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BIOL 311 Human Genetics Fall 2006 Lecture: Human Nuclear Gene Organization Reading: Chap. 9 selections Lecture outline: 1. Discoveries from human genome project 2. Human genes for RNA 3. Gene families 4. Pseudogenes 5. Tandem repeats 6. a. Interspersed repeats (transposons) b. SINES (Alu family) c. LINES Lecture: 1. Discoveries from the human genome project 3.2 megabases (3.2 billion base pairs) many fewer genes than was originally predicted ~35,000-38,000 genes increased frequency of alternately spliced genes euchromatin (pale Giemsa staining) is G+C rich and has more genes 2. Human genes for RNAs: 700-800 rRNA genes tandemly repeated clusters located in nucleolus genes for large ribosomal subunit: 28S, 5.8S, 5S genes for small ribosomal subunit: 18S tRNA genes snRNAs: small nuclear RNAs involved in pre-mRNA splicing o i.e. U1, U2, U4, U6 snRNAs snoRNAs: small nucleolar RNAs involved in rRNA processing many other genes 3. Gene families most genes code for proteins most genes contain introns, some do not o histones, interferons, heat shock proteins most eukaryotic genomes contain many duplicated genes. Individual members of gene families are free to diverge o example alpha globin and beta globin, Fig. 9.11 most gene families are dispersed, although some (such as globin genes, immunoglobulin genes) are clustered 1 4. Pseudogenes: Two types versions of genes that have become inactivated, such as some alpha and beta globin genes. Retroposons or processed pseudogenes: copies of mRNAs from functional genes that are reinserted into the chromosome. o Usually not expressed and contain multiple mutations. o Fig. 9-14 shows how processed pseudogenes originate 5. tandem repeats Satellite DNA: DNA at the centromeres of chromosomes, can be physically separated from the rest of nuclear DNA alpha-satellite DNA contains binding site for CENP-B, a centromeric protein. Minisatellite DNA: DNA near telomeres Very useful for DNA fingerprinting Location on many chromosomes means individuals have many different DNA regions that hybridize to mini-satellite probes. Microsatellite DNA: SSRs (simple sequence repeats)=small arrays of tandem repeats of simple sequence Interspersed throughout the genome Make up 2% of genome CA/TG repeats are very common Function unknown 6. Interspersed repeats derived from transposons (mobile genetic elements) Fig. 9.17 families of transposons SINE=short interspersed nuclear element Example: Alu family repeat 1,200,000 copies in human genome do not encode proteins can't transpose on their own Alu family repeat is ~300 bp long Contains site for Alu I restriction enzyme Unique to primate lineage, although other SINES found in all mammals LINE: long interspersed nuclear elements Example LINE1 6.1 kb Fully functional transposable elements Fig. 9.18 Encode protein that is a reverse transcriptase and endonuclease that allow them to hop around in the genome Many copies in genome are incomplete 2 LINES are responsible for most of the reverse transcription of the genome There are about ~6000 copies in human genome About 60-100 copies can still transpose 3