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DNA Replication (2.7/7.1)


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2.7 DNA replication, transcription and translation

Nature of science:

Obtaining evidence for scientific theories—Meselson and Stahl obtained evidence for the semi-conservative replication of DNA. (1.8)

Understandings:

The replication of DNA is semi-conservative and depends on complementary base pairing.

Helicase unwinds the double helix and separates the two strands by breaking hydrogen bonds.

DNA polymerase links nucleotides together to form a new strand, using the pre-existing strand as a template.

Transcription is the synthesis of mRNA copied from the DNA base sequences by RNA polymerase.

Translation is the synthesis of polypeptides on ribosomes.

The amino acid sequence of polypeptides is determined by mRNA according to the genetic code.

Codons of three bases on mRNA correspond to one amino acid in a polypeptide.

Translation depends on complementary base pairing between codons on mRNA and anticodons on tRNA.

Applications and skills:

Application: Use of Taq DNA polymerase to produce multiple copies of DNA rapidly by the polymerase chain reaction (PCR).

Application: Production of human insulin in bacteria as an example of the universality of the genetic code allowing gene transfer between species.

Skill: Use a table of the genetic code to deduce which codon(s) corresponds to which amino acid.

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

Skill: Use a table of mRNA codons and their corresponding amino acids to deduce the sequence of amino acids coded by a short mRNA strand of known base sequence.

Skill: Deducing the DNA base sequence for the mRNA strand.

Guidance:

The different types of DNA polymerase do not need to be distinguished.

Aims:

Aim 8: There are ethical implications in altering the genome of an organism in order to produce proteins for medical use in humans.

Essential idea: Cell respiration supplies energy for the functions of life.

7.1 DNA structure and replication

Nature of science:

Making careful observations—Rosalind Franklin’s X-ray diffraction provided crucial evidence that DNA is a double helix. (1.8)

Understandings:

Nucleosomes help to supercoil the DNA.

DNA structure suggested a mechanism for DNA replication.

DNA polymerases can only add nucleotides to the 3’ end of a primer.

DNA replication is continuous on the leading strand and discontinuous on the lagging strand.

DNA replication is carried out by a complex system of enzymes.

Some regions of DNA do not code for proteins but have other important functions.

Applications and skills:

Application: Rosalind Franklin’s and Maurice Wilkins’ investigation of DNA structure by X-ray diffraction.

Application: Use of nucleotides containing dideoxyribonucleic acid to stop DNA replication in preparation of samples for base sequencing.

Application: Tandem repeats are used in DNA profiling.

Skill: Analysis of results of the Hershey and Chase experiment providing evidence that DNA is the genetic material.

Skill: Utilization of molecular visualization software to analyse the association between protein and DNA within a nucleosome.

Theory of knowledge:

Highly repetitive sequences were once classified as “junk DNA” showing a degree of confidence that it had no role. To what extent do the labels and categories used in the pursuit of knowledge affect the knowledge we obtain?

Aims:

Aim 6: Students could design models to illustrate the stages of DNA replication.

Guidance:

Details of DNA replication differ between prokaryotes and eukaryotes. Only the prokaryotic system is expected.

The proteins and enzymes involved in DNA replication should include helicase, DNA gyrase, single strand binding proteins, DNA primase and DNA polymerases I and III.

The regions of DNA that do not code for proteins should be limited to regulators of gene expression, introns, telomeres and genes for tRNAs.

Essential idea: Information stored as a code in DNA is copied onto mRNA.

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