Notes on Chromosome Structure and DNA Packaging

6.2 BACKGROUND KNOWLEDGE: The role of chromosomes as structures that package DNA

6.2.1 Why are chromosomes important?

  • Located in the nucleus in almost all cells; chromosomes are the thread-like structures present in almost all eukaryotic and prokaryotic cells.
  • Structure: chromosomes are composed of a single molecule of deoxyribonucleic acid (DNA) and proteins called histones; DNA contains the four bases adenine, guanine, cytosine and thymine.
  • The presence of chromosomes across many cell types indicates their importance for life.
  • DNA packaging and chromosomes:
    • A typical human cell contains approximately 2 m2\ \text{m} of DNA.
    • This enormous length must be packaged into a nucleus of average size about 30 μm30\ \mu\text{m} in diameter.
    • Packaging must preserve the chemical makeup and the position of genes.
  • Gene concept:
    • A chromosome contains sections of DNA that code for proteins; a chromosome that codes for a protein does so via the order of nucleotide bases.
  • Nucleosomes and chromatin:
    • DNA wraps around a core of eight histones to form a nucleosome.
    • Nucleosomes combine with other nucleosomes to form supercoiled, tightly packed DNA called chromatin.
  • Chromosome vs chromatin terminology:
    • Condensed DNA for cell division is referred to as chromatin; a common exam mistake is to call it chromatid (which is half of a replicated chromosome).
  • Analogy to aid understanding:
    • Coiling DNA around histones is like coiling a long hose around a reel to make it easier to manage and store.
  • Key functions and significance of chromosomal packaging:
    • Enables packing of large amounts of genetic information into a tiny nucleus.
    • Maintains DNA integrity and preserves chemical composition during packaging.
    • Facilitates movement and segregation of chromosomes to the poles during cell division.
  • Terminology and implications:
    • Gene: a chromosome segment that codes for a protein via the nucleotide sequence.
    • Histone: the protein around which DNA is wrapped to form nucleosomes.
    • Chromatin: the condensed DNA–protein complex that forms chromosomes during cell division.

6.2.2 Eukaryotic chromosome structure

  • Eukaryotic chromosomes are composed of two main ingredients: DNA and proteins called histones.
  • Nucleosome formation:
    • A short section of DNA wraps tightly around a core of histones (an octamer of histones) to form a nucleosome.
    • The nucleosome plus linker DNA between nucleosomes forms the higher-order structure known as chromatin.
  • Coiling and condensation:
    • The coiling of DNA around histones to form nucleosomes enables the enormous amount of DNA to condense into a much smaller volume.
    • This condensation reduces the physical space that the DNA occupies and helps maintain the DNA’s chemical integrity.
  • Chromosome dynamics:
    • The condensed DNA (chromatin) can be easily maneuvered to the cell’s poles during cell division.
  • Important distinctions and cautions:
    • A common exam mistake is referring to condensed DNA as chromatid; the correct term for the condensed form is chromatin.
  • Quantitative context:
    • The human cell contains approximately 2 m2\ \text{m} of DNA within a nucleus of about 30 μm30\ \mu\text{m} in diameter.
  • Gene structure:
    • The chromosome contains a gene, which codes for a protein through the order of nucleotide bases.
  • Visual references in course materials (Figure 6.3 and related explanations):
    • Coiling and supercoiling of DNA form chromosomes; nucleosome structure, linker DNA, and the progression from DNA to chromatin to chromosome.
  • Implications for regulation:
    • The degree of chromatin condensation can influence gene expression; tightly packed regions are generally less accessible for transcription than loosely packed regions.

6.2.3 Prokaryotic chromosome structure

  • Prokaryotic chromosomal organization:
    • Prokaryotes also package their DNA into chromosomes, but typically as a single circular chromosome rather than multiple linear chromosomes as in many eukaryotes.
  • Condensation and histones:
    • The prokaryotic chromosome is less condensed than eukaryotic chromosomes and does not use histones for supercoiling; however, the DNA is still highly coiled and forms characteristic loops and twists due to supercoiling.
  • Consequences of improper packaging:
    • If DNA is not properly packaged, it will not physically fit into the cell, leading to loss of genetic information.
    • Improper packaging can prevent correct distribution of DNA during cell division, so daughter cells may not receive the proper genetic information.

CASE STUDY: Mitochondrial DNA

  • Mitochondria contain their own DNA, supporting the endosymbiotic theory.
  • Characteristics of mitochondrial DNA (mtDNA):
    • mtDNA is approximately 16 500 bp16\ 500\ \text{bp} (base pairs) in length.
    • It codes for 13 proteins13\ \text{proteins}.
    • mtDNA is circular, and mitochondria do not contain proteins in their DNA; this makes them resemble prokaryotic chromosomes.
  • Significance:
    • The distinct, circular, protein-free nature of mtDNA provides evidence for the endosymbiotic theory, which posits that mitochondria originated from ancient symbiotic bacteria.
  • Context in the cell:
    • Mitochondria are organelles responsible for the major site of ATP production in eukaryotic cells.
    • Singular form: one mitochondrion; plural: mitochondria.

SAMPLE PROBLEM 1: Comparing and contrasting eukaryotic and prokaryotic chromosomes

  • Task: Compare and contrast features of chromosomes in eukaryotes and prokaryotes.
  • THINK steps:
    1) Identify what the question is asking (compare and contrast similarities and differences).
    2) Contrast the two cell types, addressing both, not just one.
    3) Synthesize the two aspects into a unified answer.
  • WRITE answer (3 marks):
    • Both involve supercoiling to condense the DNA molecule, reducing the physical space the DNA occupies.
    • Eukaryotic DNA is coiled around histones to produce nucleosomes; this condensed DNA is called chromatin. Prokaryotic chromosomes do not contain histones and therefore do not contain nucleosomes or chromatin.
    • Eukaryotic chromosomes are linear; prokaryotic chromosomes are circular.
    • The DNA in both cell types is highly twisted, producing supercoiled DNA, which reduces the physical space the DNA can fit into (1 mark).
    • Eukaryotic DNA is wrapped around histones to form nucleosomes; condensed DNA is chromatin (1 mark). Prokaryotic DNA lacks histones and nucleosomes/chromatin (1 mark).

INVESTIGATION 6.1: Extraction of DNA from kiwi fruit (online resource)

  • Aim:
    • To extract DNA from within the nucleus of cells in a kiwi fruit.
  • Key ideas:
    • Chromosomes are condensed single molecules of DNA with associated proteins.
    • Condensing DNA allows a large amount of genetic information to be stored inside a cell.
    • Prokaryotic chromosomes do not contain proteins within their structure (as described in the context of this topic).
    • Highly condensed chromosomes can be safely moved around the cell during cell division.
  • Context:
    • This investigation reinforces the concepts of DNA packaging, chromatin, and chromosome behavior in a model plant tissue.

Notes:

  • Throughout, be mindful of terminology: chromatin refers to condensed DNA–protein material; chromatid refers to a replicated chromosome half.
  • Key quantities to remember: the approximate length of DNA in a human cell (~2 m2\,\text{m}), the size of the nucleus (~30 μm30\,\mu\text{m}), and mtDNA length (~16 500 bp16\,500\,\text{bp}) with coding for ~13 proteins13\,\text{proteins}.
  • Conceptual links to broader topics: chromatin structure relates to gene expression regulation; the endosymbiotic theory is supported by mtDNA characteristics; proper DNA packaging is essential for accurate inheritance during cell division.