Download 16. Nuclear gene organization

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
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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:
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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
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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
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