Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 21.3 Genomes Vary in Size, Number of Genes, and Gene Density
Genome sequencing reveals extensive variability among organisms in genome size, number of genes, and gene density. These characteristics differ significantly across the domains of life, highlighting the complexity and diversity in genomic organization.
Genome Size
- Genome size is measured in base pairs (bp), typically million base pairs (Mb).
- Prokaryotic genomes (bacteria and archaea):
- Generally smaller (1 to 6 million base pairs).
- E. coli genome size is approximately 4.6 Mb.
- Eukaryotic genomes:
- Much larger than prokaryotes; yeast genome ~12 Mb.
- Animal and plant genomes often exceed 100 Mb.
- Human genome ~3,000 Mb, significantly larger than typical bacterial genomes.
- Not always correlated with complexity: single-celled amoeba (Polychaos dubium) genome estimated at ~670,000 Mb.
- Genome sizes vary dramatically even within groups (e.g., insects, plants).
Number of Genes
- Bacteria and archaea typically have 1,500–7,500 genes.
- Eukaryotes usually have more genes, ranging from around 5,000 (yeast) to over 40,000 (multicellular organisms).
- Surprisingly, humans (~21,300 genes) have fewer genes than initially expected and similar to simpler organisms like nematodes (C. elegans).
- Gene numbers don't directly correlate with genome size.
Reasons for Gene Number Discrepancies
- Alternative splicing significantly increases protein diversity:
- One human gene can produce multiple protein products.
- About 90% of human genes undergo alternative splicing.
- Post-translational modifications (cleavage, chemical modifications) further expand protein variety.
- Noncoding RNAs (miRNAs, regulatory RNAs) add regulatory complexity, enhancing organismal sophistication beyond mere gene count.
Gene Density and Noncoding DNA
- Gene density: number of genes per Mb; prokaryotes have high gene density compared to eukaryotes.
- Bacterial genomes mostly contain protein-coding DNA, with minimal noncoding regions.
- Eukaryotic genomes include vast amounts of noncoding DNA:
- Introns (noncoding segments within genes) significantly increase gene length (human genes ~27,000 bp, bacterial genes ~1,000 bp).
- Intergenic regions (between genes) are extensive and mostly noncoding, especially in humans.
- Humans have approximately 10,000 times more noncoding DNA than bacteria, reflecting more complex regulation and structural organization.
In a Nutshell
Genomes vary greatly in size, gene number, and gene density across species. While prokaryotic genomes are small, gene-rich, and straightforward, eukaryotic genomes are large, gene-sparse, and filled with extensive noncoding DNA, including introns and regulatory elements, reflecting higher regulatory complexity.