VCE Biology Unit 2 AOS 1 – From Chromosomes to Genomes

Genes, DNA & Alleles

• Gene
  • A gene is a specific length of DNA that contains the coded instructions for building a gene product, usually a polypeptide (protein).
  • Information is stored as a sequence of nucleotide bases read in triplets (codons). Each codon specifies one amino-acid monomer and, in sequence, determines the primary structure of the polypeptide.
  • Metaphor used: A gene is like one recipe in a large recipe collection (the genome).
• DNA (Deoxyribonucleic Acid)
  • A nucleic acid polymer built from nucleotide monomers (deoxyribose sugar, phosphate group, nitrogenous base).
  • Carries hereditary information in all known living organisms.
• Alleles
  • Definition: alternative forms/versions of the same gene that differ in nucleotide sequence.
  • Theory summary: If the base sequence is altered, the amino-acid sequence may change → altered protein function → possible phenotypic change.
  • Example: Cystic fibrosis (CF)
  • Caused by mutant alleles of the CFTR gene.
  • Normal CFTR protein length: 1480 amino acids.
  • Researchers have documented 1700 distinct CF-causing mutations; the most common involves the deletion of just one amino acid.
  • In diploid organisms, each individual carries two alleles for every autosomal gene – one maternal, one paternal.

The Genome

• Coined by Hans Winkler in 1920; originally defined as one haploid chromosome set.
• Modern usage: the complete genetic complement of a cell, an individual, or a species (context dependent).
• Size variation examples
  • Smallest known genome: bacterium Pelagibacter ubique with 1\,389 genes.
  • Human somatic cell genome: about 30\,000 protein-coding genes.

Chromosome Structure & Packaging

• Chromosome
  • One very long DNA molecule extensively wrapped around histone proteins → nucleosomes → further coiling/super-coiling.
  • Chromatin fibre condenses to form the visible chromosome during cell division.
• Homologous chromosome pair
  • Two chromosomes (one maternal, one paternal) with the same length, centromere position and gene loci although they may carry different alleles.
• Autosomes vs Sex Chromosomes
  • Autosomes: non-sex chromosomes; occur as homologous pairs in both sexes.
  • Sex chromosomes: determine genetic sex; may be homologous (ZZ) or heterologous (XY, WZ) depending on species.

Karyotypes

• Definition: a laboratory photograph of all chromosomes in a dividing cell, arranged in descending size order and matched as homologous pairs.
• Preparation steps: arrest mitosis (metaphase), isolate chromosomes, stain, photograph, digitally/physically cut, align.
• Diagnostic / analytical uses
  • Determine species identity.
  • Identify chromosomal sex (e.g., presence/absence of Y or W).
  • Detect large-scale chromosomal abnormalities (e.g., trisomies, translocations, deletions, duplications).
• Illustrated practice questions (slides)
  • Example 1: Karyotype of a domestic cat (used to practise species identification).
  • Example 2: Human karyotype prompting recognition of Down syndrome, sex determination and autosome count.

Chromosome Diversity Across Species

• Chromosome number is NOT conserved between species.
  • Scarlet macaw: 22 macrochromosomes + 40 microchromosomes.
  • Jack Jumper ant: only 1 pair (haploid males, diploid females).
  • Certain ferns: up to 720 pairs.
• Size terminology
  • Megachromosome: > 40 megabases (MB).
  • Microchromosome: < 20 MB.
• Sex-determination systems vary
  • Birds: ZZ male, WZ female (no X/Y).
  • Crocodiles & turtles: temperature-dependent sex determination, no dedicated sex chromosomes.

Concept Checks (Multiple-Choice Activities)

• Definition of a gene → Correct answer: "sequence of nucleotides that code for a protein or tRNA".
• Composition of chromosomes → DNA + proteins (histones).
• Interpreting a human karyotype → recognise 44 autosomes + XX or XY; identify Down syndrome if trisomy 21 present.
• True statement about other species → Male green sea turtles lack sex chromosomes (temperature determines sex).

Key Take-Home Points

• Genes (DNA segments) encode RNA/proteins; variants are called alleles.
• The genome is the full set of genetic information in a cell or organism.
• DNA interacts with histones to form chromosomes; homologous pairs carry the same loci but may differ in alleles.
• Karyotyping visually analyses chromosome number/structure for species ID, sexing and clinical genetics.
• Chromosome number, size and sex-determination systems vary dramatically across taxa, illustrating biological diversity.
• These foundational ideas precede deeper study of meiosis, inheritance patterns and genetic variation.

Looking Ahead

• Upcoming topic: Meiosis – specialised cell division producing gametes and reshuffling alleles, thereby contributing to genetic diversity.