Genes! (and wins Nobel Prize, 1933)

Transcription

1 Today: Mendelian Genetics Wrap-up Adding Chromosomes to the Mix Lunch Inoculate Cultures The Eukaryotic Genome Morgan Discovers Sex-Linked Genes! (and wins Nobel Prize, 1933)? Sex Determination Happens in a Variety of Ways An Aside: X Inactivation in Female Mammals Sex chromosomes (especially the X chromosome) carry genes for many other characters. In humans, the term sex-linked generally refers to genes on the X chromosome. In females, one X chromosome is inactivated (at random) and condenses into a compact Barr body along the inside of the nuclear envelope. Most genes on this X chromosome are not expressed. In females, one X chromosome is inactivated (at random) and condenses into a compact Barr body along the inside of the nuclear envelope. Most genes on this X chromosome are not expressed. Because it is random which X chromosome forms the Barr body during development, females are Mosaics of the two cell types. Because it is random which X chromosome forms the Barr body during development, females are Mosaics of the two cell types. 1

2 Practice Question: Sex-Linked Chromosomal Inheritance If you see the number 74, then you do not have red-green color blindness. If you see the number 21, you are color blind to some extent. A totally color-blind person will not be able to see any of the numbers. Practice Questions: Sex- Linked Chromosomal Inheritance (Part 2) If a color blind man has children with a wild-type woman, what are the chances that a daughter of theirs will be colorblind? What are the chances that their son will be colorblind? Can females be colorblind? What would the genotype of the parents have to be? More of Morgan s Complications Morgan does further crosses between wild type flies (gray bodies and normal wings) and mutant flies (black bodies and vestigial wings) b + = gray b = black vg + = normal wings vg = vestigial wings His Results: High Frequency of Parental Phenotypes!?? An Understanding of Linkage Groups allows for LINKAGE MAPPING One of Morgan s students, Alfred Sturtevant, develops a method to construct a genetic map. He hypothesizes that recombination frequencies reflect the distances between genes on chromosomes. One map unit (or centrimorgan) = 1 % recombination frequency 2

3 A sample Genetic Map of a Drosophila Chromosome Thinking Back to Meiosis: Complications of Chromosomal Inheritance- Non-Disjunction Note that a linkage map is based on recombination frequencies. As the frequency of cross-over is NOT uniform over the length of the chromosome, it portrays the sequence of genes but not their precise locations. Nondisjunction can result in ANEUPLOIDY- an abnormal chromosome number Aneuploidy results in several human disorders: 3 copies of Chromosome 21 =Down s Syndrome (1:700 children born in US) Monosomic- the aneuploid cell has a single copy of a chromosome Trisomic- the aneuploid cell has three of a given chromosome Red viscacha rat from Argentina = 4n If an entire organism has more than two complete chromosome sets it is POLYPLOID (triploid=3n, tetraploid=4n, etc.) XYY = no general traits, except tall XXX = healthy, normal XXY = Klinefelter Syndromephenotypically male with normal intelligence, sterile XO = Turner syndromephenotypically female, sterile, usually normal intelligence Chromosomal Alterations can also cause Human Disorders: One Other Notable Exception: Extranuclear Genes Genes contained in the chloroplasts or mitochondria are inherited maternally and do NOT display Mendelian inheritance patterns! In Plants, plastid genes typically responsible for variegation in leaves. In animals, defects in proteins involved in the ETC or ATP synthase affect ATP Synthesis (i.e. mitochondrial myopathy) Skeletal Muscle: Mitochondrial myopathy, electron microscopy. Number, size and shape of mitochondria are increased and abnormal. 3

4 Thinking About Genomes Next: What s a Genome?? Understanding Genome Structure and Function! Why is genome structure/ function important? Remembering Structure Remembering Structure Nucleosomes are formed of DNA winding around 8 histone proteins, two each of H2A, H2B, H3, and H4. The N-terminus of each protein extends outward forming a histone tail. Nucleosomes condense into 30nm fibers due to interactions between the histone tails of one nucleosome, the linker DNA, and the nucleosomes on either side. Remembering Structure Thinking About Genomes During prophase, chromosomes condense further! In metaphase chromosomes, the same genes always end up at the same locations. What does this tell us about chromosome packing?? Photo: V. Miszalok, U. Klingbeil, I. Chudoba, V. Smolej 4

5 The Importance of Gene Expression Cell Differentiation! Differences in cell types are due to differential gene expression. How might a cell regulate gene expression?? Regulating Chromatin Histone acetylation (-COCH 3 ) prevents adjacent nucleosomes from binding to one another Lysine Residues (amino acids) in the Histone Tails have an Acetyl group added to them Regulating Chromatin How does this change the structure of the tail? Staining of Acetylated H3 Throughout the Cell Cycle. A field of cells containing interphase, prophase(p), prometaphase (PM) and metaphase (M); Michael J. Hendzel and Michael J. Kruhlak Other Modifications to Histone Tails Histones may also be Methylated (CH 3 ) New Model: Histone Code Hypothesis! Figure: two different metaphase spreads (human female) with preferential staining. Barbara A. Boggs, Peter Cheung, Edith Heard, David L. Spector, A. Craig Chinault & C. David Allis DNA Methylation Proteins that bind to methylated DNA may recruit histone deacetylases! What would this enzyme do?? 5

6 Gene Regulation at the Level of Chromatin Structure Heterochromatin vs Euchromatin?? New Field: Epigenomics! Figure: Epigenomics.com Next Up: Regulating at the Level of Transcription! Regulating Transcription with Transcription Factors A single protein, TFIID, binds to the TATA box in the promoter. The correct combination of other small proteins must then bind to TFIID before the RNA Polymerase can bind and initiate transcription. Eukaryotes Also Package Regulator Protein Binding Sites within the Promoter Upstream sequences known as Enhancers, may also bind proteins and fold over tohelp initiate transcription Proteins that Bind DNA are (relatively) Easy to Find! Understanding DNA and Protein Structure allows us to recognize motifs, or structures that allow a protein to interact with DNA 6

7 Combinatorial Control of Gene Expression Post Transcriptional Regulation: Small RNAs We carry 250+ genes for micro RNAs (~20 base pairs long). How might these micro RNA s affect translation? Post-Transcription: RNA Interference (RNAi) Post-Transcription: Alternative Splicing Experimental Observation: Injecting dsrna into a cell can silence the corresponding gene! Post-Transcription: mrna Editing?!! Post- Transcription: mrna Stability Figure: Analysis of H-ferritin mrna stability in control and PMA-treated THP-1 cells; Biochemical Journal (1996) Volume 319,

8 Post-Translation: Ubiquitin What We ve Learned from Our Own Genome, and Comparing Genomes Chamber of Doom?!? Comparing Prokaryotic and Eukaryotic Genomes Genome length (base pairs) 4,640,000 12,068,000 Number of protein-coding genes 4,300 5,800 Proteins with roles in: Metabolism Energy production/storage Membrane transport DNA rep./repair/recombination Transcription Translation Protein targeting/secretion Cell structure Essential Components of Multicellular Genomes Lessons from Genomics: Many Repetitive Sequences Examples: Minisatellits (10-40 bp) repeats Microsatellites (1-3 bp) repeats Lessons from Genomics: We have transposons!! Transposons make up >40% of the human genome! CODIS, our national DNA database, uses Short Tandem Repeats from 13 loci to identify individuals. 8