Chapter10_SSM - 65781_CH10_198_221.qxd 8/1/08 1:01 PM Page...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
Chapter 10: Molecular Biology of Gene Expression Chapter Summary The normal flow of information from a gene to its product is from DNA to RNA to protein. The properties of the different protein products of genes are determined by the sequence of amino acids of the polypeptide chain and by the way in which the chain is folded. Each protein-coding gene is usually responsible for the synthesis of a single polypeptide chain. Gene expression begins with the enzymatic synthesis of an RNA molecule complementary to one strand of the DNA. This process is called transcription, and it is carried out by the enzyme RNA polymerase. This enzyme joins ribonucleoside triphosphates by the same chemical reaction used in DNA synthesis. RNA polymerase differs from DNA polymerase in that a primer is not needed to initiate synthesis. Transcription is initiated when RNA polymerase binds to a promoter sequence. The product of transcription is the pri- mary transcript. In prokaryotes, the primary transcript is used directly as a messenger RNA (mRNA) in polypeptide synthesis. In eukaryotes, the primary transcript is processed into mRNA: Noncoding sequences (introns) are removed, the exons are spliced together, and the termini are modified by formation of a 5 cap and usually by addition of a poly-A tail at the 3 end. After mRNA is formed, polypeptide chains are synthesized by translation of the mRNA molecule. Translation includes initiation, elongation, and release of the polypeptide chain. In eukaryotes, the initiation complex scans from the 5 cap to locate the first AUG; in prokaryotes, the initiation complex forms at any ribosome-binding site in the mRNA. In elongation, the process of translocation ratchets the 40S subunit along the mRNA as each new peptide bond is formed at the tRNA-binding site called P. Then the 40S sub- unit moves one notch (codon) farther along the mRNA, the 60S subunit moves up to align with the 40S subunit, the used tRNA is ejected from the E site, and the next charged tRNA moves into line at the A site. Elongation makes possible the successive reading of the nucleotide sequence of an mRNA molecule in groups of three nucleotides (the codons). When a stop codon is encountered, one or more release factors bind with the ribosome and cause release of the finished polypeptide and dissociation of the ribosome from the mRNA. 198
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
199 Several ribosomes can translate an mRNA molecule simultaneously, forming a polysome. In prokaryotes, translation often begins before synthesis of mRNA is completed (coupled transcription–translation); in eukaryotes, this does not occur because mRNA is made in the nucleus, whereas the ribosomes are located in the cytoplasm. Prokaryotic mRNA molecules are often polycistronic, encoding several different polypeptides.
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 09/29/2011 for the course GENETICS 380 taught by Professor Glodowski during the Spring '08 term at Rutgers.

Page1 / 24

Chapter10_SSM - 65781_CH10_198_221.qxd 8/1/08 1:01 PM Page...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online