D1.1  DNA Replication

Theme:  Continuity and Change

Continuity of the genetic code of DNA occurs when a daughter cell is genetically identical to its parent. This is achieved through the mechanism of semi-conservative DNA replication.

• Each strand of the original molecule serves as a template for a new one. The result is two DNA molecules, each consisting of one "old" and one "new" strand, ensuring that the genetic information is conserved through every cell division.
  
• The interaction between adenine-thymine and cytosine-guanine is the mechanical basis of continuity. Because of complementary base pairing via hydrogen bonding, the sequence of the template strand codes for the correct sequence in the new strand.
  
• DNA Polymerase III "checks" its work as it adds nucleotides. If an incorrect base is added, it can be removed and replaced, maintaining the integrity of the genetic code.
  

Mutations are an inevitable and essential part of DNA replication. Without change, there would be no evolution or adaptation.

• Mutations occur if DNA polymerase adds a mismatched nucleotide or as the result of environmental factors. These changes in the DNA base sequence is the primary source of new alleles.
  

Guiding Questions:  
Guiding questions help students view the content of the syllabus through the conceptual lenses of both the themes and the levels of biological organization.

• How is new DNA produced? 
• How has knowledge of DNA replication enabled applications in biotechnology?

​
​Linking Questions:  
Linking questions strengthen students’ understanding by making connections between topics.  The ideal outcome of the linking questions is networked knowledge.

• How is genetic continuity ensured between generations? 
• What biological mechanisms rely on directionality?

Key Terms to Know: * higher level only

3' Terminal*
5' Terminal*
Amplify
Base Sequence
Complementary Base Pair
Deoxyribonucleic Acid (DNA)
DNA Ligase*
DNA Marker
DNA Polymerase

DNA Polymerase I*
DNA Polymerase III*
DNA Primase*
DNA Profiling
DNA Proofreading*
DNA Replication
DNA Strand
Gel Electrophoresis
Helicase

Hydrogen Bond
Lagging Strand*
Leading Strand*
Nucleotide*
Okazaki Fragment*
Polymerase Chain Reaction
Primer
Ribonucleic Acid (RNA)*
Semi-Conservative
Taq Polymerase

D1.1.1— DNA replication as production of exact copies of DNA with identical base sequences.

• State that DNA replication is the production of exact copies of DNA with identical base sequences. 
• Outline the purposes of DNA replication.​

D1.1.2— Semi-conservative nature of DNA replication and role of complementary base pairing.

• Describe the meaning of “semi conservative” in relation to DNA replication.
• Explain the role of complementary base pairing in DNA replication.

D1.1.3— Role of helicase and DNA polymerase in DNA replication.

• State why DNA strands must be separated prior to replication.
• Outline the role of helicase in DNA replication. 
• Outline the role of DNA polymerases in DNA replication.
• State that there are separate DNA polymerases for each stand of template DNA.

D1.1.4-- Polymerase chain reaction and gel electrophoresis as tools for amplifying and separating DNA.

• State the function of the PCR.
• Outline the process of the PCR, including the use of primers, temperature changes and Taq polymerase.
• Deduce the number and relative size of DNA fragments from the number of bands in an electrophoresis gel. 
• Outline the procedure for DNA electrophoresis.
• Describe how and why DNA fragments separate during electrophoresis.​ ​

D1.1.5— Applications of polymerase chain reaction and gel electrophoresis.  

• ​Outline the process of DNA profiling.
• List applications of DNA profiling.
• Analyze a DNA profile to determine relatedness or forensic guilt. 
• List example sources of DNA that can be used in DNA profiling.​

AHL ​​​​D1.1.6- Directionality of DNA polymerases.

• Identify the 5’ ends and 3’ ends of a strand of DNA.
• Describe the formation of the covalent bond between adjacent nucleotides during DNA replication. 
• State that DNA polymerases can only add free nucleotides to an existing DNA strand. 
• State that DNA polymerases can only add the 5’ phosphate of a free nucleotide to the 3’ deoxyribose of the elongating strand.

AHL ​​​​D1.1.7- Differences between replication on the leading strand and the lagging strand.

• Explain why replication is different on the leading and lagging strands of DNA.
• Compare the pace and direction of replication on the leading and lagging strands of DNA.
• Outline the formation of Okazaki fragments on the lagging strand.

AHL ​​​​D1.1.8- Functions of DNA primase, DNA polymerase I, DNA polymerase III and DNA ligase in replication.

• Explain the need for RNA primers in DNA replication.
• Compare the number of RNA primers on the leading and lagging strands.
• Outline the function of the enzyme DNA primase.
• Outline the function of the enzyme DNA polymerase III. 
• Outline the function of the enzyme DNA polymerase I. 
• Explain why there are gaps between adjacent Okazaki fragments on the lagging strand. 
• Outline the function of the enzyme DNA ligase.

AHL ​​​​D1.1.9- DNA proofreading.

• State the function of DNA proofreading. 
• Outline the process of DNA proofreading by DNA polymerase III.