Essential Idea: Chromosomes carry genes in a linear sequence that is shared by members of a species

Prokaryotic Chromosomes

Figure 1: Structure of a prokaryote

  • Prokaryotes have a single, long, circular thread of DNA
  • Coiled up and concentrated in the nucleoid region
  • 1 Chromosome, 1 copy of each gene, no histone  proteins
  • Prokaryotic bacteria may have plasmids (not found in eukaryotes)
  • Plasmids = small loops of DNA that are extra copies of some genetic material (Not connected to main chromosome)
  • Replicate independently of chromosomal DNA
  • Not needed for daily life of cell – can give advantages to cell in unusual conditions (i.e. antibiotic resistance)

Figure 2: Prokaryotic cell with plasmid

  • Plasmids can be passed between prokaryotes (conjugation – horizontal gene transfer) and can be incorporated into the chromosomal DNA
  • Due to this, scientists use them for genetic engineering and gene transfer (ex. The gene for insulin)

 

Figure 3: Conjugation – horizontal gene transfer

Cairns’ Technique

  • Cairns supplied cell with a radioactive version of thymidine – a component of a DNA nucleotide (pentose sugar bonded to thymine)
  • This radioactive version is called 3H-Thymidine (3H = radioactive isotope of H)
  • 3H-Thymidine is used because it will selectively label only DNA, not RNA. Why?
  • The cells were broken open by enzymes and the cell contents were applied to a photographic emulsion and placed in the dark (~2 months)
  • Radioactive 3H Thymidine reacted and emitted particles that exposed the film, creating an autoradiogram.
  • The results shows the location of the 3H thymidine and therefore the presence/location of the thymine in the DNA strand.
  • Cairns found that prokaryotic DNA also replicates semi-conservatively.

Figure 4: Outcome of Autoradiography of bacterial DNA

Eukaryotic Chromosomes

  • Eukaryotic chromosomes are linear and are made up of DNA and histone proteins
  • Histones act as spools in which the DNA winds around

    Figure 5: Nucleosome structure

  • Chromatin = strands of DNA and histones tightly packed together
  • Nucleosome = DNA wrapped around 8 histone proteins. (DNA wraps twice around the histone protein core)
  • DNA is attracted to histones because DNA is negatively charged and histones are positively charged.
  • Eukaryotes have multiple chromosomes
  • All individuals of a species have same number of chromosomes

Chromosomes vary by:

  • Length (# of base pairs)
  • Position of the centromere
  • Genes present (Each gene is at a specific locus – always found at the same position on the same chromosome)

Figure 6: Representation of chromosomes 1-3

Homologous chromosomes are chromosomes within each cell that carry the same genes.
  • Although homologous chromosomes carry the same genes they are not identical because the alleles could be different.

    Figure 6: Representation of homologous chromosomes – notice how they can contain different alleles

     

  • Sister chromatids – two identical copies (chromatids) formed by the replication of a single chromosome, with both copies joined together by a centromere

Figure 7: Chromosome representation

  • Diploid (2n) = a cell that has pairs of homologous chromosomes (Most cells in human body are diploid)
  • Haploid (n) = a cell that has a single chromosome from each pair (Sex cells or gametes)
  • Haploid gametes (sperm and egg) fuse to create a diploid zygote.
N number = number of different chromosomes possessed by a species.
  • The N number is a characteristic feature of species members (organisms with different numbers are unlikely to be able to interbreed)
  • Humans have 22 autosomal pairs of chromosomes and 1 pair of sex chromosomes
  • This 23rd pair determines the sex of the offspring – females are XX and males are XY.
  • The X chromosome is longer and contains more genes essential to human development.
  • The Y chromosome has a smaller number of genes, many of which are only necessary for male development.

Karyotyping

  • Karyotype = Number and appearance of chromosomes in a cell.
  • Karyogram = Diagram/photo of chromosomes in a cell arranged into homologous pairs and in sequence.
  • Chromosomes are arranged in pairs according to their size and structure (largest = chromosome pair 1, smallest = 22)
  • The 23rd pair are the sex chromosomes.

Figure 8: A karyogram

Comparing Prokaryotic and Eukaryotic chromosomes

PROKARYOTE EUKARYOTE
# of Chromosomes 1 2 or more*
Shape Circular Linear
Histones Not present** Present
Presence of plasmids Sometimes Never
Organized into pairs No Yes

*- Rare for eukaryotes to have one chromosome but some can (ex. Male bees)

** – Histones present in archean DNA but not bacterial