Gene Transcription and Translation Overview

These flashcards cover key vocabulary terms related to gene transcription and translation processes in molecular biology.

Transcription: Detailed Steps

The process of synthesizing RNA from a DNA template. In eukaryotes, it involves several key steps:

• Initiation:
  • RNA polymerase binds to the promoter region of the DNA, signaling the start point for transcription.
  • The DNA strands unwind and separate, forming a transcription bubble.
• Elongation:
  • RNA polymerase moves along the DNA template strand in the 3' \to 5' direction.
  • It synthesizes an RNA strand by adding complementary RNA nucleotides to the growing 5' end.
  • Uracil (U) pairs with Adenine (A) in the new RNA strand.
• Termination:
  • RNA polymerase encounters a specific terminator sequence on the DNA.
  • The RNA polymerase detaches from the DNA, and the newly synthesized RNA molecule is released.
  • The DNA double helix reforms.
• RNA Processing (in eukaryotes):
  • The pre-mRNA undergoes modifications including the addition of a 5' cap and a poly-A tail to the 3' end.
  • Splicing occurs: Introns (non-coding sequences) are removed, and exons (coding sequences) are ligated together to form mature mRNA.

RNA Polymerase

An enzyme essential for transcription:

• Synthesizes RNA by joining together RNA nucleotides.
• Binds to the DNA template strand.
• Unwinds the DNA double helix locally.
• Adds complementary RNA nucleotides to the growing RNA chain, following the base-pairing rules (A with U, T with A, C with G, G with C).

Promoter

A specific DNA sequence that:

• Signals the start point for transcription.
• Serves as the binding site for RNA polymerase.
• Determines which DNA strand will be used as the template.

Intron

A non-coding sequence within a pre-mRNA molecule:

• Is transcribed but not translated.
• Must be removed during RNA processing (splicing) in eukaryotes.
• Allows for alternative splicing, producing different proteins from a single gene.

Exon

A coding sequence within a pre-mRNA molecule:

• Ultimately expressed and translated into protein.
• Spliced together with other exons to form mature mRNA after intron removal.

Codon

A sequence of three RNA nucleotides that:

• Corresponds to a specific amino acid.
• Is found on the mRNA molecule.
• The genetic code is read in codons during translation.

Anticodon

A three-nucleotide sequence:

• Located on a tRNA molecule.
• Complementary to a specific codon on the mRNA.
• Ensures the correct amino acid is brought to the ribosome during translation.

Peptide Bond

A chemical bond:

• Formed between the carboxyl group of one amino acid and the amino group of another.
• Crucial for linking amino acids together to form polypeptide chains during protein synthesis.

Translation: Detailed Steps

The process by which mRNA is decoded to produce a specific polypeptide. This occurs in the ribosome and involves three main stages:

• Initiation:
  • The small ribosomal subunit binds to the mRNA molecule, recognizing the 5' cap (in eukaryotes) or a specific sequence (in prokaryotes).
  • The initiator tRNA, carrying the amino acid methionine (or N-formylmethionine in prokaryotes), binds to the start codon (AUG) on the mRNA.
  • The large ribosomal subunit then joins, forming a complete ribosome with the initiator tRNA situated in the P-site.
• Elongation:
  • Codon Recognition: A new tRNA, carrying its specific amino acid, enters the A-site of the ribosome, where its anticodon pairs with the complementary mRNA codon.
  • Peptide Bond Formation: A peptide bond is formed between the amino acid in the A-site and the growing polypeptide chain held by the tRNA in the P-site. This reaction is catalyzed by rRNA (a ribozyme).
  • Translocation: The ribosome moves one codon along the mRNA in the 5' \to 3' direction. The tRNA in the P-site shifts to the E-site and is released, while the tRNA from the A-site moves to the P-site, leaving the A-site vacant for the next incoming tRNA.
• Termination:
  • Elongation continues until one of the three stop codons (UAA, UAG, or UGA) enters the A-site of the ribosome.
  • A release factor protein binds to the stop codon in the A-site.
  • This binding causes the hydrolysis of the bond between the polypeptide and the tRNA in the P-site, releasing the newly synthesized polypeptide.
  • The ribosomal subunits then dissociate from the mRNA and from each other, completing translation.

Quaternary Structure

The highest level of protein structure, involving:

• The arrangement of multiple polypeptide chains (subunits).
• These subunits come together to form a single, functional protein complex.
• Not all proteins possess a quaternary structure; it is only present in multi-subunit proteins.