Biology Chapter 5: Genetics and Classification

DNA Structure and Properties

• DNA is the hereditary material in humans and almost all other organisms
• It carries the genetic material of an organism
• Most DNA is located in the cell nucleus, but a small amount is found in mitochondria (mitochondrial DNA) and chloroplasts
• The components of a nucleotide include a phosphate group, a pentose sugar (deoxyribose), and a nitrogenous base

Components of a Nucleotide

• Phosphate group: A phosphorus atom bonded with four oxygen atoms
• Pentose sugar (deoxyribose): A five-carbon sugar
• Nitrogenous base: A nitrogen-containing molecule; can be classified as either a purine (adenine, guanine) or a pyrimidine (thymine, cytosine)

The Nitrogenous Bases (DNA)

• Classified into two groups

  • Pyrimidines: Thymine and Cytosine
  • Purines: Adenine and Guanine

• Adenine pairs with Thymine (2 hydrogen bonds)

• Guanine pairs with Cytosine (3 hydrogen bonds)

Salient Features of the Double-Helix Structure of DNA

• Composed of two polynucleotide chains
• Backbone is formed by sugar-phosphate units
• Bases project inwards
• Chains have anti-parallel polarity (5' to 3' and 3' to 5')

DNA Replication

• The process of producing two identical replicas from one original DNA molecule
• DNA replication is semi-conservative
• Each newly synthesized molecule contains one original "parent template" strand and one new "daughter" strand

Key Enzymes in DNA Replication

• Helicase: Unwinds the DNA double helix
• Primase: Provides the starting point of RNA
• DNA Polymerase: Adds new nucleotides to the 3' end of the elongating strand. It also proofreads and corrects errors.
• DNA Ligase: Re-anneals the semi-conservative strands and joins Okazaki fragments of the lagging strands
• Topoisomerase: Relaxes the supercoiled nature of the DNA
• Single-strand Binding Proteins (SSBs): Bind to ssDNA to prevent re-annealing after DNA helicase unwinds, maintaining strand separation

The Central Dogma of Molecular Biology

• A theory stating that genetic information flows in one direction from DNA to RNA to protein
• The steps of the process involve Replication, Transcription, and Translation

Transcription

• The process of synthesizing mRNA from DNA
• Involves the transfer of the DNA code to mRNA within the nucleus

Translation

• The process of reading the RNA sequence of an mRNA to create the amino acid sequence of a protein

Variations

• Naturally occurring genetic differences among individuals of the same species

Mendel's Laws of Inheritance

• Mendel's First Law (Law of Dominance): In a heterozygote, one trait will conceal another trait for the same characteristic.
• Mendel's Second Law (Law of Segregation): Alleles segregate from each other during gamete formation (meiosis) such that only one allele is present in each gamete.
• Mendel's Third Law (Law of Independent Assortment): Alleles of different genes are sorted independently into gametes.
• Monohybrid Cross: A cross between individuals heterozygous for a single trait (e.g., monohybrid cross for flower color).
• Dihybrid Cross: A cross between individuals heterozygous for two traits (e.g., dihybrid cross for seed color and seed shape).

Non-Mendelian Patterns of Inheritance

• Multiple Alleles: A single gene with more than two normal versions (e.g., ABO blood groups)
• Codominance: Both alleles for a gene are expressed equally in the phenotype (eg. AB blood type).
• Incomplete Dominance: The dominant allele is not completely dominant over the recessive allele; the resultant phenotype is intermediate (eg. pink flowers)
• Polygenic Traits: Traits controlled by more than one gene, with the alleles of each gene having a small additive effect on the phenotype (eg. human height).
• Pleiotropy: A single gene that affects multiple phenotypic traits.
• Epistasis: One gene influences/affects the expression of one or more other genes.

Earth's Origin

• Earth formed approximately 4.6 billion years ago
• Early Earth conditions were extremely harsh
  • Constant bombardment from space debris
  • Extremely high temperatures
  • Toxic atmosphere (containing nitrogen oxides, methane, ammonia, and hydrogen sulfide) without much oxygen
  • Frequent volcanic eruptions
  • Intense lightning and radiation

Geologic Time Scale

• The geologic time scale (GTS) is a record of Earth's history and the order of life from 2500 Mya to the present
• It divides Earth's history into eons, eras, periods, and epochs
• The divisions are based on changes in life-forms over time

Early Invertebrates

• Cambrian Period (488-444mya) and Ordovician Period (448-444mya)
  • Brachiopods, anomalocaris, pikaia gracilens were abundant
• The Ordovician period saw the appearance of many animals with jointed external skeletons, known as arthropods

Early Coral Reefs, Fishes, Vascular Plants

• Silurian Period (444-416 mya)
  • Jawless fish, first known freshwater fish, vascular plants (Cooksonia)
• Devonian Period (416-359 mya)
  • Placoderms (armored fishes), bony fishes (ray-finned fishes), lobe-finned fish, early vascular plants, lycophytes, horsetails, ferns
  • First tetrapods (four-limbed vertebrates) and insects

Amphibians, Origins of Reptiles, First Seed Plants

• Carboniferous Period (359-299 mya):
  • vast swamp forests, ferns, lycophytes, early amphibians, earliest reptiles

Reptiles (Dinosaurs), First Angiosperms, First Mammals

• Mesozoic Era (251-65.5 mya), Triassic, Jurassic, and Cretaceous Periods
  • Marine reptiles, dinosaurs, first angiosperms (flowering plants), first mammals

Mammals

• Cenozoic Era (65.5 -present)
  • First primates, rodents, apes, and humans

Evolution

• The change in heritable characteristics of biological populations over successive generations
• Natural selection is a key mechanism of evolution

Evidence of Evolution

• Direct (fossils):
  • Fossils provide a record of how organisms have changed over time
• Indirect evidence:
  • Comparative Anatomy
    • Homologous structures, analogous structures, vestigial structures
  • Molecular Biology
    • Examining the similarities of biochemical molecules (DNA, ATP, amino acids, enzymes)
  • Biogeography
    • Studying the distribution of organisms
  • Embryological Development
    • Examining the similarities in the embryonic development of various organisms

How evolution or change in gene pool occurs

• Nonrandom mating
• Mutation, changes in gene and chromosome composition
• Genetic drift, change in gene pool due to chance alone
• Gene flow, when individuals migrate from one population to another

Test your knowledge on genetics and classification concepts with this quiz focusing on natural selection, blood group inheritance, and Linnaean taxonomy. Questions cover key principles like gene flow, codominance, and patterns of inheritance. Perfect for students looking to review important biological concepts.