From Chromosomes to Genomes – Comprehensive Study Notes

Genes

• A gene is a specific length of DNA that contains the coded instructions for building a gene product, usually a polypeptide (protein).
  • DNA itself is a nucleic acid made of nucleotide monomers.
  • The nucleotide sequence is read in triplets (codons); each triplet specifies one amino-acid monomer in the resulting polypeptide.
  • If even a single base in the sequence changes, the amino-acid sequence—and therefore protein structure/function—can change.
  • Connection to later topics: Understanding gene structure underpins how meiosis shuffles genetic information and how mutations introduce variation.

Alleles

• Allele: an alternative form of the same gene; arises when the base sequence is altered.
  • Different alleles may encode polypeptides with altered primary structures, leading to varied phenotypes.
  • Cystic fibrosis example:
  • Normal CFTR protein length = 1480 amino acids.
  • Scientists have identified \approx1700 separate mutations in the CFTR gene that can cause disease.
  • The most common pathogenic allele deletes a single amino acid.
• Significance:
  • Allelic variation explains intra-species diversity (eye colour, enzyme efficiency, hormonal regulation, disease susceptibility).

The Genome

• Term origin: Coined by Hans Winkler (1920); initially meant a haploid chromosome set.
• Modern definition: All genetic information in a cell, individual or species.
  • Smallest known genome: Pelagibacter ubique – 1389 genes.
  • Human genome: \approx30\,000 genes.
• Practical note: Whole-genome knowledge enables comparative genomics, personalised medicine and conservation genetics.

Chromosomes & Chromatin

• Genome is partitioned into separate, extremely long DNA molecules.
• Each DNA molecule is wrapped around histone proteins → forms chromatin → further coiling produces a chromosome.
  • Definition: A molecule of DNA, coiled and recoiled around histones.
• A pair of homologous chromosomes carries identical gene loci but may bear different alleles.
• Chromosome categories in humans:
  • Autosomes: pairs 1\text{–}22.
  • Sex chromosomes: X and Y.

Karyotypes

• Karyotype: a photograph of an individual’s chromosomes lined up by size after isolation, staining, and cutting.
• Diagnostic uses:
  • Identify species.
  • Determine sex.
  • Detect some chromosomal abnormalities (e.g., trisomy 21, XXY, monosomy X).
• Steps in preparation link cytogenetics to medical genetics.

Chromosome Diversity Across Species

• Chromosome number & size vary widely:
  • Scarlet macaw: 22 macro- + 40 micro-chromosomes.
  • Jack Jumper ant: only 1 pair.
  • Some fern species: 720 pairs.
• Sex-determining systems differ:
  • Birds: males ZZ, females ZW (no XY).
  • Crocodiles & turtles: temperature-dependent sex determination; no dedicated sex chromosomes.
• Size terminology:
  • Megachromosome: > 40 megabases (MB).
  • Microchromosome: < 20 MB.
• Implication: Caution is needed when inferring genetic relationships solely from chromosome counts.

Sample Multiple-Choice Review (with answers)

• Best definition of a gene: A sequence of nucleotides that codes for a protein or tRNA molecule.
• Chromosome composition: DNA and proteins (histones).
• Human karyotype example question: Human females show 44 autosomes + XX.
• True cross-species statement: Male green sea turtles have no sex chromosomes.

Numerical & Statistical References

• 1480 – length of CFTR polypeptide.
• \approx1700 – number of known CFTR disease-causing mutations.
• 1389 – genes in Pelagibacter ubique (smallest genome recorded).
• \approx30\,000 – genes in Homo sapiens.
• Chromosome size thresholds:

Ethical, Philosophical & Practical Implications

• Karyotyping and genome sequencing raise privacy and discrimination concerns (insurance, employment).
• Early detection of chromosomal abnormalities allows informed reproductive choices but also triggers debate on selective termination.
• Conservation genetics leverages genome data to protect biodiversity, e.g., managing small populations with low allelic diversity.

“Big Picture” Summary (slide 25 synthesis)

• Genes → alleles → genome establish genetic blueprint.
• DNA + histones → chromatin → chromosomes organise that blueprint.
• Karyotypes visualise chromosomes, revealing number, structure and sex.
• Inter-species variation in chromosome architecture underpins evolutionary diversity and adaptive strategies.
• These foundational ideas set the stage for meiosis, which creates gametes and fuels genetic variation.