Collisions and Rate Worksheet

General description

This lesson investigates the structure of the nucleic acids DNA and RNA. As well as examining individual components, there is the opportunity to watch an animation of the formation of polynucleotides, and an exercise involving manipulating nucleotides to form complementary pairs is provided. The structural differences between DNA and RNA are explained and an outline of the formation and structure of the three forms of RNA is included. The replication of DNA is then dealt with through a detailed animation of the process, and the idea of whether replication is conservative or semi-conservative is considered by an investigation of the classic experiment by Meselson and Stahl. An exercise designed to test understanding of how this evidence supported the semi-conservative hypothesis is also provided.

Assumed prior knowledge

Students should already be able to:

outline the main features of DNA;
state the location of DNA in cells;
describe a condensation reaction.

Objectives

Having completed this lesson, students should be able to:

describe, in detail, the structure of the nucleic acids DNA and RNA;
explain why nucleotide bases pair the way they do;
describe the formation of polynucleotides;
distinguish between the structure of DNA and RNA;
distinguish between the three types of RNA;
describe the process of DNA replication;
explain the role of the enzymes DNA polymerase and DNA ligase in the replication process;
distinguish between conservative and semi-conservative replication;
explain how the results of the Meselson and Stahl experiment provide evidence for semi-conservative replication.

Activity descriptions

Activity 1

Nucleic acids

In this activity simplified diagrams of the components of a nucleotide are provided. These provide access to more detailed structural diagrams and an animation that shows the formation of a polynucleotide. An interactive exercise allows students to explore the correct base pairing in DNA at a simple level. The detailed structure of the nitrogenous bases is accessed later in the activity, with an exercise during which students must select the correct bases in order to form complementary pairs.

The structural differences between DNA and RNA are then explored, with emphasis placed upon the replacement of thymine with uracil and the single-stranded nature of RNA. Students can then explore, in brief, the three different forms of RNA within the context of the process of protein synthesis.

Activity 2

DNA replication

In this activity an animation of the process of replication is viewed and additional details can be added concerning the roles of the two main enzymes, DNA polymerase and DNA ligase. An exploration of whether replication is conservative or semi-conservative is then introduced, with an explanation of these two terms provided visually through diagrams of both options.

A brief explanation of the classic experiment by Meselson and Stahl is given and students can explore the results and conclusions of their work by looking at representations of 'heavy' and 'light' DNA strands. A final exercise involves students selecting DNA molecules in order to create the same bands of DNA in test tubes from the original experiment. This will enable them to link the results of the experiment with the idea of semi-conservative replication.

Common misconceptions

Students often confuse DNA and RNA. This lesson addresses this problem by emphasising their structural differences and by visual representations of the two types of nucleic acid for comparison purposes. The confusion that can arise from thymine being replaced by the uracil in RNA is highlighted in the accompanying text, and on the screen in the detailed diagrams of the nitrogenous bases.

Differentiation

The simple representations of the components of the two nucleic acids are built upon using more detailed structural diagrams. This is then related specifically to the pairing of complementary nitrogenous bases in DNA.

From understanding this, students are in a position to appreciate the processes involved in DNA replication. Once the process has been understood, the ideas of conservative and semi-conservative replication can be appreciated and the evidence from Meselson and Stahl's experiment evaluated.

The structure of RNA is explored in a similar way to that of DNA, with simple representations of the various components built upon to provide more complex diagrams. The differences between the two nucleic acids are highlighted. A brief overview of the three types of RNA is then given, with their roles in protein synthesis.

Less able students may be confused by the names of the different components of the nucleic acids. The option of labelling the components shown on the screen throughout the activities is one way to address the problem of identification. Such students may have difficulty understanding the antiparallel nature of the opposing strands in each DNA molecule. This is specifically addressed in the second activity and the teacher can draw attention to this using the 5' and 3' labelled ends on the visual representations of DNA.

The experiment performed by Meselson and Stahl may be difficult for less able students to follow. A simple outline of the procedure is provided, alongside easily accessible visual representations of the results, to address this.

Although it is not essential for students to know the detailed structures of the components of nucleotides, more able students could be encouraged to investigate these further (access to these detailed structures is provided in the activities). The procedure of the Meselson and Stahl experiment is not covered in detail in the lesson, but teachers may wish to challenge more able students by giving them access to the experimental protocol used.

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