Transcription (Protein Synthesis)

• Transcription


  Making RNA copies from DNA instructions
• RNA polymerase


  • The key player, unwinding DNA, guiding nucleotide assembly, and linking them into an RNA strand
• Single-stranded messenger (m) RNA


  Built using only one strand of DNA as a template
• Complementary base pairing


  RNA nucleotides pair with DNA bases (A-U, C-G), except uracil replaces thymine in RNA
• Hydrogen bonding and complementary base pairing in transcription

  • Adenine (A) on the DNA template strand pairs with Uracil (U) on the RNA strand
• Transcription


  1. RNA nucleotides are added to a growing RNA strand based on the template DNA strand

  2. Accuracy relies on complementary base pairing
• Hydrogen bonding


  Specific base pairs form weak attractions called hydrogen bonds. Adenine (A) pairs with Uracil (U), Cytosine (C) pairs with Guanine (G)
• Template (antisense) strand and Sense Strand

  Only one DNA strand, called the template strand, is used for copying. The other strand, the sense strand, has a base sequence complementary to the template
• RNA vs. DNA


  The resulting RNA molecule has the same base sequence as the sense strand of DNA, except Uracil (U) replaces Thymine (T)
• The precise copying mechanism ensures that the information encoded in DNA is accurately transferred to RNA molecules, which are crucial for protein synthesis and other cellular functions
• Single DNA strands can be used as a template for transcribing a base sequence, without the DNA base sequence changing. In somatic cells that do not divide, such sequences must be conserved throughout the life of a cell
• Transcription


  DNA separates briefly for RNA copying, but rejoins quickly to minimize mutation risks
• Stable DNA is vital as genes get copied often. Mutations would pile up, harming proteins made from these genes
• Fast strand separation and rejoining during transcription keeps DNA templates safe for accurate copying
• Transcription


  The first stage of gene expression, is a key stage at which expression of a gene can be switched on and off
• Gene expression


  Turning gene instructions into observable traits requires two steps: transcription (copying DNA to RNA) and translation (using RNA to build proteins)
• Selective activation


  Only needed genes are transcribed in a cell at a specific time, allowing for control over protein production
• Cellular needs


  Different cell types have unique sets of active genes (transcriptomes) to fulfill their specialized functions
• Transcription


  1. DNA is unwound/strands are separated by RNA polymerase

  2. New nucleotides attached to DNA strand by RNA polymerase

  3. Complementary base pairing/base pairing with an example: adenine with thymine/uracil with adenine/cytosine with guanine/guanine with cytosine

  4. mRNA detaches from template

  5. DNA rewinds
• mRNA also has a poly(A) tail at the other end, which helps with translation.
• mRNA has a cap at one end, which protects it from degradation by nucleases.
• In eukaryotes, there are three main stages of transcription: initiation, elongation, and termination.
• Initiation involves the binding of RNA polymerase to the promoter region of the gene, where it recognizes the start site for transcription.
• Elongation occurs when RNA polymerase moves along the DNA molecule, synthesizing new RNA molecules as it goes.
• The mRNA molecule is complementary to the DNA template strand.
• Termination happens when RNA polymerase reaches the end of the gene and releases the newly formed mRNA molecule.
• During transcription, RNA polymerase reads the sequence of bases on the DNA template strand and creates an antisense RNA molecule that is complementary to the DNA sequence.
• During transcription, RNA polymerase reads the genetic code on the DNA template strand and creates a complementary copy of that information in the form of messenger RNA (mRNA).
• What is gene expression?


  Using and selecting specific genes for transcription
• Promoter region functions as a recognition site for RNA polymerase to bind.
• Promoter either permits of blocks an area of the DNA to RNA polymerase
• Binding of RNA polymerase causes the double helix to unwind and open up, allowing the DNA to be read
• RNA polymerase links nucleotides to the 3' end
• 5' cap and 3'-poly-A-tail are added after the mRNA is produced in the nucleus
• Introns are non-coding sections of the mRNA, that have to be removed
• Exons are coding regions of the mRNA, which remain intact
• The spliced exons are then joined together to form mature mRNA
• Splicing removes introns from pre-mRNA molecules
• A spliceosome are RNA and proteins formed together that coil introns together and remove them
• Splicing allows for the formation of mature mRNA