Why is transcription before translation

The pre-messenger RNA thus formed contains introns which are not required for protein synthesis. In alternative splicing, individual exons are either spliced or included, giving rise to several different possible mRNA products. Each mRNA product codes for a different protein isoform; these protein isoforms differ in their peptide sequence and therefore their biological activity.

Several different mechanisms of alternative splicing are known, two of which are illustrated in Figure 6. Alternative splicing contributes to protein diversity - a single gene transcript RNA can have thousands of different splicing patterns, and will therefore code for thousands of different proteins: a diverse proteome is generated from a relatively limited genome.

Splicing is important in genetic regulation alteration of the splicing pattern in response to cellular conditions changes protein expression. Perhaps not surprisingly, abnormal splicing patterns can lead to disease states including cancer. This process, catalyzed by reverse transcriptase enzymes, allows retroviruses, including the human immunodeficiency virus HIV , to use RNA as their genetic material. The mRNA formed in transcription is transported out of the nucleus, into the cytoplasm, to the ribosome the cell's protein synthesis factory.

Here, it directs protein synthesis. The ribosome is a very large complex of RNA and protein molecules. Each three-base stretch of mRNA triplet is known as a codon , and one codon contains the information for a specific amino acid. This tRNA molecule carries an amino acid at its 3'-terminus, which is incorporated into the growing protein chain.

The tRNA is then expelled from the ribosome. Figure 7 shows the steps involved in protein synthesis. Transfer RNA adopts a well defined tertiary structure which is normally represented in two dimensions as a cloverleaf shape, as in Figure 7.

The structure of tRNA is shown in more detail in Figure 8. The reaction of esters with amines is generally favourable but the rate of reaction is increased greatly in the ribosome. Each transfer RNA molecule has a well defined tertiary structure that is recognized by the enzyme aminoacyl tRNA synthetase, which adds the correct amino acid to the 3'-end of the uncharged tRNA.

The presence of modified nucleosides is important in stabilizing the tRNA structure. Some of these modifications are shown in Figure The genetic code is almost universal.

It is the basis of the transmission of hereditary information by nucleic acids in all organisms. In theory only 22 codes are required: one for each of the 20 naturally occurring amino acids, with the addition of a start codon and a stop codon to indicate the beginning and end of a protein sequence. Many amino acids have several codes degeneracy , so that all 64 possible triplet codes are used. For example Arg and Ser each have 6 codons whereas Trp and Met have only one. In this period I have to teach them all of biology they need for their non-science majors, plus leave enough time for each student to give a presentation on the science of their favorite plant and animal and for two exams.

Thus I have to strip the lectures to the bare bones, and hope that those bare bones are what non-science majors really need to know: concepts rather than factoids, relationship with the rest of their lives rather than relationship with the other sciences.

Thus I follow my lectures with videos and classroom discussions, and their homework consists of finding cool biology videos or articles and posting the links on the classroom blog for all to see. A couple of times I used malaria as a thread that connected all the topics - from cell biology to ecology to physiology to evolution. I think that worked well but it is hard to do.

They also write a final paper on some aspect of physiology. Another new development is that the administration has realized that most of the faculty have been with the school for many years. We are experienced, and apparently we know what we are doing. Thus they recently gave us much more freedom to design our own syllabus instead of following a pre-defined one, as long as the ultimate goals of the class remain the same.

I am also worried that, since I am not actively doing research in the lab and thus not following the literature as closely, that some of the things I teach are now out-dated. Not that anyone can possibly keep up with all the advances in all the areas of Biology which is so huge, but at least big updates that affect teaching of introductory courses are stuff I need to know.

I need to catch up and upgrade my lecture notes. And what better way than crowdsource! So, over the new few weeks, I will re-post my old lecture notes note that they are just intros - discussions and videos etc.

If I got something wrong or something is out of date, let me know but don't push just your own preferred hypothesis if a question is not yet settled - give me the entire controversy explanation instead. If something is glaringly missing, let me know. If something can be said in a nicer language - edit my sentences.

If you are aware of cool images, articles, blog-posts, videos, podcasts, visualizations, animations, games, etc. And at the end, once we do this with all the lectures, let's discuss the overall syllabus - is there a better way to organize all this material for such a fast-paced class. See the previous lectures:. Biology and the Scientific Method. BIO - Cell Structure. Here is the third BIO lecture from May 08, Again, I'd appreciate comments on the correctness as well as suggestions for improvement.

DNA is a long double-stranded molecule residing inside the nucleus of every cell. It is usually tightly coiled forming chromosomes in which it is protected by proteins. Each of the two strands of the DNA molecule is a chain of smaller molecules. Each link in the chain is composed of one sugar molecule, one phosphate molecule and one nucleotide molecule.

The two strands of DNA are structured in such a way that an adenine on one strand is always attached to a thymine on the other strand, and the guanine of one strand is always bound to cytosine on the other strand. Thus, the two strands of the DNA molecule are mirror-images of each other. The exact sequence of nucleotides of all of the DNA on all the chromosomes is the genome. Each cell in the body has exactly the same chromosomes and exactly the same genome with some exceptions we will cover later.

In prokaryotes and eukaryotes, the basics of elongation of translation are the same. The P peptidyl site binds charged tRNAs carrying amino acids that have formed peptide bonds with the growing polypeptide chain but have not yet dissociated from their corresponding tRNA. The E exit site releases dissociated tRNAs so that they can be recharged with free amino acids. Elongation proceeds with single-codon movements of the ribosome each called a translocation event.

During each translocation event, the charged tRNAs enter at the A site, then shift to the P site, and then finally to the E site for removal. Peptide bonds form between the amino group of the amino acid attached to the A-site tRNA and the carboxyl group of the amino acid attached to the P-site tRNA. The formation of each peptide bond is catalyzed by peptidyl transferase , an RNA-based ribozyme that is integrated into the 50S ribosomal subunit. The amino acid bound to the P-site tRNA is also linked to the growing polypeptide chain.

Several of the steps during elongation, including binding of a charged aminoacyl tRNA to the A site and translocation, require energy derived from GTP hydrolysis, which is catalyzed by specific elongation factors. Amazingly, the E. On aligning with the A site, these nonsense codons are recognized by release factors in prokaryotes and eukaryotes that result in the P-site amino acid detaching from its tRNA, releasing the newly made polypeptide.

The small and large ribosomal subunits dissociate from the mRNA and from each other; they are recruited almost immediately into another translation init iation complex. In summary, there are several key features that distinguish prokaryotic gene expression from that seen in eukaryotes. These are illustrated in Figure 6 and listed in Table 1. Figure 6. Post-translational modifications include:. During elongation in translation, to which ribosomal site does an incoming charged tRNA molecule bind?

Which of the following is the amino acid that appears at the N-terminus of all newly translated prokaryotic and eukaryotic polypeptides? Skip to main content. Mechanisms of Microbial Genetics. Search for:. Protein Synthesis Translation Learning Objectives Describe the genetic code and explain why it is considered almost universal Explain the process of translation and the functions of the molecular machinery of translation Compare translation in eukaryotes and prokaryotes.

Think about It How many bases are in each codon? What amino acid is coded for by the codon AAU? What happens when a stop codon is reached? Think about It Describe the structure and composition of the prokaryotic ribosome.

In what direction is the mRNA template read? Describe the structure and function of a tRNA. Think about It What are the components of the initiation complex for translation in prokaryotes? What are two differences between initiation of prokaryotic and eukaryotic translation? What occurs at each of the three active sites of the ribosome? What causes termination of translation? The genetic code is degenerate in that several mRNA codons code for the same amino acids. The genetic code is almost universal among living organisms.

Prokaryotic 70S and cytoplasmic eukaryotic 80S ribosomes are each composed of a large subunit and a small subunit of differing sizes between the two groups. Each subunit is composed of rRNA and protein. Organelle ribosomes in eukaryotic cells resemble prokaryotic ribosomes. Some 60 to 90 species of tRNA exist in bacteria. Each tRNA has a three-nucleotide anticodon as well as a binding site for a cognate amino acid.

All tRNAs with a specific anticodon will carry the same amino acid. Initiation of translation occurs when the small ribosomal subunit binds with initiation factors and an initiator tRNA at the start codon of an mRNA, followed by the binding to the initiation complex of the large ribosomal subunit.