NOTES FOR CH. 13

13.1 RNA

Lesson Objectives

Contrast RNA and DNA.

Explain the process of transcription.

Lesson Summary

The Role of RNA RNA (ribonucleic acid) is a nucleic acid like DNA. It consists of a long chain of nucleotides. The RNA base sequence directs the production of proteins. Ultimately, cell proteins result in phenotypic traits. The main differences between RNA and DNA are:

The sugar in RNA is ribose instead of deoxyribose.

RNA is generally single-stranded and not double-stranded like DNA.

RNA contains uracil in place of thymine.

RNA can be thought of as a disposable copy of a segment of DNA. Most RNA molecules are involved in protein synthesis. The three main types of RNA are:

Messenger RNA (mRNA) carries copies of instructions for polypeptide synthesis from the nucleus to ribosomes in the cytoplasm.

Ribosomal RNA (rRNA) forms an important part of both subunits of the ribosomes, the cell structures where proteins are assembled.

Transfer RNA (tRNA) carries amino acids to the ribosome and matches them to the coded mRNA message.

RNA Synthesis Most of the work of making RNA takes place during transcription. In transcription, segments of DNA serve as templates to produce complementary RNA molecules. In prokaryotes, RNA synthesis and protein synthesis takes place in the cytoplasm. In eukaryotes, RNA is produced in the cell’s nucleus and then moves to the cytoplasm to play a role in the production of protein. The following focuses on transcription in eukaryotic cells.

The enzyme RNA polymerase binds to DNA during transcription and separates the DNA strands. It then uses one strand of DNA as a template from which to assemble nucleotides into a complementary strand of RNA.

RNA polymerase binds only to promoters, regions of DNA that have specific base sequences. Promoters are signals to the DNA molecule that show RNA polymerase exactly where to begin making RNA. Similar signals cause transcription to stop when a new RNA molecule is completed.

RNA may be “edited” before it is used. Portions that are cut out and discarded are called introns. The remaining pieces, known as exons, are then spliced back together to form the final mRNA.

13.2 Ribosomes and Protein Synthesis

Lesson Objectives

Identify the genetic code and explain how it is read.

Summarize the process of translation.

Lesson Summary

The Genetic Code A specific sequence of bases in DNA carries the directions for forming a polypeptide, a chain of amino acids. The types and order of amino acids in a polypeptide determine the properties of the protein. The sequence of bases in mRNA is the genetic code. The four bases, A, C, G, and U, act as “letters.”

The code is read three “letters” at a time, so that each “word” is three bases long and corresponds to a single amino acid. Each three-letter “word” in mRNA is known as a codon.

Some codons serve as “start” and “stop” signals for protein synthesis.

Translation Ribosomes use the sequence of codons in mRNA to assemble amino acids into polypeptide chains. The process of decoding of an mRNA message into a protein is translation.

Messenger RNA is transcribed in the nucleus and then enters the cytoplasm.

On the ribosome, translation begins at the start codon. Each codon attracts an anticodon, the complementary sequence of bases on tRNA.

Each tRNA carries one kind of amino acid. The match between the codon and anticodon ensures that the correct amino acid is added to the growing chain.

The amino acids bond together, each in turn. The ribosome moves along the mRNA, exposing codons that attract still more tRNAs with their attached amino acids.

The process concludes when a “stop code” is reached. The newly formed polypeptide and the mRNA molecule are released from the ribosome.