BIOL 1st Edition Lecture 21 Outline of Last Lecture I The Cell Cycle II Meiosis and Sexual Cell Cycles III Mendel and Chromosomes IV The Molecular Basis of Inheritance Outline of Current Lecture I Split Genes and RNA Processing II MicroRNAs and Gene Expression III Mutations IV Viruses V Types of Viruses Current Lecture Mutations and Viruses I Split Genes and RNA Processing a Occurs in eukaryotes b Coding regions in DNA i 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 noncoding iv Most coding regions are interrupted by long noncoding regions c 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 together i Splicing occurs in the nucleus by complex particles called spliceosomes e mRNA is modified by adding 5 cap and 3 poly A tail f Mature mRNA after splicing and modification leaves nucleus for translation II MicroRNAs and Gene Expression a The genome carries the code to make all of the proteins the cell is capable is making b The cell does not make all the proteins it is capable of at any given time c 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 cells i 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 RNA 2 Dicer the enzyme that processes the initial precursor RNA 3 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 rules 4 When RISC binds to target mRNA the mRNA is either destroyed or its translation is blocked no protein e miRNA control is common about 60 of human genes are controlled by a microRNA III IV Mutations a Mutations a change in the nucleotide sequence in the DNA b Base pair substitutions i One nucleotide pair is replaced by another ii 2 types 1 Missense alters a codon in a gene new codon still codes for an amino acid a Depending on how different the new amino acid is and where it is located in 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 gene 2 Nonsense changes a codon to a STOPcodon a Stops translation prematurely b Most nonsense mutations result in nonfunctional proteins c Base pair deletions or insertions i One or more nucleotide pairs are inserted or deleted from gene ii 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 3 Viruses a Virus simplest of genetic systems that exists at the border of living and nonliving i Not really cells so not alive ii Lack many cell structures and metabolic machinery iii Consist only of viral genes enclosed in a protein shell iv A virus is inert by itself v It can enter a cell and take over the cell machinery and reprogram the cell to make lots of virus particles b Virus particles consists of 2 basic parts i Viral genome set of genes a type of nucleic acid V 1 Genome is either DNA or RNA but never both 2 Genome can be double stranded DNA dsDNA or single stranded ssDNA or it can be ssRNA cells or dsRNA ii Virus shell made of a protein called capsid orcoat arranged in a particular shape around the viral genome 1 Shape is often simple rod or polyhedron 2 Bacterial viruses often have very complex shell 3 Some animal viruses have a membrane that surrounds the particle c Virus replication i Viruses obligate intracellular parasites only replicate inside a cell ii Viruses use many cellular materials 1 Nucleotides 2 Amino acids 3 Protein synthesis machinery 4 ATP iii Virus infected cell makes thousands of viral genomes and thousands of protein shells and these self assemble to make particles iv Once particles have assembles they emerge from the host cell burst and kill the cell d Viral host range nearly all living things have viruses i Viruses that infect very different organisms are often similar in many ways can be classified into super groups according to their replication strategies ii 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 Types of Viruses a Bacterial viruses also known as bacteriophage i Have 2 alternative life cycles 1 Lytic Cycle a Bacteriophage enters cell b Produces a protein that destroys host cell DNA c Makes viral parts genome and shell and assembles them to make thousands of viral particles d Makes a protein that destroys the bacterial cell wall e Cell bursts releasing all the newly made bacteriophage 2 Lysogenic Cycle a Bacteriophage co exists peacefully with host cell b Prophage genome of the phage inserts into the host genome c Prophage replicates along with host DNA and is passed to all daughter cells d Sometimes the prophage excises from the host DNA and enters a lytic cycle killing the cell e Excision and entry to lytic cycle is often triggered by environmental stress b Animal viruses i Diverse group of viruses some with DNA genomes some with RNA genomes some with membranes wide variation in life cycles ii Few of interest c Herpesvirus i dsDNA viruses causative agent of fever blisters and cold sores and gential herpes 1 Caused by different related viruses ii Primary herpesvirus infection is lytic iii 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 excises and goes into a lytic cycle causes a recurrence of the disease c Recurrence of disease outbreak d Outbreaks are associated with host stress d Retroviruses backward viruses replication is the reverse of transcription in cells i ssRNA genome of virus catalyzed by enzyme reverse transcripts dsDNA copy ii Once a