RNA Learning Objectives Contrast RNA and DNA. Explain the process of transcription. Comparing RNA and DNA The sugar in RNA is ribose instead of deoxyribose. RNA is generally single-stranded, not double-stranded. RNA contains uracil in place of thymine. What can be found in both…. Phosphate groups, guanine, and cytosine The Role of RNA The roles played by DNA and RNA molecules in directing protein production are like the two types of plans builders use: A master plan A blueprint Types of RNA The three main types of RNA are: Transfer RNA Ribosomal RNA Messenger RNA Messenger RNA An mRNA molecule is a copy of the portion of DNA that will be used to make a protein. After being made in the nucleus, mRNA travels to the cytoplasm, the site of protein synthesis. The complementary strand of RNA that is produced during transcription is called MRNA Ribosomal RNA Protein synthesis occurs on ribosomes, which are made up of two subunits. These subunits consist of several molecules of ribosomal RNA (rRNA) and as many as 80 different proteins. Transfer RNA During protein synthesis, transfer RNA molecules (tRNA) carry amino acids from the cytoplasm to the mRNA. RNA Synthesis: Transcription • In transcription, segments of DNA serve as templates to produce complementary mRNA molecules. • Takes place in the nucleus RNA Synthesis: Promoters RNA polymerase binds only to regions of DNA that have specific base sequences. These regions are called promoters. RNA Synthesis: RNA Editing New RNA molecules sometimes require a bit of editing before they are ready to be read. The pieces of pre-mRNA molecules that are cut out, or “edited out,” and discarded Exons are called introns. Cap The remaining pieces are known as exons. Introns Tail Ribosomes and Protein Synthesis Learning Objectives Identify the genetic code and explain how it is read. Summarize the process of translation. Describe the central dogma of molecular biology. The Genetic Code RNA has four bases: adenine, cytosine, guanine, and uracil. These bases form a “language”: A, C, G, and U. The Genetic Code: Codons The genetic code is read in three-letter groupings called codons. A codon is a group of three nucleotide bases in messenger RNA that specifies a particular amino acid. AUG AAC UCU Genetic Code Table There are 64 possible three-base codons in the genetic code. Several different codons can specify the same amino acid Reading Codons Start at the middle of the circle with the first letter of the codon and move outward. CAC = Histidine Start and Stop Codons The methionine codon AUG serves as the “start” codon for protein synthesis. There are three “stop” codons. UAA, UAG, and UGA are “stop” codons AUG = methionine = “start” codon Translation Transcribed mRNA directs the translation process. Translation is the process that produces proteins by decoding the sequence of mRNA codons. During translation, the type of amino acid that is added to the growing polypeptide depends on the codon on the mRNA and the anticodon on the tRNA. Takes place in the cytoplasm Translation: Transfer RNA • Translation starts when a ribosome attaches to an mRNA molecule. Then, tRNA molecules, carrying amino acids with them, bind to mRNA codons. • Transfer RNA is important in adding amino acids to polypeptide anticodon Translation: The Polypeptide Assembly The ribosome helps form a peptide bond. It breaks the bond holding the first tRNA molecule to its amino acid. Translation: Completing the Polypeptide Third step: The ribosome reaches a stop codon, releasing the newly synthesized polypeptide and the mRNA molecule, completing the process of translation. Roles of RNA in Translation All three major forms of RNA—mRNA, tRNA, and rRNA—are involved in the process of translation. The Molecular Basis of Heredity • The central dogma of molecular biology is that information is transferred from DNA to RNA to protein. • An exception to the central dogma of molecular is that viruses sometimes transfer information from RNA to DNA Gene Expression When a gene (segment) of DNA code is used to build a protein, scientists say that gene has been expressed. Mutations Learning Objectives Define mutations and describe the different types of mutations. Describe the effects mutations can have on genes. Mutations Mutations are heritable changes in genetic information. Types of Mutations Mutations fall into two basic categories: Gene mutations Chromosomal mutations Gene Mutations: Point Mutations A point mutation is a change in a single nucleotide. There are three types of point mutations: Point Mutations: Substitutions In a substitution, one base is changed to a different base. Point Mutations: Insertions and Deletions • Insertion mutation: when a single extra base is added into the code • Deletion mutation: when a single base is removed from the code • Frameshift mutations usually produces a more drastic change than a point mutation Chromosomal Mutations Deletion Duplication Inversion Translocation Effects of Mutations Mutations can harm, help, or have no effect on an organism. Some mutations arise from mutagens—chemical or physical agents in the environment. Effects of Mutations: Harmful Some of the most harmful mutations are those that dramatically change protein structure or gene activity. Example: Sickle cell disease affects the shape of red blood cells. Sickle cell Normal red blood cell Effects of Mutations: Beneficial • Mutations often produce proteins with new or altered functions that can be useful to organisms in different or changing environments. • For example, when a complete set of chromosomes fails to separate during meiosis, the gametes that result may produce triploid (3N) or tetraploid (4N) organisms. The condition in which an organism has extra sets of chromosomes is called polyploidy. • Polyploid plants are often larger and stronger than diploid plants Gene Regulation and Expression Learning Objectives Describe gene regulation in prokaryotes. Explain how most eukaryotic genes are regulated. Relate gene regulation to development in multicellular organisms. Prokaryotic Gene Regulation DNA-binding proteins in prokaryotes regulate genes by controlling transcription. An operon is a group of genes that are regulated together One of the keys to gene transcription in bacteria is the organization of genes into operons. The Lac Operon When lactose is not present, the lac genes are turned off by regulatory proteins that bind to DNA and block transcription. Promoters and Operators Located in front of the operon’s three genes are two regulatory regions: A promoter - an RNA polymerase binding site on a typical gene An operator- where a DNA-binding protein known as the lac repressor can bind to DNA. The Lac Repressor Blocks Transcription When the lac repressor binds to the O region, RNA polymerase cannot reach the lac genes to begin transcription. Repressor protein Lactose Turns On the Operon When lactose is added to the medium, it diffuses into the cell and attaches to the lac repressor. RNA polymerase Lactose Repressor protein with changed shape Eukaryotic Gene Regulation A typical eukaryotic gene has a TATA box binds a protein that helps position RNA polymerase by marking a point just before the beginning of a gene. Transcription Factors By binding DNA sequences in the regulatory regions of eukaryotic genes, transcription factors control the expression of those genes. Cell Specialization Complex gene regulation in eukaryotes is what makes differentiation and specialization possible. RNA Interference Small RNA molecules that do not belong to any of the major groups of RNA play a powerful role in regulating gene expression. They do so by interfering with mRNA. RNA Interference The small interfering RNA molecules fold into double-stranded hairpin loops. The dicer enzyme cuts the double strands into microRNA (miRNA). RNA Interference The two strands of the loops separate. One of the miRNA pieces attaches to a cluster of proteins, forming a silencing complex. RNA Interference Blocking gene expression by means of an miRNA silencing complex is known as RNA interference. The Promise of RNAi Technology The discovery of RNAi has made it possible for researchers to switch genes on and off at will, simply by inserting doublestranded RNA into cells. It may provide new ways to treat and perhaps even cure diseases. Genetic Control of Development • Regulating gene expression is important in shaping how a multicellular organism develops. • Transcription factors regulate the expression of most eukaryotic genes • Each of the specialized cell types found in the adult originates from the same fertilized egg cell. Homeotic, Homeobox, and Hox Genes Homeotic genes regulate organ development. Homeobox genes code for transcription factors. Hox genes determine the identities of each body segment. Environmental Influences Environmental factors can affect gene regulation. Metamorphosis is an example of how organisms can regulate gene expression in response to change in their environment.
