Chapter 17: From Gene to Protein

DNA makes RNA makes Protein Summary of Transcription and Translation A triplet code in DNA is transcribed into a codon sequence in mRNA This pre-RNA is processed in the nucleus The codon sequence is then translated into an amino acid sequence at the ribosome

DNA makes RNA makes Protein If the strand of DNA triplets to be transcribed is: 5’ – AAA TAA CCG GAC – 3’ Then the strand of mRNA codons that forms is: 3’ – UUU AUU GGC CUG – 5’ The tRNA anticodon strand complementary to the mRNA strand is: AAA UAA CCG GAC

Transcription Transcription is the process by which DNA makes RNA mRNA: messenger; carries messages directly from DNA to cytoplasm tRNA: shaped like cloverleaf; carries amino acids to the mRNA at the ribosome rRNA: ribosomal; makes up ribosome

3 Stages: initiation, elongation & termination RNA polymerase recognizes and binds to DNA at the promoter region (facilitates transcription of a gene) A collection of proteins called transcription factors recognize a key area within the promoter called the TATA box and mediates the binding of RNA polymerase to DNA Transcription factors + RNA polymerase + promoter = transcription initiation complex

3 Stages: initiation, elongation & termination Now that RNA polymerase has attached to the promoter, transcription of the DNA template can begin

3 Stages: initiation, elongation & termination RNA polymerase adds nucleotides to the 3’ end of a growing chain RNA polymerase pries the two strands of DNA apart and attaches RNA nucleotides according to the base pairing rules: C with G, A with U Stretch of DNA transcribed into mRNA molecule is called a transcription unit

3 Stages: initiation, elongation & termination Each unit consists of triplets of bases called codons (AAU, CGA), which code for specific amino acids

3 Stages: initiation, elongation & termination Final stage of transcription Elongation continues for a short distance after the RNA polymerase transcribes the termination sequence (AAUAAA) mRNA is now cut free from the DNA template

RNA Processing Pre-RNA that has just formed cannot be shipped to the ribosome until it is processed by a series of enzymes A 5’ cap consisting of modified guanine nucleotide is added to the 5’ end and helps RNA bind to the ribosome

RNA Processing A poly (A) tail, consisting of a string of adenine nucleotides, is added to the 3’ strand Protects RNA from degradation, helps ribosomes attach to RNA, facilitates release of RNA into cytoplasm Noncoding regions of mRNA called introns or intervening sequences are removed by snRNPs and spliceosomes Allows exons, or expressed sequences, to only leave the nucleus

Translation Translation is the process by which the codons of an mRNA sequence are changed into an amino acid sequence Amino acids are present in a pool in the cytoplasm They are carried by tRNA molecules to the codons of the mRNA strand at the ribosome according to base pairing rules

Translation One end of the tRNA molecules holds the specific amino acid; the other end bears a nucleotide triplet called the anticodon Energy for this process is provided by GTP AUG is the start codon; UAA, UGA and UAG are the stop codons

Translation Some tRNA molecules have anticodons that can recognize two or more different codons This happens because pairing rules for the third base of a codon are not as strict as they are for the first two bases ; this is called wobble Example: UCU, UCC, UCA and UCG all code for serine

Initiation, Elongation & Termination Initation mRNA attaches to a subunit of the ribosome The first codon is always AUG, which codes for methionine Elongation tRNA brings amino acids to the ribosome and a polypeptide chain is formed

Initiation, Elongation & Termination The ribosome reaches one of three termination or stop codons A release factor breaks the bond between the tRNA and the last amino acid of the polypeptide chain Polypeptide is then freed and mRNA is broken down

Gene Mutation Mutations are changes in genetic material that occur spontaneously and randomly Mutagenic agents include toxic chemicals and radiation Genetic disorders or hereditary diseases occur when mutations have adverse effects in somatic cells

Gene Mutation Mutations occurring in gametes can be passed on to offspring and change the gene pool of a population Mutations are the raw material for natural selection

Point Mutation Simples mutation caused by a base-pair substitution Example: THE FAT CAT SAW THE DOG THE FAT CAT SAW THE HOG Remember wobble If the final base in a sequence is different, it may not call for a harmful mutation

Insertion or Deletion Deletion = loss of a letter Insertion = addition of a letter Both result in a frameshift Example: Deletion THE FAT CAT SAW THE DOG THF ATC ATS AWT HED OG

Insertion or Deletion Example: Insertion THE FAT CAT SAW THE DOG THE FTA TCA TSA WTH EDO G A mutated polypeptide is formed or no polypeptide is formed When point mutations or frameshifts change a codon within a gene into a stop codon, a missense or nonsense mutation occurs