Gene Expression

• The regulatory gene of the lac operon produces an mRNA that produces a repressor protein, which can bind to the operator of the lac operon. The general term for the product of a regulatory gene is a regulatory
• protein. The Lac regulatory protein is called a repressor because it keeps RNA polymerase from transcribing the structural genes. Thus the
• repressor inhibits transcription of the lac operon.

• When lactose is present, it binds to the repressor and changes its shape. As a result of this change, the repressor can no longer bind to the
• operator region. RNA polymerase can then bind to the promoter and transcribe the lac genes.

• DNA wrapped around protein molecules (histones) makes a nucleosome
• Nucleosomes make up chromosomes

• A regulatory protein that binds to DNA and stimulates transcription of specific genes. Some transcription factors, called activators, bind to enhancer regions some distance from the gene. The DNA bends, bringing the activators closer to the gene, where the activators interact with other transcription factors and RNA polymerase
• to initiate transcription.

A DNA segment containing multiple control elements that may be located far away from the gene it regulates.

By regulating several processes that happen after transcription. In RNA processing, a cap and tail are added, introns removed, and remaining exons spliced together. Alternative splicing of RNA may create different mRNA molecules from the same primary RNA transcript.

The repression of the lac operon is relieved in the presence of glucose if excess cAMP is added. As the level of glucose in the medium falls, the level of cAMP increases. Simultaneously there is an increase in inducer binding to the lac repressor. The net result is an increase in transcription from the operon.

• The following describes the situation where there is lactose but no glucose available to the cell:
• No glucose means high levels of cAMP.
• cAMP binds to a molecule known as CAP.
• CAP, when in association with cAMP, can bind to the promoter at the CAP binding site.
• Here, the cAMP-CAP complex stimulates transcription by helping RNA polymerase bind to the promoter.
• RNA polymerase has a weak affinity for the Lac promoter and will not bind without this help.

• With glucose present, there is very little or no cAMP.
• It cannot bind to the CAP binding site.
• Without this complex, RNA polymerase cannot bind to the promoter and transcription cannot occur.
• Even though allolactose is present and blocks the
• action of the repressor, there is no transcription of the lac genes
• because glucose is present.

• With lactose present so is allolactose.
• Allolactose binds to the repressor and prevents it from binding to the operator.
• Therefore, transcription and translation of the genes can occur.

As an mRNA strand slides though a ribosome, triplets of RNA bases spell out the amino acid sequence of a polypeptide. It is the job of transfer RNA molecules to match RNA bases with the correct amino acids.

A specialized base triplet at one end of a tRNA molecule that recognizes a particular complementary codon on an mRNA molecule.

• One of a ribosome’s three binding sites for tRNA during translation.
• The P site holds the tRNA carrying the growing polypeptide chain. (P
• stands for peptidyl tRNA.)

• Modification of RNA before it leaves the nucleus, a process unique to eukaryotes.
• RNA processing begins with alteration of the ends of the RNA. A 5' cap consisting of a modified guanine nucleotide is added to the 5' end. A poly-A tail made of 30 to 200 adenine nucleotides is added to the 3' end. The processing continues with the removal of portions of the coding segment that do not actually code for protein. This process is carried out by small nuclear ribonucleoproteins— snRNPs, which join with other proteins to form large structures called spliceosomes.

A complex assembly that interacts with the ends of an RNA intron in splicing RNA, releasing the intron and joining the two adjacent exons

Upon encountering the RNA primer, DNA pol III falls off the DNA strand and is replaced by DNA pol I.