IB Biology SL

Revision Notes

2.6.4 Skills: DNA, RNA & Protein Synthesis

Drawing DNA & RNA Nucleotides & DNA Double Helix

Drawing simple diagrams of the structure of single nucleotides of DNA and RNA

  • Simple shapes can be used to draw the main building blocks of nucleotides and the DNA double helix
    • Advanced drawing skills are not required!
  • Pentagons can represent pentose sugars
  • Circles can represent phosphates
    • Often shown as a circle with the letter P inside: ℗
  • Rectangles can represent bases
  • Covalent bonds can be shown with solid lines
  • Hydrogen bonds can be shown with dashed lines
    • Or with complementary shapes that fit together (see diagrams)

Using Shapes to Represent Nucleotides, downloadable IB Biology revision notes

Simple shapes can be used to represent parts of nucleotide molecules

Drawing DNA Nucleotides bonded together, downloadable IB Biology revision notes

Two nucleotides shown bonded together covalently within a strand

Drawing Base Pairing, downloadable IB Biology revision notes

When drawing the base pairing, the opposite strand should be antiparallel to the first. The presence of hydrogen bonding is shown, but the numbers/lengths of bonds is not required

Exam Tip

Simple, hand-drawn shapes will suffice in an exam. Expert tip – a large drawing is always easier for an examiner to read (and award marks for) than a small one!

Read the question carefully; examiners often want a whole nucleotide to be identified in your diagram and to ensure your diagram includes all 4 complementary bases.

You don’t have to remember the number of hydrogen bonds between the bases.

Also, remember to draw DNA strands as antiparallel (one upside-down versus the other) but you don’t have to be able to draw a helix shape!

Meselson & Stahl's Experiments

Analysis of Meselson and Stahl’s results to obtain support for the theory of semi-conservative replication of DNA

  • Crick and Watson, as they defined the shape of DNA, suggested a credible explanation of how the DNA molecule replicates itself
    • This was the theory of semi-conservative replication
  • Like any scientific theory, this explanation required evidence to back up the claims
  • Five years after their discovery, two other scientists, Matthew Meselson and Franklin Stahl, provided data to prove Crick and Watsons’ theory

Meselson and Stahls’ Experiment

  • Bacteria were grown in a broth containing the heavy (15N) nitrogen isotope
    • DNA contains nitrogen in its bases
    • As the bacteria replicated, they used nitrogen from the broth to make new DNA nucleotides
    • After some time, the culture of bacteria had DNA containing only heavy (15N) nitrogen
  • A sample of DNA from the 15N culture of bacteria was extracted and spun in a centrifuge
    • This showed that the DNA containing the heavy nitrogen settled near the bottom of the centrifuge tube
  • The bacteria containing only 15N DNA were then taken out of the 15N broth and added to a broth containing only the lighter 14N nitrogen. The bacteria were left for enough time for one round of DNA replication to occur before their DNA was extracted and spun in a centrifuge
    • If conservative DNA replication had occurred, the original template DNA molecules would only contain the heavier nitrogen and would settle at the bottom of the tube, whilst the new DNA molecules would only contain the lighter nitrogen and would settle at the top of the tube
    • If semi-conservative replication had occurred, all the DNA molecules would now contain both the heavy 15N and light 14N nitrogen and would therefore settle in the middle of the tube (one strand of each DNA molecule would be from the original DNA containing the heavier nitrogen and the other (new) strand would be made using only the lighter nitrogen)
  • Meselson and Stahl confirmed that the bacterial DNA had undergone semi-conservative replication.
    • The DNA from this second round of centrifugation settled in the middle of the tube, showing that each DNA molecule contained a mixture of the heavier and lighter nitrogen isotopes
    • If more rounds of replication were allowed to take place, the ratio of 15N:14would go from 1:1 after the first round of replication, to 3:1 after the second and 7:1 after the third
  • This experiment proved Crick and Watsons’ theory correct

NOS: Obtaining evidence for scientific theories; Meselson and Stahl obtained evidence for the semi-conservative replication of DNA

  • Meselson and Stahl’s experiment is a great example of how scientists can obtain evidence to back up a theory about a biological process

Meselson & Stahl's Experiments 1, downloadable IB Biology revision notesMeselson & Stahl's Experiments 2, downloadable IB Biology revision notes

Meselson and Stahls’ experiment provided unequivocal proof that DNA replicates via semi-conservative DNA replication

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.
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