Section 8

Question 1

What is chromosome remodeling, and what essential cellular processes rely on it?

Answer 1

Chromosome remodeling, also referred to as chromatin remodeling, is the modification of chromatin structure to “open up” or modify the organization of the condensed genomic DNA. This modification is necessary to gain access to specific DNA sequences for important cellular processes. Essential cellular processes that rely on chromosome remodeling include regulating gene expression, DNA replication, DNA editing and repair, recombination events, and preserving epigenetic tags.

Question 1

What role do nucleosomes play in controlling gene expression?

Answer 1

Nucleosomes influence gene expression by regulating the accessibility of DNA to specific proteins, such as transcriptional activators or repressors. Nucleosome arrangements that result in an “open” chromatin state allow transcription in that region of the chromosome, while those leading to a “closed” chromatin state repress transcription.

Question 2

What are the two main classes of enzymes responsible for regulating the arrangement of nucleosomes in an ‘open’ or ‘closed’ state?

Answer 2

The two main classes of enzymes are chromatin remodeling complexes and histone modifying enzymes. These enzymes control the accessibility of DNA and gene expression by modifying chromatin structure.

Question 3

What are the three main functions of chromatin remodeling complexes?

Answer 3

1. Repositioning or sliding the nucleosome to a different location along the DNA strand.
2. Ejecting the nucleosome from the DNA.
3. Replacing the nucleosome with one that contains a histone variant, which imparts special properties to the chromatin.

These functions of chromatin remodeling complexes allow them to open DNA binding sites, enabling the binding of transcription factors and regulating gene expression.

Question 4

How do chromatin remodeling complexes affect the movement of nucleosomes along the DNA strand?

Answer 4

• Chromatin remodeling complexes physically move the nucleosome.
• This movement can either block or expose promoters, leading to transcription repression or activation, or it can eject or replace histones.
• The process of nucleosome movement by chromatin remodeling complexes requires energy in the form of ATP.
• These proteins typically have an ATPase domain.

Question 5

How do some chromatin remodeling complexes mobilize nucleosomes?

Answer 5

Some chromatin remodeling complexes create a DNA loop within the nucleosome. This loop is propagated around the histone octamer, causing the nucleosome to slide to a new segment of the DNA. This action can enhance DNA accessibility, such as by exposing a previously blocked promoter.

Question 6

Chromatin remodelling complexes alter nucleosome structure along the DNA strand by repositioning, ejecting, or replacing the nucleosome. This can result in the blocking or exposing of a promoter on a gene. What is the result of blocking or exposing the promoter?

Answer 6

Blocking the promoter will repress transcription, while exposing the promoter will activate transcription and therefore the gene will be transcribed and translated into a functional protein.