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Transcription. The Central Dogma. Translation. Transcription. The Central Dogma. Transcription. Transcription is the synthesis of an RNA strand from a DNA template DNA transcribed to messenger RNA (mRNA) mRNA carries the code from DNA to the ribosomes where translation occurs.

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  1. Transcription

  2. The Central Dogma Translation Transcription

  3. The Central Dogma

  4. Transcription • Transcription is the synthesis of an RNA strand from a DNA template • DNA transcribed to messenger RNA (mRNA) • mRNA carries the code from DNA to the ribosomes where translation occurs

  5. DNA vs. RNA DNA RNA BOTH Double stranded Dexoyribose sugar Contains thymine Single stranded Ribose sugar Contains uracil (NOT thymine) Both have adenine (A), cytosine (C), and guanine (G) Both are nucleic acids

  6. DNA vs. RNA RNA DNA Single stranded CGAU Ribose sugar Double stranded CGAT Deoxyribose sugar

  7. mRNA tRNA rRNA

  8. Transcription • Transcription is the process of forming a strand of RNA from a strand of DNA • This process occurs in the nucleus • The cell must make RNA to send to the cytoplasm to tell the ribosomes how and which proteins to make • The RNA molecule is a copy of a gene’s protein building instructions. This type of RNA is called mRNA (messenger RNA) • An enzyme called RNA polymerase catalyzes the reaction • The purpose of transcription is to copy one gene from the DNA molecule

  9. Three Stages of Transcription Initiation Elongation Termination RNA polymerase binds to DNA at a specific sequence of nucleotides called the promoter The promoter contains an intiation site where transcription begins RNA polymerase then unwinds DNA at the beginning of the gene RNA polymerase synthesizes a single strand of RNA complementary to the DNA template strand adding to the 3’ end of the RNA chain As RNA polymerase moves along the DNA, it continues to untwist the double helix RNA polymerase continues to elongate until it reaches the terminator, a specific sequence of nucleotides that signals the end of transcription Transcription stops and RNA polymerase and the new mRNA transcript are released from DNA The double helix reforms

  10. Steps of Transcription • RNA Polymerase binds to a site of the DNA molecule called the promoter TAC • Separates the DNA strand for transcription • One strand of DNA is used as a template • New nucleotides are inserted according to the base pairing rules. When transcribing RNA, adenine pairs with uracil; cytosine pairs with guanine • This continues until the terminator is reached • As the RNA polymerase moves along the DNA molecule, hydrogen bonds between the two stands of DNA are reformed • A single stranded RNA molecule has been transcribed

  11. Steps of Transcription • Remember: The purpose of transcription is NOT to copy the entire length of DNA, but to only copy small portions – a gene’s worth – to be sent to the ribosome as the instructions for protein synthesis

  12. RNA Processing and Editing • The RNA is not yet ready to be sent to the cytoplasm for translation. It must be modified first. • The mRNA is a copy of a small section of DNA. This contains sections called introns and other sections called exons • Introns are not involved in the making of the protein. These are cut out of the RNA before the RNA goes to the ribosome. • Exons are the sequences that are involved in the making of the proteins. These are pieced back together to form the final mRNA.

  13. RNA Processing and Editing • When mRNA is formed, both the introns and exons are copied from the DNA. • The introns are cut out of the RNA and remain in the nucleus. • Exons will then exit the nucleus. Introns: in the nucleus; not involved in protein synthesis Exons: exit the nucleus; involved in protein synthesis

  14. The Code of DNA • Proteins are made of building blocks called amino acids • There are 20 different amino acids and only 4 nucleotides (4 bases) • Three nucleotides in a sequence must specify each amino acid. This provides 64 possible combinations of amino acids • Each triplet of nucleotides is called a codon

  15. The Code of DNA • Each codon calls for a specific amino acid. When many amino acids are linked together a protein is made • A few codons do not call for any amino acids. One codon acts as a start codon to tell where the sequence of amino acids is to begin. Three other codons are “stop” codons and act as signals for the end of a protein chain • A gene on a chromosome is many codons long, and codes for a particular protein • Genes provide the instructions for making specific proteins, but a gene does not build a protein directly. The bridge between DNA and protein synthesis is RNA

  16. Practice Transcribing • Create the complementary mRNA strand for each DNA strand • Remember to use U instead of T when pairing A • DNA strand 5’ ACTAGCTA 3’ • mRNA strand • DNA strand 5’ TTCCAGCA 3’ • mRNA strand 3’ UGAUCGAU 5’ 3’ AAGGUCGU 5’

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