Topic 7.2: Transcription and Gene Expression Flashcards
Gene
Sequence of DNA which is transcribed into RNA
Sections of a Gene
1) Promoter (transcription initiation site)
2) Coding sequence (the region transcribed)
3) Terminator (transcription termination site)
DNA Double strand and transcription
1) The antisense strand is transcribed into RNA (complementary)
2) The sense strand is not transcribed into RNA
(identical)
Transcription
Process by which a DNA sequence is copied into a complementary RNA sequence by RNA polymerase
Steps in transcription
1) Initiation
2) Elongation
3) Termination
Initiation
1) RNA polymerase binds to a promoter and unwinds DNA
2) It breaks the H bonds between complementary bases
3) Nucleoside triphosphates bind to complementary bases
Elongation
1) RNA polymerase covalently joins the nucleotides together. The two extra phosphates are released (provides energy)
2) Transcription occurs in a 5’ → 3’ direction (antisense strand)
Termination
At the terminator site, RNA polymerase is detached and the RNA sequence is released (and the DNA rewinds).
Modifications of mRNA after transcription in eukaryotic cells (3)
1) Capping
2) Polyadenylation
3) Splicing
Capping
1) Addition of a methyl group to the 5’-end of the transcribed RNA
2) The methylated cap provides protection against degradation by exonucleases
3) Allows the transcript to be recognized by the cell’s translational machinery
Polyadenylation
1) Addition of a long chain of adenine nucleotides (a poly-A tail) to the 3’-end of the transcript
2) The poly-A tail improves the stability of the RNA transcript and facilitates its export from the nucleus
Splicing
Non-coding regions within genes are removed
Introns
Non-coding regions in genes
Exons
Coding regions of genes
Alternative Splicing
Exons can be selectively removed to form different proteins from the same gene
Regulators of transcription activity
1) Transcription factors
2) Regulatory proteins
Transcription factors
Form a complex with RNA polymerase at the promoter
Without these factors RNA Pol cannot initiate transcription
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
Repressor proteins
Bind to silencer sequences and decrease the rate of transcription
The environment of a cell has an impact on gene expression
Chemical signals within the cell can trigger changes in levels of regulatory proteins or transcription factors in response to stimuli
Examples of organisms changing their gene expression patterns in response to environmental changes
1) Hydrangeas change colour depending on the pH of the soil
2) The Himalayan rabbit produces a different fur pigment depending on the temperature
3) Humans produce different amounts of melanin depending on light exposure
Nucleosomes and Transcription Regulation in Eukaryotes
Histones proteins have protruding tails that determine how tightly the DNA is packaged within nucleosomes
Acetylation and DNA package
Makes DNA less tightly packed
Methylation and DNA package
Makes DNA more tightly packed
Types of Chromatin
1) Heterochromatin (Supercoiled | No transcription)
2) Euchromatin (Loosely packed | Transcription)
DNA Methylation
1) Increased methylation of DNA decreases gene expression
2) Genes that are not transcribed tend to exhibit more DNA methylation than genes that are actively transcribed
3) Environmental factors (e.g. diet, pathogen exposure, etc.) influences DNA methylation.
