1 - DNA and Chromosome structure

Question 1

What does chromatin facilitate

Answer 1

• Packing and protection of DNA
• Regulation of gene expression

Question 2

Nucleosome

Answer 2

DNA coiled around a core of histone proteins

Question 3

Histone core

Answer 3

Octamer (8 protein complex) - 2 of each subunits

Question 4

Subunits of histone core

Answer 4

• H2A
• H2B
• H3
• H4

Question 5

Role of H1

Answer 5

Stabilises internucelosome region. binds DNA at nucleosome entry and exit points

Question 6

Heterochromatin

Answer 6

Tightly condensed, no transcription, recombination, replication, repair

Question 7

Constitutive heterochromatin

Answer 7

Remains condensed throughout cell cycle. Not transcribed

Question 8

Facultative heterochromatin

Answer 8

Can be unpacked when needed and become transcriptionally active

Question 9

Open chromatin

Answer 9

DNA is accessible and transcriptionally active

Question 10

Closed chromatin

Answer 10

DNA is not accessible, gene expression is repressed

Question 11

Topoisomerase

Answer 11

reduces the torsional stress, prevents tangling

Question 12

Chromatin conformation

Answer 12

DNA –> nucleosome –> Chromatin –> loop –> domain –> compartment –> chromosome

Question 13

Cohesins in interphase

Answer 13

Maintain chromatin conformation by bringing together distant regions of DNA. Facilitate gene expression

Question 14

When is chromatin less condensed

Answer 14

Interphase. Extremely condensed during metaphase and anaphase, allowing segregation of chromosomes

Question 15

Chromosome territories

Answer 15

Arrangement of chromosomes within nucleus. Within a chromosome territory, positions at edge are expressed and positions in centre are repressed

Question 16

where is Euchromatin found and give example

Answer 16

Active X chromosome, towards centre of nucleus

Question 17

where is Heterochromatin found and give example

Answer 17

inactivated X chromosome, periphery of nucleus

Question 18

Do chromosome territories vary

Answer 18

Yes, within cell type and disease state

Question 19

Examples of post translational modification

Answer 19

• Acetylation
• Methylation
• Phosphorylation
• Ubiquitination

Question 20

Histone lys acetylation

Answer 20

• Negative charge modification (histones become less positive)
• DNA is less tightly wound around histones (starts to unravel, allowing transcriptional machinery to access DNA)

Question 21

Enzymes involved in histone modification

Answer 21

• Histone acetyl transferase (HAT) adds acetyl groups
• Histone deacetylase (HDAC) removes acetyl groups
• Histone methyl transferase (HMT) adds methyl groups

Question 22

Chromodomain

Answer 22

Recognises methylated lysines

Question 23

Chromoshadow domain

Answer 23

Aggregates with other chromoshadow domains, bringing nucleosomes close together

Question 24

Hinge domain

Answer 24

Joins chromodomain and chromoshadow domain together

Question 25

Boundary elements

Answer 25

Prevent condensation spreading to active chromatin regions

Question 26

Mechanism of constitutive heterochromatin formation

Answer 26

• A HP1 chromodomain binds to H3K9me3
• HP1 chromoshadow domains self aggregate resulting in condensation
• HP1 chromoshadow domain interacts with HMT tri-methylates adjacent H3K9s, causing chain reaction

Question 27

H3K9me3

Answer 27

maintains closed chromatin conformation