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Learning Objectives

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Learning Objectives

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Now · 1. DNA Motifs

Learning Objectives

Understand DNA motifs as regulatory elements
Apply motif discovery algorithms (MEME, Gibbs sampling)
Interpret Position Weight Matrices

1. DNA Motifs

Short, recurring patterns in DNA with biological function. Examples: transcription factor binding sites, promoter elements, splice sites. Position Weight Matrix (PWM): 4xL matrix (4 nucleotides, L = motif length). Each cell = probability or log-odds score. More informative than consensus sequence.

2. Motif Discovery Algorithms

Word Enumeration: Count all possible k-mers, find overrepresented ones. Simple, works for short motifs. Expectation-Maximization (MEME): Alternates: E-step (estimate motif occurrences given current PWM) and M-step (update PWM from estimated occurrences). Popular tool. Gibbs Sampling: Iteratively find motifs by sampling from current motif model. Removes one sequence, rebuilds PWM from others, samples new position for held-out sequence.

3. Applications

Identify transcription factor binding sites in promoter regions
Discover regulatory elements
Find RNA binding protein motifs
Identify splice site signals

Q1: What is a Position Weight Matrix (PWM)?

4xL matrix. Each cell = probability (or log-odds score) of that nucleotide at that position. Captures variation at each position, more informative than a consensus sequence. Q2: How does Gibbs sampling find motifs?

Randomly choose starting positions in each sequence. 2) Remove one sequence, build PWM from others. 3) Sample new position for held-out sequence based on PWM. 4) Repeat until convergence. Q3: What is information content in a motif?

Measure of conservation. Ranges from 0 (all bases equally likely) to 2 (completely conserved, one base only). Formula: 2 - sum(p*log2(p)) per position. Visualized as sequence logos. Q4: Why is de novo motif discovery challenging?

Motifs are short (6-20 bp), have variation, occur in large genomes (3 billion bp), signal is weak compared to background, many false positives. Statistical significance is critical. Q5: What is the TATA box?

Promoter element (consensus TATAAA) ~25-35 bases upstream of transcription start site in eukaryotes. Binding site for TATA-binding protein (TBP). One of the first discovered regulatory motifs. Join Discord PreviousPhylogeneticsNextGenome Assembly