Overview
This lecture covers the structure and function of nucleic acids (DNA and RNA), the organization of chromosomes, and the different types of chromosome-linked traits including sex-limited, sex-influenced, and sex-linked traits.
Nucleic Acids and Their Structure
- Nucleic acids (DNA and RNA) store genetic information and are one of four major biomolecules (others: carbohydrates, lipids, proteins).
- DNA stores genes in most organisms; some viruses use RNA.
- Nucleotides are the monomers (building blocks) of nucleic acids; each contains a nitrogenous base, a pentose sugar, and a phosphate group.
- Nitrogenous bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C); RNA uses Uracil (U) instead of Thymine.
- Purines (double ring): Adenine and Guanine; Pyrimidines (single ring): Cytosine, Thymine, Uracil.
- DNA contains deoxyribose sugar (no oxygen at carbon 2); RNA contains ribose sugar (oxygen present at carbon 2).
- Nucleotides connect in a 5' to 3' direction via polymerization, forming long chains.
- DNA is typically double-stranded (antiparallel), while RNA is usually single-stranded; structure leads to the double helix shape in DNA.
DNA Packaging and Chromosome Structure
- DNA wraps around histone proteins to form nucleosomes, which coil further into chromatin.
- Chromatin condenses into chromosomes during cell division (mitosis and meiosis).
- Chromosomes can be double rod (sister chromatids attached by centromere, seen in metaphase) or single rod (seen in anaphase).
- Key chromosome parts: centromere (attachment point), kinetochore (spindle fiber attachment), telomere (ends), p arm (short), q arm (long).
- Chromosomes classified by centromere position: metacentric (centered), submetacentric, acrocentric, telocentric.
Human Karyotype
- Humans have 46 chromosomes (23 pairs): pairs 1-22 are autosomes (body chromosomes), pair 23 are sex chromosomes (X and/or Y).
- Karyotype arranges chromosomes by size; sex chromosomes differ in size (X is larger than Y).
Chromosome-Linked Traits
Sex-Limited Traits
- Genes located on autosomes (chromosomes 1–22).
- Trait appears in only one sex due to physiological or hormonal differences (e.g., milk production in females).
Sex-Influenced Traits
- Genes also on autosomes (chromosomes 1–22).
- Trait can appear in both sexes but is more common or severe in one (e.g., baldness more common in males).
Sex-Linked Traits
- Genes located on sex chromosomes (pair 23: X or Y chromosome).
- X-linked traits can affect both sexes; Y-linked traits affect only males.
- Example of X-linked recessive trait: Hemophilia—affects blood clotting, more likely in males.
- Example of Y-linked trait: Hypertrichosis (hairy ears)—exclusively affects males.
- Pedigree charts track inheritance patterns, showing only males inherit Y-linked traits from affected fathers.
Key Terms & Definitions
- Nucleic acid — macromolecule (DNA or RNA) storing genetic information.
- Nucleotide — monomer of nucleic acids; consists of nitrogenous base, pentose sugar, phosphate group.
- Purine — nitrogenous base with double-ring structure (adenine, guanine).
- Pyrimidine — nitrogenous base with single-ring structure (cytosine, thymine, uracil).
- Histone — protein that DNA wraps around to form nucleosomes.
- Nucleosome — DNA-histone complex, basic unit of chromatin structure.
- Chromatin — coiled form of DNA and protein present during interphase.
- Chromosome — highly condensed DNA structure visible in cell division.
- Autosome — non-sex chromosomes, pairs 1–22 in humans.
- Sex chromosome — chromosome determining sex (X or Y in humans).
- Centromere — chromosome region joining sister chromatids.
- Kinetochore — protein at the centromere, spindle fiber attachment site.
- Telomere — repetitive DNA at chromosome ends, protects chromosomes.
- Sex-limited trait — trait expressed in only one sex.
- Sex-influenced trait — trait appearing in both sexes, but more commonly/severely in one.
- Sex-linked trait — trait determined by genes on sex chromosomes.
Action Items / Next Steps
- Review textbook sections on DNA/RNA structure and chromosome organization.
- Practice analyzing pedigree charts for sex-linked inheritance patterns.
- Prepare questions about chromosome-linked traits for further discussion.