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How are genes expressed in eukaryotic cell?
Genes are composed of DNA and RNA sequences called introns and exons. The exons carry instructions needed to make a protein - the coding part and are expressed. Introns contain sections of DNA/RNA that are not required - the non coding part and are not expressed. Splicing removes the introns before translation of DNA occurs.
How are genes expressed in prokaryotic cells?
Genes are contolled by an operon, where several genes are under one regulatory control. An operon is comprised of a promotor, strutural genes and the operator.
State 2 other modifications to the DNA sequence which occur during transcription before it can complete translation on ribosomes.
Addition of 5' cap on the mRNA
Addition of poly (A) tail at the 3' end of the mRNA
Describe the role of the repressor protein.
Binds to the operator. This prevents the RNA polymerase transcribing. The RNA polymerase can't continue to move forward on the DNA.
Describe the role of the inducer.
The inducer molecule lactose (allolactose) binds to the repressor inactivating it, which frees the operator. RNA polymerase can then transcribe genes.
Describe the role of the promotor
Binding site for RNA polymerase
Describe the role of the operator
Sequence of nucleotides located between the promotor and the structural genes that can be activated or inactivated in gene regulation.
When a repressor is bound to the operator it obstructs the promotor. What is the outcome of this?
This blocks RNA polymerase from binding to the DNA molecule and effectivlely switches the gene off. Or if already bound, prevents its movement along the molecule. The result is the same - no mRNA transcription.
What does RNA polymerase enzyme do?
Unzips the DNA strands
During the elongation phase of DNA transcription, one strand of DNA is the template or antisense strand. What does this mean?
The antisense or template strand acts as a template for RNA polymerase. It is the non coding DNA strand of a gene. A cell uses antisense DNA strand as a template for producing mRNA that is resposible for synthesis of a protein. When replicated Thymine nuleotides are replaced with Uracil.
What happens to the sense or coding strand of DNA during transcription?
This is the segment of DNA strand which carries the translatable code in the 5' to 3' direction. It does not have the RNA polymerase moving along it but protects exposed regions and it is complimentary to the antisense or template strand of DNA.
Describe the role of the enhancers.
Enhancers speed up transcription and make it occur faster and/or more often.
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