2.25 Watson and Crick Model of DNA

Introduction

DNA (deoxyribonucleic acid) is the molecule that carries genetic information in all living organisms. The structure of DNA was determined by James Watson and Francis Crick in 1953. Their model, known as the Watson and Crick Model of DNA, explained how DNA stores and transmits genetic information. This discovery is considered one of the most important milestones in biology.

Definition

Watson and Crick Model of DNA: According to this model, DNA is made up of two polynucleotide chains that are attached together by base pairs. The two chains are antiparallel to each other and form a double helix structure.

In simple words: DNA is a double helix made of two strands that are complementary to each other and run in opposite directions.

Core Concept

• In 1951, Erwin Chargaff found that the nitrogenous bases in DNA show specific ratios.
• He observed that the amount of adenine (A) is always equal to the amount of thymine (T).
• He also observed that the amount of guanine (G) is always equal to the amount of cytosine (C).
• This implies that the total purines and total pyrimidines are in 1:1 ratio in any DNA.
• This conclusion is known as Chargaff's rule.
• In those days, the X-ray diffraction analysis of DNA by Maurice Wilkins and Rosalind Franklin was published.
• They first time claimed that DNA is a duplex (double helix) molecule.
• The width of the duplex is 2 nm while the length of each turn is 3.4 nm.
• In 1953, on the basis of these observations, a graduate student Francis Crick and a research fellow James Watson of Cambridge University proposed a physical model of DNA which is now called the Watson and Crick Model of DNA.

Detailed Explanation

Historical Background

Note: Rosalind Franklin's X-ray diffraction images (especially Photo 51) were crucial in determining the structure of DNA. She died in 1958 and was not eligible for the Nobel Prize.

Key Features of the Watson and Crick Model

• DNA is made up of two polynucleotide chains.
• These chains are attached together by base pairs.

• The two polynucleotide chains are opposite in direction.
• One chain runs from 5' to 3' downward.
• The other chain runs from 5' to 3' upward.
• Therefore, both chains are antiparallel to each other.

3. Constant Width: - Both chains show a constant width of 2 nm.

• The base pairing is very specific:
• Adenine (A) makes the pair with Thymine (T).
• Guanine (G) makes the pair with Cytosine (C).

• The base pairs are held together by hydrogen bonds.
• There are three hydrogen bonds between Guanine and Cytosine.
• There are two hydrogen bonds between Adenine and Thymine.

6. Number of Base Pairs per Turn: - Each turn of the duplex consists of 10 base pairs.

• Both polynucleotide chains are complementary to each other.
• This means the sequence of one chain determines the sequence of the other.

• There is no restriction on the sequence of nucleotides along the length of a DNA strand.
• The sequence can vary in countless ways.
• The sequence is specific for different species, organisms, and even individuals.

Dimensions of DNA

Chargaff's Rule

Definition: In 1951, Erwin Chargaff found that the nitrogenous bases in a DNA show specific ratios.

• **A = T**
• **G = C**
• **Purines = Pyrimidines**
• **A + G = T + C**

In simple words: In DNA, the amount of adenine equals thymine, and the amount of guanine equals cytosine.

• Chargaff's rule helped Watson and Crick determine the structure of DNA.
• It showed that A pairs with T and G pairs with C.

Base Pairing in Detail

• A–T: Two hydrogen bonds form between the amino group of A and the keto group of T.
• G–C: Three hydrogen bonds form between the amino and keto groups of G and C.
• The shapes of the bases allow them to fit together perfectly.

• Purines are double-ringed (larger).
• Pyrimidines are single-ringed (smaller).
• A purine + pyrimidine pair maintains a constant width of 2 nm.
• If two purines paired, the helix would be too wide.
• If two pyrimidines paired, the helix would be too narrow.

X-ray Diffraction Analysis

• Used X-ray diffraction to study the structure of DNA.
• Rosalind Franklin's Photo 51 was a key piece of evidence.
• The X-ray diffraction pattern showed:
• DNA is a helix.
• The helix has a constant width.
• The helix has repeating units every 3.4 nm.

• This data helped Watson and Crick build their model.
• Watson and Crick used this data along with Chargaff's rule.

Science Tidbits (from textbook)

• Watson and Crick assembled the molecular model and published their two-page article on their molecular model of DNA in the journal "Nature" in April 1953.
• Few milestones in the history of biology have as broad an impact as their double helix.
• They were awarded Nobel Prize in 1962 for their model of DNA.

Gene (from textbook)

• A gene is a region of DNA which is made up of nucleotides.
• It is the physical and functional unit of heredity.
• Each gene contains the information required to build specific proteins needed in an organism.
• Genes contain the instructions for our individual characteristics — like eye and hair colour.
• In order to make proteins, the gene from the DNA is copied into messenger RNA.
• The mRNA moves out of the nucleus and uses ribosomes to form the polypeptide that finally folds and configures to form the protein.

Intermediate Level

Watson and Crick Model in Detail

Chargaff's Rule in Detail

• Helped determine the structure of DNA.
• Showed that A pairs with T and G pairs with C.

Base Pairing in Detail

• G–C has 3 hydrogen bonds.
• A–T has 2 hydrogen bonds.
• More hydrogen bonds = stronger pairing.
• DNA with high G–C content has a higher melting temperature.

Antiparallel Strands

• The two strands of DNA run in opposite directions.
• One strand runs from 5' to 3'.
• The other strand runs from 3' to 5'.
• This is called antiparallel arrangement.
• This arrangement is essential for DNA replication and transcription.

Complementary Base Pairing

• The two strands are complementary to each other.
• If one strand has the sequence 5'-ATGC-3', the other strand has the sequence 3'-TACG-5'.
• This means the sequence of one strand determines the sequence of the other.
• This is essential for DNA replication and transcription.

X-ray Diffraction Analysis

Nobel Prize

• Watson, Crick, and Wilkins were awarded the Nobel Prize in Physiology or Medicine in 1962.
• Rosalind Franklin died in 1958 and was not eligible for the prize.

Important Points

• 1. In 1951, Erwin Chargaff found that the nitrogenous bases in a DNA show specific ratios.
• 2. He observed that the amount of adenine is always equal to the amount of thymine and the amount of guanine is always equal to the amount of cytosine in DNA.
• 3. This implies that the total purines and total pyrimidines are in 1:1 in any DNA.
• 4. This conclusion is known as Chargaff's rule.
• 5. In those days the X-ray diffraction analysis of DNA by Maurice Wilkins and Rosalind Franklin was published.
• 6. They first time claimed that DNA is a duplex (double helix) molecule.
• 7. The width of duplex is 2 μm while the length of each turn is 3.4 m.
• 8. In 1953, on the basis of these observations a graduate student Francis Crick and a research fellow James Watson of Cambridge University proposed a physical model of DNA which is now called Watson and Crick Model of DNA.
• 9. According to this model a DNA is made up of two polynucleotide chains which are attached together by base pairs.
• 10. In order to make base pairing the two polynucleotide chains are opposite in direction i.e., one chain runs from 5' to 3' downward and the other chain runs from 5' to 3' upward.
• 11. Both chains show a constant width of 2 nm.
• 12. Therefore, both chains are supposed to be antiparallel to each other.
• 13. The base pairing is very specific i.e., Adenine makes the pair with Thymine and Guanine with Cytosine.
• 14. The base pairs are held together by the hydrogen bond.
• 15. There are three hydrogen bonds between Guanine and Cytosine and two hydrogen bonds between Adenine and Thymine.
• 16. Each turn of the duplex consists of 10 base pairs.
• 17. Both polynucleotide chains are complementary to each other.
• 18. There is no restriction of the sequence of nucleotides along the length of a DNA strand.
• 19. The sequence can vary in countless ways.
• 20. The sequence is specific for different species, organisms, and even individuals.
• 21. Science Tidbits: Watson and Crick assembled the molecular model and published their two-page article on their molecular model of DNA in the journal "Nature" in April 1953.
• 22. Few milestones in the history of biology have as broad an impact as their double helix.
• 23. They were awarded Nobel Prize in 1962 for their model of DNA.
• 24. Gene: A gene is a region of DNA which is made up of nucleotides.
• 25. It is the physical and functional unit of heredity.
• 26. Each gene contains the information required to build specific proteins needed in an organism, such as they contain the instructions for our individual characteristics — like eye and hair colour.
• 27. In order to make proteins, the gene from the DNA is copied into messenger RNA.
• 28. The mRNA moves out of the nucleus and uses ribosomes to form the polypeptide that finally folds and configures to form the protein.

Remember

• - **Chargaff's rule:** A = T, G = C
• - Watson and Crick Model: Double helix
• - Two strands: Antiparallel (5' → 3' and 3' → 5')
• - Width: 2 nm
• - Turn length: 3.4 nm
• - Distance between base pairs: 0.34 nm
• - Base pairs per turn: 10
• - Base pairing: A–T (2 H-bonds), G–C (3 H-bonds)
• - Purine pairs with pyrimidine
• - Complementary strands
• - X-ray diffraction: Wilkins and Franklin
• - Nobel Prize: 1962 (Watson, Crick, Wilkins)
• - Gene: Region of DNA; unit of heredity

Common Mistakes

• 1. Confusing Chargaff's rule with base pairing – Chargaff's rule: A = T, G = C. Base pairing: A pairs with T, G pairs with C.
• 2. Thinking the two strands are parallel – The two strands are antiparallel (opposite directions).
• 3. Confusing the width and turn length – Width = 2 nm; Turn length = 3.4 nm.
• 4. Forgetting the number of base pairs per turn – 10 base pairs per turn.
• 5. Confusing the number of hydrogen bonds – A–T = 2; G–C = 3.
• 6. Thinking A pairs with G – A pairs with T; G pairs with C.
• 7. Thinking purines pair with purines – Purines pair with pyrimidines.
• 8. Forgetting the contribution of Rosalind Franklin – Her X-ray diffraction Photo 51 was crucial.
• 9. Thinking Watson and Crick discovered DNA – DNA was discovered earlier (1869); Watson and Crick determined its structure.
• 10. Forgetting the year of the model – 1953.
• 11. Thinking the Nobel Prize was awarded in 1953 – The Nobel Prize was awarded in 1962.
• 12. Forgetting that the model describes B-DNA – The Watson-Crick model describes B-DNA, the most common form.
• 13. Confusing the major and minor grooves – Major groove is wider; minor groove is narrower.
• 14. Thinking DNA is always a double helix – DNA can also exist in other forms (A-DNA, Z-DNA), but B-DNA is the most common.

Quick Revision

• In 1951, Erwin Chargaff found that the nitrogenous bases in a DNA show specific ratios.
• He observed that amount of adenine is always equal to the amount of thymine and amount of guanine is always equal to the amount of cytosine in DNA.
• This implies that the total purines and total pyrimidines are in 1:1 in any DNA.
• This conclusion is known as Chargaff's rule.
• In those days the X-ray diffraction analysis of DNA by Maurice Wilkins and Rosalind Franklin was published.
• They first time claimed that DNA is a duplex (double helix) molecule.
• The width of duplex is 2 μm while the length of each turn is 3.4 m.
• In 1953, on the basis of these observations a graduate student Francis Crick and a research fellow James Watson of Cambridge University proposed a physical model of DNA which is now called Watson and Crick Model of DNA.
• According to this model a DNA is made up of two polynucleotide chains which are attached together by base pairs.
• In order to make base pairing the two polynucleotide chains are opposite in direction i.e., one chain runs from 5' to 3' downward and the other chain runs from 5' to 3' upward.
• Both chains show a constant width of 2 nm.
• Therefore, both chains are supposed be antiparallel to each other.
• The base pairing is very specific i.e., Adenine makes the pair with Thymine and Guanine with Cytosine.
• The base pairs are held together by the hydrogen bond.
• There are three hydrogen bonds between Guanine and Cytosine and two hydrogen bonds between Adenine and Thymine.
• Each turn of the duplex consist of 10 base pairs.
• Both polynucleotide chains are complementary to each other.
• There is no restriction of the sequence of nucleotides along the length of a DNA strand.
• The sequence can vary in countless ways.
• The sequence is specific for different species, organisms and even individuals.
• Science Tidbits: Watson and Crick assembled the molecular model and published their two-page article on their molecular model of DNA in the journal "Nature" in April 1953.
• Few milestones in the history of biology have as broad an impact as their double helix.
• They were awarded Nobel Prize in 1962 for their model of DNA.

60-Second Revision

• **Chargaff's rule:** A = T, G = C
• Watson and Crick Model: Double helix
• Two strands: Antiparallel (5' → 3' and 3' → 5')
• Width: 2 nm
• Turn length: 3.4 nm
• Distance between base pairs: 0.34 nm
• Base pairs per turn: 10
• Base pairing: A–T (2 H-bonds), G–C (3 H-bonds)
• Purine pairs with pyrimidine
• Complementary strands
• X-ray diffraction: Wilkins and Franklin
• Nobel Prize: 1962 (Watson, Crick, Wilkins)
• Gene: Region of DNA; unit of heredity
• Warning: The two strands are antiparallel, not parallel. Width = 2 nm; turn length = 3.4 nm. A–T has 2 hydrogen bonds; G–C has 3. Chargaff's rule: A = T, G = C. Watson and Crick proposed the model in 1953; Nobel Prize in 1962. Rosalind Franklin's X-ray diffraction was crucial. The model describes B-DNA. Do not confuse the major and minor grooves.