Notes

MBB 222

Lecture 23 & 24

Interphase:
>G1 phase- Synthesize histones (packaged DNA)
>S phase- DNA is replicated (daughter cells)
>G2 phase- at end of G2 phase (second gap phase, it is ready to enter mitosis)

Mitosis:
>Prophase
>Metaphase
>Anaphase
>Telophase
>Cytokinesis (Cell splitting)

>Chromatin=DNA and Protein with a little RNA
>Histone proteins hold together chromatin-> order the DNA into nucleosomes -> first unit of chromatin (beads on a string)
>Histones composed of 25% lys and arg to neutralize negatively charged DNA
Assembly of histones need "chaperone" - nucleoplasmin
4 types of histones, 2 of each make and octamer
Nucleosome=2 loops of DNA wrapped around a histone octamer clamped shut by H1 histone protein

>Linker DNA b/w the nucleosomes can be digested by a nuclease and the resulting nucleosome core particle can be disassociated with "High concentration of salt" into the histone proteins and a 147 bp DNA double helix
>Higher order chromatin -> Beads on a string-> Solenoid->Chromatin associated scaffold (loops of DNA may have related genes)
>Transcription can only occur in areas with low H1 concentration = less ordered state = more easily accessible

>Histone 2A and 2B form a dimer in a "handshake"
>H3 and H4 form a scaffold which H2A/H2B connect to form an octamer

>N-terminal tails of histones are positively charged and attach to negative sugar-phosphate backbones on neighbouring nucleosomes to help with condensing.
>Chromatin "remodelling" to change the structure of nucleosomes to become more accessible
>ATP powers chromatin remodelling complexes to change the structure of nucleosomes

N-terminal Histone tail modifications:
>Reversible phosphorilation of series (PO3)->adds negative charge to histone (effect unknown)
>Rev methylation of lysine (CH3) -> gene activation or silencing
>Reversible acetylation of lysine-> can make DNA more/less accessible

>Histones are modified to regulate "State of compaction" and "gene activity (transcription)

>Histone code: Specific combinations of histone modifications of different structures can cause specific effects on chromatin structure/gene activity.

Chromosome replication occurs at:
>replication origin->where DNA replication begins, in Eukaryotes can have many of these for quicker replication
>centromere->attachment site for mitotic spindle
>telomere->formed at end of chromosomes, repeated sequences that protects from nucleases and being recognition as breaks

Problem with DNA replication at ends of chromosomes:
>At the lagging strand of the daughter RNA, the DNA is synthesized starting at 3' OH groups. The "segments" each have an RNA primers for that need to be replaced with a 3' OH after polymerization. Usually an adjacent okazaki fragment adds and OH but at the ends of chromosomes there are gaps left because there no 3'OH groups can be donated -> Enzyme telomerase helps with this by adding extra sequences at the end of telomeres.

Telomeres have repeat sequences of T and G's, one strand is longer due to 'incomplete translation'
Telomerases have repeated sequences of A and C sequences (RNA template)->used to ELONGATE the PARENTAL strand a few times over.
Then DNA polymerase Alpha has an RNA primer which attaches to the now elongated parental strand, and starting with this primer the rest of the RNA can be relocated to remove any gaps of important information.

>Also 3' overhang can cause unusual Guanine quartets!