DNA Transcription and Translation

Revisiting the Nucleus

• Site of DNA location
  • DNA is organized into chromosomes.
  • Chromatin:
  • Composed of DNA and a complex of proteins.
  • Contains the nucleolus (may have multiple in some species).
    • rRNA (ribosomal RNA) is synthesized using DNA instructions.
    • Proteins from the nucleolus assemble with rRNA to form ribosomal units.
  • Nuclear Pores:
  • Structure that regulates the passage of molecules in and out of the nucleus.
• Overview of Transcription and Translation:
  • Transcription results in pre-mRNA.
  • Pre-mRNA undergoes modifications to produce the final mRNA.
  • mRNA exits the nucleus and enters the cytoplasm for translation.

The Central Dogma

• Described by Francis Crick.
• Information flow: DNA → RNA → Protein.
  • Transcription: DNA is converted into RNA.
  • Translation: RNA is translated into protein.

The Genetic Code

• Composed of only four nucleotide bases to encode 20 different amino acids.
• Codons:
  • A triplet code derived from the nucleotide sequence.

Transcription

• Synthesis of RNA from DNA (copying the original code).
• Occurs in the nucleus.
• Both RNA and DNA utilize the same basic language; however, there are key differences:
  • Uracil (U) in RNA replaces thymine (T) in DNA.
  • RNA contains ribose while DNA contains deoxyribose.
• Resulting RNA is called messenger RNA (mRNA):
  • Carries genetic information from the nucleus to the cytoplasm for translation.

Why Use mRNA?

• Enables the transfer of genetic information while protecting DNA from damage during protein synthesis.

RNA Polymerases and Transcription Factors

• RNA Polymerases are enzymes initiating transcription at the promoter site.
  • Transcription Factors:
  • Protein complex facilitating the binding of RNA polymerase to the DNA.

Promoter Details

• TATA Box:
  • A fixed promoter region ~25 nucleotides upstream from the start site.
• Transcription Unit:
  • The part of DNA that is transcribed into RNA.

Stages of Transcription

1. Initiation:

   • The promoter acts as a recognition site for sigma factors which assist in RNA polymerase binding.
   • DNA unwinds into a structure known as the open complex.

2. Elongation:

   • RNA polymerase synthesizes the RNA transcript as it moves along the DNA strand.

3. Termination:

   • Occurs when RNA polymerase reaches the terminator, leading to the release of the RNA transcript.

Additional Note on mRNA Development

• Directionality: mRNA is synthesized in the 5’ to 3’ direction.
• RNA Splicing: Removal of non-coding regions:
  • Exons: Coding regions of final mRNA.
  • Introns: Intervening non-coding sequences.
  • Catalyzed by small nuclear ribonucleoproteins (snRNPs) forming a spliceosome.

Translation

• Occurs under the direction of mRNA where DNA sequences are translated into amino acid sequences forming a polypeptide.

tRNA - The Translator

• Each tRNA is specific to a codon via an anti-codon that reads the mRNA codons and delivers the correct amino acid.

Ribosomes - The Translation Center

• Composed of two subunits (large and small) facilitating translation.

Stages of Translation

1. Initiation:

   • mRNA binds to the ribosome, starting with the “Met” codon (AUG).
   • Both ribosomal subunits assemble around the mRNA.

2. Elongation:

   • Codon Recognition: Charged tRNA binds to the A site.
   • Peptide Bond Formation: A bond forms between the polypeptide chain and the new amino acid.
   • Translocation: Ribosome moves along the mRNA, releasing uncharged tRNA from the E site.
   • Process repeats until reaching a stop codon.

3. Termination:

   • Recognized by stop codons (UAG, UAA, UGA).
   • A release factor protein binds to the A site, freeing the last amino acid.
   • Ribosomal subunits dissociate, ending the translation process.