Transcription and Translation

RNA DNA stores and transmits the information needed to make proteins, but it does not actually use that information to synthesize proteins— RNA, ribonucleic acid, does this.

Structure of RNA Like DNA, RNA is made of nucleotides. Differs from DNA in three ways. 1. An RNA molecule consists of only one strand of nucleotides instead of two strands. 2. RNA has ribose as its five-carbon sugar rather than deoxyribose. 3. RNA has the nitrogen base uracil instead of thymine.

Structure of RNA RNA exists in three forms—each has a different role in protein synthesis. 1. Messenger RNA (mRNA)—a single uncoiled strand that transmits information from DNA during protein synthesis. Serves as a template for the assembly of amino acids. 2. Transfer RNA (tRNA)—single strand of RNA folded back on itself—which allows complimentary bases to pair. Has 20 or more varieties—each bonds to only one specific type of amino acid. 3. Ribosomal RNA (rRNA)—RNA in globular form— major component of the ribosomes.

Transcription RNA is produced from DNA through a process called transcription. 1. An enzyme called RNA polymerase binds to a DNA molecule and causes the separation the complementary strands of DNA. 2. The enzyme directs the formation of hydrogen bonds between the bases of a DNA strand and the complementary bases of the RNA nucleotides that are floating in the nucleus. 3. RNA polymerase then moves along the section of DNA to create bonds between the sugar and phosphate sections of the RNA nucleotides. 4. RNA polymerase reaches the sequences of bases on the DNA that trigger a stop signal; the newly created RNA is released.

Transcription All three types of RNA are transcribed in this manner. RNA will now move from the nucleus into the cytoplasm where it will be involved in the synthesis of proteins.

Protein Synthesis Protein synthesis is the formation of proteins using information coded on DNA and carried by RNA.

Protein Structure Organisms may have from several hundred to several thousand proteins as part of its make up. Proteins are made from polypeptides—a group of amino acids linked together. 1. There are twenty different amino acids. 2. A protein may be made up of hundreds or thousands of amino acids arranged in a particular sequence.

Codons The assembly of amino acids into a specific protein is the task accomplished during protein synthesis. Genetic code is the system that contains information needed by cells for proper functioning.

Codons 1. DNA contains the information needed to put amino acids together in a proper sequence. 2. The code in the DNA is reflected in the bases of the mRNA 3. A specific group of three bases of mRNA is called a codon. a. Each codon codes for a specific amino acid using tRNA. b. There is 64 possible codons

Codons 1. Each amino acid has several codons—usually these differ from each other only the base in the third position. 2. Some codons do not encode amino acids at all— instead, they are start and stop signals—telling the ribosome to start or stop reading the mRNA a. Many codons are universal—all organisms use those codes. EXAMPLES: 1. UUU—codon for phenylalanine 2. AUG—codon that is a start code

Translation 1. The process of assembling protein molecules from information encoded in mRNA is called translation. A. Translation begins when the mRNA moves out of the nucleus. B. The mRNA then migrates to a group of ribosomes where the actual synthesis of proteins takes place.

Translation C. The tRNA transfers amino acids that are floating freely in the cytoplasm to the ribosomes. 1. Each kind of tRNA has a region than bonds to a specific amino acid. 2. On the opposite end of the tRNA is an anticodon a sequence of three bases that are complementary to the codon of mRNA

Translation D. The assembly begins when a ribosome attaches at an AUG codon on the mRNA and the AUG codes with the anticodon UAC on a tRNA. E. Usually several ribosomes are translating at the same time on the same mRNA. F. As the ribosomes move along the strand of mRNA and the codon is paired with the anticodon of the tRNA, the specified amino acid is added to the chain.

Translation G. Process continues with each amino acid being linked to the chain in turn. H. Eventually, the ribosome reaches a stop codon—which brings the process to an end. 1. The mRNA is released 2. Polypeptide is complete