Genetic Concepts and DNA Technology: A Comprehensive Overview
Key Genetic Terms and Concepts
Locus: The physical location of a gene on a chromosome.
Allele: Different versions of a given gene. New alleles form because of mutations, changes in the DNA. Mutations can be:
- Neutral (most of the time)
- Harmful (sometimes)
- Beneficial (occasionally)
Genotype: The combination of alleles an organism has.
Phenotype: The combination of alleles that create a physical trait.
Homozygous: If the alleles at a given locus are the same.
Heterozygous: If the alleles at a given locus are different.
Dominant: An allele that exerts a controlling influence on the phenotype of a heterozygote.
Recessive: An allele that has no effect on the phenotype when paired with a dominant allele in a heterozygote.
Autosomal: Alleles found on normal (non-sex) chromosomes.
Sex-linked: A trait associated with a gene that is carried only by the male or female parent.
Polygenic trait: Traits whose phenotype is controlled by several genes. Examples: Skin color, eye color, height.
Multifactorial trait: Genetic traits where phenotype is controlled by both genes and the environment. Examples: Skin color, height, weight. Skin color changes with exposure to sunlight. Height and weight are influenced by diet.
Exon: Transcribed portions of a gene that code for the amino acid sequence of a protein.
Intron: Transcribed portions of a gene that do not code for the amino acid sequences of proteins, and whose corresponding sequence in RNA is removed before the RNA leaves the nucleus.
Punnett Square: A tool used to show all possible ways in which two alleles can recombine through fertilization.
Transcription: The conversion of a DNA-based sequence of nucleotides in a gene to an RNA-based sequence.
Translation: The process by which ribosomes convert the genetic information in mRNA into proteins.
RNA: A single-stranded nucleic acid similar to DNA.
Messenger RNA (mRNA): Delivers the genetic information, or instructions, from DNA to the ribosomes, where proteins are made.
Complete dominance: Dominant allele completely covers up the recessive allele.
Incomplete dominance: Phenotype that is a blending of the parental traits.
Codominance: “Recessive” and “dominant” traits appear together in the phenotype of hybrid organisms.
Genetic Disorders: Symptoms and Inheritance
Achondroplasia (dwarfism): Characterized by stunted growth and disproportionately sized limbs and head. Homozygous dominant: The child is very unlikely to live to be older than a few months. No issues with having children. Autosomal dominant.
Huntington’s disease: Causes degradation of the nervous system. Leads to muscle ticks, loss of muscle control, delusion, amnesia, and a host of other physical and mental symptoms. Does not present symptoms until after child-bearing age. Autosomal dominant.
Sickle cell anemia: Causes red blood cells to be “sickle” shaped. Sickled cells get stuck to one another and cause clots. Can cause severe pain and prevent oxygen from getting to where it is needed. Reduced life expectancy. Because of the protection from malaria, the trait has remained in the population. Autosomal recessive.
Cystic fibrosis: Causes production of thick, sticky mucus. Clogs and damages the lungs, digestive system, and reproductive system. Autosomal recessive.
Red-green color blindness: X-Linked recessive trait.
Hemophilia: X-Linked recessive. Causes a lack of a clotting factor in the blood. Blood does not clot as quickly.
Nucleotides, DNA, and RNA
Components of a nucleotide: Phosphate group, sugar, nitrogenous bases (Adenine, Thymine, Guanine, Cytosine, Uracil).
Nitrogenous bases of DNA and RNA: Adenine, Thymine, Guanine, Cytosine, Uracil.
DNA Structure Discovery: In the early 1950s, Rosalind Franklin and Maurice Wilkins studied DNA using X-rays. Franklin produced an X-ray photograph that allowed James Watson and Francis Crick to determine the 3D double helix structure of DNA.
Complementary base pairing: DNA: A-T, G-C; mRNA: A-U.
Protein Synthesis
The information in DNA is used to make a protein through transcription and translation.
Three types of RNA: Messenger RNA (mRNA), Ribosomal RNA (rRNA), Transfer RNA (tRNA).
Types of Mutations
Replacement: One base is substituted for another in a DNA sequence.
Insertion: One or more bases are inserted into a DNA sequence.
Deletion: One or more bases are deleted from a DNA sequence.
Frameshift: Insertion or deletion mutations that scramble the downstream sequence of amino acids.
DNA Technology and its Applications
Restriction Enzymes: Enzymes that cut DNA into fragments, often used to protect bacteria from viral DNA.
Gel Electrophoresis: Used for comparing two or more samples of DNA, DNA fingerprinting, comparing relatedness of different species, and matching organ donors with recipients.
Polymerase Chain Reaction (PCR): Used to make large quantities of a target piece of DNA, sequencing entire genomes, and creating transgenic organisms.
DNA Sequencing: Machines can sequence entire genomes, determining ancestry, and learning more about extinct species.
Cloning: Creating a new individual that is an exact copy of another, potentially bringing extinct animals back, and making stem cells.
Stem Cells: Undifferentiated cells that can become other types of cells. Used in basic research to understand cell division and differentiation, and in biomedical applications like drug development and regenerative medicine.
Recombinant DNA: Combining different fragments of DNA to create a new DNA sequence.
Genetic Engineering: The permanent introduction of one or more genes into a cell, tissue, or organism.
Gene therapy: “Fixing” malfunctioning genes in people with genetic disorders.
Creating transgenic organisms: Organisms with genes from other organisms.
GMOs: Bacteria that produce useful materials like medicines, enzymes (lactase), hormones (insulin), livestock that fatten up more quickly, and crops that are herbicide resistant.
Stem Cell Applications and Ethical Considerations
Embryonic stem cells: Can differentiate into any type of cell.
Adult stem cells: Can only differentiate into a few specialized cell types.
Gene Therapy Process: A target gene is cut from the DNA of one organism using restriction enzymes, copied using PCR, and then inserted into the DNA of another organism, usually via a plasmid introduced into bacteria.
Ethical Controversy: The use of embryos produced through in vitro fertilization (IVF) for embryonic stem cell research is controversial. Advocates believe it will produce promising therapeutic applications, while opponents believe embryos have moral status and it is unethical to use one embryo to benefit another.
Law of Segregation
The two copies of a gene are separated during meiosis and end up in different gametes. Therefore, each offspring receives one copy of the gene (one allele) from the egg and the other copy (other allele) from the sperm.