DNA packaging in the nucleus

State 3 reasons as to why DNA needs to be packaged

• there’s a lot of it and it needs to be packaged to fit

• organised packaging allows it to be easily unpacked

• protects DNA from attack

-there’s a lot of DNA and so it has to fit- (2 points)

• each of your cells has about 2m of DNA in the nucles

• average human nucleus has a diameter of 10μm

-organised packing allows it to be easily unpacked- for.. (2 points)

• transcription

• repair

-protects DNA from attack- (2 points)

• mutagens and other DNA elements can attack uncoiled DNA

• packaging it up for safety helps protect it

What is a nucleoside?

a base with deoxyribose

What is a nucleotide?

a nucleoside with 1-3 phosphates

What is a nucleus?

the eukaryotic cellular compartment where DNA is stored

What is a nucleolus?

a dense area within the nucleus where ribosomes are assembled

What is a nucleosome?

histone octamer wrapped in 147 bp DNA and linker DNA

List 3 common DNA replication sequences

• DNA replication origin

• centromere

• telomere

DNA replication origin (2 points)

• the location at which DNA duplication begins

• eukaryotic chromosomes contain many origins of replication to ensure that the entire chromosome can be replicated rapidly

Centromere (2 points)

• link identical sister chromatids after DNA replication

• allows one copy of each duplicated chromosome to be pulled into each daughter cell

Telomeres (3 points)

• ends of a chromosome

• contain repeated nucleotide sequences that enable the ends of chromosomes to be officially replicated

• repeated telomere sequences, together with the region adjoining them, form structures that protect the ends of the chromosome from being mistaken by the cell for a broken DNA molecule in need of repair

List the arrangement of chromatin

“beads on a string“

String =

DNA

Bead =

nuclesome core particle

How can we separate a nucleosome from chromatin?

degrading the linker DNA

What do nucleosome core particles consist of?

• an octametric histone core

• 147 nucleotide double stranded DNA (wrapped in a left handed coil of 1.7 turns)

List the structural composition of the histone octamer core

2x (tetramers)

(H2A + H2B + H2A + H2B)

(H3 + H4 + H3 + H4)

Describe the bonds present between octamer core and DNA (2 points)

there are 142 hydrogen bonds formed between DNA and histone core in each nucleosome

• nearly half of these bonds are between amino acid backbone and sugar phosphate backbone of DNA

How long is the linker DNA that separates each nucleosome core particle

80 nucleotide pairs

How often do nucleosomes repeat?

intervals of 200 nucleotide pairs

Discuss amino acid composition in histone cores

>1/5 of amino acids in each histone core are either lysine or arganine, whos positive charges neutralise the negatively charged DNA backbone

Discuss histone amino acid tail composition (2 points)

each histone core has an N terminal amino acid tail

• these tails, subject to covalent modification, can determine critical aspects of chromatin structure and function

Describe the dynamic structure of nucleosomes

unwraps from each end 4x per second, remaining exposed for 10-50ms before the structure releases

Describe the dynamic structure of chromatin (2 points)

further loosening of DNA-histone contacts is required because eukaryotic cells contain a large variety of ATP dependent chromatin remodeling complexes

• this complex binds to histone core and the DNA, using the energy of ATP hydrolysis to move the DNA relative to the core, making it less tightly bound

State 2 different variations of chromatin sructure

zig zag model of 30nm fiber

• nucleosomes form a tetra-nucleosome

solenoid model

• six nucleosomes per twist of coil