Written for this site from the class slides, with textbook page references for each statement. The chapter is on pages 25–33 of the course textbook. Image credits are on slide 14.
Phosphodiester bonds link carbon 3′ of one sugar to carbon 5′ of the next, so each strand has a 5′ end (phosphate) and a 3′ end (free -OH). The two strands of DNA are antiparallel. New nucleotides join only the 3′ end, so DNA and RNA are always built 5′ to 3′. In replication, the two strands are copied in opposite directions; in transcription, RNA polymerase reads the template strand 3′ to 5′; in translation, ribosomes read mRNA from 5′ to 3′.
Purines (adenine, guanine) have two rings; pyrimidines (cytosine, thymine) have one. Each base pair is one of each, so A-T and C-G pairs are the same width and the helix keeps the same shape whatever the sequence. Hydrogen bonds (two in A-T, three in C-G) hold the pairs together.
In eukaryotes, DNA is wound about twice around a core of eight histone proteins. Linker DNA runs to the next nucleosome, and an H1 histone holds the DNA in place. Positively charged lysine and arginine in histones attract the negative phosphates of DNA. Packing fits about 2 m of DNA into each nucleus, lets chromosomes condense for division, and helps control which genes can be transcribed.
In 1952, phages were labelled with 35S (in protein) or 32P (in DNA) and allowed to infect E. coli. Blending removed the phage coats and centrifuging separated them from the cells. Most 32P went into the bacteria and some appeared in new phages; most 35S stayed outside. So DNA, not protein, carries the genetic information.
Chargaff measured base proportions in DNA from many species. In every species A = T and G = C, so purines equal pyrimidines; but the proportions differ between species. This falsified the tetranucleotide hypothesis, which said DNA was a simple repeat of the four bases and could not carry information, and it supported complementary base pairing.
Select a video to play it here. Select it again to close it.
Print this guide and complete it as you work through the slides and videos. It asks for your own explanations, so close the notes while you try each section.
The self-check needs JavaScript to run.
A printable worksheet covering all five statements, from quick recall to data questions and a longer answer. It has 40 marks and takes about 40 minutes, or you can do one section at a time.
Your teacher has the mark scheme.
You will need: Published base composition data for several species, spreadsheet or calculator
Safety: No practical hazards.
Carry this out with your teacher's supervision and follow your school's lab rules.
Avery's team pointed to DNA as the genetic material in 1944, but many scientists were only convinced after Hershey and Chase's 1952 experiment.
Many expected the genetic material to be protein. Consider how prior expectations affect the weight given to results.
Summarised in our own words. Use each code to find the exact wording in your textbook or the IB subject guide.
© 2026 Akash Parthi Bendran. Please ask before using or sharing any resources from this site: see permission to use.