Genetic Code and Transcription

The Genetic Code

• The genetic code was deciphered in the 1960s through experiments using artificial RNA molecules.
• A UUU RNA molecule produced a polypeptide of phenylalanine, confirming that the UUU codon specifies phenylalanine.
• Of the 64 codons, 61 code for amino acids, and 3 are stop codons.
• AUG codes for methionine and signals the start of a polypeptide chain.
• UAA, UGA, and UAG codons signal the end of translation.
• RNA codons are complementary to DNA codons and are arranged linearly without gaps.
• The genetic code has redundancy, where multiple codons can specify the same amino acid.
• There is no ambiguity in the genetic code, meaning each codon represents only one amino acid.
• The universality of the genetic code across organisms supports the idea that it evolved early in the history of life.

Transcription

• Transcription transfers genetic information from DNA to RNA, using one DNA strand as a template.
• RNA polymerase moves along the gene, synthesizing an RNA strand by base-pairing, with uracil (U) replacing thymine (T).
• Transcription starts at a promoter sequence and ends at a terminator sequence.

Stages of Transcription

• Initiation: RNA polymerase binds to the promoter region of the DNA, unwinds the double helix, and starts RNA synthesis.
• Elongation: The RNA strand grows as RNA polymerase moves along the gene, with the new RNA molecule peeling away from the DNA template.
• Termination: RNA polymerase reaches the terminator sequence and detaches from the RNA and DNA.
• Special DNA sequences (promoters and terminators) mark the start and end of a gene.

Eukaryotic RNA Processing

• In eukaryotic cells, mRNA is transcribed in the nucleus and must be processed before moving to the cytoplasm for translation.
• Eukaryotic transcripts undergo modifications, including adding a cap (modified G nucleotide) at the 5' end and a tail (50-250 A nucleotides) at the 3' end.
• The cap and tail facilitate mRNA export, protect from degradation, and help ribosomes bind.
• RNA splicing is a process in which noncoding regions (introns) are removed, and coding regions (exons) are joined together.
• RNA splicing is catalyzed by proteins and RNA molecules.
• RNA splicing allows multiple polypeptides to be produced from a single gene by varying the included exons.

Translation

• Translation converts the nucleic acid language of mRNA into the protein language of amino acids.
• Translation requires mRNA, enzymes, chemical energy, ribosomes, and transfer RNA (tRNA).
• Eukaryotic genes are longer than mRNA because of the presence of introns.

Transfer RNA (tRNA)

• tRNA acts as an interpreter, converting mRNA codons into amino acid sequences.
• tRNA carries amino acids to the growing polypeptide in a ribosome.
• tRNA picks up the correct amino acid and recognizes the corresponding mRNA codon.
• tRNA molecules are made from a single RNA strand of about 80 nucleotides, forming a cloverleaf structure.
• tRNA contains an anticodon, a triplet of bases complementary to a codon on mRNA.
• At the other end of tRNA, a specific amino acid attaches.
• Each amino acid is joined to the correct tRNA by a specific enzyme, using ATP.
• An anticodon is the base triplet of a tRNA molecule that allows it to couple to a complementary codon in mRNA.

Ribosomes

• Ribosomes coordinate mRNA and tRNA function and catalyze polypeptide synthesis.
• A ribosome consists of a large subunit and a small subunit, made of proteins and ribosomal RNA (rRNA).
• Bacterial and eukaryotic ribosomes differ in size and structure, which is medically significant.
• Antibiotics like tetracycline and streptomycin can target and inactivate bacterial ribosomes.
• A ribosome binds mRNA on the small subunit and tRNA on the large subunit (P and A sites).
• Ribosomes holds mRNA and tRNAs together and connects amino acids from the tRNAs to the growing polypeptide chain.

Elongation

• Incoming tRNA anticodon pairs with the mRNA codon in the ribosome's A site.
• The polypeptide detaches from the tRNA in the P site and forms a peptide bond with the amino acid on the tRNA in the A site.
• The ribosome moves the tRNA with the growing polypeptide from the A site to the P site.
• Elongation continues until a stop codon (UAA, UAG, or UGA) is reached, which signals termination.
• If mutation caused the UUA codon in the middle of an mRNA to mutate to UAA, then translation would halt prematurely.

Flow of Genetic Information

• Genes encode RNA molecules used to produce proteins, controlling structures and functions.
• Transcription (DNA to RNA) synthesizes mRNA from a DNA template.
• Translation (RNA to protein) occurs in the cytoplasm.
• Polypeptides fold into proteins, which determine the cell's and organism's appearance and functions.
• This whole flow allows genes to influence an organism's traits.
• DNA does not participate directly in translation.

Mutations

• Sickle-cell disease is caused by a single amino acid change in hemoglobin due to a single nucleotide difference.
• A mutation is an alteration of one nucleotide pair in the DNA double helix.
• A nucleotide substitution replaces one nucleotide pair with another.
• Silent mutations have no effect on the protein product because the new codon codes for the same amino acid.
• Missense mutations change one amino acid to another in a protein, potentially impairing function.
• Nonsense mutations convert an amino acid codon into a stop codon, leading to a prematurely terminated protein.
• Frameshift mutations occur when nucleotides are added or subtracted, altering the reading frame and producing nonfunctional polypeptides.
• Mutations can arise spontaneously or be caused by mutagens.
• High-energy radiation and chemical agents are physical and chemical mutagens.
• A single nucleotide substitution can result in a shortened protein if it changes an amino acid codon into a stop codon.