DNA packaging and the nucleus Flashcards
What is histone proteins function?
Structural component of chromatin - packages DNA
Why is packing DNA important?
Pack lots in small space and makes DNA less vulnerable
Describe the electrostatic attractions in chromatin?
Histone has cationic surface (+ charged AA) attracted to -ve DNA charged phosphate groups - allow tight wrapping of DNA
What is the structure of a histone?
Octamer entailing 2 sets of H2A, H2B, H3 and H4 with DNA wrapped around (nucleosome) - stabilised by H1 protein and joined by linker DNA
Describe different levels of organisation of chromatin?
Bead and string model, 10nm fibre.
Fibril supercoiled. Nucleosomes are folded and stacked creating 30nm solenoid fibril
(2m DNA into 5micrometer wide nucleus)
What bonds underpin the histone protein?
Hydrogen bonds between the DNA backbone and the amide group of histone proteins.
Nonpolar interactions between the histone and deoxyribose sugars on DNA.
Salt bridges and hydrogen bonds between side chains of basic amino acids (especially lysine and arginine) and phosphate oxygens on DNA.
When does highest degree of compaction of genome occur?
In mitosis - not transcriptionally active
What are the functions of nucleus?
Gene replication + repair
Gene transcription
Ribosome production in nucleoli
What are nucleoli, what’s the function?
Dense regions of DNA, RNA and protein (nucleoprotein)
Site of ribosome production -exported via nuclear pores
How many homolgous pairs of chromosomes does nucleus have?
22 (+1 pair of sex chromosomes)
Structure of nucleus (inc size)?
7-8 micrometer
Nuclear envelope (defines eukaryotes)
Double membrane contains pores allow macromolecules to cross (perinuclear space in between)
Inner nuclear membrane (nuclear lamina) contains scaffold like network of protein filaments - maintain nuclear shape
Describe how condensed DNA is in interphase, why?
Chromatin less compact allow enzymes to access DNA
Describe difference between faculative and constitutive heterochromatin?
Constitutive heterochromatin: always condensed and inactive. Found at chromosomal centromeres and telomeres
Faculative heterochromatin: condensed but can be actively transcribed -euchromatin e.g. one X chromosome packaged as Barr body - silenced (other is packaged as euchromatin)
Explain how mitotic and interphase chromosomes differ under light microscope?
Mitotic chromosomes visible but interphase ones are not
Describe structure and appearance of euchromatin and heterochromatin?
Euchromatin: less comapct and stains lightly containing most active genes
Heterchromatin: chromatin more tightly packed dark staining
What is a chromatin loop?
Stretches of genomic code, that lie on the same chromosome, are in closer physical proximity to each other than to the base sequence between the two points
What is the role of chromatin loops?
Allows regulation
Describe how chromosome territories are characterised?
Territories are determined by genomic regions, such as, the lamina-associated domains (LADs), and the topologically associating domains (TADs)
What is involved in a LAD?
Anchoring specific parts of DNA to the nuclear lamina.
What type of chromatin do lads consist of?
Transcriptionally silent
What type of DNA sequence is packaged as constitutive chromatin?
High copy number tandem repeats: satellite repeats, mini-satellite and microsatellite repeats, and transposon repeats.
How can facultative chromatin be silenced?
Histone deacetylation or Piwi-interacting RNA (piRNA) through RNAi
What is the diameter of the DNA double helix?
2nm
~ what % of the cell volume does the nucleus make up?
10%
What is the role of the nuclear envelope?
Separates the cell’s genetic material from the surrounding cytoplasm, serving as a barrier to prevent macromolecules from diffusing freely between the nucleoplasm and the cytoplasm
What is the outer nuclear membrane continuous with?
RER
What is the name of the area between the inner and outer nuclear envelope?
Perinuclear space
What can pass through the nuclear pore?
Small water-soluble molecules
What is the role of the nucleolus?
Synthesis of rRNA and assemble ribosomes
Does the nucleolus have a membrane?
No
The number of helical turns per ~10 nucleotides in natural B-form DNA…
1
The nitrogenous base in a nucleotide is attached to the sugar via a covalent…
C-N bond
The presence of a nuclear envelope around the core genetic material defines a eukaryotic cell.
True
Chromatin is composed of which macromolecules?
Protein, RNA and DNA
What is DNA helical periodicity?
Base pairs per turn on the nucleosome
10 for DNA
