From DNA to Protein Ch 9

RNA Ribonucleic Acid Nucleic acid, single strand Contains the sugar ribose RNA has the same base pairing rule as DNA, but it has Uracil instead of Thymine A – U C – G Three major types of RNA mRNA, rRNA, tRNA

Ribonucleic Acid Messenger RNA (mRNA): forms complementary to one strand of DNA, contains amino acid sequence (protein- building message) and carries it from the nucleus to the ribosome to direct the synthesis of a specific protein Ribosomal RNA (rRNA): main component of ribosomes, assembles amino acids into polypeptide chains Transfer RNA (tRNA): delivers amino acids to a ribosome during protein synthesis

Gene Expression Transcription and translation are part of gene expression: Multistep process by which information from genes guides the assembly of an RNA or protein product DNA is TRANSCRIBED into RNA, RNA is TRANSLATED into proteins

Transcription Transcription: synthesis of DNA code into mRNA The DNA is unzipped in the nucleus An enzyme called RNA polymerase moves along the template DNA strand It reads the template strand and a complementary mRNA strand is synthesized [uracil is substituted for thymine] When the polymerase reaches the end of the gene, the mRNA is released and moves into the cytoplasm through nuclear pores

Transcription mRNA code is significantly shorter than DNA Introns: interrupting sequences that DO NOT code for anything, these are removed before the mRNA leaves the nucleus Exons: The coding regions where the genes are located, stay in the mRNA In a process called alternative splicing, exons are rearranged and spliced together in different combinations producing multiple versions of a gene

Transcription Codon: sequence of three nucleotide bases that code for an amino acid (or stop signal) Amino acids make up proteins, proteins are building blocks of cells and enzymes The sequence of bases in a triplet determines which amino acid the codon specifies Codons occur one after another in an mRNA, and when it is translated, the order of codons determines the order of amino acids in the resulting polypeptide Some signal the beginning or end of a protein-coding sequence

Translation Translation: sequence of codons in mRNA is read and translated to make a protein Anticodon: a triplet of tRNA nucleotides that base-pairs with an mRNA codon Once in the cytoplasm, mRNA moves to a ribosome The tRNA anticodon binds to the mRNA codon in the ribosome Additional tRNA’s continue to move into the ribosome, binding to additional mRNA codons as the mRNA moves through the ribosome like a ribbon.

Translation The ribosome forms peptide bonds between the amino acids, growing polypeptide (protein) chain As the ribosome moves the next codon, it releases the tRNA This continues until a stop codon is reached and then protein synthesis stops The amino acid chain (polypeptide) folds into a protein

Mutations When a mutation occurs in a protein-coding region, the redundancy of the genetic code offers a margin of safety a mutation that changes a CCC codon to CCG may not have further effects, both of these specify the amino acid serine Other mutations may change an amino acid in a protein or result in a premature stop codon that shortens it Mutations that alter a protein can have drastic effects on an organism

Mutations Base-Pair Substitution: chemical change in just 1 base pair  Insertion: addition of a nucleotide to the DNA sequence Deletion: removal of a nucleotide to the DNA sequence Insertions and deletions result in frameshifts Mutations can lead to genetic disorders or can be harmless

Mutations Sickle cell anemia occurs because of a base-pair substitution Causes hemoglobin molecules to clump together and red blood cells to form a crescent (sickle) shape Sickled cells clog tiny blood vessels, disrupting blood circulation throughout the body

Ricin Ricin is a ribosome-inactivating protein (RIP) RIPs remove adenine bases from rRNAs which halts protein synthesis Death from ricin exposure occurs in days due to low blood pressure and respiratory failure A dose of ricin as small as a few grains of salt can kill an adult human

Ricin Bulgarian spy’s weapon: an umbrella modified to fire a tiny pellet of ricin into a victim. An umbrella like this one was used to assassinate Georgi Markov on the streets of London in 1978.