Objective VIII

What is heterochromatin?

Heterochromatin is tightly packed chromatin that is transcriptionally inactive and typically gene-poor. It is inaccessible to the transcription machinery.

What is euchromatin?

Euchromatin is loosely packed chromatin that is transcriptionally active and gene-rich. It is more open and accessible to RNA polymerase and transcription factors.

How do heterochromatin and euchromatin differ structurally?

• Heterochromatin is densely packed with histones, making DNA less accessible.

• Euchromatin has a relaxed structure, allowing access to transcription machinery.

How does cytosine methylation play a role in transcription regulation?

• Cytosine methylation generally represses gene expression by promoting heterochromatin formation.

How does the probable formation of Z-DNA play a role in transcription regulation?

Z-DNA, a left-handed DNA helix, may form in methylated regions and is associated with transcriptional silencing.

What is chromatin remodeling?

Chromatin remodeling refers to the dynamic alteration of chromatin architecture to expose or hide specific DNA regions, regulating access to the transcription machinery.

What do histone acetyltransferases (HATs) do?

HATs acetylate lysine residues on histone tails, reducing their positive charge. This weakens the histone-DNA interaction, making DNA more accessible for transcription.

What are basal transcription factors?

Proteins required for the transcription of all genes. They help position RNA Polymerase II at the core promoter and initiate transcription.

What are DNA-binding transactivators?

Transcription factors that bind specific enhancer sequences and promote transcription by recruiting coactivators and the transcription machinery.

What are coactivator protein complexes?

Large multiprotein complexes that do not bind DNA directly but link transactivators to the transcription apparatus. They often include chromatin remodeling activities.

What is the TATA box?

A core promoter DNA sequence (~25 bp upstream of the start site) that binds TBP (TATA-binding protein) and helps recruit RNA Polymerase II.

What is the CAAT box?

A promoter element located ~75 bp upstream of the start site. It binds transcription factors and increases the efficiency of transcription initiation.

What is the GC box?

A DNA sequence rich in guanine and cytosine. It binds transcription factor Sp1 and contributes to promoter activity, especially in housekeeping genes.

What are enhancers?

Regulatory DNA sequences that bind activator proteins and increase transcription levels, often functioning at great distances from the promoter.

What are silencers?

Regulatory DNA sequences that bind repressor proteins, which inhibit transcription by preventing activator binding or recruiting corepressors.

What are response elements?

Short DNA sequences within promoters or enhancers that respond to specific stimuli (e.g., hormones, stress) by binding activated transcription factors.