Before cells may enter mitosis [or meiosis for that matter] they need to duplicate – replicate – their DNA. The process of replication occurs during ‘S’ phase of the cell cycle.
Review DNA structure:
Note: The video fails to highlight ALL the HL objectives.
How do we know about semi-conservative replciation? TOK
The meselson- stahl experiment was a brilliant example of creative science. Using cetrifugation, they were able to demonstrate the manner in which DNA is replicated.
IB expectations : SL
3.3 DNA structure (1) hour
3.3.1 Outline DNA nucleotide structure in terms of sugar (deoxyribose), base and phosphate. (2)
Chemical formulas and the purine/pyrimidine subdivision are not required. Simple shapes can be used to represent the component parts. Only the relative positions are required.
3.3.2 State the names of the four bases in DNA. (1)
3.3.3 Outline how DNA nucleotides are linked together by covalent bonds into a single strand. (2)
Only the relative positions are required.
3.3.4 Explain how a DNA double helix is formed using complementary base pairing and hydrogen bonds. (3)
3.3.5 Draw and label a simple diagram of the molecular structure of DNA. (1)
An extension of the diagram in 3.3.3 is sufficient to show the complementary base pairs of A–T and G–C, held together by hydrogen bonds and the sugar–phosphate backbones. The number of hydrogen bonds between pairs and details of purine/pyrimidines are not required. TOK: The story of the elucidation of the structure of DNA illustrates that cooperation and collaboration among scientists exists alongside competition between research groups. To what extent was Watson and Crick’s “discovery” of the three- dimensional structure of DNA dependent on the use of data generated by Rosalind Franklin, which was shared without her knowledge or consent?
3.4 DNA replication (1) hour
3.4.1 Explain DNA replication in terms of unwinding the double helix and separation of the strands by helicase, followed by formation of the new complementary strands by DNA polymerase. (3)
It is not necessary to mention that there is more than one DNA polymerase.
3.4.2 Explain the significance of complementary base pairing in the conservation of the base sequence of DNA. (3)
3.4.3 State that DNA replication is semi- conservative. (1)
IB Objectives: HL
7.1 DNA structure (2) hours
7.1.1 Describe the structure of DNA, including the antiparallel strands, 3’–5’ linkages and hydrogen bonding between purines and pyrimidines. (2)
Major and minor grooves, direction of the “twist”, alternative B and Z forms, and details of the dimensions are not required.
7.1.2 Outline the structure of nucleosomes. (2)
Limit this to the fact that a nucleosome consists of DNA wrapped around eight histone proteins and held together by another histone protein.
7.1.3 State that nucleosomes help to supercoil chromosomes and help to regulate transcription. (1)
7.2 DNA replication (2) hours
7.2.1 State that DNA replication occurs in a 5’ → 3’ direction. (1)
The 5’ end of the free DNA nucleotide is added to the 3’ end of the chain of nucleotides that is already synthesized.
7.2.2 Explain the process of DNA replication in prokaryotes, including the role of enzymes (helicase, DNA polymerase, RNA primase and DNA ligase), Okazaki fragments and deoxynucleoside triphosphates. (3)
The explanation of Okazaki fragments in relation to the direction of DNA polymerase III action is required. DNA polymerase III adds nucleotides in the 5’ → 3’ direction. DNA polymerase I excises the RNA primers and replaces them with DNA.
7.2.3 State that DNA replication is initiated at many points in eukaryotic chromosomes. (1)