Mastering Biology: Biotech And Gene Expression

RNA polymerase: RNA polymerase untwists a portion of the DNA double helix.

the promoter: The promoter is the region of DNA at which the process of transcription begins.

DNA: DNA is a double helix.

GTTACG CAAUGC: In RNA, uracil takes the place of thymine.

5' -> 3': Nucleotides are added to the 3' end of RNA.

RNA polymerase binds to the promoter region to begin transcription.

RNA polymerase can initiate RNA synthesis, but DNA polymerase requires a primer to initiate DNA synthesis.

modified guanine nucleotide: The 5' cap consists of a modified guanine nucleotide.

a long string of adenine nucleotides: A poly-A tail is added to the 3' end of the RNA.

snRNPs and other proteins: These are the component of spliceosomes.

exons: Exons are expressed regions.

cytoplasm: Ribosomes, the sites of translation, are found in the cytoplasm.

exons

many noncoding stretches of nucleotides are present in mRNA.

excision of introns

their presence allows exons to be shuffled.

can allow the production of proteins of different sizes from a single mRNA.

* UGAGCC Why? There are three principles to keep in mind when predicting the sequence of the mRNA produced by transcription of a particular DNA sequence. The RNA polymerase reads the sequence of DNA bases from only one of the two strands of DNA: the template strand. The RNA polymerase reads the code from the template strand in the 3' to 5' direction and thus produces the mRNA strand in the 5' to 3' direction. In RNA, the base uracil (U) replaces the DNA base thymine (T). Thus the base-pairing rules in transcription are A→U, T→A, C→G, and G→C, where the first base is the coding base in the template strand of the DNA and the second base is the base that is added to the growing mRNA strand.

* the base sequence of the gene's promoter Why? In eukaryotes, binding of RNA polymerase II to DNA involves several other proteins known as transcription factors. Many of these transcription factors bind to the DNA in the promoter region (shown below in green), located at the 3' end of the sequence on the template strand. Although some transcription factors bind to both strands of the DNA, others bind specifically to only one of the strands. Transcription factors do not bind randomly to the DNA. Information about where each transcription factor binds originates in the base sequence to which each transcription factor binds. The positioning of the transcription factors in the promoter region determines how the RNA polymerase II binds to the DNA and in which direction transcription will occur.

* A cap consisting of a modified guanine nucleotide is added to the 5' end of the pre-mRNA. * Noncoding sequences called introns are spliced out by molecular complexes called spliceosomes. * A poly-A tail (50-250 adenine nucleotides) is added to the 3' end of the pre-mRNA. Why? Once RNA polymerase II is bound to the promoter region of a gene, transcription of the template strand begins. As transcription proceeds, three key steps occur on the RNA transcript: Early in transcription, when the growing transcript is about 20 to 40 nucleotides long, a modified guanine nucleotide is added to the 5' end of the transcript, creating a 5' cap. Introns are spliced out of the RNA transcript by spliceosomes, and the exons are joined together, producing a continuous coding region. A poly-A tail (between 50 and 250 adenine nucleotides) is added to the 3' end of the RNA transcript. Only after all these steps have taken place is the mRNA complete and capable of exiting the nucleus. Once in the cytoplasm, the mRNA can participate in translation.

B

aminoacyl-tRNA synthetase: This enzyme matches a particular tRNA with a particular amino acid.

CUG: In RNA uracil replaces thymine.

initiation (of translation): The diagram illustrates the initiation of translation.

P: The initiator tRNA attaches to the ribosome's P site.

proline-threonine.