IBDP Biology Study Notes - All Subjects
- IB-style question banks with solution
- IB DP Biology SL- IB Style Practice Questions with Answer- Wise-Paper Topic 1
- IB DP Biology HL- IB Style Practice Questions with Answer- Wise-Paper Topic 1
- IB DP Biology SL- IB Style Practice Questions with Answer- Wise-Paper Topic 2
- IB DP Biology HL- IB Style Practice Questions with Answer- Topic Wise-Paper 2
2.6 Structure of DNA and RNA
Main idea:
The structure of DNA allowsefficient storage of genetic information
Understanding:
- The nucleic acids of DNA and RNA are polymers of nucleotides.
- DNA differs from RNA in the number of strands present, the composition of the bases, and the type of pentose.
- DNA is a double helix molecule composed of two antiparallel strands of nucleotides connected by hydrogen bonds between complementary base pairs.
Forms:
- Crick and Watson's Elucidation of DNA Structure Through Modeling
Capabilities:
- Draw simple diagrams of the structure of individual DNA and RNA nucleotides using circles, pentagons and rectangles to represent phosphates, pentoses and bases
2.6.U1 The nucleic acids of DNA and RNA are polymers of nucleotides.
- Name the two types of nucleic acids.
- Group the parts of a nucleotide.
- Identify and label the carbons by their number (eg C1, C2, C3) in a nucleotide drawing.
- Explain how nucleotides can be joined together to form a nucleic acid polymer.
- Name the nitrogenous bases found in DNA and RNA.
- Identify the nitrogenous bases as pyrimidine or purine.
- Give rules for complementary base pairing.
2.6.U2-DNA differs from RNA in the number of strands present, in the composition of the bases and in the type of pentose.
- Compare the structure of DNA and RNA.
2.6.U3 DNA is a double helix formed by two antiparallel nucleotide strands connected by hydrogen bonds between complementary base pairs.
- Define antiparallel in terms of the structure of DNA.
- Describe the formation of a DNA double helix by hydrogen bonding between nitrogenous bases.
- Identify the four DNA bases by the number of rings (purines or pyrimidines) and the number of hydrogen bonds they can form.
- Give the number of nitrogenous bases per complete turn of the DNA double helix.
2.6.A1 Crick and Watson's elucidation of the structure of DNA through modeling.
- Describe the role of Chargaff, Watson, Crick, Franklin, and Wilkins in discovering the structure of DNA.
- Explain how Watson and Crick used models to determine the structure of DNA.
2.6.S1 Draw simple diagrams of the structure of individual nucleotides in DNA and RNA using circles, pentagons, and rectangles to represent phosphates, pentoses, and bases.
- Draw the basic structure of a single nucleotide (with circle, pentagon and rectangle).
- Draw a simple diagram of the structure of RNA.
- Draw a simple diagram of the structure of DNA,
- Identify and label the 5' and 3' ends on a DNA or RNA diagram
2.6.NOS Using models to represent the real world: Crick and Watson used modeling to discover the structure of DNA.
- Name the types of models used in science.
- Name a common feature of models in science.
- List how the models differ from the structure or process they represent.
Structure of double-stranded DNA
DNA are of different types like:
DNAB
- Two long polynucleotide chains are wound around the axis.
- The strands are antiparallel to each other.
- Guanine bases pair with cytosine and adenine bases pair with thymine.
- Guanine forms 3 hydrogen bonds with cytosine, while adenine forms 2 hydrogen bonds with thymine.
ADN-Z
- They are thinner than DNA B.
- They have alternating purine and pyrimidine bases.
- They are stabilized by a high concentration of salt.
DNA denaturation
When DNA is exposed to changes in pH, temperature, etc., two DNA strands separate. That is, the DNA is said to be denatured. When denaturants such as temperature and pH are removed, DNA recovers its original or native structure. This is known asrenaturation.
Fig. 7. Denaturation process
ARN
RNA known asribonucleic acid. It usually exists as a single strand. RNA exists in cells in different forms as explained below:
Boten-RNAcarries genetic information from DNA in the form of codons (three nucleotides make up a codon) that code for amino acids or proteins.
RNA transferplays an important role in protein synthesis. They act as an interface between the language of nucleic acids and the language of proteins. They are located in the cytosol of the living cell.
ribosomal RNAIt is a component of ribosomes. They have important roles in protein synthesis in eukaryotes and prokaryotes.
Topic 2.6: Structure of DNA and RNA
In the DNA structure unit, students learn about the structure of DNA. The structure of the DNA molecule is critical to understanding replication and how the molecule works.
The unit is designed for 2 school days.
Main idea:
- The structure of DNA allows efficient storage of genetic information.
Nature of Science:
- Using models to represent the real world: Crick and Watson used modeling to discover the structure of DNA. (1.10)
- Name the types of models used in science.
- Name a common feature of models in science.
- List how the models differ from the structure or process they represent.
Understanding
2.6.U1 The nucleic acids of DNA and RNA are polymers of nucleotides.
- Name the two types of nucleic acids.
- Name the two types of nucleic acids.
- Group the parts of a nucleotide.
- Identify and label the carbons by their number (eg C1, C2, C3) in a nucleotide drawing.
- Explain how nucleotides can be joined together to form a nucleic acid polymer.
- Name the nitrogenous bases found in DNA and RNA.
- Identify the nitrogenous bases as pyrimidine or purine.
- Give rules for complementary base pairing.
Nucleotides consist of three parts:
- Sugar has five carbon atoms, which means it is a pentose sugar.
- a phosphate group, acidic and negatively charged part of nucleic acids
- Base that contains nitrogen and has one or two atomic rings in its structure
Both the phosphate group and the nitrogenous base are attached to the central pentose sugar.
The nitrogenous base is attached to the 1' carbon atom (right dot)
The phosphate base is attached to the 5' carbon atom (left dot)
The base and phosphate are covalently bonded to the pentose sugar.
- Covalent bonds are formed between the phosphate of one nucleotide and the pentose of the next nucleotide, creating a strong backbone for the alternating sugar molecule and phosphate groups, with a base attached to each sugar.
- different bases in DNA and RNA = four different nucleotides
The four different nucleotides can be joined together in any order because the phosphate and sugar used to join together are the same in each nucleotide. Any sequence of bases is possible along the length of a DNA or RNA molecule.
This is fundamental for nucleic acids to act as storehouses of genetic information. Base string = information store. Sugar phosphate structure = provides stability and security for the camp
2.6.U2-DNA differs from RNA in the number of strands present, in the composition of the bases and in the type of pentose.
- Compare the structure of DNA and RNA
Two types of nucleic acids are found in cells: DNA and RNA.
- DNA (deoxyribonucleic acid) is a more stable double-stranded form that stores the genetic blueprint of cells.
- RNA (ribonucleic acid) is a more versatile single-stranded form that carries genetic information for decoding
2.6.U3 DNA is a double helix consisting of two antiparallel strands of nucleotides connected by hydrogen bonds between complementary base pairs.
- Define antiparallel in terms of the structure of DNA.
- Describe the formation of a DNA double helix by hydrogen bonding between nitrogenous bases.
- Identify the four DNA bases by the number of rings (purines or pyrimidines) and the number of hydrogen bonds they can form.
- Give the number of nitrogenous bases per complete turn of the DNA double helix.
Nucleic acids are composed of nucleotide monomers that are joined together into a single strand by condensation reactions.
- The phosphate group of one nucleotide is attached to the sugar of another nucleotide (at the 3'-hydroxyl (-OH) group).
- This leads to the formation of a phosphodiester bond between the two nucleotides (and water is formed as a by-product).
- Successive condensation reactions result in the formation of long polynucleotide chains.
Each chain has a chain of nucleotides connected by covalent bonds, two chains are parallel but run in opposite directions, so they are antiparallel. As the antiparallel chains grow, the atoms arrange themselves into the most stable energetic configuration. This atomic arrangement results in double-stranded DNA forming a double helix (~10 to 15 bases per turn).
The chains are held together by hydrogen bonds between the nitrogenous bases.
- Adenine (A) and Thymine (T)
- Guanine (G) and Cytosine (C)
This is called complementary base pairing: A and T complement each other by forming base pairs, and the same with G and C.
Application:
2.6.A1 Crick and Watson's elucidation of the structure of DNA through modeling.
- Describe the role of Chargaff, Watson, Crick, Franklin, and Wilkins in discovering the structure of DNA.
- Explain how Watson and Crick used models to determine the structure of DNA.
The structural organization of the DNA molecule was correctly proposed by James Watson and Francis Crick in 1953. These scientists built models to quickly visualize and assess the feasibility of potential structures. Their efforts were guided by an understanding of molecular distances and bond angles developed by Linus Pauling and were based on some important experimental discoveries:
- DNA is made up of nucleotides consisting of a sugar, a phosphate and a base - Phoebus Levene, 1919
- DNA is composed of equal parts purines (A + G) and pyrimidines (C + T) - Erwin Chargaff, 1950
- DNA is arranged in a helical structure - Rosalind Franklin, 1953 (data shared without permission)
Create DNA Templates
Through trial and error, Watson and Crick were able to assemble a DNA model that showed:
- The DNA strands are antiparallel and form a double helix.
- DNA strands are paired by complementary base pairing (A = T; C Ξ G)
- Outer edges of bases remain exposed (allows access to replicative and transcriptional proteins)
As Watson and Crick's model building was based on trial and error, some of the early models were flawed:
- The first model generated was a triple helix.
- Early models had bases on the outside and sugar and phosphate residue in the middle.
- Nitrogenous bases were initially misconfigured and, therefore, did not show complementarity.
The Rosalind Franklin Controversy
The eventual construction of a correct DNA molecule was largely due to the X-ray crystallographic data generated by Rosalind Franklin. These data confirmed the arrangement of the DNA strands in a helical structure. The data was shared without Franklin's knowledge or permission and was critical to the final design. Therefore, Franklin is now considered one of the most important contributors to the elucidation of the structure of DNA.
Watson and Crick article
Capacity
2.6.S1 Draw simple diagrams of the structure of individual nucleotides in DNA and RNA using circles, pentagons, and rectangles to represent phosphates, pentoses, and bases.
- Draw the basic structure of a single nucleotide (with circle, pentagon and rectangle).
- Draw a simple diagram of the structure of RNA.
- Draw a simple diagram of the structure of DNA.
- Identify and label the 5' and 3' ends on a DNA or RNA diagram
Both the phosphate group and the nitrogenous base are attached to the central pentose sugar.
The nitrogenous base is attached to the 1' carbon atom (right dot)
The phosphate base is attached to the 5' carbon atom circles (left dot) for phosphate
- Pentagons for pentose sugar
- rectangles for bases
- C, the carbon atom is on the right side of the pentose sugar
- the phosphate is attached to C5, the carbon atom in the side chain on the upper left side of the pentose sugar
3.3.1 Describe the nucleotide structure of DNA in terms of sugar (deoxyribose), base, and phosphate.
A nucleotide consists of the sugar deoxyribose, a base (which can be adenine, guanine, cytosine or thymine) and a phosphate group. Below is a representation of a nucleotide.
3.3.2 Name the four bases of DNA.
Adenine, Guanine, Cytosine and Thymine.
Exercise 3.3.3 Sketch how the nucleotides in DNA are linked by covalent bonds in a single strand.
Below is a diagram showing how nucleotides are joined together to form a strand. A covalent bond is formed between the sugar of one nucleotide and the phosphate group of another nucleotide.
3.3.4 Explain how a DNA double helix is formed by hydrogen bonds and complementary base pairing.
DNA consists of two strands of nucleotides. Nucleotides are linked together within each strand by covalent bonds. The sugar of one nucleotide forms a covalent bond with the phosphate group of another. The two strands themselves are connected by hydrogen bonds. Hydrogen bonds are found between the bases of the two nucleotide chains. Adenine forms hydrogen bonds with thymine, while guanine forms hydrogen bonds with cytosine. This is called complementary base pairing. Below is a diagram showing the molecular structure and bonds within DNA.
Related Links
- IB Biology Study Guide and Notes for SL/HL
- IB Physics Study Guide and Notes for SL/HL