A Level Biology: Lesson 25

The Structure of DNA

Task 1. Check Where this Lesson fits into your Exam Specification!

Task 2. Watch the Revision Notes Lesson in Full.

00:00 Intro Screen / Learning Outcomes

01:30 A Super Brief History of DNA

03:09 DNA contains Purines and Pyrimidines

03:27 1950 - Chargaff's Rule

03:53 - Wilkins and Franklin

04:40 1953

05:16 The DNA Nucleotide

06:03 Complementary Base Pairing requires DNA polynucleotide chains to run in Antiparallel directions...

07:30 The % of A is equal to T...

08:00 Antiparallel Structure of DNA

08:26 Hydrogen bonding...

09:14 All this leads Watson and Crick to Build their 3D model of DNA...

09:29 - Summary of DNA Structure.

Task 3. 

Download, Print out & complete the accompanying Interactive Worksheet.


Check Your Spec!

★ AQA Specification Reference: - 3.1.5 Nucleic acids are important information-carrying molecules. Structure of DNA. Deoxyribonucleic acid (DNA) is an important information-carrying molecules. In all living cells, DNA holds genetic information. DNA is a polymer of nucleotides. Each nucleotide is formed from a pentose, a nitrogen containing organic base and a phosphate group. The components of a DNA nucleotide are deoxyribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or thymine. A condensation reaction between two nucleotides forms a phosphodiester bond. A DNA molecule is a double helix with two polynucleotide chains held together by hydrogen bonds between specific complementary base pairs. Students should be able to appreciate that the relative simplicity of DNA led many scientists to doubt that it carried the genetic code.


★ CIE Specification Reference: - 6 Nucleic acids. 6.1 Structure of DNA. Understanding the structure of nucleic acids allows an understanding of their role in the storage of genetic information. a) describe the structure of nucleotides. b) describe the structure of DNA and explain the importance of base pairing and the different hydrogen bonding between bases (include reference to adenine and guanine as purines and to cytosine, thymine and uracil as pyrimidines. Structural formulae for bases are not required but the recognition that purines have a double ring structure and pyrimidines have a single ring structure should be included).


★ Edexcel (Biology A – Salters-Nuffield) Specification Reference: - Topic 2: Genes and Health. 2.5 i) Know the basic structure of mononucleotides (deoxyribose linked to a phosphate and a base, including thymine, cytosine, adenine or guanine) and the structures of DNA (polynucleotide composed of mononucleotides linked through condensation reactions). ii) Know how complementary base pairing and the hydrogen bonding between two complementary strands are involved in the formation of the DNA double helix.


★ Edexcel (Biology B) Specification Reference: - Topic 1: Biological Molecules. 1.4 DNA and protein synthesis. i) Know the structure of DNA, including the structure of the nucleotides (purines and pyrimidines), base pairing, the two sugar-phosphate backbones, phosphodiester bonds and hydrogen bonds.


★ OCR (Biology A) Specification Reference: - 2.1.3 Nucleotides and nucleic acids. (a) the structure of a nucleotide as the monomer from which nucleic acids are made (the identification of the purines and pyrimidines and the type of pentose sugar and the formation of phosphodiester bonds (the sugar phosphate backbone. (b) the synthesis and breakdown of polynucleotides by the formation and breakage of phosphodiester bonds. (d) (i) the structure of DNA (deoxyribonucleic acid) To include how hydrogen bonding between complementary base pairs (A to T, G to C) on two antiparallel DNA polynucleotides leads to the formation of a DNA molecule, and how the twisting of DNA produces its ‘double-helix’ shape.


★ OCR (Biology B) Specification Reference: - 2.1.4 Nucleic acids. (a) the structure of a nucleotide as the monomer from which nucleic acids are made (the identification of the purines and pyrimidines and the type of pentose sugar and the formation of phosphodiester bonds (the sugar phosphate backbone). (c) (i) the structure of the DNA molecule, including a review of the evidence for complementary bases pairing “Chargaff’s rules”).


★ WJEC Specification Reference: - Core Concepts 5. Nucleic acids and their functions. (a) the structure of nucleotides (pentose sugar, phosphate, organic base). (e) the structure of nucleic acids: DNA bases: purines and pyrimidines; complementary base pair rule; hydrogen bonding and the double helix; antiparallel strands.


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