Gene Expression and Protein Synthesis Essentials
Genetic Code Essentials
The genetic code defines the relationship between nucleotide triplets (codons) and amino acids (AA). It operates in the 5′ to 3′ direction. Most codons are synonymous, meaning multiple codons can code for the same amino acid. There are 64 possible codons, with 61 coding for amino acids.
Regulation of Gene Expression
Cells regulate gene expression to control when proteins are produced.
DNA Replication
DNA replication is the process of duplicating DNA in each cell generation. It occurs bidirectionally from specific origins, with DNA polymerases synthesizing new strands in the 5′ to 3′ direction. Primers are required to initiate synthesis. Replication is semi-conservative, meaning each new DNA molecule contains one original and one new strand.
Transcription
Transcription is the process of creating an RNA molecule complementary to a DNA strand. In prokaryotes, translation can begin before transcription is complete. In eukaryotes, RNA must mature before translation. Steps include:
- Initiation: RNA polymerase recognizes a promoter sequence, causing the DNA duplex to open.
- Elongation: RNA polymerase adds nucleotides complementary to the DNA template.
- Termination: RNA polymerase reaches a termination sequence (e.g., UAA).
- Maturation (Eukaryotes): A 5′ cap and a 3′ poly-A tail are added, and introns are removed (splicing).
Translation (Protein Biosynthesis)
Translation is the process of reading mRNA to create proteins. It follows the genetic code, where each codon corresponds to an amino acid.
- Initiation: mRNA binds to a ribosome subunit, and a tRNA carrying methionine (AUG) binds to the start codon.
- Elongation: The ribosome moves along the mRNA, reading codons and adding amino acids to the growing polypeptide chain.
- Termination: A stop codon (UAA, UAG, UGA) signals the end of translation, and the ribosome complex disassembles.