2.6 Structure of DNA & RNA

Nucleic Acids

• What do we know already?

i. The nucleic acids DNA and RNA are polymers of nucleotides

• Nucleotides consist of 3 parts• A 5-carbon sugar –

ribose or deoxyribose

• A phosphate group (acidic & negatively-charged)

• A nitrogen-containing base – 1 or 2 rings

i. The nucleic acids DNA and RNA are polymers of nucleotides

i. The nucleic acids DNA and RNA are polymers of nucleotides

• To create a nucleic acid the phosphate of one nucleotide is covalently bonded to the sugar of another nucleotide

ii. DNA differs from RNA in the number of strands present, the base

composition and the type of pentose.

o Sugar is different:

ii. DNA differs from RNA in the number of strands present, the base

composition and the type of pentose.

o Bases are different:Adenine (A) Guanine (G) Thymine (T) Cytosine (C)

DNA: Adenine pairs with Thymine and Guanine with CytosineRNA Shares the same bases except that Uracil (U) replaces Thymine

Purines

adenineGuanine

Double ring structures

Pyrimidines

Thymine Cytosine

Single ring structures

Base pairing

Thymine and adenine (2 hydrogen bonds)

Cytosine and guanine (3 hydrogen bonds)

2.6.U1 The nucleic acids DNA and RNA are polymers of nucleotides.

http://lucasmind.tumblr.com

What is the relevance of this coffee cup to this topic?

ii. DNA differs from RNA in the number of strands present, the base

composition and the type of pentose.

o Strands are different:

SKILL: Drawing simple diagrams of the structure of single nucleotides of

DNA and RNA.

SKILL: Drawing simple diagrams of the structure of single nucleotides of

DNA and RNA.

Label the phosphate, sugar, base.

Is this DNA or RNA?

SKILL: Drawing simple diagrams of the structure of single nucleotides of

DNA and RNA.Use this simple, but very effective You Tube video to learn how to draw the nucleotides making up a short section of a DNA molecule.

To make sure you have learn this skill you need to practice it repeatedly.

http://youtu.be/kTH13oI8BSI

iii. DNA is a double helix made of two antiparallel strands of nucleotides

linked by hydrogen bonding between complementary base pairs.

• Strands are parallel but run in opposite directions (antiparallel) – one in 5’ to 3’ direction & other in 3’ to 5’

• Strands are held together by hydrogen bonds between the bases – A always pairs with T & C with G (complimentary base pairing rule)

http://i-biology.net/

http://i-biology.net/

Application: Crick and Watson’s elucidation of the structure of DNA

using model making.

https://www.youtube.com/watch?v=JiME-W58KpU

https://www.youtube.com/watch?v=fBWPzdyyPic watsons word

https://www.youtube.com/watch?v=sf0YXnAFBs8The secret of life

2.6.A1 Crick and Watson’s elucidation of the structure of DNA using model making.

http://scarc.library.oregonstate.edu/coll/nonspcoll/catalogue/picture-dnamodel-900w.jpg

Whilst others worked using an experimental basis Watson and Crick used stick-and-ball models to test their ideas on the possible structure of DNA. Building models allowed them to visualize the molecule and to quickly see how well it fitted the available evidence.

It was not all easy going however. Their first model, a triple helix, was rejected for several reasons:• The ratio of Adenine to Thymine was not 1:1 (as

discovered by Chargaff)• It required too much magnesium (identified by

Franklin)

From their setbacks they realized:• DNA must be a double helix.• The relationship between the bases and base pairing• The strands must be anti-parallel to allow base

pairing to happen

Because of the visual nature of their work the second and the correct model quickly suggested:• Possible mechanisms for replication• Information was encoded in triplets of bases

2.6.A1 Crick and Watson’s elucidation of the structure of DNA using model making.

http://scarc.library.oregonstate.edu/coll/nonspcoll/catalogue/picture-dnamodel-900w.jpg

http://www.nobelprize.org/educational/medicine/dna_double_helix/readmore.html

http://youtu.be/sf0YXnAFBs8

Find out more about the discovery of DNA:

Watson and Crick gained Nobel prizes for their discovery. It should be remembered that their success was based on the evidence they gained from the work of others. In particular the work of Rosalind Franklin and Maurice Wilkins, who were using X-ray diffraction was critical to their success.

The discovery of the structure of DNA shows the importance of collaboration between scientists. However, both cooperation and competition can occur between research groups….

Before we watch the video clip, discuss and write a brief response to these questions in your group:1. To what extent is keeping research

discoveries secret ”anitiscientific?”2. How are shared (KD) and personal

knowledge (KA) related in scientific research?

Rosie: 3:46

https://www.youtube.com/watch?v=JiME-W58KpU

Watson’s words: 2:09 https://www.youtube.com/watch?v=fBWPzdyyPic

Discuss and Answer these questions as a group:

-Do you think what Watson & Crick did to Franklin was morally/ethically wrong? Why or why not?

-Do you think discrimination is present in the scientific research community today? Have you ever witnessed or heard stories similar to this one in todays world?

Discovery of DNA• http://www.youtube.com/watch?v=sf0Y

XnAFBs8

• The secret of Life: discovery of DNA structure. 9:16 min

• As a group, answer these questions: How is the discovery of DNA an example of cooperation, competition, and internationalism?

covalent bond

covalent bond

A nucleotide: a single unit of a nucleic acid

• five carbon atoms = a pentose sugar• If the sugar is Deoxyribose the polymer is

Deoxyribose Nucleic Acid (DNA)• If the sugar Ribose the polymer is Ribose

Nucleic Acid (RNA)

• acidic• negatively charged

• contains nitrogen• has one or two rings in it’s

structure

i. The nucleic acids DNA and RNA are polymers of nucleotides