DNA Replication, Transcription, and Translation

• DNA replication, transcription, and translation are fundamental biological processes.

DNA Replication

• DNA replication involves building an mRNA strand to match a DNA message
• It occurs in three phases which are initiation, elongation, and termination
• Helicase separates DNA strands while binding proteins prevent strands from rejoining
• Primase makes a short stretch of RNA on the DNA template
• DNA polymerase adds nucleotides to the RNA primer
• There is a leading strand synthesis that continues in the 5' to 3' direction
• The Okazaki fragments result, as DNA synthesis transpires in the 5' to 3' direction, but the template strand reads in the 3' to 5' direction.
• The RNA primers are then replaced with DNA
• DNA ligase then seals the nicks (gaps) between the Okazaki fragments
• DNA replication results in copying one DNA double helix into two identical double helices.

Replication Proteins and Steps

• Helicase unwinds the double helix
• Binding proteins stabilize the single strands
• Primase adds an RNA primer to the template strand
• DNA polymerase binds nucleotides to create new strands
• Ligase forms covalent bonds between DNA segments

Transcription

• Transcription starts when the helicase unwinds the DNA
• It then involves proteins called transcription factors that bind to the TATA box and recruit an enzyme called RNA polymerase
• This synthesizes RNA from DNA
• Adding nucleotides occurs only to the 3' end of the growing RNA molecule
• Termination happens when the RNA polymerase separates from the DNA, releasing the newly produced RNA
• The RNA polymerase stops
• Next newly synthesized mRNA is made
• Hydrogen bonds are formed

Transcription Modifications

• A 5’ cap and a poly A tail is created
• RNA splicing removes silent introns and leaves behind expressed exons
• Mature mRNA consists of only exons

Ribosomes

• A site (Aminoacyl-tRNA binding site): incoming aminoacyl-tRNA binds to the ribosome
• P Site (Peptidyl-tRNA binding site): the peptidyl-tRNA carries the growing polypeptide chain
• E Site (Exit site): this is where the tRNA exits

Translation

• AUG is the start codon and methionine
• UAG, UAA, UGA are all stop codons

Protein Synthesis

• mRNA moves into the cytoplasm
• Then it becomes associated with ribosomes
• tRNAs that have anticodons carry amino acids to mRNA
• Anticodon-codon complementary base pairing then happens
• Polypeptide synthesis later occurs one amino acid at a time
• First tRNA releases its amino acid and goes off into the cytoplasm
• A peptide bond then joins the second and third amino acids to form a polypeptide chain