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KEY CONCEPT Transcription converts a gene into a single-stranded RNA molecule.

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KEY CONCEPT Transcription converts a gene into a single-stranded RNA molecule. - PowerPoint PPT Presentation

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KEY CONCEPT Transcription converts a gene into a single-stranded RNA molecule. DNA and the Genetic Code George Beadle and Edward Tatum began with a hypothesis: each gene causes the production of a single enzyme, and that enzyme catalyzes a biochemical reaction within an organism.

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DNA and the Genetic Code

  • George Beadle and Edward Tatum began with a hypothesis: each gene causes the production of a single enzyme, and that enzyme catalyzes a biochemical reaction within an organism.
  • Beadle and Tatum set out to provide experimental proof of the connection between genes and enzymes. They hypothesized that if there really was a one-to-one relationship between genes and specific enzymes, it should be possible to create genetic mutants that are unable to carry out specific enzymatic reactions. To test this theory, they exposed spores of Neurospora crassa (a bread mold) to X-rays or UV radiation and studied the resulting mutations. The mutant molds had a variety of special nutritional needs. Unlike their normal counterparts, they could not live without the addition of particular vitamins or amino acids to their food. For example, normal Neurospora requires only one vitamin (biotin), but mutants were created that also required thiamine or choline.
  • Genetic analysis showed that each mutant differed from the original, normal type by only one gene. Biochemical studies showed that the mutants seemed to be blocked at certain steps in the normal metabolic pathways. Their cells contained large accumulations of the substance synthesized just prior to the blockage point.
  • As Beadle and Tatum had predicted, they were able to create single gene mutations that incapacitated specific enzymes, so that the molds with these mutations required an external supply of the substance that the enzyme normally produced, and the substance that the enzyme normally used, piled up in the cell. These results led them to the one gene/one enzyme hypothesis, which states that each gene is responsible for directing the building of a single, specific enzyme.
  • Subsequent work has led to further refinement of this hypothesis. We now appreciate that not all genes code for enzymes - they may instead direct the building of structural proteins, such as the collagen in our skin, or the keratin in our hair. Also, many proteins are made of more than one polypeptide chain - hemoglobin consists of four polypeptide chains of two different types, and each of the two chain types is controlled by a different gene. Thus, given what we know now, a more accurate way to summarize Beadle's and Tatum's results is: one gene-one polypeptide.
rna carries dna s instructions
RNA carries DNA’s instructions.
  • The central dogma states that information flows in one direction from DNA to RNA to proteins.




  • The central dogma includes three processes.
  • Replication
  • Transcription
  • Translation
  • RNA is a link between DNA and proteins.

RNA differs from DNA in three major ways.

  • RNA has a ribose sugar.
  • RNA has uracil instead of thymine.
  • RNA is a single-stranded structure.
transcription makes three types of rna
Transcription makes three types of RNA.
  • Transcription copies DNA to make a strand of RNA.

transcription complex

start site


  • Transcription is catalyzed by RNA polymerase.
  • RNA polymerase and other proteins form a transcription complex.
  • The transcription complex recognizes the start of a gene and unwinds a segment of it.


RNA polymerase moves along the DNA

  • Nucleotides pair with one strand of the DNA.
  • RNA polymerase bonds the nucleotides together.
  • The DNA helix winds again as the gene is transcribed.


  • The RNA strand detaches from the DNA once the gene is transcribed.

Transcription makes three types of RNA.

  • Messenger RNA (mRNA) carries the message that will be translated to form a protein.
  • Ribosomal RNA (rRNA) forms part of ribosomes where proteins are made.
  • Transfer RNA (tRNA) brings amino acids from the cytoplasm to a ribosome.
the transcription process is similar to replication



growing RNA strands


The transcription process is similar to replication.
  • Transcription and replication both involve complex enzymes and complementary base pairing.
  • The two processes have different end results.
    • Replication copiesall the DNA;transcription copiesa gene.
    • Replication makesone copy;transcription canmake many copies.


  • RNA- ribonucleic acid
  • Protein synthesis- manufacture of proteins
  • Takes place on ribosomes in cytoplasm
  • Instructions for protein synthesis are transferred from genes on DNA to the ribosomes by RNA
  • Three types of RNA are involved
    • Messenger RNA (mRNA)- carries the coded instructions for protein synthesis from the DNA in the nucleus to the ribosome
    • Transfer RNA (tRNA) brings the amino acid to the ribosome in the correct order so that they can be assembled into proteins
  • Ribosomal RNA (rRNA)- along with several proteins it makes up the structure of a ribosome

The main difference between DNA and RNA is the sugar present in the molecules. While the sugar present in a RNA molecule is ribose, the sugar present in a molecule of DNA is deoxyribose. Deoxyribose is the same as ribose, except that the former has one more OH.

  • DNA does not usually exist as a single molecule, but instead as a tightly-associated pair of molecules. These two long strands entwine like vines, in the shape of a double helix. This arrangement of DNA strands is called antiparallel. The asymmetric ends of DNA strands are referred to as the 5′ (five prime) and 3′ (three prime) ends. One of the major differences between DNA and RNA is the sugar, with 2-deoxyribose being replaced by the alternative pentose sugar ribose in RNA. The four bases found in DNA are adenine (abbreviated A), cytosine (C), guanine (G) and thymine (T). A fifth pyrimidine base, called uracil (U), usually takes the place of thymine in RNA and differs from thymine by lacking a methyl group on its ring


  • Transcription- the manufacture of mRNA- the process of transferring information from a strand on DNA to a strand of RNA

DNA strand unwinds and separates

  • Transcription starts at the region of DNA that has the gene needed to be transcribed
  • Only one of the two strands of DNA contains the gene for the particular protein
  • This strand acts as a template directing the sequence of nucleotides in the new mRNA
  • The created mRNA will carry the instructions for protein synthesis
  • Loose RNA nucleotides are in the nucleus waiting to be assembled into mRNA
  • RNA polymerase (enzyme) matches the RNA base with the complementary base on the DNA template strand
  • RNA has the same nucleotide bases with one exception. Uracil (RNA) replaces Thymine (DNA)
  • This means that the complementary bases are Adenine↔Uracil and Cytosine↔Guanine IE>
    • If first three DNA bases are CGT the complementary RNA bases will be GCA
    • If the first three DNA bases are AAG then the complementary RNA bases will be UUC
  • After transcription the mRNA leaves the nucleus and moves into the cytoplasm to a ribosome where protein synthesis will occur