This preview shows page 1. Sign up to view the full content.
Unformatted text preview: Biology in the News [see folder on BB] • •
• • • •
• The slime mold Dictyostelium discoideum (also known as a type of amoeba) has been shown to harbor beneﬁcial bacterial as a food source This is poten?ally costly because harmful pathogenic bacteria could take advantage of this system Individual Dictyostelium cells aggregate into a mul?cellular structure known as a "slug" which then moves around looking for food The slug also produces a frui?ng body which produces spores; the bacteria are released with the spores, providing a growing source of food for the oﬀspring amoebae This type of "farming" was only found in some but not all strains of D. discoideum ‐ the "farmer" strains also don't completely deplete bacterial food sources like non‐farmer strains (instead, they save some for the spores) and disperse lower distances than non‐
farmers the farmer trait was found to be gene?cally encoded but the gene(s) involved are not known yet this interac?on with bacteria has implica?ons for the evolu?on of human pathogens ‐ how are some pathogens able to evade host defense systems to get inside cells and cause disease? hIp://www.nsf.gov/news/news_summ.jsp?
cntn_id=118407&WT.mc_id=USNSF_51&WT.mc_ev=click hIp://ucsdnews.ucsd.edu/newsrel/science/mcamoeba.asp Natural Selec+on: Evidence Rapid Selec+on Bacterial popula?on An?bio?c treatment Purple cell has a muta?on conferring resistance: pre‐
exis?ng varia?on, Ini?ally not detectable. An?bio?c‐
resistant cells selected in an an?bio?c‐rich environment in the an?bio?c/microbe example: what happens if you remove an?bio?c from the environment? • popula?on likely to go back to: • but the resistant gene will s?ll be present – in a few slow growing cells – in other species – as DNA • it only takes one copy plus an+bio+cs to breed an en?re popula?on of resistant cells a biology in the news story from Spring 2011 •
• • new research is ﬁnding that a few resistant bacterial cells can help an en?re bacterial popula?on survive exposure to an?bio?cs a cri?cal problem for human infec?ous diseases caused by bacteria "supermutant" cells are altruis+c; they help other cells survive at a cost to themselves (slower growth) in addi?on to producing gene products directly associated with resistance, "supermutant" cells also produce indole compounds that signal to the other cells and generally help cells survive harsh environments (probably by triggering stress responses) i.e. bacteria are ac?ng as a popula?on or possibly even a community (if mul?ple species are involved), not all as selﬁsh individual cells, as previously thought hIp://www.nsf.gov/news/news_summ.jsp?
cntn_id=117596&WT.mc_id=USNSF_51&WT.mc_ev=click hIp://www.topnews.in/healthcare/general/health‐news?page=5 mul?drug approaches • usually work against microbes, viruses – e.g. HIV • resistance is costly to the cell – resistance to more than one drug usually physiologically diﬃcult • but many DNA plasmids encode “mul?drug resistance” • in the absence of an?microbial drugs, suscep?ble strains quickly outgrow resistant strains drug resistance is a rela?vely simple trait; what about “complex” traits? hIp://www.ﬂ[email protected]/3819789968/ Adapta+ons are o7en exquisite • It is oken hard to conceive how such complexity evolved in a gradual or stepwise manner. • Each step must be adap?ve, or at leastnot disadvantageous – some traits may be “neutral” – we will see in a future lecture that a great deal of DNA sequence varia?on is thought to be neutral Adapta+ons of life forms: the vertebrate eye Re?na (photoreceptor cells) Cornea (lens) Evolu?on of eyes • Some eyes are complex, some are simpler • Speciﬁc eye types (cell and ?ssue conﬁgura?ons) evolved independently • Photoreceptor cells – May have evolved just once • The ﬁrst eyes ‐ probably just a few photorecep?ve cells, like planarian worms • Photorecep?ve pigments (opsins) in these cells – Transform energy from photons into an neural signal – Probably evolved just once – Many diﬀerent types in diﬀerent animal species • Gene duplica?on • Molecular evolu?on Dugesia
dorotocephala some types of animal eyes compound eye (arthropod)
nautilus (mollusk), planarian (flatworm) scallop (mollusk),
http://www.schuchman.com/SCROC/3D-Character-Design/Projects/ProjectEyeball-Anatomicaly-Correct/Animal-Eye-Types.jpg Evolu?on can produce “design ﬂaws” • e.g. human eye (compared with octopus) – same basic “design” – Photoreceptor cells point away from light • Light must pass through blood vessels and other cells to reach photoreceptors – Physiological blind spot (scotoma) • Corresponds to part of visual ﬁeld for which the re?na does not have photoreceptors because op?c nerve must pass through • All mammal eyes have this scotoma, cephalopods (octopus and squid) do not • Natural selec?on modiﬁes what is already there (“?nkering” not “engineering”) How does evolu?on work? • To understand this, we need to understand inheritance, i.e. Gene?cs hIp://fergusonbiology.homestead.com/gene?cs.jpg Darwin and inheritance • Darwin was unaware of Mendel’s work • Chromosome theory of inheritance was not yet developed • Meiosis was not yet understood • Reconciling Darwinian natural selec?on with new gene?c knowledge was one of the main pursuits of early 20th century biology Chromosomes Human mito?c chromosomes Drosophila larval salivary gland chromosomes • First discovered in plants and worms in 1842, using microscopy on ?ssue prepara?ons stained with various dyes • Later named “chromosomes” (color bodies) Chromosomes Human mito?c chromosomes Drosophila larval salivary gland chromosomes • Linear molecule of DNA – Normally not visible (interphase cells) • Visible in some organisms in some ?ssues: – Mitosis (various cell types) and meiosis • Highly condensed and packaged with proteins – In some ?ssues when chromosomes are massively over‐replicated (many thousands of copies aligned side‐by‐side) Chromosome theory of inheritance Human mito?c chromosomes Drosophila larval salivary gland chromosomes • Theory: chromosomes are linear arrangements of genes ‐ combined cell biology and gene?c data • Established in the early 20th century from many observa?ons in several organisms – Peas • William Bateson; 1st to the term “gene?cs” • Hugo DeVries – Drosophila • Thomas Hunt Morgan, Calvin Bridges – Salamander (Batrachoseps a4enuatus) • F.A. Janssens California slender salamander Batrachoseps a4enuatus Mitosis • Mitosis: DNA replica?on/cell division during growth and development (in mul?cellular plants and animals) • Takes place in all soma?c ?ssues One diploid (2N) cell replicates its chromosomes (“4N”) and then divides once, resul?ng in two diploid (2N) daughter cells that are exact copies of the original cell • [N refers to the haploid chromosome number. N for humans = 23, N for Drosophila melanogaster = 4] Mitosis Pre‐mito?c cell 2N chromosome replica?on (S‐phase: synthesis) in this example N=2 chromosomes 4N Chromosomes from mother: red Chomosomes from father: blue pole pole Daughter cells 2N 2N Mito?c metaphase Meiosis • Meiosis: DNA replica?on/cell division during the produc?on of gametes (sperm and egg) – Takes place only in “germ line” ?ssues in the gonad • “soma” versus “germ line” dis?nc?on – One diploid cell (2N) duplicates its chromosomes (“4N”) and then undergoes two divisions, resul?ng in four haploid (N) gametes Meiosis Pre‐meio?c cell 2N in this example N=2 chromosomes chromosome replica?on (S‐phase: synthesis) 4N Chromosomes from mother: red Chomosomes from father: blue pole Independent 1st Meio?c division “Meiosis I” assortment also occurs in meiosis 2N Process determining blue and red 2N pairs in each 4N tetrad going to one 2nd Meio?c division “Meiosis II” pole or other is random pole Meiosis I metaphase Crossing over (recombina?on) Some chromosomes are “recombinant” Four haploid gametes Mitosis vs. Meiosis • What happens in meiosis that does not happen in mitosis? – Products are haploid – Homologous chromosomes pair (Meiosis I metaphase) – Recombina?on between homologs occurs (“crossing over”) – => single chromosomes in gametes are not guaranteed to be exact copies of parental chromosomes Mendel: Par+culate inheritance Previous hypothesis: Blending inheritance Mendelian inheritance
(semi-dominant A,a alleles
also called “additive” alleles) Gregor Mendel (1822‐1884) 100% 100%
Pea plant Pisum sa9vum 100% 1:2:1 Mendelian inheritance
(semi-dominant A,a alleles
also called “additive” alleles) 1:2:1 Mendelian inheritance
(completely dominant A
allele) 1:2:1 widespread misconcep?ons are a star?ng point for science • “blending inheritance” – oﬀspring usually do look intermediate (or mosaic) between their parents – suggests that en?re popula?on should become uniform (for some intermediate trait value) • dominance of a trait – suggests that en?re popula?on should become uniform (for the dominant trait value) • Mendelian par?culate inheritance resolves both problems Consider one locus with two alleles: • “the A locus” – Can also be called “the A gene” • Two alleles – “A” – “a” – Alleles are also called “genes” • “I inherited my height gene(s) from my father, who is tall.” – In what ways is this statement misleading or incomplete? Chromosomes and genes Allele = alterna?ve form of a gene An individual can be: AA Aa or aa Aker replica?on at the beginning of meiosis AAaa AA A aa A a a Remember: there are typically thousands of genes on each chromosome. The A locus is just one of them. Genes and alleles • "gene" in general can refer to any inherited gene?c material, including “alleles” • "gene" speciﬁcally refers to a certain address in the genome of a species, a "locus"; usually containing a "transcrip?on unit"‐> transcribed to RNA, then to protein – Except for genes encoding RNA products, such as… Genes and alleles • Alleles are alterna+ve forms of the same gene (can be single nucleo?des or a whole locus) • Each diploid individual has one allele of each locus from each parent – (except for X‐ and Y‐linked loci in males) What is a polymorphism? hIps://sites.google.com/site/
members/post‐docs/jeremy • At the phenotype level ‐ observable varia?on in a popula?on (e.g. plumage color in a bird species) – Can be discrete – or con+nuous • At the genotype level ‐ any DNA sequence or allelic varia?on at a gene – e.g. AACTGGTA, AACAGGTA,
AACCGGTA – e.g. high or low activity
alleles of a gene that codes
for an enzyme hIp://www.ethno‐botanik.org/Startseite_en.html ...
View Full Document
This note was uploaded on 09/23/2011 for the course BIO 201 taught by Professor True during the Spring '08 term at SUNY Stony Brook.
- Spring '08