{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

ch03.4 - Body Tissues - Chapter 3 Cells and Tissues...

Info icon This preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
molecules. Ribosomal RNA (rRNA) helps form the ribosomes, where proteins are built. Messenger RNA (mRNA) molecules are long, single nu- cleotide strands that resemble half of a DNA mole- cule and carry the “message” containing instructions for protein synthesis from the DNA gene in the nu- cleus to the ribosomes in the cytoplasm. Protein synthesis involves two major phases: transcription, when complementary mRNA is made at the DNA gene, and translation, when the information carried in mRNA molecules is “decoded” and used to assemble proteins. These steps are summarized simply in Figure 3.16, and described in more detail next. Transcription The word transcription often refers to one of the jobs done by a secretary converting notes from one form (shorthand notes or an audiotape record- ing) into another form (a typewritten letter, for ex- ample). In other words, the same information is transformed from one form or format to another. In cells, transcription involves the transfer of information from DNA’s base sequence into the complementary base sequence of mRNA (Figure 3.16, step 1). Only DNA and mRNA are involved in transcription. Whereas each three-base sequence specifying a particular amino acid on the DNA gene is called a triplet, the corresponding three- base sequences on mRNA are called codons. The form is different, but the same information is being conveyed. Thus, if the (partial) sequence of DNA triplets is AAT-CGT-TCG, the related codons on mRNA would be UUA-GCA-AGC. Translation A translator takes words in one language and re- states them in another language. In the trans- lation phase of protein synthesis, the language of nucleic acids (base sequence) is “translated” into the language of proteins (amino acid sequence). Translation occurs in the cytoplasm and involves three major varieties of RNA. As illustrated in Figure 3.16, steps 2–5, translation consists of the following events. Once the mRNA attaches to the ribosome (step 2), tRNA comes into the picture. Its job is to transfer, or ferry, amino acids to the ribo- some, where they are bound together by enzymes in the exact sequence specified by the gene (and its mRNA). There are about 45 common types of tRNAs, each capable of carrying one of the 20 or so common types of amino acid to the protein syn- thesis sites. But that is not the only job of the tiny tRNAs. They also have to recognize the mRNA codons “calling for” the amino acid they are toting. They can do this because they have a special three-base sequence called an anticodon on their “head” that can bind to the complementary codons (step 3). Once the first tRNA has maneuvered itself into the correct position at the beginning of the mRNA message, the ribosome moves the mRNA strand along, bringing the next codon into position to be read by another tRNA. As amino acids are brought to their proper positions along the length of mRNA, they are joined together by enzymes (step 4). As an amino acid is bonded to the chain, its tRNA is released and moves away from the ribo- some to pick up another amino acid (step 5).
Image of page 1

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

View Full Document Right Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern