Central Dogma and mRNA Processing

• Central Dogma of Molecular Biology describes the flow of information in cells: DNA → RNA → Protein.
• A gene is a sequence of DNA with a specific function.
• For each gene, only one of the two strands is transcribed into RNA.
• RNA serves as an intermediate between the DNA code and the proteins that determine phenotype.
• There are three kinds of RNA: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).

Messenger RNA (mRNA)

• mRNA carries the DNA message from the nucleus to the cytoplasm.
• mRNA is read in triplets called codons.
• 64 different codons code for 20 different amino acids.
• "Wobble" refers to the fact that codons for some amino acids can differ in the 3rd base.
• mRNA must be processed/modified before translation.
• A guanine cap is added to the 5’ end, and a polyA tail is added to the 3’ end for stability, which helps with passage through the nuclear membrane.
• Pre-mRNA contains introns (non-coding DNA segments) and exons (coding DNA segments).
• Splicing can produce different proteins from the same gene by editing mRNA in different ways.

Transfer RNA (tRNA)

• tRNA has a cloverleaf-like secondary structure.
• tRNA brings amino acids to the ribosome.
• tRNA attaches amino acids in the proper place.
• The anticodon region of tRNA matches the codon on mRNA.

Ribosomes (rRNA)

• Ribosomes are made up of rRNA and proteins.
• rRNA is made in the nucleolus in eukaryotes and assembled with proteins imported from the cytoplasm.
• rRNA in ribosomes reads mRNA.
• Large and small ribosomal subunits join to form a functional ribosome only when attached to mRNA.

Transcription

• Occurs in three stages: initiation, elongation, and termination.
• Initiation: RNA polymerase binds to DNA at the promoter, separating the DNA strands to begin transcription.
• Elongation: The RNA chain grows as nucleotides are added to the 3’ end and base pair with the template strand. The noncoding strand of DNA reforms a double helix by pairing with the coding strand.
• Termination: Transcription proceeds until RNA polymerase reaches a terminator site on the DNA, and the RNA molecule is released.

Translation

• Occurs in three stages: initiation, elongation, and termination.
• Initiation: The small ribosomal subunit attaches to the mRNA (AUG). Then, tRNA carries the 1st amino acid (methionine) to the mRNA, and the large ribosomal subunit attaches to the mRNA.
• Elongation: The ribosome moves along the mRNA, matching tRNA anticodons with mRNA codons. tRNA with a new amino acid arrives at the ribosome. Amino acids bond with peptide bonds at the catalytic site. The empty tRNA is recycled, and brings in a new amino acid. A new tRNA arrives at the ribosome repeating the process, increasing the length of the polypeptide chain.
• Termination: Occurs when the ribosome encounters a stop codon. Ribosome subunits detach, the polypeptide chain is released, and mRNA can be read multiple times or nucleotides get recycled.

The central dogma explains information flow in cells: DNA to RNA to protein. Messenger RNA (mRNA) carries genetic code from the nucleus to the cytoplasm for translation into proteins. mRNA undergoes processing before translation, including the addition of a guanine cap and a polyA tail.