Karyograms: Skills (SL IB Biology)

Chromosomes

• Chromosomes are strands of DNA, along which are sections known as genes
  • A gene is a section of DNA that codes for one polypeptide
• Chromosomes that have undergone DNA replication have the appearance of an 'X' shape, where the 'legs' of the X are made up of two strands of DNA attached at a region known as the centromere
  • Chromosomes with a centromere located roughly in the middle are known as metacentric chromosomes
  • Chromosomes with the centromere near the end are acrocentric
• Chromosomes that have been stained with a dye have a banded appearance

Metacentric and acrocentric chromosomes diagram

Chromosomes with a central centromere and arms of equal length are metacentric, and chromosomes with a near-terminal centromere and arms of unequal length are acrocentric

Karyotypes

• A karyogram is an image that shows all of the chromosomes in a cell, arranged by size, shape, and banding pattern, and placed with their homologous pairs
• A karyogram shows the karyotype of an individual, which can be defined as

The appearance of a complete set of an individual's chromosomes, including their number, size, shape, and banding

• Note that chromosome pair 23 often does not fit the size-order pattern, as pair 23 contains the sex chromosomes and the X chromosomes is very large

Human karyogram diagram

A karyogram contains an individual's chromosomes arranged in homologous pairs. It shows a karyotype; the appearance of a complete set of chromosomes arranged by size, shape, and banding pattern

Making a karyogram

• A karyogram can be produced as follows
  • Cells are stained and viewed under a light microscope
  • Photographs are taken of the contents of the nucleus during metaphase of cell division
  • The photographs of the chromosomes are cut up and arranged by size, shape, and banding pattern
    • This can be done with paper and scissors or on a computer

Evaluating chromosome evidence relating to human evolution

• As you will know from the section on chromosome number, humans have 46 chromosomes, while chimpanzees have 48
  • Gorillas and bonobos also have 48 chromosomes
• Given that evidence shows that humans share a recent common ancestor with these species, this raises the question of how these extra chromosomes were lost during evolution
  • A whole pair of chromosomes must be removed for 48 chromosomes (24 pairs) to become 46 (23 pairs)
• The possible mechanisms by which this loss could have occurred when humans split from chimpanzees and gorillas include:
  • A pair of chromosomes disappeared from the genome
  • A pair of chromosomes fused with another pair to form a single pair
• The loss of an entire pair of chromosomes would have had a significant effect on the characteristics of human ancestors, and may have put their survival at risk, so the first mechanism is unlikely to be correct, but scientists have been able to use karyograms to test the following hypothesis:

Chromosomes in pairs 12 and 13 in a common ancestor fused to form the chromosomes in human pair 2

(Note that from this point onwards the notes will only refer to one chromosome from each pair, but the same will also be true for the other member of the pair)

• Evidence that supports this hypothesis includes:
  • Chimpanzee chromosomes 12 and 13, when placed end-to-end, match the length of human chromosome 2
  • The location of the centromere of chimpanzee chromosome 12 matches that of human chromosome 2
  • Human chromosome 2 contains a region of non-coding DNA known as satellite DNA that corresponds to the location of the centromere in chimpanzee chromosome 13; this could be a remnant of a centromere
  • The banding of the long arms of acrocentric chimpanzee chromosomes 12 and 13 corresponds to the banding of metacentric human chromosome 2

  • Human chromosome 2 contains telomeric DNA in the middle of the chromosome

• Evidence that does not support this hypothesis includes:
  • The length of chimpanzee chromosomes 12 and 13 combined is not a perfect match for human chromosome 2; there is a slight overlap

  • The location of the centromere of chimpanzee chromosome 13 does not match that of human chromosome 2

• Note that chimp chromosomes 12 and 13 are sometimes referred to as chromosomes 2A and 2B, in acknowledgement of the fusion event discussed above

Chromosome fusion evidence diagram

Comparing the structures of chimp chromosomes 12 and 13 with human chromosome 2 provides evidence both for and against the chromosome fusion hypothesis

NOS: Distinguishing between testable hypotheses and non-testable statements

• The hypothesis above, relating to the origin of chromosome 2, is an example of a testable hypothesis

• For a hypothesis to be testable, it must have the following characteristics:

  • There needs to be access to evidence that supports it or refutes it
    • The hypothesis above about the fusion of ancestral chromosomes 12 and 13 can be tested by examining chromosome evidence from humans and modern chimps

    • A hypothesis such as 'the common ancestor of humans and chimps enjoyed singing' would not be testable, as there is no available evidence to support or refute it

  • The hypothesis needs to be a testable statement:
    • The hypothesis written above is 'chromosomes in pairs 12 and 13 in a common ancestor fused to form the chromosomes in human pair 2'; this statement can be accepted or rejected
    • Another example might be 'organisms with a large surface area to volume ratio lose heat more quickly'; an investigation can be carried out to determine whether to accept or reject this statement
    • A hypothesis should not contain vague statements that use terms like 'may' or 'could'; this makes it difficult to entirely accept or reject a statement, e.g. 
      • 'The fusion of ancestral chromosomes may have reduced the chromosome number'
      • 'Surface area to volume ratio could affect the rate of heat loss'
    • A hypothesis should not make predictions, e.g.
      • 'Surface area to volume ratio will affect the rate of heat loss'
    • A hypothesis should not draw causal conclusions, e.g.
      • 'A larger surface area to volume ratio causes increased heat loss'