Molecular Biology of the Gene Chapter 12 (part 2) BIO1000 Dr. Mohamad H. Termos

Objectives Following this lecture, you should be able to describe: Central Dogma DNA molecules DNA Replication Covered in Unit-3 Mutations Mitosis Transcription to RNA Translation into proteins

Central Dogma The central dogma of molecular biology is the process of going from the DNA storage material into functional cellular proteins. However, this is not a direct process: RNA is an intermediate molecule. So the central dogma is

DNA and RNA DNA is located in the nucleus, but recall that proteins are assembled on ribosomes either floating in the cytoplasm or attached to the rough ER. Thus, you need a transporter molecule: RNA… RNA (ribonucleic acid) transports the info from the DNA inside the nucleus out to the site of protein synthesis. RNA molecules are similar to DNA molecules, except that they are single stranded, contain ribose sugars, and have the uracil (U) instead of thymine (T) base.

Transcription and Translation Transfer of info from DNA to RNA is called transcription. Since it is in the same “language” of nucleotides, it is considered transcribed. Transfer of info from RNA to proteins is called translation, since you are translating to a different “language”: from nucleotides to amino acids.

TCo Synthesis of RNA occurs by direct ribonucleotide base pairing with the DNA. First, the DNA double-helix separates to expose the deoxyribonucleotide template bases. RNA polymerase binds to the DNA. RNA polymerase is the enzyme that can produce RNA from reading a DNA template strand. Then, the exposed template DNA pairs with free RNA molecules.

RNA This newly made RNA has a specifically ordered sequence of A, U, G, and C bases that were copied from the DNA. The strand of the RNA that has been transcribed is what becomes mRNA. The order of the RNA letters is very important, because every 3 letters forms a codon. A codon is the code for an amino acid. Since there are 4 different kinds of bases, this means that there are 43, or 64, different codons. You can think of codons like 3-letter words, where each word has a different meaning.

Genes “code for” protiens The “triplet code” of DNA determines which amino acid will be placed in each position of the protein (note: this diagram is missing the RNA intermediate)

RNA Codons Although the RNA bases can be arranged in 64 3-letter combinations, remember that there are only 20 different amino acids found in proteins. This means that the system is redundant, and there is more than 1 codon for each amino acid. In other words, there is more than one way to say the same thing: just as “physiology is fun!” and “I love physiology!” have the same basic meaning

Translation Translation is the process of reading the RNA transcript and creating the new protein chain of amino acids. Remember, the cell organelle we use to do this is called ribosome. Ribosomes are either out in the cytoplasm, or attached to the rough ER. Up until this point, while we have been transcribing DNA into RNA, we have remained in the nucleus. Now we must shuttle the RNA out of the nucleus and bring it to a ribosome.

Ribosome, or rRNA is a 2-subunit system that translates the nucleotide “language” of mRNA into the amino acid “langauge” of proteins. The large ribosomal subunit has two pockets: the A-site and the P-site. Each can hold an amino acid. The ribosome binds amino acids together by means of peptide bonds. However, the ribosome cannot do translation by itself, it needs help. You see, ribosomes can recognize the codons on the RNA strand, and it knows what amino acid to put there, but the ribosome cannot actually grab the amino acid and bring it in: this is where tRNA comes in… Translation

tRNA tRNA or transfer RNA is a special kind of RNA strand that has an anticodon on one end, and an amino acid attachment site on the other end. The anticodon portion base pairs to the codon on the mRNA. The other end is attached to the appropriate amino acid. So the tRNA has become the link between the 3-nucleotide codon and the amino acid.

Translation Now we are ready to make a protein: First, the mRNA gets to the ribosome Next, a tRNA brings the amino acid that corresponds to the codon on the ribosome Now, we have to attach the amino acids together. This is where the ribosome goes into action: the ribosome can create the bond between two amino acids, thereby linking them into a protein chain.

Translation tRNA’s will keep bringing in amino acids, and the ribosome will keep linking them together. But eventually, we will need to end the amino acid sequence. Well, the end is also coded for by a codon on the mRNA: there are “stop codons” that tell the ribosomes and tRNA to quit linking amino acids, and to just let go. At this time, the new protein chain, ribosome, mRNA, and tRNA’s all let go and go their separate ways…

Control of Protein Synthesis So you may now be wondering why all our genes are not always transcribed and translated. Who controls this process? After all, the ribosomes don’t have a brain and can’t really say NO! to an mRNA that wants to be translated… Well, most of the control happens at the DNA transcription level. In front of each gene, there is a controller, kind of like a gatekeeper. This gatekeeper can be influenced by activators and repressors (transcription factors), and can increase or decrease the transcription rate of RNA accordingly.