Lecture 10 Flashcards
1
Q
Interphase vs mitotic chromosomes
A
- Interphase chromosomes are much less compact than mitotic chromosomes
2
Q
What are the 5 packing levels of DNA
A
- DNA double helix (2nm)
- “Beads on a string” form of chromatin (11nm)
- Chromatin fiber of packed nucleosomes (30nm)
- Chromatin fiber folded into a loop (700nm)
- Entire mitotic chromosome (1400nm)
3
Q
Nucleosomes
A
- The DNA in chromatin is tightly associated with protein called histones.
• The histones package and order the DNA into structured units called nucleosomes
• Nucleosomes are the fundamental unit of chromatin
• Also known as “beads on a string”
4
Q
Histones
A
- Histones are small proteins that associate with DNA
- They are a major component of chromatin
- Histones are rich in basic amino acids particularly lysine and arginine
5
Q
5 types of histones:
A
- H2A
- H2B
- H3
- H4
- H1 – clamps DNA wrapped around nucleosomes
6
Q
Histones core:
A
- Histones core is composed of two copies of each histone to form an octamer (8 subunits)
- DNA is wrapped around the histone core to form the nucleosomes
- Histone H1 clamps around DNA
7
Q
Histone N-terminal tails
A
- They protrude from the octameric disk
- They are highly conserved between histones
- They contain positively charged lys and arg residues
- They are highly flexible
8
Q
How to determine the structure of a nucleosome?
A
- Treat the DNA with a nuclease (it digests linker DNA)
- DNA wrapped around histone protects against degradation
- The core can be dissociated into individual subunits by high concentration of salts
9
Q
Describe the 30nm chromatin fiber of packed nucleosomes:
A
- Also known as solenoid
- “Beads on a string” coil to form 30nm chromatin fiber
10
Q
H1 and the 30nm fiber
A
- H1 holds the DNA on the histone core and is necessary for formation of 30nm fiber (contains a pair of long flexible tails keeping the DNA more tightly compact)
- H1 is located in the interior of the 30nm fiber
11
Q
Describe the relationship between the amount of histone H1 present in a 30nm fiber and the transcriptional activity of that DNA.
A
- Regions of DNA that are transcriptionally active are less well ordered and contain very little H1 histone
- Compaction makes the DNA inaccessible to transcription and replication factors
- Transcription doesn’t generally occur in 30 nm fiber, but it does in “beads on a string”
12
Q
What is the relationship between N-terminal tails and the 30nm fiber?
A
- Histone tails help pack the nucleosomes into the 30nm fiber
- The positive charged N-termini bind to the negative charged sugar phosphate backbone on DNA of neighbouring nucleosomes
13
Q
How are the 30-nm fiber is held together by:
A
How are the 30-nm fiber is held together by:
- Histone H1 proteins that pull nucleosomes together
- N-terminal tails
14
Q
Describe the higher order of chromatin organization
A
- Further looping/coiling of solenoid DNA into highly compacted, transcriptionally silent form of chromatin which occurs during mitosis
15
Q
Chromatin remodeling process
A
- Protein machine that uses ATP to change the position of DNA wrapped around the nucleosomes
- Makes DNA less or more accessible
