chapter 13

The flow of genetic information in eukaryotic cells can be summarized as the central : information encoded in is converted (expressed) into in two steps.
- • Central dogma. Gene expression involves 2 steps:
  1. messenger RNA (mRNA) is produced by transcription from DNA.
  2. proteins are synthesized by translation from the mRNA.
  Other types of RNA (ribosomal RNA and transfer RNA) can also be transcribed.
Transcription takes place in the .
- RNA assembles a primary RNA transcript.
  - • Transcription
    The enzyme RNA polymerase binds to DNA at the promoter region and unwinds the double helix.
    One of the DNA strands is used as a template on which RNA nucleotides are assembled, forming a hybrid helix.
    The same base pairing rules for DNA replication is used, except that Uracil (U) substitutes for Thymine (T) and pairs with Adenine (A).
    The immediate product of transcription is a primary mRNA transcript.
    Review:
  - • RNA polymerase binds to a specific region on the DNA called the promoter.
    This requires the DNA double helix to be open and accessible to the enzyme.
    However, binding cannot occur if the DNA is in a condensed state, such as seen in mitotic chromosomes.
- The primary RNA transcript is to form mature RNA (mRNA) which exits the nucleus.
  - • RNA splicing. The primary RNA transcript in eukaryotes contains introns (non-coding regions) as well as exons (coding regions). Enzymes called spliceosomes remove the introns and splice together the exons to produce the mature mRNA transcript, which exits the nucleus.
    Spliceosome splicing:
- Sequences of 3 nucleotides serve as to translate the mRNA into sequences of acids, using a genetic .
- • The genetic code. A codon is a triplet of 3 nucleotides on the mRNA. Ribosomes translate each codon to an amino acid; most amino acids are specified by more than one codon. The codon AUG is both the "start" codon and also a codon for the amino acid Methionine. There are 3 "stop" codons.
Translation takes place in the , on ribosomes.
- • Protein synthesis takes place on ribosomes which can be found free in the cytoplasm or associated with rough ER.
  A ribosome consists of a large and a small subunit.
  Each subunit is an aggregate of proteins and ribosomal RNA (rRNA) molecules.
- Molecules called RNA (tRNA) bind to specific amino acids.
  - • A transfer RNA (tRNA) molecule is single-stranded and forms regions of helices with hydrogen bonds, separated by loops.
    The 3' end carries a specific amino acid.
    One of the loops contains a triplet anticodon, which binds to a complementary codon on the mRNA.
    Types of RNA exercise:
- The ribosome reads the and uses molecules to build a polypeptide chain,
- • Ribosomes guide the translation process by binding to mRNA and loading tRNAs.
  The anticodons on the tRNA bind to complementary codons on the mRNA.
  A polypeptide chain (protein) is synthesized, using the genetic code as a "dictionary".
  Codon pairing exercise:
Summary of the central dogma: the code must be "read" in the correct reading for the message to be interpreted correctly.