BIO151 CH. 15

Discuss the role of promoters in prokaryotic transcription

A promoter is a DNA sequence onto which the transcription machinery, including RNA polymerase, binds and initiates transcription. In most cases, promoters exist upstream of the genes they regulate. The specific sequence of a promoter is very important because it determines whether the corresponding gene is transcribed all the time, some of the time, or infrequently.

Describe the different steps in RNA processing

Addition of a 5' cap to the beginning of the RNA. Addition of a poly-A tail (tail of A nucleotides) to the end of the RNA. Chopping out of introns, or "junk" sequences, and pasting together of the remaining, good sequences (exons)

Describe how and when transcription is terminated

Depending on the gene being transcribed, there are two kinds of termination signals. One is protein-based and the other is RNA-based. Rho-dependent termination is controlled by the rho protein, which tracks along behind the polymerase on the growing mRNA chain. Rho-independent termination is controlled by specific sequences in the DNA template strand.

Understand the significance of exons, introns, and splicing for mRNAs

Eukaryotic genes are composed of exons, which correspond to protein-coding sequences (ex-on signifies that they are expressed), and intervening sequences called introns (int-ron denotes their intervening role), which may be involved in gene regulation but are removed from the pre-mRNA during processing. Intron sequences in mRNA do not encode functional proteins.

List the different steps in prokaryotic transcription

Initiation Prokaryotic RNA Polymerase Prokaryotic Promoters Elongation and Termination Termination Signals

List the steps in eukaryotic transcription

Initiation Three Eukaryotic RNA Polymerases RNA Polymerase II Promoters and Transcription factors Promoter Structures for RNA Polymerases I and III Elongation and Termination

Compare and contrast the three RNA polymerases

RNA polymerase I synthesizes all of the rRNAs from the tandemly duplicated set of 18S, 5.8S, and 28S ribosomal genes. RNA polymerase II is located in the nucleus and synthesizes all protein-coding nuclear pre-mRNAs. RNA polymerase III is also located in the nucleus. This polymerase transcribes a variety of structural RNAs that includes the 5S pre-rRNA, transfer pre-RNAs (pre-tRNAs), and small nuclear pre-RNAs.

Discuss the role of ribosomes in protein synthesis

The small subunit is responsible for binding the mRNA template, whereas the large subunit sequentially binds tRNAs. Each mRNA molecule is simultaneously translated by many ribosomes, all synthesizing protein in the same direction: reading the mRNA from 5′ to 3′ and synthesizing the polypeptide from the N terminus to the C terminus. The complete mRNA/poly-ribosome structure is called a polysome.

Explain how tRNAs and rRNAs are processed

The tRNAs and rRNAs are structural molecules that have roles in protein synthesis; however, these RNAs are not themselves translated. Pre-rRNAs are transcribed, processed, and assembled into ribosomes in the nucleolus. Pre-tRNAs are transcribed and processed in the nucleus and then released into the cytoplasm where they are linked to free amino acids for protein synthesis.

Explain the significance of transcription factors

Transcription factors that bind to the promoter are called basal transcription factors. The other transcription factors systematically fall into place on the DNA template, with each one further stabilizing the pre-initiation complex and contributing to the recruitment of RNA polymerase II.

Describe the different steps in protein synthesis

Translation requires the input of an mRNA template, ribosomes, tRNAs, and various enzymatic factors

Discuss the role of RNA polymerases in transcription

mRNA synthesis