A1.2:  Nucleic Acids

Theme:  Unity and Diversity

Unity:
The unity demonstrated by nucleic acids stems from fundamental similarities across all organisms:​

• All living organisms use DNA as the genetic code and molecule of heredity. 
• All nucleic acids are polymers built from repeating monomeric units called nucleotides.
• The sugar-phosphate backbone, formed by phosphodiester bonds between the 3' carbon of one sugar and the 5' carbon of the next, is a universal feature of all nucleic acid strands.
• The fundamental principle of base pairing (A with T/U, and C with G) through hydrogen bonds is conserved across all life. This precise pairing is crucial for DNA replication and transcription. 
• The flow of genetic information from DNA to RNA to protein (DNA -> RNA -> Protein) occurs in all living things. 
• With minor exceptions, the genetic code, which dictates which sequence of three nucleotides (a codon) codes for a specific amino acid, is nearly identical across all organisms.

Diversity:
While sharing fundamental similarities, nucleic acids also exhibit remarkable diversity, leading to the vast array of life forms:

• The unique sequence of A, T, C, and G (or U) creates distinct genetic information for each species and for each individual within a species. This variation in DNA sequence is the direct basis for differences in traits, adaptations, and the evolutionary history of life.
• Different organisms have different sets of genes encoded in their DNA due to variations in nucleotide sequences, leading to their unique characteristics and functionalities. 
  
• The amount of DNA and its organization varies greatly among organisms. From small viral genomes to the vast and complex genomes of multicellular eukaryotes with multiple chromosomes, the packaging and arrangement of nucleic acids, ultimately derived from their sequence content, contribute to biological diversity.
  

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 does the structure of nucleic acids allow hereditary information to be stored?
• How does the structure of DNA facilitate accurate replication?

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

• What explains the use of certain molecular building blocks in all living cells? (A2.2)
• What makes RNA more likely to have been the first genetic material, rather than DNA? 
• How can polymerization result in emergent properties?

Resources:

• ​Quizlet study set for this topic.  ​​

A1.2.1— DNA as the genetic material of all living organisms.

• State the two primary functions of nucleic acids.
• State the two types of nucleic acids used in cells.
• Outline the meaning and implication of DNA being the genetic material of all living organisms.
• State why RNA viruses do not falsify the claim that all living things use DNA as the genetic material. ​​

A1.2.2— Components of a nucleotide.

• ​List the three components of a nucleotide. 
• Identify and label the carbons of a pentose sugar.
• Draw the basic structure of a single nucleotide (using circle, pentagon and rectangle).

A1.2.3— Sugar–phosphate bonding and the sugar–phosphate “backbone” of DNA and RNA.

• Define “backbone” as related to nucleic acid structure.
• Explain how nucleotides connect to form a nucleic acid polymer.

A1.2.4— Bases in each nucleic acid that form the basis of a code.

• State the names of the nitrogenous bases found in DNA and RNA.
• State a similarity and a difference between the nitrogenous bases.
• Outline how the sequence of bases in a nucleic acid serves as a ‘code.’ 
• Define gene.

A1.2.5— RNA as a polymer formed by condensation of nucleotide monomers. 

• Describe the condensation reaction that forms a polymer of RNA from RNA nucleotides.
• Identify the monomer and polymer of an RNA molecule.
• Draw a short section of an RNA polymer (using circle, pentagon and rectangle)

A1.2.6— DNA as a double helix made of two antiparallel strands of nucleotides with two strands linked by hydrogen bonding between complementary base pairs.

• Describe the structure of a DNA double helix.
• Outline the complementary base pairing rule, including the type and number of bonds between bases. 
• Define antiparallel in relation to DNA structure.

A1.2.7— Differences between DNA and RNA.

• Compare and contrast the structures of DNA and RNA.​
• Compare and contrast the functions of DNA and RNA.
• Compare and contrast the location of DNA and RNA in prokaryotic and eukaryotic cells.

A1.2.8- Role of complementary base pairing in allowing genetic information to be replicated and expressed. 

• Outline the role of complementary base pairing in maintaining the DNA sequence during DNA replication.
• ​Outline the role of complementary base pairing in transmitting the genetic code in transcription and translation.

A1.2.9— Diversity of possible DNA base sequences and the limitless capacity of DNA for storing Information.  

• Outline why there is a limitless diversity of DNA base sequences.

A1.2.10— Conservation of the genetic code across all life forms as evidence of universal common ancestry. 

• Define universal in relation to the genetic code.
• Outline why conservation of the genetic code across all forms of life is evidence of common ancestry. 
  

AHL A1.2.11— Directionality of RNA and DNA.

• Identify and label the 5’ and 3’ ends on a diagram of DNA.
• Identify and label the 5’ and 3’ ends of the daughter DNA strands on a diagram of the DNA replication fork.
• Outline the impact of DNA directionality on DNA replication.
• Identify and label the 5’ and 3’ ends of RNA on a diagram of the transcription bubble.
• Outline the impact of DNA directionality on transcription.
• Identify and label the 5’ and 3’ ends of mRNA on a diagram the ribosome bound to mRNA.
• ​Outline the impact of RNA directionality on transcription.

AHL A1.2.12— Purine-to-pyrimidine bonding as a component of DNA helix stability. 

• Compare and contrast the structures of purines and pyrimidines.
• State that in DNA, a purine forms hydrogen bonds with a pyrimidine.
• Given a diagram of DNA, identify the four bases of DNA based on purine or pyrimidine and the number of hydrogen bonds.​
• State two consequences of purine-to-pyrimidine bonding on the structure of DNA.  
  

AHL A1.2.13— Structure of a nucleosome.

• Describe the structure of eukaryotic DNA and associated histone proteins during interphase (chromatin).
• Draw and label the structure of a nucleosome, including the H1 protein, the octamer core proteins, linker DNA and two wraps of DNA.​
• ​Identify nucleosome structures using molecular visualization software. 
• Outline the mechanism of histone-DNA association.

AHL A1.2.14— Evidence from the Hershey–Chase experiment for DNA as the genetic material.

• State the experimental question being tested in the Hershey and Chase experiment.
• Outline the procedure of the Hershey and Chase experiment.
• Explain how the results of the Hershey and Chase experiment supported the notion of nucleic acids as the genetic material.
• Outline the use of radioisotopes as research tools. 

AHL A1.2.15— Chargaff’s data on the relative amounts of pyrimidine and purine bases across diverse life forms.

• Explain the role of falsifiability in determining the structure and function of DNA.
• Describe implications of Chargaff’s data that showed a 1:1 ratio of purine to pyrimidine in a sample of DNA.