DNA Transcription PDF

DNA transcription In the nucleus DNA is the code for inherited traits. Genes code for these inherited traits. Proteins such as enzymes, hormones and antibodies. Genes direct the production of proteins. The absence or the presence in altered form manifest...

DNA transcription In the nucleus DNA is the code for inherited traits. Genes code for these inherited traits. Proteins such as enzymes, hormones and antibodies. Genes direct the production of proteins. The absence or the presence in altered form manifest genetic disorders. Proteins drive all cellular processes. It was the studying of such defects that led scientists to discover how genes encode for proteins. 1 2 Replication is a duplicate of DNA for cell reproduction Transcribing DNA using RNA is transcription. This RNA is translated into proteins. 3 During interphase the cell goes about its processes. Making proteins for metabolic processes requires DNA transcription and translation. About 20 seconds to several minutes to make a protein. Overview 2:40 mins* 4 One gene-one polypeptide hypothesis: each gene is unique and codes for the synthesis of a single polypeptide Transcription is the making an RNA copy of one DNA strand. Translation is the decoding of the RNA strand into amino acids. Each amino acid is linked by an enzyme, and each enzyme is coded by a different gene. Enzyme 1 Enzyme 3 gene gene 5 RNA - the single stranded cousin of DNA RNA nucleotides consist of phosphate, ribose sugar and the nitrogenous bases (G, C, A, and U uracil) single stranded Extra oxygen on the sugar 3 forms of this nucleotide polymer ○ mRNA -messenger; a copy of DNA taken to the ribosome to make a protein (hundreds of nucleotides) ○ tRNA - transfer; retrieves amino acids from cytoplasm and delivers to ribosome to make proteins (about 80 nucleotides) ○ rRNA - ribosomal; used to synthesize ribosomes, site of protein synthesis 6 A codon is a group of three base pairs Since there are 20 amino acids scientists that code for an individual amino acid. knew that the code had to be 3 letters. 4 letters so 43 allows 64 different amino acids whereas 42 only allows for 16. The start codon indicates the beginning of the translation. It is usually AUG for the amino acid methionine and is the same for both prokaryotes and eukaryotes. 5’ 3’ The stop codon signals the end of the polypeptide chain and signals translation to stop. The codes are UAA, UGA, and UAG and AUG is used for 3542 genes GUG is used for 612 genes. do not code for an amino acid. UAA is used for 2705 genes. UUG is used for 130 genes. UGA is used for 1257 genes. AUU is used for 1 gene. UAG is used for 326 genes. CUG may be used for 1 gene. 7 The codons are universal with some minor exceptions. Notice: Only Met and Try have a single codon. The others have synonym codons. The 3rd nucleotide (letter) can change. 8 “Wobble” is term used for the variability in the 3rd nucleotide of the codon and the 1st position of the anticodon. These are non-Watson-Crick base pairing. This allows for some tRNAs to pair with 2 or 3 codons, thereby reducing the number of tRNAs required for translation. It’s economical. 61 codons can be read by as few as 31 anticodons or tRNAs. Chemical similar amino acids have similar codons. I.e. hydrophilic amino acids have U in the second position so a change in the third position is fine. 9  P318 #8 & 12 The following mRNA strand is being used to assemble a polypeptide strand by a ribosome: 5’-AUGCUUGCUCAUCGGGGUUUU-3’ a. Write out the amino acids that will be assembled, in their correct order b. Compare with another group. Same, different? Is that possible? Suppose that you are studying a protein with the primary sequence Met-Leu-His-Asn-Ala. a. Write out one double-stranded DNA sequence for this peptide. Include the start codon and a stop codon. Indicate the 3’ and 5’ ends of the DNA. Which is the template strand? b. Compare with another group. Are they different? Is that possible? 10 DNA transcription 2 mins* The central dogma DNA makes RNA makes Protein 11 Adding complexity Transcription happens in the nucleus and has 3 sequential processes: 1. Initiation (start) 2. Elongation (make) 3. Termination (stop) This happens continuously in order to have enough code to make enough proteins. 12 Initiation is universal RNA polymerase unwinds the DNA helix. RNA polymerase binds to the TATA box on a section of the promoter which a special sequence on one strand of DNA. Bind and regulate In eukaryotes the binding sequence is TATA and in prokaryotes it is TATAAT. Suggest why C and G are not used here. A and T have 2 H Bonds to break whereas C and G have 3. So RNA polymerase requires an even lower activation energy. 13 Elongation RNA polymerase can make the RNA strand in the 5’ to 3’ direction without a primer. An mRNA strand is made from the DNA template strand running 3’ to 5’ The polymerase unwinds the DNA as it moves along the strand. The RNA strand elongates (gets longer) as nucleotides are added. More polymerases can start right behind the first one in order to mass-produce the required protein. It would be too slow otherwise. Only one strand of DNA is copied. The coding strand 5’ to 3’ is not copied but has the same sequence as the RNA copy (but with T for U) 14  1 min* Termination RNA recognizes a signal to stop transcribing It could be… A termination sequence, in eukaryotes this could be a string of adenines transcribed as a string of uracils on the RNA A protein binding to the mRNA, in prokaryotes The mRNA could make a loop The RNA polymerase is free to be used again. 15  Some modifications to the mRNA are needed before leaving the nucleus and entering the cytoplasm. Capping and tailing Capping is the addition of 7 Gs to the 5’ cap. This functions as the attachment site for mRNAs to ribosomes Tailing is the addition of a poly (A) G GG tail, polyadenylation; a chain of adenine nucleotides is added 1 by 1 to the 3’ end by enzyme poly-A polymerase. This tail protects against RNA-digesting There is a lot of nonsense code in enzymes in the cytosol DNA that needs to be removed before making a protein, splicing ○ Introns (intervening segments) are non coding sections to be removed and exons (expressed) 30 secs* 16 code for proteins. Why introns? It allows for alternative splicing The exons can be joined in different combinations to make a family of related proteins This is why humans have about 20 000 genes coding for 100 000 different proteins 17 mutations? https://scitechdaily.com/disproving-a-60-year-old-hypothesis-most-silent-mutations -are-actually-harmful/ Synonymous (last codon changes but codes for the same amino acid i.e. ACU or ACA = thr…thought to be a silent mutations but a new study shows proof of significant involvement in disease) vs non-synonymous where the last codon affects the sequence of amino acids or adds a stop codon i.e. UGG to UGA making a stop codon) 18 P323 Read and make notes on the comparison of transcription in eukaryotic cells vs. prokaryotic cells P324 #8 As a graduate student in a university laboratory, you have been challenged with the problem of determining whether a sample of mRNA is from a eukaryotic cell or a prokaryotic cell. You have been provided with a nucleotide sequencer, which will help you determine the DNA sequence. What features in the sequence will you look for to determine whether the mRNA is eukaryotic or prokaryotic? Sample answer: I would look for non-coding regions in the mRNA, a 5' cap Click containing to reveal seven sample guanine residues, and a 3' adenine tail, answer all of which would indicate the mRNA is from a eukaryotic organism. 19