DNA Replication and Protein Synthesis

Stages of DNA Replication

Introduction

The primosome primases open strands and start helicases, unwinding the double helix. The rest of the replisome gets together with the polymerase, forming replication forks. After opening, the DNA strands bind to SSB proteins that prevent the DNA from renaturing or forming secondary structures. Other enzymes called topoisomerases prevent kinks and the formation of super-enrollments. The DNA polymerase catalyzes the first phosphodiester bond.

Elongation

In this step, enzymes called DNA polymerases catalyze the synthesis of new chains, rapidly adding hundreds of nucleotides to the template strand.

Termination

When a DNA polymerase reaches the end of another adjacent Okazaki fragment (or other replicon), the RNA primer is removed. DNA ligase connects the two newly synthesized DNA fragments by catalyzing the binding of the phosphate and sugar groups, respectively, of contiguous nucleotides. Once all the pieces are together, it completes the double helix of DNA.

Protein Synthesis: RNA Translation

Protein synthesis, or RNA translation, is an anabolic process by which proteins are formed from amino acids. It is the next step after the transcription of DNA into RNA. As there are 20 different amino acids and only four nucleotides in RNA (adenine, uracil, cytosine, and guanine), the colinearity must occur between each amino acid and nucleotide triplets. Since there are sixty-four different triplets (combinations of four nucleotides taken three at a time with repetition), it is obvious that some amino acids must correspond with several different triplets. The amino acid-coding triplets are called codons.

Initiation of Protein Synthesis

mRNA binds to the small subunit of ribosomes. Aminoacyl-tRNA is associated with these; the tRNA has, in one of its loops, a triplet of nucleotides called the anticodon, which attaches to the first triplet codon of the mRNA according to the complementarity of the bases. This group of molecules binds to the larger ribosomal subunit, forming the ribosomal complex or active complex. The resulting first codon is AUG, which corresponds to the amino acid methionine in eukaryotes (and formylmethionine in prokaryotes).

Elongation of the Polypeptide Chain

The ribosomal complex has a peptidyltransferase center (P site), where the first aminoacyl-tRNA is located, and an acceptor center (A site) for the new aminoacyl-tRNA. The carboxyl radical (-COOH) of the initiating amino acid joins the amino radical (-NH2) of the following amino acid through a peptide bond. The P site is then occupied by a tRNA without an amino acid. The free amino acid tRNA leaves the ribosome. Ribosomal translocation occurs. The dipeptidyl-tRNA is now at the P site. This is catalyzed by elongation factors. According to the third codon, the third aminoacyl-tRNA appears and occupies the A site. Then the tripeptide forms in A, and subsequently, its second ribosome translocation is made.

Completion of Polypeptide Chain Synthesis

The end of the synthesis is signaled by stop codons. There are three: UAA, UAG, and UGA. There is no tRNA whose anticodon is complementary to them, and therefore, the synthesis of the polypeptide is disrupted. This process is regulated by release factors, proteins in nature, which are located at the A site and cause peptidyl-transferase to separate the polypeptide chain from the tRNA by hydrolysis. If the mRNA is long enough, it can be read or translated by several ribosomes simultaneously.