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How is the genetic material organised within a genome, and what can genomics tell us about an organism?

Describe how DNA is organised into chromosomes and genomes and outline the applications of genome sequencing

A focused answer to the H2 Biology Molecular Genetics outcome on genome organisation. Histones and chromosome packaging, coding and non-coding DNA, the difference between the genome and the proteome, and the applications of genome sequencing.

Generated by Claude Opus 4.88 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
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What this dot point is asking

SEAB wants you to describe how DNA is organised in the cell (histones, nucleosomes and chromosomes), to distinguish coding from non-coding DNA, to understand the terms genome and proteome, and to outline the applications of genome sequencing (genomics). It connects DNA structure to the practical power of modern biology.

The answer

Packaging DNA into chromosomes

A eukaryotic cell holds a length of DNA far greater than the diameter of its nucleus, so the DNA must be condensed. The double helix wraps around histone proteins to form nucleosomes (beads on a string), and the nucleosome chain coils and folds repeatedly into increasingly condensed fibres, reaching the compact chromosome seen at cell division. Packaging fits the DNA into the nucleus, protects it, and (through the degree of condensation) helps control gene access.

Coding and non-coding DNA

Only part of the genome codes for proteins. Genes contain coding exons and non-coding introns, and large stretches between genes are non-coding. Some non-coding DNA is regulatory (binding transcription factors) or codes for functional RNA; much has no known function.

Genome and proteome

The genome is the complete set of DNA of an organism. The proteome is the complete set of proteins it can make. The proteome is larger than the gene count because one gene can yield several proteins through alternative splicing and post-translational modification.

Genomics and its applications

Sequencing a genome reads its entire base sequence. Applications include diagnosing genetic disease, identifying disease-risk alleles, designing drugs tailored to a patient (personalised medicine), tracing evolutionary relationships, and tracking pathogens during an outbreak.

Examples in context

Example 1. Personalised medicine. Sequencing a patient's genome can reveal alleles that affect how they respond to a drug, allowing the dose or the choice of medicine to be tailored. This makes treatment safer and more effective and is one of the most important applications of genomics in healthcare.

Example 2. Evolutionary relationships. Comparing genome sequences across species reveals how closely related they are, because more closely related species share more sequence. Genomics has refined the tree of life and confirmed relationships that anatomy alone could not resolve.

Try this

Q1. Name the proteins that DNA wraps around during packaging. [1 mark]

  • Cue. Histones.

Q2. Explain why one gene can give rise to more than one protein. [2 marks]

  • Cue. Alternative splicing can join the exons of a gene in different combinations to make different mRNAs and therefore different proteins; post-translational modification adds further variety.

Q3. State one application of whole genome sequencing in medicine. [1 mark]

  • Cue. Diagnosing genetic disease, identifying disease-risk alleles, or tailoring drug treatment to a patient (personalised medicine).

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original4 marksDescribe how a long DNA molecule is packaged into a chromosome in a eukaryotic cell, and explain why such packaging is necessary.
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The answer needs the packaging mechanism and a reason.

The DNA double helix wraps around proteins called histones to form structures called nucleosomes, which resemble beads on a string. The nucleosome string coils and folds further, becoming increasingly condensed, until at cell division it forms the highly compact chromosome visible under the microscope.

Packaging is necessary because the total length of DNA in a cell is far greater than the diameter of the nucleus, so it must be condensed to fit. Packaging also protects the DNA from damage and tangling, and the degree of condensation helps control which genes are accessible for transcription.

Markers reward the role of histones and nucleosomes, the progressive coiling into a condensed chromosome, and at least one reason such as fitting into the nucleus or controlling access for transcription.

Original4 marksDistinguish between the genome and the proteome of an organism, and explain why the proteome is larger than the number of genes might suggest.
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The answer should define both terms and account for the discrepancy.

The genome is the complete set of genetic material (all the DNA, including coding and non-coding sequences) of an organism. The proteome is the complete set of proteins that an organism can produce.

The proteome is larger than the number of protein-coding genes because one gene can give rise to several different proteins. Alternative splicing can join the exons of a single gene in different combinations to produce different mRNAs and therefore different proteins. Proteins can also be modified after translation in different ways.

Markers reward correct definitions of genome and proteome and a valid mechanism (alternative splicing or post-translational modification) explaining how one gene yields multiple proteins.

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