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 (nonfunctional gene copies) and gene fragments.
Repetitive DNA: largest portion (~58%), consisting mostly of sequences 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 widely in all organisms.
Two main types:
Transposons: Move by "cut-and-paste" or "copy-and-paste" mechanisms using DNA intermediates and enzyme transposase.
Retrotransposons: Move through RNA intermediates, converted back to DNA by reverse transcriptase(common in eukaryotes, leaving original copies behind).
In humans:
Alu elements (short, ~300 nucleotides) make up ~10% of genome; transcribed into RNA, possibly regulating gene expression.
LINE-1 (L1) retrotransposons (~6,500 nucleotides) constitute 17% of the genome, playing roles in early embryonic development.
Other Repetitive DNA
Simple sequence DNA: Short, tandemly repeated sequences (2–500 nucleotides).
Includes short tandem repeats (STRs, 2–5 nucleotides repeated), useful in genetic profiling.
Found prominently in telomeres and centromeres, aiding chromosome stability and segregation.
Large-segment duplications: Long DNA segments (10,000–300,000 bp) duplicated and dispersed within the genome (~5–6%).
Multigene Families
Multigene families consist of multiple identical or similar gene copies.
Two types of multigene families:
Identical genes: clustered together; often encoding rRNA molecules, enabling rapid ribosome production.
Nonidentical genes: similar but differing slightly, such as the globin gene families encoding hemoglobin subunits (α-globin on chromosome 16, β-globin on chromosome 11), which express differently at developmental stages.
Significance of Noncoding DNA
Noncoding regions have critical regulatory and structural roles, confirmed by sequence conservation across species.
Projects like ENCODE emphasize the functional importance of noncoding DNA, indicating widespread biological roles.
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.