1. Lesson 4- Gene Expression PART 2 - TRANSLATION

2. Warm-Up Name 10 differences between DNA replication and transcription

3. Learning Goals To determine how mRNA transcripts are read to build proteins by ribosomes. To identify the key molecules in the process of translation Determine the genetic code and how it determines the structure of polypeptides from a gene.

4. Gene Expression Refresh Gene expression refers to the transfer of genetic information from DNA to protein. There are two stages: transcription and translation.

5. Translation in the Cytoplasm mRNA is transported to the cytoplasm in Eukaryotes, where ribosomes will translate their template into amino acid sequences.

6. Ribosomes Ribosomes are very large molecules comprised of protein and ribosomal RNA (rRNA). There are two main subunits in Eukaryotic ribosomes, called the small (40S) and and large (60S) subunits.

7. tRNA Also needed for translation are special adapter molecules of RNA called tRNA (transfer RNA). These molecules have specific amino acids bound to an attachment site. They also have a specific three letter sequence called an anticodon at the end opposite the amino acid.

8. Molecular Events of Translation Translation is one of the most energy- consuming processes in the cell, as many protein and nucleic components must be assembled. It begins when an mRNA strand is recognized by the small subunit of the ribosome, which binds to the 5’ cap.

9. Recognition and Scanning The small subunit of the ribosome will start moving down the strand of the mRNA until it reaches a specific three base sequence: AUG. This is called the start codon, and it references the amino acid methionine. Once this tRNA is in place, the large subunit binds and protein synthesis can begin. Note that GTP is used in the process of translation. The functional part of the large subunit is rRNA, which can catalyze reactions like an enzyme due to its tertiary structure.

10. The Genetic Code Following the AUG codon, the ribosome continues down the strand of mRNA three bases at a time. Every three base sequence is called a codon, and each codon refers to an amino acid. The specific way the sequences are divided into non-overlapping triplets is called the reading frame.

11. Number Games There are 4 3 different ways of organizing the four bases in RNA into three letter sequences, but there are only 20 amino acids, so there is some overlap in the code (some amino acids are coded for by more than one codon). There are also 3 special codons that don’t code for amino acids, they are the stop codons. The three stop codons that are part of the genetic code.

12. Binding Sites on the Ribosomes There are three binding sites on the ribosome for tRNA molecules. P site - tRNA that carries the growing polypeptide A site - entry for tRNA holding the next amino acid in the sequence. E site - tRNA separates from its amino acid.

13. Elongation When a new tRNA enters the A site, a peptide bond is formed between neighbouring amino acids. The ribosome moves along to the next codon, thus shifting the position of the tRNAs (P to E, and A to P). Proteins are synthesized from the N-terminus to the C- terminus.

14. Termination Termination occurs when a stop codon is reached in the same reading frame. The polypeptide and the translation machinery are separated and the polypeptide can fold into its 3-D shape given the specific primary sequence it has.

15. Overview

16. An Example Note how there are untranslated regions on the mRNA strand at both the 5’ end of the start codon, and the 3’ end of the stop codon. These regions play a role in recognition and stability of the mRNA. http://highered.mheducation.co m/olcweb/cgi/pluginpop.cgi?it=s wf::535::535::/sites/dl/free/0072 437316/120077/micro06.swf::Pr otein%20Synthesishttp://highered.mheducation.co m/olcweb/cgi/pluginpop.cgi?it=s wf::535::535::/sites/dl/free/0072 437316/120077/micro06.swf::Pr otein%20Synthesis

17. Some Thinking 1.List the different types of RNA, and their functions 2.Differentiate between the following: ◦P site and A site ◦Codon and Anticodon ◦Start codon and stop codon 3.Why might an error in the transcription or translation process not be a big concern