Molecular Biology: Gene Expression, Transcription, Translation, and Mutations

gene

A gene is a segment of DNA that contains the instructions to produce a functional product, usually a protein (or sometimes an RNA molecule).

central dogma of molecular biology

The central dogma describes the flow of genetic information: DNA → RNA → Protein (Information is transcribed from DNA into RNA, then translated from RNA into protein).

transcription

Transcription is the process of using a DNA template to synthesize RNA.

translation

Translation is the process of reading an mRNA sequence to build a polypeptide (protein).

genetic code

The genetic code assigns groups of three nucleotides (codons) to specific amino acids, telling the ribosome how to build a protein.

RNA polymerase

Prokaryotes use one RNA polymerase that synthesizes all RNA types (mRNA, tRNA, rRNA). It requires a sigma factor to bind promoters.

synthesis of transcript in prokaryotes

RNA polymerase binds promoter, DNA unwinds → transcription bubble, RNA is synthesized 5' → 3', Termination occurs by rho-dependent or rho-independent mechanisms.

RNA polymerase II

RNA polymerase II synthesizes mRNA and some noncoding RNAs. It binds promoters with help from many transcription factors.

synthesis of transcript in eukaryotes

Transcription factors recruit RNA pol II, RNA is synthesized 5' → 3', Occurs inside the nucleus.

5' methyl-G cap

A modified guanine is added to the 5' end to protect RNA and help ribosome binding.

3' poly-A tail

A long chain of adenines is added to the 3' end to protect mRNA and aid export from nucleus.

splicing

Introns (non-coding sequences) are removed; exons (coding sequences) are joined together.

tRNA structure

Cloverleaf shape, Anticodon loop (reads mRNA codon), 3' end attaches the correct amino acid, Each tRNA is linked to an amino acid by aminoacyl-tRNA synthetase.

ribosome structure

Made of rRNA + proteins, Has large and small subunits, Contains three sites: A (aminoacyl), P (peptidyl), E (exit).

initiation of translation in prokaryotes

Small ribosomal subunit binds Shine-Dalgarno sequence on mRNA, Initiator tRNA carries fMet, Large subunit joins.

initiation of translation in eukaryotes

Small subunit binds to 5' cap, Scans to the AUG start codon, Initiator tRNA carries Met, Large subunit joins.

elongation in translation

tRNA enters A site, Peptide bond forms in P site, Ribosome shifts (translocation), Empty tRNA exits from E site.

termination in translation

Stop codon enters A site, Release factor binds, Polypeptide is released, Ribosome dissociates.

mutation

A point mutation changes a single nucleotide.

silent mutation

Changes a codon but does not change the amino acid.

nonsense mutation

Changes a codon into a stop codon, ending translation early.

missense mutation

Changes a codon so that it codes for a different amino acid.