Sequencing reads and how a genome is assembled · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Rebuilding a genome from reads

Science · Genetics & Evolution · ages 20-22
Name ______________________   Date ____________

  1. In a toy assembly, TACG and CGAA share the end sequence CG. Which join keeps that overlap once?

    • TACGAA
    • TACGCGAA
    • TACGAACG

  2. What is the raw sequence output of a sequencing instrument?

    • A complete assembly with all gaps resolved
    • Reads whose lengths depend on the method
    • A gene map showing each feature's location

  3. When reading errors are independent and random, how can extra reads improve a consensus base call?

    • They force every base to have equal coverage
    • They remove every systematic instrument error
    • They add independent support for a base

  4. A region has no difficult repeats. Both read options meet its accuracy and assembly needs at the target coverage. Short reads cost 1 unit; long reads cost 3 units. Which suitable option costs less?

    • Long reads costing 3 units
    • Short reads costing 1 unit
    • Neither option meets the stated assembly needs

  5. Five locations contain identical 1,000-base repeats. A 150-base read lies wholly inside one repeat, with no linking information. What limits its placement?

    • The read has several possible locations
    • Its short length prevents any sequence comparison
    • Its internal sequence identifies a distinctive flank

  6. A read spanning an entire repeat and reaching distinctive sequence on both sides can help resolve that repeat.

    Circle one:   True   False

  7. Unique sequence alone guarantees a complete assembly, even when usable reads leave gaps.

    Circle one:   True   False

  8. A team doubles the number of reads wholly inside identical repeats. It adds no spanning reads or linking information. Why can placement remain ambiguous?

    • More reads make the identical copies different
    • Read count determines each fragment's read length
    • Internal reads still fit several repeat copies
LightMySky · lightmysky.comW1-mt_4mUJIUAZGd-s1

Answer key

For grown-ups. Fold this page away before handing over the rest.

Rebuilding a genome from reads W1-mt_4mUJIUAZGd-s1

  1. TACGAA · TACG already contains CG. Add the unmatched AA from CGAA to obtain TACGAA.
  2. Reads whose lengths depend on the method · The instrument reports reads. Reconstructing longer regions and identifying genes are separate tasks.
  3. They add independent support for a base · Repeated observations can support the correct base over random errors. Systematic errors need not disappear.
  4. Short reads costing 1 unit · Both options meet the stated needs. One unit costs less than three units.
  5. The read has several possible locations · The read contains sequence shared by all five copies and reaches no distinctive flank.
  6. True · Distinctive flanks connect the repeated sequence to its surroundings.
  7. False · Unique sequence reduces placement ambiguity but cannot supply missing sequence or connections.
  8. Internal reads still fit several repeat copies · More observations of shared internal sequence do not add a connection to distinctive surrounding DNA.
Worksheet · LightMySky