The Structure of Chromatin • Each Chromosome about 6 cm long, 1000x longer than width of cell • Eukaryotic DNA associated w/ proteins • Chromatin undergoes changes in the course of the cell cycle. • During Interphase, chromatin fibers are diffused within the nucleus. • Various levels of packing of DNA
Histones: proteins DNA wrap around. • Positive charge, DNA negative charge • 5 types of histones (gene highly conserved) • Mass of Histone = Mass of DNA • Nucleosome (DNA/Histone) complex stays together except during DNA Replication • Nucleosome can change shape
Higher levels of DNA Packing • Heterochromatin- • nontranscribed eukaryotic chromatin • highly compacted • visible with a light microscope during Interphase. • Not transcribed • Euchromatin- (“True Chromatin”) less compacted than heterochromatin.
Tandemly Repetitive DNA • Short sequences repeated in a series (GTTACGTTACGTTAC). • 10-15% of genome • Up to 10 bp long • Several hundred thousand repeats long • Have a different density (lower) • Satellite DNA • Fragile X Syndrome • CGG repeat 30x Normal • CGG repeat 1000x Fragile X • Huntington’s Disease • CAG repeated • Translated to glutamine • Different protein • Satellite DNA located at telomeres
INTERSPERSED REPETITIVE DNA • Scattered about the genome. • thousands of base pairs long. • Copies are not identical. • 25-40% in mammalian genomes. • 5% are in a family of sequences called Alu Elements • Made into RNA but function unknown • Transposons (jumping genes)
Gene Family Duplication • Most Euk. Genes have unique sequence, one copy per haploid cell. • Some present in more than one copy, or resemble sequence. • Multigene families: • identical gene, clustered tandemly, • mostly RNA products (rRNA). • Single transcription unit repeated 100x • Gene for Histones
Non-identical gene families • Two related Families of Genes • Globins alpha and beta code for… • Hemoglobin • Chromosome 16 codes for α globin • Chromosome 11 codes for β globin • Both genes have similar sequence • Possibly from common ancestor? • Different version of hemoglobin expressed at different times • Fetal Hgb has higher affinity to O2 than adult form • Difference b/c of possible mutations that accumulate • Evidence….pseudogenes • Similar genes w/no product
GENE AMPLIFICATION & REARRANGEMENT • Gene Amplification- number of copies of a gene temporarily increases during a stage in development. (rRNA in ovum, cancer cells, does not affect gametes • DNA Rearrangements or Loss: • Gene loci changes • Retrotransposons (DNA RNADNA) • Reverse Transcriptase • Transposons (jumping genes) • McClintock purple gene in corn
Immunoglobin Genes • Example of gene rearrangment • Immune System needs to recognize many different foreign invaders • Function pieces of DNA spliced together to form functional antibody • Undifferentiated cell DNA is different from Differentiated cells DNA • Various polypeptides from the gene are used • Constant Region (C) consistent in all Ab’s • Variable Region (V) changes per Ab’s • Increases Antibody diversity
THE CONTROL OF GENE EXPRESSION • Eukaryotic genomes may contain thousands of genes. • Gene expression is controlled on a long term basis for cellular differentiation. • Highly specialized cells, (muscles or nervous tissues) express only a fraction of their genes. • Gene expression is regulated at the level of transcription by DNA-binding proteins that interact with other protein and external signals.
ORGANIZATION OF A TYPICAL EUKARYOTIC GENE • Coordinate gene expression depends on the association of a specific control element with every gene of a dispersed group. • Genes with the same control elements are activated by the same chemical signals.
Transcriptional Control • Typical EuK Gene • Enhancers (distal) • Control Elements (proximal) • Promoter • Gene (intron/exons • Terminator • Control Elements • Noncoding DNA • Regulate transcription • Binds to Transcription Factors • Enhancers need Activators to bind b4 working
Post-Transcriptional Control • Expression of functional protein regulates synthesis of RNA transcript. • Regulation of RNA degradation • Control of translation: regulatory proteins block leading region prevents ribosomes to attach. • Protein Processing/Degredation • Alternative RNA Splicing: produces different RNA molecules from same primary target.
Degradation of Protein • Polypeptide needs to be transported for function and destruction • Ubiquitin targets p.p. for destruction • Proteasomes recognize ubiquitin and degrade tagged protein • Mutation in proteasome may cause cancer
THE MOLECULAR BIOLOGY OF CANCER • Mutations that alter genes in somatic cells can lead to cancer. • Oncogenes- cancer causing genes. • Proto-oncogenes- normal cellular genes that code for proteins that stimulate normal cell growth and divisions.
Normal Signaling Pathways Growth StimulatoryGrowth Inhibitory • Ras gene codes for G protein • Stimulates cell cycle • Increase Mitosis • Proto-oncogene • Mutation in Ras may cause hyperactive G protein • Excessive cell growth • P53 gene • “Guardian Angel of the Genome • Damage to DNA causes p53 to turn on • Triggers apoptosis or cell death • Mutation in p53 • No cell death • Cell cycle not inhibited
Multiple Mutation needed to produce cancer • Incidence of cancer increases with age as mutations accumulate in cells
Operons • Multiple genes coding for an enzyme • Clustered together • Single promoter • Operator: on/off switch for all genes within the promoter (always in on position) • Regulatory Gene: produces protein that can potentially blocks Operator but must be activated • Corepressor: small molecule that activates the regulatory protein • Causes gene to be turned off • Corepressors can activate or deactivate genes as needed • Pg 347-350