DNA Replication and Protein Synthesis: From Genetic Code to Functional Proteins
DNA Replication
Genetic information must be copied to be transmitted during cell division to daughter cells. This self-duplication or replication process occurs in the synthesis phase (S) of the nucleus in eukaryotes or the cytoplasm in prokaryotes. According to the Watson and Crick theory, replication is semiconservative: each maternal DNA strand serves as a template for synthesizing a complementary strand. Consequently, each of the two resulting DNA molecules comprises one original and one newly synthesized complementary strand.
Replication Stages
Replication consists of three stages, similar in eukaryotes and prokaryotes:
- Unwinding and Opening of the Double Helix: To be copied, the two DNA strands must be separated. Enzymes like helicases and topoisomerases unwind the double helix by breaking hydrogen bonds between bases, separating the chains, relieving tension, and stabilizing the separated strands. Strand separation begins at the origins of replication. The process is bidirectional, with a helicase working in each direction, forming a replication bubble that extends along the DNA. The separated strands act as templates for synthesizing new ones.
- Synthesis of New Strands: The enzyme DNA polymerase performs two functions. First, it reads the template strand in the 3′ to 5′ direction and selects the deoxyribonucleotide triphosphate whose base is complementary to the template base. Second, it catalyzes the hydrolysis of the nucleotide into a monophosphate and pyrophosphate, incorporating the nucleotide into the daughter DNA chain via a phosphodiester bond. The strand with the free 3′ end is the leading strand, copied continuously as the replication fork opens. The other strand, the lagging strand, grows discontinuously because DNA polymerase cannot read it continuously. It is synthesized in short fragments called Okazaki fragments, which are later joined together.
- Correction of Errors: During replication, DNA polymerase is self-correcting. After attaching a nucleotide, it checks for errors in matching before proceeding. If an error is detected, the incorrect nucleotide is removed, and the correct one is inserted.
Protein Synthesis (Translation)
Protein synthesis, or translation, is an anabolic process where proteins are formed from amino acids. It follows the transcription of DNA into RNA. Since there are 20 different amino acids and only four nucleotides in RNA (Adenine, Uracil, Cytosine, and Guanine), a minimum of three nucleotides (a triplet) must code for each amino acid. With 64 possible triplets (combinations of four nucleotides taken three at a time with repetition), some amino acids are encoded by multiple triplets. These triplets are called codons.
Initiation
mRNA binds to the small subunit of ribosomes. Aminoacyl-tRNA, carrying an anticodon (a triplet of nucleotides complementary to the mRNA codon), associates with the mRNA. The large ribosomal subunit joins, forming the active ribosomal complex. The initiation codon is AUG, which codes for methionine in eukaryotes and formylmethionine in prokaryotes.
Polypeptide Chain Elongation
The ribosome has two sites: the P (peptidyl) site, where the first aminoacyl-tRNA binds, and the A (acceptor) site, where the next aminoacyl-tRNA binds. The carboxyl group (-COOH) of the initiated amino acid joins the amino group (NH2) of the following amino acid through a peptide bond. The tRNA without an amino acid leaves the ribosome. Ribosomal translocation occurs, moving the dipeptidyl-tRNA to the P site. Elongation factors catalyze this process. The next codon is read, the corresponding aminoacyl-tRNA occupies the A site, and a tripeptide is formed. The ribosome translocates again.
Termination
The synthesis ends when a stop codon (UAA, UAG, or UGA) is encountered. There is no tRNA with a complementary anticodon for these codons, disrupting the polypeptide synthesis. Release factors, proteins that bind to the A site, cause the peptidyl transferase to hydrolyze the bond between the polypeptide chain and the tRNA, releasing the completed protein. A single mRNA molecule, if sufficiently long, can be read or translated by multiple ribosomes simultaneously.
RNA Transcription
Transcription is the first step in gene expression, where DNA sequences are copied into RNA by RNA polymerase. Transcription produces messenger RNA (mRNA), which is used as a template for protein synthesis.
RNA Polymerase
RNA polymerase is a group of enzymatic proteins that polymerize ribonucleotides to synthesize RNA from a DNA template. It catalyzes the joining of ribonucleotide triphosphates, releasing phosphate groups.
Messenger RNA (mRNA)
mRNA is a ribonucleic acid that carries genetic information from DNA to be used in protein synthesis. It determines the order in which amino acids are joined together.
Transcription Process
- Initiation: RNA polymerase recognizes promoter sequences, unwinds the double helix, and begins joining ribonucleotides to the template strand.
- Elongation: RNA polymerase advances in the 3′ to 5′ direction on the DNA template, synthesizing RNA in the 5′ to 3′ direction.
- Termination: RNA polymerase recognizes a termination signal (palindromic sequences in prokaryotes) on the DNA, indicating the end of the transcript.