DNA Replication, Transcription, and Translation Processes

Posted on Jan 9, 2025 in Biology

DNA Replication

At the origin of replication, DNA is unzipped by helicase. Single-strand DNA-binding proteins prevent the double helix from forming again.
Primase adds RNA primers as a binding site for DNA polymerase.
DNA polymerase adds nucleotides to synthesize the new strand of DNA at the 3′ end.
Removal of primers by exonuclease.
Filling the gap by DNA polymerase. The leading strand is synthesized continuously in the 5′ to 3′ direction, and the lagging strand is synthesized discontinuously in fragments called Okazaki fragments.
Sealing the gap by ligase.

Initiation: Binding of RNA polymerase to regulatory elements of the gene with the help of transcription factors.
Elongation: Addition of nucleotides by RNA polymerase to newly synthesized RNA using complementary base-pairing rules.
Termination: Loop formation leading to the release of newly synthesized RNA.

mRNA goes to the cytoplasm.
tRNA molecules transport amino acids to ribosomes during protein synthesis. They have sites for amino acid attachment by the aminoacyl-tRNA synthetase enzyme. Aminoacyl-tRNA binds to the A site of the ribosome, and it has anticodons on its end, which are three nucleotides that complement the RNA strand. Each amino acid has a different anticodon.
rRNA is the ribosomal RNA that the tRNA binds to in its A site, then moves to the P site, and at the end, it exits from the E site.

Addition of a 5′ methylated guanine cap (5′ cap) for protection and the start of translation.
Addition of a 3′ poly-A (adenine) tail involved in the stability of RNA and also translation.
Removing introns (non-coding sequences) and joining exons (coding sequences of a gene).

Initiation: The small subunit of the ribosome joins mRNA and finds the start codon (AUG, universal). The correct tRNA carrying the amino acid methionine (Met) joins the start codon. The large subunit joins the complex now so that the tRNA carrying Met is located in the P site of the large subunit.
Elongation: The next tRNA carrying the correct amino acid for the adjacent codon is brought to the A site. Peptidyl transferase makes a peptide bond between the first two amino acids. The ribosome moves forward along the mRNA strand so the uncharged tRNA will be released from the E site, and the A site will be available for the next tRNA.
Termination: The ribosome reaches the stop codon, and releasing factors bind the stop codon in the A site. All the components, including mRNA, ribosome, and polypeptide, are released.

Start codon is AUG. Stop codons are UGA, UAA, and UAG.

Not all genes will be turned on at the same time.
It is how genes can be turned on and off in response to signals.
A strong rRNA makes more than a weak one, and it attracts polymerase better.
Alternative processing is about what intron is removed, depending on the cell and the region it is in.
A promoter is a DNA sequence located upstream from the gene, like the TATA and CAAT boxes. RNA polymerase cannot bind to the promoter region without the presence of transcription factors (DNA-binding proteins).
Enhancer DNA sequences can bind to activators.

siRNA: It binds to mRNA and blocks translation.
miRNA: If miRNA matches mRNA, it degrades it, but if it doesn’t match, it blocks translation.

Bacteria use operons, which are clusters of several related genes located together and controlled by one promoter.
Lac operon:
- lac Z codes for β-galactosidase to break down lactose.
- lac Y codes for galactoside permease, which transports lactose into the cell.
- lac A codes for acetyltransferase, which has a protective function.
When lactose is present, it binds to the repressor, causing it to change shape and fall off the operator, allowing the RNA polymerase to transcribe these three genes. When glucose is present, the repressor stays on.