Unit 6 Topic 3 Packet

Overview of Proteins

  • Proteins are polypeptides made up of amino acids.

  • Amino acids are linked by peptide bonds.

  • Gene Expression: Process from DNA to RNA to protein, defining gene expression as the synthesis of proteins directed by DNA.

Gene Expression

  • Two Stages: Transcription and Translation.

  • Occurs in all organisms: prokaryotes and eukaryotes.

Transcription

  • Definition: Synthesis of RNA using DNA information.

  • Location: Occurs in the nucleus.

  • mRNA can leave the nucleus to proceed to translation (protein synthesis).

Types of RNA

  1. Messenger RNA (mRNA): Synthesized during transcription using a DNA template.

    • Carries DNA information from the nucleus to ribosomes in the cytoplasm.

  2. Transfer RNA (tRNA): Important for translation, carrying specific amino acids.

    • Interacts with mRNA through anticodons.

  3. Ribosomal RNA (rRNA): Forms ribosomes and links amino acids together.

The Genetic Code

  • DNA: Contains nucleotides coding for proteins, read in triplets (triplet code).

  • Only one DNA strand (the template strand) is transcribed.

  • mRNA molecules formed are anti-parallel and complementary to the DNA nucleotides.

    • Coding Correspondence: A (adenine) with U (uracil), T (thymine) with A, C (cytosine) with G.

  • Codons: mRNA nucleotide triplets that code for amino acids.

Codon Chart

  • There are 64 codon combinations: 61 code for amino acids and 3 are stop codons.

  • Universal across all life forms.

  • Redundancy allows for mutations to potentially have no effect on protein synthesis.

Reading Frame

  • Codons must be read in correct sequence for proper protein synthesis.

  • A shift in reading frame can lead to different outcomes (example: "The fat cat ate the rat" vs. "his fat cat ate the rat").

Steps of Transcription

1. Initiation

  • RNA Polymerase attaches to a promoter region of DNA (specific regions that signal where transcription begins).

  • In eukaryotes, this region is called the TATA box and requires transcription factors for RNA polymerase binding.

  • In prokaryotes, RNA polymerase binds directly to the promoter.

2. Elongation

  • RNA polymerase opens DNA and reads the template strand from 3' to 5'.

  • mRNA transcript elongates in 5' to 3' direction.

  • Only small sections of DNA are opened to ensure specificity in complementary nucleotide pairing.

  • Multiple RNA polymerase molecules can transcribe a single gene simultaneously.

3. Termination

  • In prokaryotes: Transcription ends when crossing a termination sequence, releasing mRNA directly into translation without modifications.

  • In eukaryotes: Involves a polyadenylation signal sequence leading to the release of pre-mRNA, which undergoes modifications:

    • 5' Prime Cap: Modified guanine added to the 5' end.

    • Poly A Tail: About 50 to 250 adenine nucleotides added to the 3' end.

    • RNA Splicing: Introns (non-coding sequences) are removed and exons (coding sequences) are joined together.

    • Allows for alternative splicing, enabling a single gene to code for multiple polypeptides.

Conclusion of Transcription

  • Modified mRNA (mature mRNA) leaves the nucleus for translation at ribosomes after transcription.

  • Overall Goal: Write down DNA information and prepare it for translation into proteins.

  • Eukaryotic vs Prokaryotic Initiation: Eukaryotes require transcription factors for RNA polymerase binding, while prokaryotes do not.