DNA packaging and the nucleus

Question 1

What is histone proteins function?

Answer 1

Structural component of chromatin - packages DNA

Question 2

Why is packing DNA important?

Answer 2

Pack lots in small space and makes DNA less vulnerable

Question 3

Describe the electrostatic attractions in chromatin?

Answer 3

Histone has cationic surface (+ charged AA) attracted to -ve DNA charged phosphate groups - allow tight wrapping of DNA

Question 4

What is the structure of a histone?

Answer 4

Octamer entailing 2 sets of H2A, H2B, H3 and H4 with DNA wrapped around (nucleosome) - stabilised by H1 protein and joined by linker DNA

Question 5

Describe different levels of organisation of chromatin?

Answer 5

Bead and string model, 10nm fibre.

Fibril supercoiled. Nucleosomes are folded and stacked creating 30nm solenoid fibril
(2m DNA into 5micrometer wide nucleus)

Question 6

What bonds underpin the histone protein?

Answer 6

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.

Question 7

When does highest degree of compaction of genome occur?

Answer 7

In mitosis - not transcriptionally active

Question 8

What are the functions of nucleus?

Answer 8

Gene replication + repair

Gene transcription

Ribosome production in nucleoli

Question 9

What are nucleoli, what’s the function?

Answer 9

Dense regions of DNA, RNA and protein (nucleoprotein)

Site of ribosome production -exported via nuclear pores

Question 10

How many homolgous pairs of chromosomes does nucleus have?

Answer 10

22 (+1 pair of sex chromosomes)

Question 11

Structure of nucleus (inc size)?

Answer 11

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

Question 12

Describe how condensed DNA is in interphase, why?

Answer 12

Chromatin less compact allow enzymes to access DNA

Question 13

Describe difference between faculative and constitutive heterochromatin?

Answer 13

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)

Question 14

Explain how mitotic and interphase chromosomes differ under light microscope?

Answer 14

Mitotic chromosomes visible but interphase ones are not

Question 15

Describe structure and appearance of euchromatin and heterochromatin?

Answer 15

Euchromatin: less comapct and stains lightly containing most active genes

Heterchromatin: chromatin more tightly packed dark staining

Question 16

What is a chromatin loop?

Answer 16

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

Question 17

What is the role of chromatin loops?

Answer 17

Allows regulation

Question 18

Describe how chromosome territories are characterised?

Answer 18

Territories are determined by genomic regions, such as, the lamina-associated domains (LADs), and the topologically associating domains (TADs)

Question 19

What is involved in a LAD?

Answer 19

Anchoring specific parts of DNA to the nuclear lamina.

Question 20

What type of chromatin do lads consist of?

Answer 20

Transcriptionally silent

Question 21

What type of DNA sequence is packaged as constitutive chromatin?

Answer 21

High copy number tandem repeats: satellite repeats, mini-satellite and microsatellite repeats, and transposon repeats.

Question 22

How can facultative chromatin be silenced?

Answer 22

Histone deacetylation or Piwi-interacting RNA (piRNA) through RNAi

Question 23

What is the diameter of the DNA double helix?

Answer 23

2nm

Question 24

~ what % of the cell volume does the nucleus make up?

Answer 24

10%

Question 25

What is the role of the nuclear envelope?

Answer 25

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

Question 26

What is the outer nuclear membrane continuous with?

Answer 26

RER

Question 27

What is the name of the area between the inner and outer nuclear envelope?

Answer 27

Perinuclear space

Question 28

What can pass through the nuclear pore?

Answer 28

Small water-soluble molecules

Question 29

What is the role of the nucleolus?

Answer 29

Synthesis of rRNA and assemble ribosomes

Question 30

Does the nucleolus have a membrane?

Answer 30

No

Question 31

The number of helical turns per ~10 nucleotides in natural B-form DNA…

Answer 31

1

Question 32

The nitrogenous base in a nucleotide is attached to the sugar via a covalent…

Answer 32

C-N bond

Question 33

The presence of a nuclear envelope around the core genetic material defines a eukaryotic cell.

Answer 33

True

Question 34

Chromatin is composed of which macromolecules?

Answer 34

Protein, RNA and DNA

Question 35

What is DNA helical periodicity?

Answer 35

Base pairs per turn on the nucleosome

10 for DNA