RNA Processing and Gene Expression

RNA Processing

• Each end of a pre-mRNA molecule is modified
  • The 5’ end receives a modified guanine nucleotide cap
  • The 3’ end gets a poly-A tail
• These modifications:
  • Facilitate the export of mRNA from the nucleus
  • Protect mRNA from degradation by hydrolytic enzymes
  • Help ribosomes attach to the 5’ end

Introns and Exons

• Most eukaryotic genes and their RNA transcripts have long noncoding stretches of nucleotides that lie between coding regions
  • These noncoding regions are called intervening sequences, or introns
  • The coding regions are called exons
• RNA splicing removes introns and joins exons, creating an mRNA molecule with a continuous coding sequence

Transcription and Translation: Central Dogma

• The information content of DNA is in the form of specific sequences of nucleotides
  • This leads to specific traits by dictating the synthesis of proteins
• This process is called gene expression and it involves two major stages:
  • Transcription: synthesis of RNA under the direction of DNA
  • Translation: synthesis of a polypeptide under the direction of mRNA
• Transcription produces messenger RNA (mRNA) which carries the code from DNA to the ribosomes
  • Ribosomes are the site of translation

The Genetic Code

• The instructions for assembling amino acids into proteins are encoded into DNA
• The genetic code is a triplet code: a series of nonoverlapping, three-nucleotide words
• Each codon specifies the amino acid to be placed at the corresponding position along a polypeptide
• The mRNA codons are read in the 5’ to 3’ direction

Cracking the Code

• 61 out of 64 codons code for amino acids
• 3 codons are stop signals to end translation
• The genetic code is redundant, meaning more than one codon can specify a particular amino acid, but it is not ambiguous, meaning no codon specifies more than one amino acid
• Codons must be read in the correct reading frame to produce the specified polypeptide

Universal Genetic Code

• The genetic code is nearly universal
  • Genes from one species can be transcribed and translated in another organism
  • This is because the genetic code is nearly identical in all organisms, from the simplest bacteria to the most complex animals

Transcription: A Closer Look

• Transcription is the first stage of gene expression
• RNA synthesis is catalyzed by RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotides
• The RNA is complementary to the DNA template strand
• The DNA sequence where RNA polymerase attaches is called the promoter
• The stretch of DNA that is transcribed is called a transcription unit
• The sequence signaling the end of transcription is called the terminator (in bacteria)

Eukaryotic RNA Processing

• Eukaryotic cells modify RNA after transcription
• The primary transcript is the initial RNA transcript from any gene prior to processing
• RNA processing involves the following modifications to the pre-mRNA:
  • 5' capping
  • 3' polyadenylation
  • Splicing

Explore the fascinating processes of RNA processing, including modifications such as the guanine cap and poly-A tail. Learn about the roles of introns and exons in mRNA formation and the central dogma of molecular biology that outlines gene expression. This quiz will test your understanding of these critical biological concepts.