How an Organism’s DNA Genotype Produces Its Phenotype
• Genotype- genetic make-up; the sequence of nucleotides• Phenotype- organism’s specific traits• The chain of command is from DNA in the nucleus of the cell to RNA to protein synthesis in the cytoplasm Two Major Stages Transcription- the transfer of genetic information from the DNA into an RNA molecule Translation- the transfer of the information in the RNA into a protein
Gene- Enzyme- Protein- Polypeptide• One Gene- One Enzyme Archibald Garrod- suggested that genes dictate phenotypes through enzymes, the proteins that catalyze chemical processes (1909). George Beadle and Edward Tatum- formulated the one gene- one enzyme hypothesis (the function of an individual gene is dictated the production of a specific enzyme)• One Gene- One Protein The one gene- one protein hypothesis was modified and extended beyond enzymes to include all types of proteins.• One Gene- One Polypeptide It was discovered that many proteins have one or more different polypeptide chains, and each polypeptide is specified by its own gene.
To understand how geneticinformation passes from genotype tophenotype, we need to see how thechemical language of DNA istranslated into the different chemicallanguage of polypeptides.
From Nucleotide Sequence to AminoAcid Sequence: An Overview • DNA & RNA- polymers made of monomers in specific sequences that carry information in English. In DNA the monomers are four types of nucleotides, which differ in nitrogenous bases (A, T, C and G). In RNA the same is true although it has U instead of T.
Specific sequences of bases, each witha beginning and an end, make up thegenes on a DNA strand. A typical geneconsists of thousands of nucleotides, anda DNA molecule may contain thousands ofgenes.
Transcription• Process when DNA is transcribed and the results to RNA molecule. Because the nucleic acid language of DNA has simply been rewritten (transcribed) as a sequence of bases of RNA; the language is still that of nucleic acids.Translation• Conversion of nucleic acid into the polypeptide language polypeptides are polymers, but the monomers that make them up- the letters of the polypeptide alphabet- are 20 amino acids common to all organisms
Triplet Code• Basis of the flow of information from gene to protein.• The smallest “words” of uniform length that can specify al the amino acids.
Codons• The genetic instructions for the amino acid sequence of a polypeptide chain are written in RNA and DNA as a series of three-base words called codons.• 1st codon was deciphered in 1961 by Marshall Nirenberg
The Dictionary of the GeneticCode (Listed by RNA codons) • Triplet AUG has a dual function Codes for amino acid methionine (Met) Provide signal for the start of polypeptide chain. • Three of the other codons do not designate amino acids. They are the stop codons that instruct the ribosomes to end the polypeptide (UAA,UAG, UGA) • Tryptophan (Trp)- the only codon for the amino acid (but most amino acids are specified but two or more codons)
The Genetic Code• The set of rules relating nucleotide sequence to amino acid sequence.• Notice in the dictionary that there is redundancy but no ambiguity. For example, although codons UUU and UUC both specify phenylalanine (redundancy), neither of them ever represents any other amino acid (ambiguity).
Transcription: From DNA to RNA• An RNA molecule is transcribed from a DNA by a process similar to DNA replication.• RNA polymerase- the transcription enzyme that links the RNA nucleotides. 3 Stages Initiation of Transcription RNA Elongation Termination of Transcription
Initiation of Transcription• Promoter- nucleotide sequence signaling the start of transcription which is located at the start of the gene – a specific place where RNA polymerase attaches. – dictates which of the two strands is to be transcribed.• Initiation- 1st phase; the attachment of RNA polymerase to the promoter and the start of RNA synthesis.
RNA Elongation• Elongation- 2nd phase; RNA grows longer• The RNA strand peels away from its DNA template, allowing two separated DNA strands to come back together in the region already transcribed.
Termination of Transcription• Termination- 3rd phase; RNA polymerase reaches a special sequence of bases in the DNA template.• Terminator- term for the special sequence that the RNA reaches. – Signals the end of the gene• The polymerase molecules detaches from the RNA molecule and the gene.
The transcription of anentire gene occurs in 3phases.As RNA nucleotidesbase-pair one by onewith DNA bases on oneDNA strand (called thetemplate strand), theRNA polymerase linksthe RNA nucleotidesinto an RNA Chain. Thegreen shape in thebackground is the RNApolymerase.
The Processing of Eukaryotic RNA• Messenger RNA (mRNA)- the result of transcription in prokaryotic cells, wherein RNA transcribed from the gene immediately functions as the messenger molecule.• RNA Processing- or modification on RNA in transcribed in eukaryotic cells before translation• Thru these processes the “final draft” of the eukaryotic mRNA is ready for translation
RNA Processing• Capping- addition of extra nucleotides to the ends of the RNA transcript. Cap and Tail- the additions; protects the RNA fro attack by cellular enzymes and help ribosomes recognize the RNA as mRNA.• Tailing- made by noncoding stretches of nucleotides that interrupts the nucleotides that actually code for amino acids. introns- internal noncoding regions extrons- the coding region; the parts of the gene that are expressed.
• RNA splicing- The process before the RNA leaves the nucleus, the introns are removed and the exons are joined to produce an mRNA molecule with a continuous coding sequence.