Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 21.4 Multicellular Eukaryotes Have a Lot of Noncoding DNA and Many Multigene Families
Multicellular eukaryotes have complex genomes containing large amounts of noncoding DNA, including transposable elements and repetitive sequences. Many genes exist as multigene families, contributing to genome diversity and functionality.
Noncoding DNA in Eukaryotic Genomes
- Protein-coding DNA constitutes only about 1.5% of the human genome.
- Most DNA in multicellular eukaryotes is noncoding, previously labeled as "junk DNA," but now understood to have regulatory or structural roles.
- Noncoding DNA categories in humans:
- Introns: ~20% of genome; noncoding segments within genes.
- Regulatory sequences: ~5%; control gene expression.
- Unique noncoding DNA: ~15%; includes pseudogenes and gene fragments.
- Repetitive DNA: largest portion (~58%), repeated many times.
Transposable Elements
- Transposable elements (TEs) or "jumping genes" move within genomes via DNA recombination.
- Discovered by Barbara McClintock in maize; later found in all organisms.
- Two main types:
- Transposons: Move by "cut-and-paste" or "copy-and-paste" using transposase.
- Retrotransposons: Move through RNA intermediates, converted by reverse transcriptase, leaving original copies behind.
- In humans:
- Alu elements (~300 nt): ~10% of genome; may regulate gene expression.
- LINE-1 (L1) retrotransposons (~6,500 nt): ~17% of genome; active in early development.
Other Repetitive DNA
- Simple sequence DNA: Short, tandem repeats (2–500 nt).
- Includes STRs (2–5 nt): used in genetic profiling.
- Found in telomeres and centromeres for chromosome stability.
- Large-segment duplications: 10,000–300,000 bp duplicated across genome (~5–6%).
Multigene Families
- Groups of multiple identical or similar genes.
- Two types:
- Identical genes: Clustered; encode rRNA for ribosome production.
- Nonidentical genes: Similar but slightly different, like α- and β-globin genes (chromosomes 16 and 11), expressed at different developmental stages.
Significance of Noncoding DNA
- Noncoding DNA has vital regulatory and structural roles, proven by evolutionary conservation.
- Projects like ENCODE highlight widespread biological functions of noncoding DNA.
In a Nutshell
Eukaryotic genomes contain extensive noncoding DNA, including transposable elements and repetitive sequences. Many genes exist as multigene families, allowing complex regulation and adaptive function across developmental stages. Noncoding DNA plays essential roles in genome regulation, stability, and evolution.