Lecture 21 Outline of Last LectureI. The Cell CycleII. Meiosis and Sexual Cell CyclesIII. Mendel and ChromosomesIV. The Molecular Basis of Inheritance Outline of Current Lecture I. Split Genes and RNA ProcessingII. MicroRNAs and Gene ExpressionIII. MutationsIV. VirusesV. Types of VirusesCurrent LectureMutations and VirusesI. Split Genes and RNA Processinga. Occurs in eukaryotesb. Coding regions in DNAi. DNA has “coding regions” that carry the code for how to make a protein  “exons”ii. DNA has “noncoding regions” that don’t encode the protein  “introns”iii. Most DNA is noncodingiv. Most coding regions are interrupted by long noncoding regionsc. During the transcription of DNA to make mRNA, the noncoding regions are transcribed with the coding regions to look like:i. 5’ – exon #1  intron #1  exon #2  intron #2  exon #3 – 3’d. “RNA splicing” – After transcription, the introns are removed from the mRNA and the exons are hooked back togetheri. Splicing occurs in the nucleus by complex particles called “spliceosomes”e. mRNA is modified by adding 5’ cap and 3’ poly-A tailf. Mature mRNA (after splicing and modification) leaves nucleus for translationII. MicroRNAs and Gene Expression BIOL 1st Editiona. The genome carries the code to make all of the proteins the cell is capable is makingb. The cell does not make all the proteins it is capable of at any given timec. “Expressed genes” – when a gene is actively being used to make proteins (by transcription and then translation)d. Major control of gene expression at the “post-transcriptional” level – after transcription – in eukaryotic cellsi. “MicroRNAs” (miRNA) – main post-transcriptional control is by this set of small RNAs 1. About 21-22 nucleotides long, but are initially made as a longer precursor RNA2. “Dicer” – the enzyme that processes the initial precursor RNA3. Resulting small RNA incorporates into a large protein complex called RISC, where it acts as a guide to bring RISC to target mRNAs by base-pairing rules4. When RISC binds to target mRNA, the mRNA is either destroyed or its translation is blocked = no proteine. miRNA control is common – about 60% of human genes are controlled by a microRNAIII. Mutationsa. Mutations – a change in the nucleotide sequence in the DNAb. Base-pair substitutionsi. One nucleotide pair is replaced by anotherii. 2 types:1. Missense – alters a codon in a gene, new codon still codes for an amino acida. Depending on how different the new amino acid is and where it is locatedin the protein, a missense mutation can be either harmless or have a major impact (may even be lethal)b. Example: Sickle cell allele is die to a missense mutation is the hemoglobin gene2. Nonsense – changes a codon to a “STOPcodon”a. Stops translation prematurely b. Most nonsense mutations result in nonfunctional proteinsc. Base-pair deletions or insertionsi. One or more nucleotide pairs are inserted or deleted from geneii. Usually have a greater effect on the encoded protein than a base-pair substitution iii. May cause a change in the “reading frame”iv. “Frameshift mutation”  when the number of nucleotides inserted or deleted are in multiples of 3IV. Virusesa. Virus – simplest of genetic systems that exists at the border of living and nonlivingi. Not really cells (so not alive)ii. Lack many cell structures and metabolic machineryiii. Consist only of viral genes enclosed in a protein shelliv. A virus is inert by itselfv. It can enter a cell and take over the cell machinery and reprogram the cell to make lots of virus particlesb. Virus particles – consists of 2 basic partsi. Viral genome (set of genes) – a type of nucleic acid1. Genome is either DNA or RNA, but never both2. Genome can be double-stranded DNA (dsDNA) or single-stranded (ssDNA) or it can be ssRNA (cells) or dsRNAii. Virus shell – made of a protein called “capsid orcoat” arranged in a particular shape around the viral genome1. Shape is often simple – rod or polyhedron2. Bacterial viruses often have very complex shell3. Some animal viruses have a membrane that surrounds the particlec. Virus replicationi. Viruses obligate intracellular parasites – only replicate inside a cellii. Viruses use many cellular materials1. Nucleotides2. Amino acids3. Protein synthesis machinery4. ATP iii. Virus infected cell makes thousands of viral genomes and thousands of protein shells and these self-assemble to make particlesiv. Once particles have assembles, they emerge from the host cell, burst, and kill the celld. Viral host range – nearly all living things have virusesi. Viruses that infect very different organisms are often similar in many ways – can be classified into “super groups” according to their replication strategiesii. Each virus has a particular host range –1. Some have a very narrow host range (ex: human cold viruses infect only cells of the upper respiratory tract of humans)2. Others are broader (ex: influenza infects humans, dogs, birds, etc.)3. Rabies virus infects a number of different mammals (humans, dogs, rodents)V. Types of Virusesa. Bacterial viruses – also known as “bacteriophage”i. Have 2 alternative life cycles:1. Lytic Cyclea. Bacteriophage enters cellb. Produces a protein that destroys host cell DNAc. Makes viral parts (genome and shell) and assembles them to make thousands of viral particlesd. Makes a protein that destroys the bacterial cell walle. Cell bursts, releasing all the newly made bacteriophage2. Lysogenic Cyclea. Bacteriophage co-exists peacefully with host cellb. “Prophage” - genome of the phage inserts into the host genomec. Prophage replicates along with host DNA and is passed to all daughter cellsd. Sometimes the prophage excises from the host DNA and enters a lytic cycle, killing the celle. Excision and entry to lytic cycle is often triggered by environmental stressb. Animal virusesi. Diverse group of viruses, some with DNA genomes, some with RNA genomes, some withmembranes, wide variation in life cyclesii. Few of interestc. Herpesvirusi. dsDNA viruses: causative agent of fever blisters and cold sores, and gential herpes1. Caused by different related virusesii. Primary herpesvirus infection is lyticiii. Can insert the viral genome into the host DNA iv. Inserted viral genome is called “provirus”1. Behaves like a lysogenic phage in many ways:a. It is latent (doesn’t cause symptoms)b. Sometimes is