Transcription Regulation in Eukaryotes – A Primer

What are the molecular mechanisms of gene regulation in eukaryotes?
How do eukaryotes generate many different patterns of gene expression with a limited number of regulatory proteins?
What roles does chromatin play in eukaryotic gene regulation?
What are epigenetic marks and how do they influence gene expression?

Previous discussions focused on operons and gene regulation in prokaryotes.
Complexities in Eukaryotic Gene Regulation:
- Genes are located in the nucleus.
- Eukaryotic cells have linear chromosomes.
- Eukaryotes possess more chromosomes and more genes.
Operons:
- Not present in eukaryotes.
- Genes may form units consisting of protein-coding sequences and adjacent control sites.

Short-term Regulation:
- Genes can be rapidly turned on or off in response to environmental factors or cellular needs.
Long-term Regulation:
- Genes involved in development and differentiation of cells; may remain inactive for long periods and be activated at specific times.

Mouse Hox Gene Clusters:
- Hox genes illustrate the concept of anterior-to-posterior body axis development during early embryonic development.
- Specific genes are activated at different developmental stages (anterior to posterior transition).

Gene expression can be regulated at the following levels:

Length of DNA:
- DNA in a human cell is approximately 2 meters long, which is significantly longer than the diameter of a human cell (10-100 μm).
Compaction:
- DNA is compacted more than 10,000-fold to fit within the cellular nucleus.
- Only genes accessible by RNA polymerase can be expressed, influencing gene regulation.

Chromatin Composition:
- DNA is packaged with proteins, specifically histones, to create chromatin structure.
Nucleosome Formation:
- DNA wraps around histones, forming nucleosomes. Each nucleosome contains approximately 150 base pairs (bps) of DNA wrapped around a core histone complex (2 H2A, 2 H2B, 2 H3, 2 H4).
Linker Histones:
- Histone H1 binds to nucleosomes, linking adjacent core particles and stabilizing chromatin structure.

Single Nucleosome:
- Measures about 11 nm in diameter.
- Compacts DNA by a factor of approximately 6x.
Types of Chromatin:
- Euchromatin: Uncompacted regions capable of transcription.
- Heterochromatin: Compacted regions that cannot be transcribed.
- Constitutive Heterochromatin: Always compacted (e.g., centromeres, telomeres).
- Facultative Heterochromatin: May transition between compacted and open states.

Mechanisms for Access to DNA:
- Chromatin Modification: Chemical changes affecting histones or nucleotides can alter access for transcription machinery.
- Chromatin Remodeling: Altering nucleosome configurations to make DNA more or less accessible for transcription.

Histone Acetylation:
- Involves the addition of acetyl groups to lysine residues, reducing histone affinity for DNA, leading to less chromatin compaction and increased transcription.
- Histone Acetyltransferases (HATs): Enzymes that add acetyl groups, promoting transcription.
- Histone Deacetylases (HDACs): Enzymes that remove acetyl groups, inhibiting transcription.
Methylation of Histones:
- Different types, including monomethylation, dimethylation, and trimethylation of specific residues (e.g., lysine, arginine) affect gene regulation differently.

Process of Methylation:
- Addition of -CH3 groups to cytosine at the 5th carbon by DNA methyltransferase (DNMT).
Methylation Effects:
- Methylation can lead to closed chromatin structure, repressing gene transcription.
- Methylated regions (islands) can inhibit the transcriptional machinery from accessing genes.

Plays a crucial role in transcription regulation by altering the interactions between histones and DNA, making nucleosomes more accessible for transcription factors.
Nucleosomes can be repositioned, allowing specific regions of DNA to become available for transcription.

Requirement of Transcription Factors:
- Transcription requires various transcription factors binding to promoters or regulatory elements around transcription start sites.
Enhancers:
- Regulatory sequences that can be proximal or distal to the core promoter, aiding transcription factor binding.

These proteins interact with transcription factors to influence their function and the initiation of transcription through histone tail modifications or direct interaction with the transcription initiation complex.

Short DNA segments (6-10 base pairs each) clustered together, crucial for regulating gene expression.
Often randomly located in the genome, but their presence is essential for gene activation.

Example: The TATAA box is part of transcription initiation.
- Probability of random occurrence of TATAA in five nucleotides is 1/1024, leading to over 3 million expected occurrences in the human genome (3 billion nucleotides long).
Adding complexity, if additional sequences like GAGA are required, the probability drops significantly, ensuring much more controlled gene expression.

In eukaryotic cells:
- A. introns are removed from RNA transcripts of protein-coding genes.
- B. three different types of RNA polymerase are present.
- C. most genes are “off” or transcriptionally inactive.
- D. DNA is packaged into nucleosomes (chromatin is present).
- E. all of these.
Histone not in nucleosome core:
- A. H2A, B. H2B, C. H3, D. H4, E. H1.
Chromatin types:
- Open: __. Closed: ___.
Chromatin remodeling:
- A. movement/repositioning of nucleosomes.
- B. selective post-translational modification of nucleosome proteins.
- C. assembly of the enhanceosome.
- D. freeing of TATA box for transcription.
- E. all of these.
Functional consequence of DNA methylation:
- A. It changes the DNA sequence.
- B. Chromatin remodeling occurs, activating genes.
- C. It chemically marks DNA states.
- D. Selective degradation occurs due to modifications.
- E. none of the above.