Biology: Chap. 11

Control of Gene Expression

Overview of Control Points

• Transcription: Does RNA get made at all?

• Post-transcriptional modification: modifying the RNA after it is created.

• Translation

• Post-translational modification: Modifying the protein after it is created.

• Control of transcription is the best characterized and most understood due to the ability to perform it in vitro (in reaction tubes on a lab bench).

• The other three control points must happen inside the cell, making them more difficult to study.

Control of DNA Structure

• A meter's worth of DNA must fit inside the tiny nucleus of a cell.
• DNA is tightly wound around proteins called histones.
• DNA wraps around clusters of histone proteins.
• Clusters of histones come together to form larger assemblages, further compacting the DNA.
• The degree of DNA winding affects enzyme access and transcription.

Heterochromatin

• Tightly wrapped DNA is called heterochromatin.
• RNA polymerase cannot access the promoter, leading to less transcription.

Euchromatin

• Loosely wound DNA is called euchromatin.
• Allows more transcription to occur.

Control Mechanisms

• Cells can control whether a region of DNA is heterochromatin or euchromatin.

DNA Methylation

• An enzyme adds a methyl group (CH_3) to cytosine nucleotides.
• RNA polymerases still read methylated cytosine as cytosine.
• More methylated DNA tends to become heterochromatin.

Epigenetics and Inheritance

• In a zygote, none of the DNA is methylated, methylation patterns get created as cells develop.
• As cells divide, different lineages acquire methylation in specific areas of their chromosomes.
• Methylated areas tend to become heterochromatin, reducing gene expression in those regions.
• Once a methylation pattern is fixed, it is inherited by all descendant cells.
• Inherited changes in methylation patterns are referred to as epigenetics.
• Environmental effects can sometimes alter methylation patterns.