Genetics and Molecular Biology Overview

Genes and How They Work

• Genes' function was initially understood through the study of human diseases.
• Early 1900s linked genes to enzymes.
• Beadle and Tatum (1941) created mutations, tracked inheritance, and proposed the one-gene/one-enzyme hypothesis.
• The current understanding is the one-gene/one-polypeptide hypothesis.

Central Dogma

• Crick first described the flow of information from DNA to RNA to protein.
• DNA → Transcription → mRNA → Translation → Protein
• Proteins carry out essential cellular activities.

Transcription

• One DNA strand (template strand) guides RNA synthesis.
• DNA template is read 3' to 5', RNA is synthesized 5' to 3'.
• Thymine (T) in DNA is replaced by uracil (U) in RNA.
• New nucleic acid strands are always generated 5' to 3'.

RNA

• RNA is synthesized from a DNA template by transcription.
• Found in all organisms.
• mRNA codes for proteins.
• rRNA forms ribosomes.
• tRNA carries amino acids for translation.
• Other types of RNA include snRNA, srpRNA, miRNA, and siRNA, only found in eukaryotes.

Genetic Code

• A codon is a block of three mRNA nucleotides corresponding to an amino acid.
• The reading frame is crucial for correct protein synthesis.
• The genetic code is practically universal, a strong indicator of common ancestry.
• Stop codons terminate translation.
• Start codon (AUG) signals translation start.

Prokaryotic Transcription

• Single RNA polymerase.
• No primer needed.
• Transcription unit includes a promoter, start site, and terminator.
• The promoter is upstream of the start site; it's a recognition and binding site for RNA polymerase.
• Transcription is initiated at a promoter sequence and ends at a terminator sequence.
• mRNA translation can begin before transcription is complete.

Eukaryotic Transcription

• Three different RNA polymerases (I, II, III).
• RNA polymerase I transcribes rRNA.
• RNA polymerase II transcribes mRNA (and some snRNA).
• RNA polymerase III transcribes tRNA (and other small RNAs).
• More proteins are involved in initiation.
• Termination is somewhat different from prokaryotes.

Eukaryotic mRNA Modifications

• Primary transcripts must be modified to become mature mRNA.
• A 5' cap is added for translation initiation and protection from degradation.
• A 3' poly-A tail is added for protection from degradation.
• Non-coding sequences (introns) are removed by splicing.

Eukaryotic Pre-mRNA Splicing

• Introns are noncoding sequences.
• Exons are coding sequences that will be translated.
• Spliceosome removes introns and joins exons.

tRNA and Ribosomes

• tRNA molecules carry amino acids to ribosomes.
• The anticodon loop of tRNA matches to mRNA codons.
• Aminoacyl-tRNA synthetases "charge" tRNA with amino acids.

Ribosomes

• The large ribosomal subunit has three binding sites for tRNAs: E, P, and A.

Translation Initiation (Prokaryotes)

• Initiation complex includes initiator tRNA (with N-formylmethionine), small ribosomal subunit, and mRNA strand.

Translation Initiation (Eukaryotes)

• Similar to prokaryotes but with more complex initiation.
• Methionine is the initiator amino acid.
• Small subunit binds to the 5' cap of mRNA.

Translation Elongation

• Elongation adds amino acids to the growing polypeptide chain.
• Elongation factor EF-Tu binds to tRNA and GTP.
• A peptide bond forms aided by peptidyl transferase.
• Ribosome moves one codon (3 bases).

Translation Termination

• Elongation continues until a stop codon is reached.
• Release factors recognize stop codons, releasing the polypeptide.

Mutations

• Point mutations: alter a single base.
• Silent mutations: same amino acid inserted.
• Missense mutations: different amino acid inserted.
• Nonsense mutations: change to a stop codon.
• Frameshift mutations: addition or deletion of one or a few bases (not multiples of three), causing a major alteration in the reading frame.
• Mutations are the raw material of evolution; however, too much change can be harmful. A balance between variation and health is essential for species survival.