Overview
This lecture covers the structure and organization of DNA and chromosomes, focusing on differences between eukaryotic and prokaryotic cells, key definitions, and related exam-style math questions.
Genes, Alleles, and Chromosomes
- A gene is a short section of DNA that codes for a polypeptide and functional RNA.
- The modern definition specifies coding for polypeptides and functional RNA, not just characteristics.
- A chromosome is tightly coiled DNA that stores genetic information; in eukaryotes, chromosomes are found in the nucleus.
- Humans have 23 pairs (46 total) chromosomes in somatic (body) cells.
Locus and Homologous Pairs
- The locus is the specific location of a gene on a chromosome.
- All individuals of a species have the same genes at the same loci but may have different alleles.
- A homologous pair consists of chromosomes with identical genes but possibly different alleles, one from each parent.
- The 23rd chromosome pair determines biological sex (XX for females, XY for males).
DNA Structure in Eukaryotes vs. Prokaryotes
- In eukaryotes, DNA is linear, tightly coiled around histone proteins, and found in the nucleus.
- DNA-histone complexes are called nucleosomes.
- Prokaryotic DNA is shorter, circular, not associated with histones, and free in the cytoplasm (no nucleus).
- Mitochondria and chloroplasts in eukaryotes contain DNA that is short, circular, and not histone-bound, similar to prokaryotic DNA.
Chromosome Replication and Appearance
- Chromosomes appear as single rods when not dividing; after replication, they have two chromatids joined by a centromere, forming an X shape.
- During cell division, chromatids separate back into single chromosomes.
Math Questions & Applications
- DNA in one human cell ≈ 2.3 meters; divided among 46 chromosomes means ~0.05 meters per chromosome.
- All body cells contain the same amount of DNA.
- 2.3 meters = 2,300 millimeters.
- The human genome has ≈ 3 x 10⁹ base pairs; divided by 46 chromosomes ≈ 6.5 x 10⁷ base pairs per chromosome.
Key Terms & Definitions
- Gene — Section of DNA coding for a polypeptide and/or functional RNA.
- Allele — Different form (version) of the same gene.
- Locus — Specific location of a gene on a chromosome.
- Homologous pair — Chromosome pair with identical genes, from each parent.
- Histone — Protein DNA wraps around for packing in eukaryotic chromosomes.
- Nucleosome — Complex of DNA wrapped around histone proteins.
Action Items / Next Steps
- Practice exam questions about DNA and chromosomes.
- Review unit conversions (meters to millimeters).
- Read more about chromosome structure and cell division.