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4.1.2Genetics, Variation and Interdependence

Eukaryotic DNA — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 5 of 46 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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4.1.2 Eukaryotic DNA — AQA A Level Biology 7402 notes: Explain how eukaryotic nuclear DNA is packaged with histones into chromosomes and describe the terms homologous chromosomes, diploid and haploid.A human cell nucleus is only a few micrometres across, yet it contains about two…

  • long and linear
  • nucleosome
  • chromatids
  • centromere
  • Exam-style question 1
  • Exam-style question 2

Section 1 of 9The scale of the problem

A human cell nucleus is only a few micrometres across, yet it contains about two metres of DNA when stretched out. To fit, and to be moved accurately during cell division, the DNA must be packed with great precision. In eukaryotes the DNA molecules are long and linear (with two ends), and they are associated with proteins called histones.

Section 2 of 9Levels of packaging

DNA is negatively charged because of its phosphate groups. Histones are positively charged, so the DNA binds tightly to them.
Nucleosome
The double helix wraps around a core of histone proteins forming a bead-like unit called a nucleosome.
Nucleosomes are connected by linker DNA, and are coiled and folded into a dense fibre.
Chromatin
The fibre is further coiled and looped, and supercoiled, with the help of more proteins, forming chromatin.
During cell division, the chromatin condenses further, giving the compact chromosomes that can be seen under a light microscope.

The combination of one long DNA molecule with its histone proteins is a chromosome. Between divisions the chromatin is less condensed so that genes can be transcribed and the DNA replicated. Where the chromatin is tightly packed, genes are generally not expressed, so packaging also helps to control gene expression.

Section 3 of 9Structure of a chromosome during division

Before mitosis, each chromosome is replicated, so it consists of two identical chromatids joined at the centromere. Each chromatid contains one DNA molecule. The centromere is where spindle fibres attach during division. The ends of chromosomes contain repeated sequences called telomeres, which protect the ends from damage and loss of coding information in replication.

Section 4 of 9Homologous chromosomes

In most body cells there are two sets of chromosomes, one from each parent, so the cell is diploid (2n). Chromosomes occur in pairs, called homologous chromosomes. The members of a pair:

Homologous chromosomes
are the same size and have the same centromere position;
carry the same genes at the same loci;
may have different alleles of those genes, one inherited from each parent.

Gametes are haploid (n), with one chromosome of each pair.

Homologous chromosomes
Humans have 46 chromosomes (23 pairs), so n = 23 and 2n = 46.
  • In humans, 22 pairs are autosomes and the remaining pair are sex chromosomes.
    • (XX in females, XY in males)
The number of chromosomes varies between species: this does not relate simply to complexity of the organism, for example a fern may have hundreds,
  • whereas the fruit fly has eight.

Section 5 of 9Comparison and links

EukaryoteProkaryote
Linear DNA with endsCircular DNA
Bound to histonesNot bound to histones
Several chromosomes in the nucleusTypically one chromosome in the cytoplasm

This lesson links to the cell cycle and mitosis, to meiosis (where homologous chromosomes pair), and to gene expression. In calculations, remember that after replication the DNA content doubles but the chromosome number stays the same until the chromatids separate.

Section 6 of 9Why packaging matters and how it is studied

The association of DNA with histones is not only a storage solution.

Why packaging matters and how it is studied
Chemical changes to the histones or to the DNA (for example the addition of methyl groups) alter how tightly the DNA is held.
In loosely packed regions (euchromatin) enzymes can reach the genes and transcription can happen; in tightly packed regions (heterochromatin) the genes are usually silent.
This is one of the ways that cells with identical DNA, such as a neurone and a liver cell, can express different sets of genes.

Chromosomes can be viewed under a light microscope only during division. A karyotype is a picture of the chromosomes of a cell arranged in homologous pairs by size and centromere position; it can reveal a change in chromosome number, such as the extra chromosome 21 in Down syndrome, or a missing X chromosome.

For exam calculations, remember that the DNA content of a cell doubles during the S phase of the cell cycle, falls back at mitosis, and falls to half again at the end of meiosis.

Test yourself

  • state the order of packaging from the double helix to a metaphase chromosome, and explain why the DNA is described as negatively charged while histones are positively charged.
  • Then explain why chromatin is less condensed in a cell that is actively making many proteins.

Section 7 of 9Exam-style questions

  1. Describe how DNA is packaged to form a chromosome in a eukaryotic cell. [4]4 marks

  2. A cell from a fruit fly has 8 chromosomes. State the number of chromosomes in a gamete, and the number of DNA molecules in the cell at the start of mitosis just after DNA replication. [2]2 marks

  3. Explain what is meant by a pair of homologous chromosomes. [2]2 marks

Section 8 of 9Key terms

TermMeaning
HistoneA positively charged protein that DNA winds around.
NucleosomeA unit of DNA wrapped around a core of histone proteins.
ChromatinThe complex of DNA and proteins in the nucleus.
ChromatidOne of the two identical DNA molecules of a replicated chromosome.
CentromereThe region where two chromatids are joined and where spindle fibres attach.
Homologous chromosomesA pair of chromosomes with the same genes at the same loci.
DiploidHaving two sets of chromosomes in a cell.
HaploidHaving one set of chromosomes in a cell.

Section 9 of 9Exam technique

1
Saying that histones are in the DNA. They are proteins around which the DNA is wound.
2
Confusing a chromosome with a chromatid; after replication a chromosome has two chromatids.
3
Stating that homologous chromosomes are identical. They have the same genes but can have different alleles.
4
Writing that DNA wraps around the histones 'to protect it from enzymes' without noting that it can also control access to genes.
5
Saying that gametes have 23 pairs of chromosomes. They have 23 single chromosomes.

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