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13: De Novo Genome Assembly, Scaffolding & Quality Assessment

1 min read • Verified October 2026 • Peer-Reviewed Learning Resource
Tested Stacks: GATK 4.6.1 Samtools 1.21 Nextflow 24.10 Seurat v5.1 Python 3.12
Prerequisites: Basic Linux CLI & Sequencing Formats
📚 Chapter 13 of 13 0% Course Completed

Chapter 13: De Novo Genome Assembly, Scaffolding & Quality Assessment

When studying a novel non-model organism or a human genome with complex structural rearrangements, aligning reads to a standard reference is insufficient. De Novo Genome Assembly reconstructs the complete chromosomes of an organism from scratch without an existing reference template, culminating in true Telomere-to-Telomere (T2T) assemblies.

De Bruijn Graphs vs. Overlap-Layout-Consensus (OLC)

Short-read assemblers break reads into overlapping sub-sequences of length \(k\) (\(k\)-mers) and construct a De Bruijn Graph, where edges represent observed sequences. However, genomic repeats longer than \(k\) collapse into tangled knots. Long-read assemblers use the Overlap-Layout-Consensus (OLC) paradigm: finding all-vs-all overlaps between long reads, constructing a string graph, and collapsing unambiguous paths into contigs.

1. Haplotype-Resolved Assembly with Hifiasm

Hifiasm is the gold-standard assembler for PacBio HiFi reads, generating phased maternal and paternal haplotype assemblies:

# Assemble PacBio HiFi reads into phased unitigs
hifiasm -o novel_organism.asm -t 32 pacbio_hifi_reads.fastq.gz

# If Hi-C data is available, perform trio-free chromosomal phasing
hifiasm -o novel_organism_phased.asm -t 32 \
    --h1 hic_R1.fastq.gz --h2 hic_R2.fastq.gz \
    pacbio_hifi_reads.fastq.gz

# Convert graphical assembly (GFA) to FASTA
awk '/^S/{print ">"$2"\n"$3}' novel_organism.asm.bp.p_ctg.gfa > primary_contigs.fasta

2. Quality Assessment with QUAST & BUSCO

# 1. Contiguity metrics (N50, L50, total length, GC%)
quast.py primary_contigs.fasta -o quast_report/ -t 8

# 2. Gene completeness assessment using Benchmarking Universal Single-Copy Orthologs
busco \
    -i primary_contigs.fasta \
    -o busco_report \
    -m genome \
    -l mammalia_odb10 \
    -c 16
⚠️

Common Pitfalls & Troubleshooting Checklist

  • Java Heap Exhaustion (GATK / Picard): If processes terminate with java.lang.OutOfMemoryError, set your environment variable explicitly: export _JAVA_OPTIONS="-Xmx16g -Djava.io.tmpdir=/tmp".
  • Index Timestamp Mismatch: If tools complain that a BAM index is missing or out of date, regenerate with samtools index -@ 4 sample.bam.
  • Conda Channel Conflicts: Avoid ABI incompatibilities between bioconda and conda-forge by setting strict priority: conda config --set channel_priority strict.
  • Chromosome Naming Contamination: Always verify whether reference FASTA headers use chr1 or 1; divergent naming causes silent alignment drops.
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