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U-M BIOLOGY 172 - Transcription

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Messenger RNA carries information for the synthesis of a specific protein.BIO 172January 18, 2019TranscriptionReadings 3.1-3.41. Deoxyribonucleic acid (DNA) stores and transmits genetic information.2. DNA is a polymer of nucleotides and forms a double helix.3. Transcription is the process by which RNA is synthesized from a DNA template.4. The primary transcript is processed to become messenger RNA (mRNA).3.1-3.3 Summary3.1 Deoxyribonucleic acid (DNA) stores and transmits genetic information.o Experiments carried out by Griffith in 1928 demonstrated that bacteria can transmit genetic information from one strain to another. o Experiments performed by Avery, MacLeod, and McCarty in 1944 showed that DNAis the molecule that transmits genetic information. o DNA is copied in the process of replication. o Ribonucleic acid (RNA) is synthesized from a DNA template. o The central dogma of molecular biology states that the usual flow of genetic information is from DNA to RNA to protein. DNA is transcribed to RNA, and RNA is translated to protein. 3.2 DNA is a polymer of nucleotides and forms a double helix.o A nucleotide consists of a 5-carbon sugar, a phosphate group, and a base. o The four bases of DNA are adenine, guanine, cytosine, and thymine. o Successive nucleotides are linked by phosphodiester bonds to form a linear DNA molecule. o DNA strands have polarity, with a 5′-phosphate group at one end and a 3′ hydroxyl group at the other end. o Cellular DNA molecules consist of a helical spiral of two paired, antiparallel strands called a double helix. o In a DNA double helix, A pairs with T, and G pairs with C. o The structure of DNA relates to its function. Information is coded in the sequence of bases, and the structure suggests a mechanism for replication, in which each parental strand serves as a template for a daughter strand. o DNA in eukaryotic cells is packaged with evolutionarily conserved proteins called histones. 3.3 Transcription is the process by which RNA is synthesized from a DNA template.o RNA, like DNA, is a polymer of nucleotides linked by phosphodiester bonds.o Some types of RNA can store genetic information and other types can catalyze chemical reactions. These characteristics have led to the RNA world hypothesis, the idea that RNA played a critical role in the early evolution of life on Earth. o Unlike DNA, RNA incorporates the sugar ribose instead of deoxyribose and the base uracil instead of thymine. o RNA is synthesized from one of the two strands of DNA, called the template strand. o RNA is synthesized by RNA polymerase in a 5′-to-3′ direction, starting at a promoter and ending at a terminator in the DNA template. o RNA polymerase separates the two DNA strands, allows an RNA–DNA duplex to form, elongates the transcript, releases the transcript, and restores the DNA duplex.  3.4 Fate of the RNA Primary Transcript The RNA transcript that comes off the template DNA strand is known as the primary transcript, and it contains the genetic information of the gene that was transcribed.  For protein-coding genes, this means that the primary transcript includes the information needed to direct the ribosome to produce the protein corresponding to thegene. The RNA molecule that combines with the ribosome to direct protein synthesis is known as the messenger RNA (mRNA) because it serves to carry the genetic “message” (information) from the DNA to the ribosome.  Messenger RNA carries information for the synthesis of a specific protein. In prokaryotes, the relation between the primary transcript and the mRNA is as simple as can be: The primary transcript is the mRNA. - Even as the 3′ end of the primary transcript is still being synthesized, ribosomes bind with special sequences near its 5′ end and begin the process of protein synthesis. This intimate connection between transcription and translation can take place because prokaryotes have no nuclear envelope to spatially separate transcription from translation; the two processes are coupled, which means that they are connected in space and time. Primary transcripts for protein-coding genes in prokaryotes have another feature not shared with those in eukaryotes: - They often contain the genetic information for the synthesis of two or more different proteins, usually proteins that code for successive steps in the biochemical reactions that produce small molecules needed for growth, or successive steps needed to break down a small molecule used for nutrients or energy.- Molecules of mRNA that code for multiple proteins are known as polycistronic mRNA (a single molecule of messenger RNA that is formed by the transcription of a group of functionally related genes located next to one another along bacterialDNA) because the term “cistron” was once widely used to refer to a protein-coding sequence in a gene. Primary transcripts in eukaryotes undergo several types of chemical modification. In eukaryotes, the nuclear envelope is a barrier between the processes of transcription and translation. - Transcription takes place in the nucleus, and translation in the cytoplasm.  The separation allows for a complex chemical modification of the primary transcript, known as RNA processing, which converts the primary transcript into the finished mRNA, which can then be translated by the ribosome. RNA processing consists of three principal types of chemical modification. First, the 5′ end of the primary transcript is modified by the addition ofa special nucleotide attached in an unusual linkage. - This addition is called the 5′ cap, and it consists of a modified nucleotide called 7-methylguanosine. - An enzyme attaches the modified nucleotide to the 5′ end of the primary transcript essentially backward: in a normal linkage between two nucleotides, the phosphodiester bond forms between the 5′ carbon of one and the 3′-OH group of the next, but here the cap is linked to the RNA transcript by a triphosphate bridge between the 5′ carbons of both ribose sugars. - The 5′ cap is essential for translation because in eukaryotes the ribosome recognizes an mRNA by its 5′ cap.  Without the cap, the ribosome would not attach the mRNA and translation would not occur. The second major modification of eukaryotic primary transcripts is polyadenylation, the addition of a string of about 250 consecutive A-bearing ribonucleotides to the 3′ end, forming a poly(A)


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