DNA

• DNA is a nucleic acid made of deoxyribose (a 5-carbon sugar), a phosphate, and 4 nitrogenous bases: purines (adenine, cytosine, guanine) and pyrimidines (thymine)
• Watson and Crick, with help from Rosalind Franklin, discovered the DNA structure as a double helix made of 2 antiparallel strands with nitrogenous bases facing each other and held together by weak hydrogen bonds
• In DNA, adenine (A) only bonds with thymine (T), and guanine (G) only bonds with cytosine (C), creating complementary bases
• DNA replication process:

  1. DNA helicase unzips the double helix

  2. RNA primer is added to show DNA polymerase where to add new nucleotides

  3. DNA polymerase adds new nucleotides with complementary bases to each strand in opposite directions, resulting in 2 identical DNA molecules
• Genes are sections of DNA that code for proteins, which are essential for body processes like enzymes and structural proteins
• RNA, unlike DNA, has an extra oxygen in its 5-carbon sugar, uses uracil (U) instead of thymine (T), and is a single strand (half-ladder)
• Gene expression involves transcription (DNA to mRNA in the nucleus) and translation (mRNA, tRNA, and rRNA working together to make a protein in the cytoplasm)
• DNA mutations can be caused by mistakes in replication or mutagens like chemicals or radiation, leading to substitutions, insertions, deletions, or frame shift mutations
• Gene regulation mechanisms include operons in prokaryotes and more complex processes in eukaryotes like transcription factors and RNA interference
• Epigenetics studies how environmental factors can cause DNA methylation, affecting gene expression and potentially being passed on to offspring
• Karyotype is a photograph of chromosomes in mitosis, cut and pasted, sorted by pair and size
• Many human genes have been mapped to their location on a karyotype
• Banding patterns on a karyotype can be used to identify genetic disorders
• A pedigree chart is a diagram of family relationships used to track a trait
• Human patterns of inheritance include sex-linked traits, where alleles are present on the X chromosome but not on the Y
• In sex-linked traits, females (XX) show normal dominant/recessive patterns, while males (XY) only get one allele, making recessive traits more likely to show
• Examples of sex-linked traits are hemophilia, baldness, and color blindness
• Codominance occurs when neither allele is dominant, and both traits show up separately
• Multiple alleles are possible for a single trait, like in blood types (A, B, O)
• Polygenic traits, influenced by more than one gene, result in a wide range of phenotypes
• Epistasis is when one gene acts upon another, affecting the expression of traits like eye color
• Incomplete dominance results in a blending of alleles to form an "in-between" phenotype
• Genetic disorders are caused by mutations of DNA and are usually recessive alleles
• Genetic diseases can become widespread in populations because heterozygotes are resistant or immune to lethal infectious diseases, providing an evolutionary advantage
• Examples of genetic diseases providing resistance to infectious diseases include sickle-cell anemia and malaria in Africa, cystic fibrosis and typhus in Western Europe, and Tay-Sachs disease and tuberculosis in Eastern Europe