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November 24, 2014

Homework Help: ap biology

Posted by Cici on Wednesday, January 4, 2012 at 5:46pm.

The first step in making a protein is transcription of a gene. This occurs in the 1._______of a eukaryotic cell. An enzyme called 2.______ carrres out the process of transcribing RNA from the DNA. It starts at a specific nucleotide sequence called a 3.______ next to the gene. RNA polymerase attaches, and the two DNA strands separate. RNA polymerase moves along one strand, and as it does, RNA 4._____________ take their places one at a time along the DNA template. They hydrogenbond with complementary bases, following the same pairing rules as in DNA--{ with G, and U (replacing T in RNA) with A. As the RNA molecule elongates, it peels away from the DNA. Finally, the enzyme reaches the 5._______. a base sequence that signals the end of the gene, and the polymerase molecule lets go of the gene and the RNA molecule. In a prokaryote, the RNA transcribed from a gene, called 6.________ (mRNA), can be used immediately in polypeptide synthesis. In a eukaryotic cell, the RNA is further modified, or 7.________ before leaving the nucleus as mRNA. Extra nucleotides are added to the ends of the transcript, and noncoding regions called 8.________ are removed. The remaining 9._________ are spliced together to from a continuous coding sequence. The finished mRNA leaves the nucleus and enters the 10.________ where translation takes place.

Translation of the "words" of the mRNA message into the 11._______ quence of a protein requires an interpreter-l2.______ (tRNA)-which links the appropriate 13._______ with each 14.______ in the mRNA message. A IRNA molecule is a folded strand of RNA. At one end, a special 15._____ attaches a specific amino acid. The other end of the IRNA molecule bears three bases called the 16.________ which is complementary to a particular mRNA codon. During the translation process, the IRNA matches its amino acid with an mRNA codon.

17._______are the "factories" where the information in mRNA is translated and polypeptide chains are constructed. A ribosome consists of protein and 18.______ (rRNA). Each ribosome has a groove that serves as a binding site for nRNA. There are two binding sites for IRNA: The P site holds the IRNA carrying the growing 19._____, while the A site holds a tRNA bearing the next amino acid.

Translation begins with initiation. An mRNA and a special 20._______ tRNA bind to the ribosome and a specific mRNA codon, the 21.________ where translation begins. The initiator IRNA generally carries the amino acid methionine (Met). Its anticodon UAC binds to the start codon. AUG. The initiator IRNA fits into the P site on the ribosome.

The next step in 22._____ synthesis is elongation-adding amino acids to the growing chain. The anticodon of an incoming IRNA, carrying its amino acid, pairs with the mRNA codon at the open A site. With help from the ribosome, the polypeptide separates from its IRNA and forms a peptide bond with the 23.______ attached to the IRNA in the A site. Then the "empty" tRNA in the P site leaves the ribosome, and the IRNA in the A site, with the polypeptide chain, is shifted to the P site. The mRNA and IRNA move as a unit, allowing the next codon to enter the A site. Another IRNA, with a complementary anticodon, brings its amino acid to the A site. Its amino acid is added to the chain, the IRNA leaves, and the complex shifts again. In this way,24._______ added to the chain, one at a time.

Finally, a 25.______ reaches the A site of the 26.________ terminating the polypeptide. A stop codon causes the polypeptide to separate from the last tRNA and the 27._______ . The polypeptide folds up, and it may join with other polypeptides to form a Iarger 28.______ molecule.

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