Aim

To know the structure of DNA

The unravelling of the helical structure of DNA – the basic building blocks of life – is hailed as one of the most significant landmarks of the 20th century.

Combining evidence from biochemistry, X-ray diffraction images created by Rosalind Franklin and Maurice Wilkins, and physical clues from molecular models, they determined the three-dimensional structure of the DNA molecule to be a double helix. This discovery was published in the 25 April 1953 edition of the journal Nature.

Francis Crick shows James Watson the model of DNAthat they started building on Wednesday, 4th March and finished in the evening of Saturday, 7th March, 1953.

• In 1962, Crick, Watson and Wilkins are awarded the Nobel Prize in Physiology or Medicine "for their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material".

• Rosalind Franklin died in 1958 – the Nobel Prize is only awarded to living people.

Genetic structures

Cells

Chromosomes

Genes

DNA

Human chromosomes

22 pairs of autosomes + 2 sex chromosomes (XX or XY)

DNA. Deoxyribose Nucleic Acid/ Deoxyribonucleic Acid (on p8 HB):

• DNA is a very large, stable molecule with 2 key abilities:

1)To carry a vast amount of genetic information, the genetic code, that codes for the characteristics of the organism and from which proteins are made.

2) DNA can make exact copies of itself ensuring that when a cell divides, each daughter cell receives an identical copy of the genetic information to the parent cell. This is called DNA REPLICATION

NUCLEIC ACIDS Nucleic acids are big polymer molecules. Two

principal types are found in cells:

1. Deoxyribose Nucleic Acid (DNA)

2. Ribose Nucleic Acid (RNA)

 

DNA is confined in the nucleus, except when a cell is dividing to make new cells. RNA is mainly (but not always) found in the cytoplasm.

 

Both of these Nucleic Acids are long chain molecules. The monomer units (or ‘building blocks’) of a Nucleic Acid are called nucleotides

 

The structure of a Nucleotide

A Phosphate group (or phosphoric acid)

A pentose sugar or 5-carbon sugar called deoxyribose sugar in a DNA nucleotide (but is ribose sugar in an RNA nucleotide).

An organic base

Adenine (A)

Thymine (T)

Cytosine (C)

Guanine (G)

(on DNA)

Look at the A5 handout sheet of diagrams showing the components of a nucleotide:

Answers these quick questions:

1) How many carbon atoms are present in a pentose sugar?

2) Describe the difference between a ribose and a deoxyribose sugar molecule.

3) Which part of a nucleotide molecule contains nitrogen atoms?

4) What atoms make up the phosphate group?

The Synthesis of Nucleotides of

DNA/RNA

Ribose Sugar

Deoxyribose Sugar

The Synthesis of Nucleotides of DNA/RNAThe Synthesis of Nucleotides of DNA/RNA

OH

OH

1C

5CH2OH

4C

3C 2C

H

HOH

HH

OH

OH

1C

5CH2OH

4C

3C 2C

H

OHOH

HH

The Synthesis of Nucleotides of DNA/RNAThe Synthesis of Nucleotides of DNA/RNA

OH

P OH

OH

O

C N

C

CC

NH

Phosphate Group

A Simplified Organic Base

The Synthesis of Nucleotides of DNA/RNAThe Synthesis of Nucleotides of DNA/RNA

A Nucleotide of DNAA Nucleotide of DNA

OH

C

CH2

C

C C

OH

P OH

OH

O

C N

C

CC

N

The Synthesis of Nucleotides of DNA/RNAThe Synthesis of Nucleotides of DNA/RNA

Contains NitrogenContains

5 Carbons

Contains phosphorus

**

The 3 component molecules that join together to make a DNA nucleotide…..

…do so by means of 2 condensation reactions. This involves the loss of a water molecule for each reaction.

On your handbook diagram on p8, put stars * to

show where this happens and write a key that states

* = Condensation reaction happens here

involving the loss of a water molecule.

OH

C

CH2

C

C C

OH

P OH

OH

O

C N

C

CC

N

Single Nucleotide

The Synthesis of Nucleotides of DNA/RNAThe Synthesis of Nucleotides of DNA/RNA

A Polynucleotide is formed when lots of individual nucleotides (monomers) join together by further

condensation reactions. Eventually a full Nucleic Acid molecule (DNA or RNA) is synthesised.

The Double Helix

DNA consists of 2 polynucleotide strands running in opposite directions i.e. anti-parallel to each other.

Anti-parallel strands of DNA (see middle of Textbook p131 + HB p8)

DNA has a DOUBLE HELIX STRUCTURE i.e. Two long polynucleotide chains in a ‘twisted ladder’ shape, held together by lots of weak hydrogen bond ‘rungs’.

There is one complete twist of the double helix for every 10 nucleotides down one side.

DNA bases

You do not have to remember the molecular structures, just the names and relative sizes. Double rings bases A + G are called PURINES, single ring bases C +T called PYRIMIDINES

Specific + Complementary base pairing

Along a length of DNA, there is ALWAYS THE SAME NUMBER OF Adenine and Thymine organic bases. This is because A always binds with T to make complementary base pairs.

Also, along DNA there is ALWAYS THE SAME NUMBER OF Guanine and Cytosine organic bases as G always binds with C to make complementary base pairs.

This complementary base pairing keeps the width of the DNA molecule the same along its length.

DNA structure

The Double Helix

Good DNA animation link:

• http://www.bbc.co.uk/science/0/22199991