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Algorithms in Computational Biology (236522) Spring 2002 Lecturer: Shlomo Moran, Taub 639, tel 4363 Office hours Wednesday 1630-1730 TA: Ydo Wexler, Taub 431, tel 4927 Office hours Monday 1030-1130 Lecture: Tuesday 11:30-13:30, Taub 2 Tutorial: Monday 9:30-10:30, Taub 4 . Course Information Requirements & Grades: 15-25% homework, in five theoretical question sets. [Submit in two weeks time]. Homework is obligatory. 75-85% test. Must pass beyond 55 for the homework’s grade to count Exam date: 7.7.04. 2 Bibliography Biological Sequence Analysis, R.Durbin et al. , Cambridge University Press, 1998 Introduction to Molecular Biology, J. Setubal, J. Meidanis, PWS publishing Company, 1997 Phylogenetics, C. Semple, M. Steel, Oxford press, 2003 url: www.cs.technion.ac.il/~cs236522 3 Course Prerequisites Computer Science and Probability Background Data structure 1 (cs234218) Algorithms 1 (cs234247) Probability (any course) Some Biology Background Formally: None, to allow CS students to take this course. Recommended: Molecular Biology 1 (especially for those in the Bioinformatics track), or a similar Biology course, and/or a serious desire to complement your knowledge in Biology by reading the appropriate material (see the course web site). Studying the algorithms in this course while acquiring enough biology background is far more rewarding than ignoring the biological context. 4 Biological Background First home work assignment: Read the first chapter (pages 1-30) of Setubal et al., 1997. (a copy is available in the Taub building library, and one for loan at Fishbach). Solve questions 1-3, p. 30 (to be on the course web site) Due time: Tutorial class of 22.3.04 (~2 weeks from today), or earlier in the teaching assistant’s mail slot. This class has been edited from Nir Friedman’s lecture which is available at www.cs.huji.ac.il/~nir. Changes made by Dan Geiger, then Shlomo Moran. . Computational Biology Computational biology is the application of computational tools and techniques to (primarily) molecular biology. It enables new ways of study in life sciences, allowing analytic and predictive methodologies that support and enhance laboratory work. It is a multidisciplinary area of study that combines Biology, Computer Science, and Statistics. Computational biology is also called Bioinformatics, although many practitioners define Bioinformatics somewhat narrower by restricting the field to molecular Biology only. 7 Examples of Areas of Interest • • • • • Building evolutionary trees from molecular (and other) data Efficiently constructing genomes of various organisms Understanding the structure of genomes (SNP, SSR, Genes) Understanding function of genes in the cell cycle and disease Deciphering structure and function of proteins _____________________ SNP: Single Nucleotide Polymorphism SSR: Simple Sequence Repeat 8 Exponential growth of biological information: growth of sequences, structures, and literature. 9 Course Goals Learning about computational tools for (primarily) molecular biology. Cover computational tasks that are posed by modern molecular biology Discuss the biological motivation and setup for these tasks Understand the kinds of solutions that exist and what principles justify them 12 Topics I Dealing with DNA/Protein sequences: Genome projects and how sequences are found Finding similar sequences Models of sequences: Hidden Markov Models Transcription regulation Protein Families Gene finding 13 Topics II Models of genetic change: Long term: evolutionary changes among species Reconstructing evolutionary trees from sequences Short term: genetic variations in a population Finding genes by linkage and association 14 Topics III (if time allows) Protein World: How proteins fold - secondary & tertiary structure How to predict protein folds from sequences data How to analyze proteins changes from raw experimental measurements (MassSpec) 15 Human Genome Most human cells contain 46 chromosomes: 2 sex chromosomes (X,Y): XY – in males. XX – in females. 22 pairs of chromosomes named autosomes. 16 Source: Alberts et al DNA Organization 17 Source: Alberts et al The Double Helix 18 DNA Components Four nucleotide types: Adenine Guanine Cytosine Thymine Hydrogen bonds (electrostatic connection): A-T C-G 19 Genome Sizes E.Coli (bacteria) Yeast (simple fungi) Smallest human chromosome Entire human genome 4.6 x 106 bases 15 x 106 bases 50 x 106 bases 3 x 109 bases 20 Genetic Information Genome – the collection of genetic information. Chromosomes – storage units of genes. Gene – basic unit of genetic information. They determine the inherited characters. 21 Genes The DNA strings include: Coding regions (“genes”) E. coli has ~4,000 genes Yeast has ~6,000 genes C. Elegans has ~13,000 genes Humans have ~32,000 genes Control regions These typically are adjacent to the genes They determine when a gene should be “expressed” “Junk” DNA (unknown function - ~90% of the DNA in human’s chromosomes) 22 The Cell All cells of an organism contain the same DNA content (and the same genes) yet there is a variety of cell types. 23 Example: Tissues in Stomach How is this variety encoded and expressed ? 24 Central Dogma שעתוק Transcription Gene תרגום Translation mRNA Protein cells express different subset of the genes In different tissues and under different conditions 25 Transcription sequences can be transcribed to RNA Source: Mathews & van Holde Coding RNA nucleotides: Similar to DNA, slightly different backbone Uracil (U) instead of Thymine (T) 26 Transcription: RNA Editing 1. Transcribe to RNA 2. Eliminate introns 3. Splice (connect) exons * Alternative splicing exists Exons hold information, they are more stable during evolution. This process takes place in the nucleus. The mRNA molecules diffuse through the nucleus membrane to the outer cell plasma. 27 RNA roles Messenger RNA (mRNA) Encodes protein sequences. Each three nucleotide acids translate to an amino acid (the protein building block). Transfer RNA (tRNA) Decodes the mRNA molecules to amino-acids. It connects to the mRNA with one side and holds the appropriate amino acid on its other side. Ribosomal RNA (rRNA) Part of the ribosome, a machine for translating mRNA to proteins. It catalyzes (like enzymes) the reaction that attaches the hanging amino acid from the tRNA to the amino acid chain being created. ... 28 Translation Translation is mediated by the ribosome Ribosome is a complex of protein & rRNA molecules The ribosome attaches to the mRNA at a translation initiation site Then ribosome moves along the mRNA sequence and in the process constructs a sequence of amino acids (polypeptide) which is released and folds into a protein. 29 Genetic Code There are 20 amino acids from which proteins are build. 30 Protein Structure Proteins are polypeptides of 70-3000 amino-acids This structure is (mostly) determined by the sequence of amino-acids that make up the protein 31 Protein Structure 32 Evolution Related organisms have similar DNA Similarity in sequences of proteins Similarity in organization of genes along the chromosomes Evolution plays a major role in biology Many mechanisms are shared across a wide range of organisms During the course of evolution existing components are adapted for new functions 33 Evolution Evolution of new organisms is driven by Diversity Different individuals carry different variants of the same basic blue print Mutations The DNA sequence can be changed due to single base changes, deletion/insertion of DNA segments, etc. Selection bias 34 Source: Alberts et al The Tree of Life 35 Characters in Species A (discrete) character is a property which distinguishes between species (e.g. dental structure, a certain gene) A characters state is a value of the character (human dental structure). Problem: Given set of species, specified by their characters, reconstruct their evolutionary tree. 38 Species ≡ Vertices States ≡ Colors Characters ≡ Colorings Evolutionary tree ≡ A tree with many colorings, containing the given vertices 39 Evolutionary trees should avoid reversal transitions A species regains a state it’s direct ancestor has lost. Famous examples: Teeth in birds. Legs in snakes. 40 Evolutionary trees should avoid convergence transitions Two species possess the same state while their least common ancestor possesses a different state. Famous example: The marsupials. 41 42 Common Assumption: Characters with Reversal or Convergent transitions are highly unlikely in the Evolutionary Tree A character that exhibits neither reversals nor convergence is denoted homoplasy free. 43 A character is Homoplasy Free ↕ The corresponding coloring is convex (each color induces a block) 44 A partial coloring is convex if it can be completed to a (total) convex coloring 45 The Perfect Phylogeny Problem Input: a set of species, and many characters, each assign states (colors) to the species. Question: is there a tree T containing the species as vertices, in which all the characters (colorings) are convex? 46 The Perfect Phylogeny Problem (combinatorial setting) Input: Some colorings (C1,…,Ck) of a set of vertices (in the example: 3 colorings: left, center, right, each by (the same) two colors). RRB BBR RRR RBR Problem: Is there a tree T which includes these vertices, s.t. (T,Ci) is convex for i=1,…,k? NP-Hard In general, in P for some special cases 47