving chromosome number to 23 (haploid).

  • Results in genetic variation among offspring.

Genetics and Heredity

• Genetics: Study of the human genome and interactions of genes.

  • Goes beyond individual genes, analyzing gene functions, interactions, and environmental impacts.

• The Human Genome Project:

  • Launched in 1990, completed in 2003 to map every human gene and sequence DNA.

  • Ongoing efforts analyze data for gene location identification and disease understanding.

• Applications of Genomics:

  • Nutritional genomics: Studies effects of nutrients on health.

  • Pharmacogenomics: Examines drug interactions with the genome.

  • Gene-environment interactions help address health conditions like hypertension, diabetes, etc.

• Genetic Screening: May help identify risk groups for diseases, such as through newborn screening initiated in the 1960s.

  • Involves understanding genotype and phenotype distinctions:

    • Genotype: Genetic makeup of an individual.

    • Phenotype: Observable traits, e.g., blood type, eye color.

Genetic Variation and Mutation

• Alleles: Different forms of a gene occupying the same locus on a chromosome.

• Polygenic Traits: Traits influenced by multiple genes and environmental interactions.

• Mutations: Errors in genetic information that can occur during replication, influenced by environmental factors:

  • Type of mutations can include:

  • Spontaneous mutations: Natural errors in DNA replication.

  • Environmental mutations: Damage caused by outside factors.

  • DNA repair mechanisms usually correct errors, but if they fail, mutations can persist across generations.

• Types of Mutations:

  1. Substitution: Base pair changes, affecting codon and protein.

  2. Insertion: Added DNA segments, leading to abnormal proteins.

  3. Deletion: Lost segments that interrupt normal function.

  4. Duplication: Repetitive segments causing potential overproduction of proteins.

  5. Inversion: Flipped DNA segments altering gene function.

  6. Translocation: DNA segment moved to different chromosome sites, disrupting normal expression.

Mendelian Genetics

• Mendelian Inheritance: Refers to Gregor Mendel's principles:

  • Punnett Square: Visual representation predicting genetic outcomes of crosses.

  • Single-gene traits: Governed by a single gene, can be homozygous or heterozygous:

  • Dominant alleles manifest in the phenotype, while recessive alleles require two copies to express.

• Examples of Traits:

  • Dominant traits expressed with one dominant allele.

  • Recessive traits require homozygosity to express (e.g., blue eyes).

Inheritance Patterns

• Autosomal Dominant Disorders:

  • One heterozygous gene from a parent results in 50% inheritance chance.

  • Examples: Huntington's disease, Marfan syndrome.

• Autosomal Recessive Disorders:

  • Require two mutated copies to express the disease.

  • Carriers do not show symptoms. Example disorders: Sickle cell anemia, cystic fibrosis.

• Sex-linked Disorders:

  • Resulting from mutations on sex chromosomes, often X-linked, affecting males more due to single X.

  • Examples: Hemophilia, X-SCID.

Nondisjunction and Chromosomal Disorders

• Nondisjunction: Failure of chromosomes to separate properly during cell division, causing abnormalities.

  • Results in conditions like monosomy (Turner syndrome - female with a single X) and trisomy (e.g., Down syndrome - trisomy 21).

Epigenetics

• Definition: Changes in gene expression caused by mechanisms not involving DNA sequence alteration.

  • Modifications can be inherited across generations. Factors influencing epigenetic changes include:

  • Environmental influences, diet, toxins.

• Genomic Imprinting: Gene expression determined by parental origin.

• Conditions influenced by imprinting: Prader-Willi syndrome, Angelman syndrome.

Genetic Counseling and Screening

• Important for assessing inherited disorder risks before conception.

  • Factors for evaluation: maternal age, family history, recurrent miscarriages.

• Various screening methods available for genetic disorders through maternal blood tests, ultrasounds; confirmed by diagnostic testing.

  • Family history analysis through pedigrees and karyotyping.

• Prenatal Counseling: Guides families in making informed reproductive choices and preparing for potential genetic disorders in offspring.

• Ethical, Social, and Legal Considerations:

  • Rapid advancements in genetics raise concerns about privacy, discrimination, and medical ethicality in genetic testing.