Chromatin and Nuclear Structure

Question 1

Describe structure and function of nucleosome

Answer 1

10nm diameter beads wrapping ~200bp of DNA. Made of octamer of histones

Question 2

Describe histones

Answer 2

Basic, highly conserved proteins found only in eukaryotes
5 families
Play chromatin regulatory role

Question 3

Euchromatin vs Heterochromatin

Answer 3

Heterochromatin: tightly condensed so NO transcription
Euchromatin: les tightly condensed so allows transcription

Question 4

Describe structure and function of fibers

Answer 4

30nm diameter curled string of nucleosomes
Form in high salt conditions
6 nucleosomes/turn = ~1,200bp DNA/turn
Form solenoid fiber (heterochromatin)
Requires H1 (linker histone) to form

Question 5

Describe structure and function of loops

Answer 5

~50 solenoid fibers to make ~60kbp loop
Attached to nuclear matrix
Regulated by ‘cis’ interactions through adjacent loop interactions
Part attached to matrix is ‘active gene’

Question 6

Describe structure and function of miniband

Answer 6

Loops wrapped around a mitotic chromosome
~1 million bp/band
Building blocks of chromosomes

Question 7

Describe structure and function of chromosome

Answer 7

~75 minibands (~75 million bp DNA)
Have centromere and telomeres
Fully packed DNA

Question 8

What are centromeres?

Answer 8

Repetitive sequence of DNA near center of chromosome which supplies anchor for chromosome separation

Question 9

What are telomeres?

Answer 9

Repetitive sequence of DNA at ends of chromosome
Gets shorter with each sequential division
Long in sex cells, short in senescent cells (non-dividing)

Question 10

What is the chromatin hierarchy?

Answer 10

Nucleosomes –> solenoid fibers –> loops –> minibands –> chromosomes

Question 11

Function of nuclear matrix

Answer 11

To organize chromatin for replication

Question 12

Describe the nucleolus

Answer 12

Site of ribosome synthesis during the G1 phase
Tips of 10 chromosomes each containing their rDNA are in nucleolus for transcription

Question 13

Describe nuclear envelope

Answer 13

Outer membrane: ER and RER
Inner membrane: lamina and marginal heterochromatin

Question 14

Describe the nuclear pore complex

Answer 14

Nucleoporins present to transport things in and out of the nucleus
Made of octameric rings
Symmetric Nup: transverse both membrane to transport things
Asymmetric Nup: 1 side of nucleus, establish directionality
NOTE: pores are not specific to import vs export

Question 15

Describe import through the NPC

Answer 15

Things tagged with K+ and R+ rich sequences which constitute an NLS (nuclear localization sequence)
NLS binds to import receptor and is brought to pore where the receptor binds the Nups

Question 16

Describe export through the NPC

Answer 16

Things tagged with Leu rich sequences which constitute an NES (nuclear export sequence)
NES binds to export receptor and is brought to pore where the receptor binds the Nups

Question 17

Function of FG-Nups

Answer 17

Innermost layer of NPC channel
Docking sites for nuclear transport receptors

Question 18

Describe the nuclear lamina

Answer 18

Between envelope and heterochromatin
Anchors NPC to matrix
Contributes to nuclear structure and reformation of daughter nucleus

LBR binds IM of envelope, B binds LBR, A/C bind B, A/C binds heterochromatin telomerases
Laminopathies = mutation in A

Question 19

Describe the Internal Nuclear Matrix

Answer 19

Important for transcription activity and DNA replication
Orders chromosomes during replication with each having a specific location

Question 20

Steps of ribosome synthesis

Answer 20

RNA Pol I/II taken rDNA –> rRNA
RNPs fold rRNA into subunits to be escorted through pores