Unit 6: Gene Expression and Regulation

Three useful Terms to know for this Unit


Helicase unwinds the DNA so replication can occur. This is the first step for DNA replication and it is a crucial step because if the two pieces of DNA strands are not separated they can not be copied/ Duplicated. When Helicase unwinds the DNA it creates the Replication fork, that is where you can usually find helicase and is where DNA replication occurs. Helicase unwinds the DNA by breaking the hydrogen bonds that go down the center of the two interlocked strands. Without helicase, DNA replication would come to a halt. Of course we need to replicate DNA so that is why this enzyme is so important for every organism.


Transcription is the most common control point of gene expression for all organisms. It is the first step in gene expression. Transcription is done by enzymes called RNA Polymerases (as you can see in the picture below). RNA polymerases link nucleotides to form an RNA strand from the DNA template strand. Transcription has 3 stages; initiation, elongation, and termination. In initiation the RNA polymerase binds to the promoter (a sequence of DNA) . Then the RNA polymerase separates the DNA strands and they use one of the single strands as the DNA template. Next is Elongation; RNA polymerase reads the template strand and builds the complementary RNA nucleotides that will form a chain from 5’ to 3’. The last stage is Termination, in this stage sequences called terminators signal that the RNA transcript is complete. When they are transcribed they allow the RNA transcript to be released from the RNA polymerase. By the end of transcription you have a function product such as a protein. The purpose of transcription is to create an mRNA copy of a gene so the genetic information can leave the nucleus and be used to assemble a protein.

Nonsense Mutation:

A nonsense mutation is when the change of a single nucleotide causes a stop codon to be formed. This causes the protein that is being formed with amino acids to stop its complete formation, creating a shortened protein. Often these proteins are completely nonfunctional because they do not have enough coding. Since this mutation often leads to non functional proteins it is more harmful than the average mutation with a single nucleotide. Mutations are more common than I originally thought, most of them go unnoticed because they are silent mutations that have no visible effect although nonsense mutations often have more observable results. All mutations increase with age because we are exposed to more carcinogens the longer we are alive.

About Mr. Mohn

Biology Teacher

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