Algorithms in Computational Biology (236522) spring 2008 Lecture #1

Transcription

1 Algorithms in Computational Biology (236522) spring 2008 Lecture #1 Lecturer: Shlomo Moran, Taub 639, tel 4363 Office hours: 15:30-16:30/by appointment TA: Ilan Gronau, Taub 700, tel 4894 Office hours:?? Lecture: Monday 12:30-14:30, Taub 5 Tutorial: Tuesday 9:30-10:30, Taub 4 url: webcourse.cs.technion.ac.il/~cs This class has been initially edited from Nir Friedman s lecture at the Hebrew University. Changes made by Dan Geiger, then by Shlomo Moran. 1

2 Course Information Requirements & Grades: 15-25% homework, in five assignments. [Submit in two weeks time]. Homework is obligatory % test. Must pass beyond 55 for the homework s grade to count Exams dates: Moed A 7/9/2008 Moed B 3/11/2008 2

3 Bibliography Biological Sequence Analysis, R.Durbin et al., Cambridge University Press, 1998 Introduction to Molecular Biology, J. Setubal, J. Meidanis, PWS publishing Company, 1997 Bioinformatics, A. Polanski & M. Kimmel, Springer (2007) 3

4 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). 4

5 Relations to Some Other Courses Bioinformatics Software (cs236523). The course Introduction to Bioinformatics covers practical aspects and hands on experience with many web-based bioinformatics programs. Albeit not a formal requirement, it is recommended that you look on the web site and examine the relevant software. Bioinformatics algorithms (cs236522). This is the current course which focuses on modeling some bioinformatics problems and presents algorithms for their solution. Bioinformatics project (cs236524). Developing bioinformatics tools under close guidance. 5

6 Biological Background First home work assignment: Read the first chapter (pages 1-30) of Setubal et al., (copies are available in the Taub building library, and in the central library). Answer the questions of the first assignment in the course site (more details in the course site) 6

7 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. 7

8 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

9 Exponential growth of biological information : growth of sequences, structures, and literature. 9

10 Four Aspects Biological What is the task? Algorithmic How to perform the task at hand efficiently? Learning How to adapt/estimate/learn parameters and models describing the task from examples Statistics How to differentiate true phenomena from artifacts 10

11 Example: Sequence Comparison Biological Evolution preserves sequences, thus similar genes might have similar function Algorithmic Consider all ways to align one sequence against another Learning How do we define similar sequences? Use examples to define similarity Statistics When we compare to ~10 6 sequences, what is a random match and what is true one 11

12 Course Goals Learning about computational tools for (primarily) molecular biology. Describe 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

13 Topics I Dealing with DNA/Protein sequences: Informal biological background. (1 week) Finding similar sequence (~3 weeks) Models of sequences: Hidden Markov Models (~2 weeks) Parameter estimation: ML methods and the EM algorithm (~4 weeks) 13

14 Topics II Reconstructing evolutionary trees: Background: Darwin s theory of evolution Distance based methods (~2 weeks) Character based methods (~2 weeks) The presentations are similar to these given in the spring 2007 Semester, and can be found in the site of that semester. Updated presentations will be uploaded to the course site before the lectures. 14

15 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

16 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

17 DNA Organization 17 Source: Alberts et al

18 The Double Helix 18 Source: Alberts et al

19 Four nucleotide types: Adenine Guanine Cytosine Thymine DNA Components Hydrogen bonds (electrostatic connection): A-T C-G 19

20 Genome Sizes E.Coli (bacteria) 4.6 x 10 6 bases Yeast (simple fungi) 15 x 10 6 bases Smallest human chromosome 50 x 10 6 bases Entire human genome 3 x 10 9 bases 20

21 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

22 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

23 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

24 Example: Tissues in Stomach How is this variety encoded and expressed? 24

25 Central Dogma Transcription תרגום שעתוק Translation Gene mrna Protein cells express different subset of the genes In different tissues and under different conditions 25

26 Transcription Coding sequences can be transcribed to RNA RNA Similar to DNA, slightly different nucleotides: different backbone Uracil (U) instead of Thymine (T) Source: Mathews & van Holde 26

27 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

28 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

29 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

30 Genetic Code There are 20 amino acids from which proteins are build. 30

31 Protein Structure Proteins are polypeptides of amino-acids This structure is (mostly) determined by the sequence of amino-acids that make up the protein 31

32 Protein Structure 32

33 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

34 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

35 The Tree of Life 35 Source: Alberts et al

36 Example of a graph theoretic problem related to evolution trees: the perfect phylogeny problem 36

37 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. 37

38 Species Vertices Characters Colorings States Colors Each species is identified by its states Evolutionary tree A tree with many colorings (one coloring for each character) - containing the given vertices = No teeth = teeth A B C D 38

39 Another tree for the same character Which tree is more reasonable? A C B D = No teeth = teeth 39

40 Evolutionary trees should avoid reversal transitions A species regains a state it s direct ancestor has lost. Famous (and rare) examples: Teeth in birds. Legs in snakes. 40

41 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

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43 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

44 A character is Homoplasy Free The corresponding coloring is convex (each color induces a connected subtree) 44

45 A partial coloring is convex if it can be completed to a (total) convex coloring 45

46 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

47 The Perfect Phylogeny Problem (combinatorial setting) Input: Some colorings (C 1,,C k ) 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,C i ) is convex for i=1,,k? NP-Hard In general, in P for some special cases 47