CHAPTER 10 notes

Chapter 10 Notes: Bacteriophages contain: Lipid bilayer – picked up when virus leaves host Protein coat Nucleic acid found inside the protein coat Nucleic acid and capsid proteins self-assemble *Virus that affects a mammal has a similar genome to the mammal Bacterial Chromosomes: Circular chromosome DNA (1 copy of genome) Genes Intergenic Regions – regions between genes Repetitive Sequences – found throughout the chromosome DNA folding, replication, regulation, & recombination Origin of Replication – a few hundred nucleotides in length To fit in a bacterial cell, the chromosomal DNA must be compacted – formation of LOOP DOMAIN DNA Supercoiling (another way to compact the bacterial chromosome) B-form DNA (Right handed helix – clockwise) Overwinding (turn to right): Positive Supercoil Underwinding (turn to left): Negative Supercoil Z-form DNA (Left handed helix – counter clockwise) Overwinding (turn to left): Positive Supercoil Underwinding (turn to right): Negative Supercoil B-form/Positive Supercoil/Negative Supercoil confirmations Topoisomers of each other *Bacteria always have negative supercoiling 1. Helps in the compaction of the chromosome 2. Creates tension that may be released by DNA strand separation The control of supercoiling = accomplished by two main enzymes, which can relax the supercoils found in DNA: 1. DNA topoisomerase I Cleaves just one strand of DNA and winds it around the other strand Relaxes negative supercoils and introduces positive supercoils 2. DNA topoisomerase II Cleaves both strands of DNA Relaxes positive supercoils and introduces negative supercoils Eukaryotic Chromosomes: have more than one set of chromosomes usually greater than that of bacterial cells higher eukaryotes = longer genes and more introns Have many origins of replications approx 100,00bp apart Centromere/Kinetochore: required for proper segregation during mitosis/meiosis Telomere: prevent 1) chromosome sticky ends 2) shortening * Repetitive sequences commonly found near the centromere & telomeres - 50% of DNA are repeated sequences - 50% Unique Sequences – contains info for controlling gene expression 25% non-repetitive DNA (not codons, not introns) 404812598107520% introns: incorporated into mRNA and then spliced out, create variety 2% protein-coding regions Eukaryotic Chromatin Compaction: Involves proteins + DNA 1) Wrap DNA around HISTONE proteins Histones are positively charged 2(H2A, H2B, H3, H4) Nucleosomes: DNA + Histones (Linker region – easily accessible) H1 histone helps keep nucleosome in place Histones have TAILS things can stick to it like: Phosphate functional groups 36766503771900Acetyl groups Modification of histone protein tails Affect structure & function of nucleosomes HISTONE CODE HISTONE CODE controls compaction the compaction of euchromatin is too high to easily access and transcribe genes Chromatin remodeling changes chromatin structure regulates ability of transcription factors to access genes Histone core protein tails are modified over 50 different enzymes identified which modify tails modifications include acetylation, methylation and phosphorylation-all covalent changes Histone code hypothesis is that the pattern of modification is a code specifying alterations 2) nucleosomes join to form 30nm fibers Two types: Solenoid Model (spiral) Zig-Zag – has little face – to – face contact 3) 30 nm Fiber + Nuclear Matrix = radial loop domains (300nm fibers) *(Go phase cells rest at this level) 2752725-66675Nuclear Matrix: Nuclear Lamina Internal Matrix Protiens MAR’s (matrix-attachment regions): anchored to the nuclear matrix creating radial loops Gene Expression: *Genes expressed at HIGH LEVELS Middle * Genes are silenced anchored to the sides through DIFFUSION The compaction level is not completely uniform: Euchromatin: radial loop domains Less condensed regions of chromosomes Transcriptionally active During interphase, most chromosomal regions are euchromatic Heterochromatin (centromere & telomere) – 700 nm Tightly compacted regions of chromosomes Transcriptionally inactive (in general) Two Types: (1) Constitutive heterochromatin Regions that are always heterochromatic Permanently inactive with regard to transcription (2) Facultative heterochromatin Regions that can interconvert between euchromatin and heterochromatin Example: Barr body Metaphase Chromosome 1,400 nm Two SMC proteins (SMC proteins use energy from ATP and catalyze changes in chromosome structure) help to form and organize metaphase chromosomes CONDENSIN: chromosome condensation 47625006524625The condensation of a metaphase chromosome by condensin (300nm 700nm) Function: binds to chromosomes and compacts the radial loops Interphase: condensin is in the CYTOPLASM Mitosis/Meiosis: condensin is in the NUCLEUS COHESION: sister chromatid alignment Spindle microtubles can push/pull chromosomes to keep towards the center of the cell (balance between pushing and pulling) Protiens at the kinetochore can sense how much pulling/pushing is necessary S phase/G2 phase: Cohesion is found in between the sister chromatids holding them together Prophase: cohesions along the chromosome are released EXCEPT at the centromere Anaphase: cohesion at the centromere is degraded Anaphase Promoting Complex (APC) – has ubiquitin ligase ( tags proteins for death/destruction) Securin ---- Separase (Cleaves cohesion) Summary: Once cells senses metaphase plates – the APC stick ubiquitin onto securing and releases seperase which cleaves cohesion from the sister chromatids during anaphase