IB Bio - 7.2 Transcription and Gene Expression
Describe the role fo the environment of the cell or organism on gene expression.
Changes in the external or internal environment can result in changes to gene expression patterns. Chemical signals within the cell can trigger changes in levels of regulatory proteins or transcription factors in response to stimuli. This allows gene expression to change in response to alterations in intracellular and extracellular conditions.
Explain how DNA methylation regulates gene expression and analyze DNA methylation patterns in genes with high and low rates of expression.
Direct methylation of DNA can affect gene expression patterns. Increased methylation of DNA decreases gene expression patterns.
Recall the role of nucleosomes in DNA packaging and explain how they regulate which regions of DNA are transcribed.
Eukaryotic DNA is wrapped around histone proteins to form compact nucleosomes. These histone proteins have protruding tails that determine how tightly the DNA is packaged. Modification of the Histone Tails typically the histone tails have a positive charge and hence associate tightly with the negatively charged DNA. Adding an acetyl group to the tail neutralizes the charge making DNA less tightly coiled and increasing transcription. Adding a methyl group to the tail maintains the positive charge, making DNA more coiled and reducing transcription. Types of Chromatin When DNA is supercoiled and not accessible for transcription, it exists as condensed heterochromatin. When the DNA is loosely packaged and therefore accessible to to the transcription machinery, it exists as euchromatin. - Different cell types will have varying segments of DNA packaged as heterochromatin and euchromatin. - Some segments of DNA may be permanently supercoiled, while other segments may change over the life cycle of the cell.
Describe the post-transcriptional modification of mRNA, explaining how alternative splicing of mRNA increases the number of proteins an organism can make.
In eukaryotes, there are 3 post-transcriptional events that must occur in order to form mature mRNA. 1. Capping capping involves the addition of methyl group to the 5' end of the transcribed RNA. The methylated cap provides protection against degradation by exonuclease. It also allows the transcript to be recognized by the cells translational machinery. 2. Polydenylation polydenylation describes the addition of a long chain of adenine nucleotides to the 3'end of the transcript. The poly-A tail improves the stability of the RNA transcript and facilitates export from the nucleus. 3. Splicing within eukaryotic genes are non-coding sequences called introns which must be removed prior to forming mature mRNA. The coding regions are called exons and these are fused together when introns are removed to form a continuous sequence. Introns are intruding sequences whereas exons are expressing sequences. The process by which introns are removed is called splicing. Splicing can also result in the removal of exons- a process of alternative splicing. The selective removal of specific exons will result in the formation of different polypeptides from a single gene sequence.
Describe DNA transcription including the role of the promoter, the direction of transcription and the role of RNA polymerase.
The process of transcription can be divided into 3 main steps: initiation, elongation and termination. During initiation RNA polymerase binds to the promoter and causes the unwinding and separating of the DNA strands. Elongation occurs as the RNA polymerase moves along the coding sequence, synthesizing RNA in a 5' to 3' direction. Elongation occurs as the RNA polymerase moves along the coding sequence, synthesizing RNA in a 5' to 3' direction. When RNA polymerase reaches the terminator, both the enzyme and nascent RNA strand detach and the DNA rewinds. Many RNA polymerase enzymes can transcribe an RNA sequence sequentially, producing a large number of transcripts. In eukaryotes, post-transcriptional modification of the RNA sequence is necessary to form mature mRNA.
Explain how gene expression is regulated in prokaryotes and eukaryotes.
Transcriptional activity is regulated by two groups of proteins that mediate binding of RNA polymerase to the promoter. Transcriptional factors form a complex with RNA polymerase at the promoter. - RNA polymerase cannot initiate transcription without these factors hence their levels regulate gene expression Regulatory proteins bind to DNA sequences outside of the promoter and interact with the transcription factors. - Activator proteins bind to enhancer sites and increase the rate of transcription by mediating complex formation. - Repressor proteins will bind to silencer sequences and decrease the rate of transcription (by preventing complex formation) The presence of certain transcription factors or regulatory proteins may be tissue - specific. - Additionally, chemical signals (eg horomones) can moderate protein levels and hence mediate a change in gene expression. Control Elements The DNA sequence that regulatory proteins bind to are called control elements. Some control elements are located close to the promoter while others are more distant. Regulatory proteins typically bind to distal control elements whereas transcription factors usually bind to proximal elements. Most genes have multiple control elements and hence gene expression is a tightly controlled and coordinated process.
