Transcription and Translation - Human Biology | 002 021, Study notes of Human Biology

Download Transcription and Translation - Human Biology | 002 021 and more Study notes Human Biology in PDF only on Docsity! 1 LECTURE 10 TRANSCRIPTION AND TRANSLATION A. General Considerations Transcription: Formation of a message using DNA as the source or code. Translation: Synthesis of a protein by translating the message. B. Protein Structure String of 20 amino acids - primary structure 20 amino acids are used. There is no restriction on the sequence of usage of particular amino acids. The actual sequence of the protein is determined by DNA nucleotide sequence of a gene. C. Transcription Messenger RNA (mRNA) is synthesized DNA is used as a template to produce mRNA. The part of the DNA to be used will depend on the stretch of DNA [= gene] which is to be transcribed. mRNA is complimentary to the DNA of gene. That is, the nucleotides of mRNA pair with the DNA of the gene to be transcribed. This means that the mRNA is not an identical sequence to the gene, but is a complementary sequence to that gene. 2 D. Nature of RNA All types of RNA are made the same way and have the same characteristics. 5 SO: 3 RNA bases in mRNA code for an amino acid. Thus, each amino acid must have at least one 3 RNA nucleotide code (codon). But, most amino acids are have more than one triplet code, i.e. has more than one codon. Look at the chart of codons and find the codons for GLY (glycine). I has four codons. Some codons do not code for an amino acid and these do not add an amino acid to the growing chain. These are called STOP codons, and they stop the addition of amino acids. They stop the formation of the protein, it is completed. Also, one codon is the start codon at which point protein synthesis starts. The same start codon is used for all protein synthesis. This codon is the triplet code AUG. This codon is actually the codon for the amino acid methionine. However, when a protein is being produced by translating the RNA, the FIRST AUG in the sequence of the mRNA will code for methionine AS WELL AS IT WILL SERVE AS THE START CODON. This means that all proteins begin synthesis with the amino acid methionine. 6 F. The message Message is a molecule of messenger RNA mRNA contains all codons necessary for a protein sequence. The mRNA sequence is determined by the sequence of one strand of DNA at a gene 1 LECTURE 10 TRANSCRIPTION AND TRANSLATION A. General Considerations Transcription: Formation of a message using DNA as the source or code. Translation: Synthesis of a protein by translating the message. B. Protein Structure String of 20 amino acids - primary structure 20 amino acids are used. There is no restriction on the sequence of usage of particular amino acids. The actual sequence of the protein is determined by DNA nucleotide sequence of a gene. C. Transcription Messenger RNA (mRNA) is synthesized DNA is used as a template to produce mRNA. The part of the DNA to be used will depend on the stretch of DNA [= gene] which is to be transcribed. mRNA is complimentary to the DNA of gene. That is, the nucleotides of mRNA pair with the DNA of the gene to be transcribed. This means that the mRNA is not an identical sequence to the gene, but is a complementary sequence to that gene. 2 D. Nature of RNA All types of RNA are made the same way and have the same characteristics. 5 SO: 3 RNA bases in mRNA code for an amino acid. Thus, each amino acid must have at least one 3 RNA nucleotide code (codon). But, most amino acids are have more than one triplet code, i.e. has more than one codon. Look at the chart of codons and find the codons for GLY (glycine). I has four codons. Some codons do not code for an amino acid and these do not add an amino acid to the growing chain. These are called STOP codons, and they stop the addition of amino acids. They stop the formation of the protein, it is completed. Also, one codon is the start codon at which point protein synthesis starts. The same start codon is used for all protein synthesis. This codon is the triplet code AUG. This codon is actually the codon for the amino acid methionine. However, when a protein is being produced by translating the RNA, the FIRST AUG in the sequence of the mRNA will code for methionine AS WELL AS IT WILL SERVE AS THE START CODON. This means that all proteins begin synthesis with the amino acid methionine. 6 F. The message Message is a molecule of messenger RNA mRNA contains all codons necessary for a protein sequence. The mRNA sequence is determined by the sequence of one strand of DNA at a gene 7 mRNA is therefore complementary to 1 of the DNA strands mRNA , therefore has the same sequence as the other DNA strand. except that it has U where the DNA has a T. G. Synthesis of the message DNA separates at a gene. a. Ttwo strands come apart in at the gene. RNA nucleotides pair up with exposed DNA bases Same pairing rules as in DNA replication. . U pairs with A instead of T pairing with A Only one DNA strand is used as template for mRNA RNA nucleotides covalent bond one at a time RNA separates from DNA DNA rebinds into a 2 stranded structure 10 Transfer RNA [tRNA], a type of RNA used in protein synthesis. tRNA has two functions: it carries an amino acid. There is a specific tRNA for each amino acid. It contains, as part of its RNA nucleotide sequence, a small sequence which is complimentary to a codon = anticodon. An anticodon is recognizes codon by complimentary base Pairing. The anti-codon is specific for the amino acid that tRNA carries. This codon-anticodon pairing allows amino acids to be placed in the protein in the right order. 11 Codons and anticodons Remember that codons are found in the sequence of mRNA. Codons are triplet codes. In mRNA, one codon follows another in the RNA strand. So, if one starts from the first AUG, every 3 triplet following that start codon is another codon. These codons, which code for the amino acid sequence of a protein, can be followed, 3 nucleotides at a time, clear to the end of the mRNA. So, if you know the genetic code, you can easily predict the amino acid sequence of the protein which is being coded for. The place to start synthesis [= start codon] using an mRNA is at the first AUG codon in the strand. It does not usually come at the end, but several codons from the end of the mRNA. AUG codes for methionine, so every protein being synthesized starts with methionine (met). Also, AUG may occur in other places, but it only the first one that is the start codon. For Example, the following sequence will code for 9 amino acids. Remember that producing a protein starts at the first AUG, and 12 this codes for methionine as well as being the start codon. UUA AUG GCA GUG GGG UUA AAC AAA AAG AUG start 2 3 4 5 6 7 8 9 15 Triplet code for next amino acid to be added is already in A site. Now tRNA of the amino acid of the next codon (here the codon is AGC, the anticodon of the tRNA of this codon will be UCG enters the A site and sticks to the codon AGC. There are now two tRNA and two amino acids in the ribosome. The amino acid from the P site (AUG) will be covalently bonded to the amino acid in the A site (AGC). AUG AGC Now, the bond to the tRNA in the P site is broken. The two amino acids bound together are now attached to the tRNA in the A site. THEN: The ribosome moves down the mRNA by 3 nucleotide bases (=1 codon). The tRNA in the P site moves to the E site (exit site). tRNA in the A site moves to P site, still with the short two amino acid chain attached to it. The tRNA for next amino acid enters A site and attaches via anticodon. 16 The two amino acid chair is covalently bonded to this new amino acid. This make a three amino acid chain. This three amino acid chain is released from the tRNA in the P site and remains connected to the tRNA in the A site. This empty tRNA will enter the E site (exit site) when the ribosome moves 1 codon. Break this bond 17 Reactions repeat until stop codon enters A site At stop, polypeptide is released from tRNA to become free. Finished protein