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CHAPTER 10 notes
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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
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