Central Dogma: Transcription and Translation

Overview: The Flow of Genetic Information

• Gene expression is how DNA directs the synthesis of proteins, involving transcription and translation.
• The central dogma states that cells are governed by a cellular chain of command which is replication, transcription, and then translation.

Basic Principles of Transcription and Translation

• RNA acts as the intermediate between genes and the proteins they encode.
• Transcription involves the synthesis of RNA using DNA as a template.
• Messenger RNA is the molecule that transcription produces.
• Translation is the synthesis of a polypeptide, using the instructions in the mRNA.
• Ribosomes are the location where translation occurs.
• Translation of mRNA can happen before transcription finishes in prokaryotes.
• The nuclear envelope separates transcription from translation in eukaryotes.
• Eukaryotic RNA transcripts undergo changes through RNA processing to become finished mRNA.
• A primary transcript is the initial RNA transcript from a gene prior to any processing.

Codons: Triplets of Nucleotides

• The flow of information from a gene to protein relies on a triplet code which is a series of nonoverlapping, three-nucleotide words
• Words of a gene are transcribed into complementary, nonoverlapping, three-nucleotide words of mRNA.
• These mRNA words are then translated into a chain of amino acids, which make up a polypeptide.
• During transcription, one of the two DNA strands known as the template strand provides the pattern for the order of complementary nucleotides in an RNA transcript.
• The template strand doesn't vary for a particular gene.
• mRNA base triplets which are called codons, are read from the 5' end to the 3' end.
• Translation machinery reads codons along an mRNA molecule from the 5' end to the 3' end.
• An mRNA codon specifies which of the 20 amino acids will be added to the corresponding position in a polypeptide.
• There are 64 codons.
• Some codons are redundant.
• Codons must be read in the correct reading frame which is the correct groupings to produce the intended polypeptide.

Molecular Components of Transcription

• Transcription is the initial step in gene expression.
• RNA polymerase catalyzes the synthesis of RNA, separating DNA strands and linking RNA nucleotides together.
• The promoter is where RNA polymerase attaches to a DNA sequence, while in bacteria, the terminator sequence signals the end of transcription.
• The DNA stretch that gets transcribed is known as a transcription unit.

Synthesis of an RNA Transcript

• The three stages of transcription include initiation, elongation, and termination.
• Promoters mark the transcriptional start point, often extending a few dozen nucleotide pairs upstream from it.
• Transcription factors are key for the binding of RNA polymerase and the start of transcription.
• A transcription initiation complex is the complete assembly of transcription factors and RNA polymerase II bound to a promoter.
• The TATA box promoter is crucial for forming the initiation complex in eukaryotes.

Alteration of mRNA Ends

• Each end of a pre-mRNA molecule is modified uniquely.
• A modified nucleotide 5' cap receives the 5' end.
• The 3' end receives a poly-A tail.
• These structural modifications facilitate the export of mRNA, protect mRNA from hydrolytic enzymes, and help ribosomes attach to the 5' end.

Split Genes and RNA Splicing

• Most eukaryotic genes and their RNA transcripts have long noncoding nucleotide stretches.
• These regions are called introns.
• Exons are regions that are eventually expressed, and usually translated into amino acid sequences.
• RNA splicing cuts out introns and combines exons, resulting in a continuous coding sequence of an mRNA molecule.

Transcription in Prokaryotes vs. Eukaryotes

• Prokaryotes
  • A single RNA polymerase synthesizes three types of RNA (mRNA, tRNA, rRNA).
  • Coupled transcription-translation is the rule.
  • RNAs released are processed in the cytoplasm.
  • Pre RNA molecules released are processed in the cytoplasm.
• Eukaryotes
  • The RNA polymerases I, II and III synthesizes rRNA, mRNA and tRNA respectively.
  • Coupled transcription translation is not.
  • RNAs released are processed in nucleas.
  • Pre RNA released are processed in nucleas.

RNA Directed Synthesis

• Genetic information flows from mRNA to produce proteins through the process of translation.

The Structure and Function of Transfer RNA

• tRNA molecules aren't identical.
• Each tRNA carries a specific amino acid on one end.
• Each tRNA has an anticodon on the other end; this anticodon pairs with a complementary codon on mRNA.

Ribosomes

• Ribosomes consist of two subunits namely large and small, these are made of proteins and ribosomal RNA (rRNA).
• A ribosome has three binding sites for tRNA.
• P site holds the tRNA carrying the growing polypeptide chain.
• A site holds the tRNA containing the next amino acid to be added to the chain.
• E site is the exit site where discharged tRNAs leave the ribosome.

Building a Polypeptide

• The stages of translation include initiation, elongation, and termination.