Biology 9.2 Structure of DNA

DNA’s Structure

DNA’s winding staircase By the early 1950s, most

scientists were convinced that genes were made of DNA. They hoped the mystery of heredity could be solved by understanding the structure of DNA.

Watson and Crick, two researchers at Cambridge University, determined that a DNA molecule is a double helix; two strands twisted around each other like a winding staircase.

DNA’s winding staircase

Each strand of DNA is made of linked nucleotides; the subunits that make up DNA.

DNA’s winding staircase Each nucleotide has three parts;

a phosphate group, a five carbon sugar molecule, and a nitrogen containing base.

The nucleotides link by a weak hydrogen bond.

DNA’s winding staircase The five-carbon sugar molecule in the backbone is

called deoxyribose.

Deoxyribose is the D in DNA.

DNA’s full name is deoxyribosenucleic acid

DNA’s winding staircase

While the sugar molecule and phosphate group that make up the backbone spirals are the same for each nucleotide molecule, the nitrogen bases will be one of four types.

DNA’s winding staircase•The four nitrogen bases may be one of thefollowing

• Adenine• Guanine • Cytosine• Thymine

DNA’s winding staircase In 1949, Erwin Chagraff, a

biochemist at Colombia University, made a discovery.

She discovered that the amount of adenine always equaled the amount of thymine.

Also, the amount of guanine always equaled the amount of cytosine.

DNA’s winding staircase In 1952, the first high-

quality photographs of DNA, high quality X-ray diffraction photographs, revealed images of DNA’s structure.

Soon after this , Watson and Crick used the information from these first images to create their first model of DNA’s spiral form.

DNA’s winding staircase

Watson and Crick discovered that a purine on one strand of DNA is always paired with a pyrimide on the opposite strand.

We call these pairings base-pairs.

Each rung of the ladder of the staircase design is made up of one of these base-pairs.

DNA’s winding staircase

Watson and Crick discovered as well that these pairings of the four nitrogen bases are always in specific base-pairs.

Adenine always pairs with thymine.

Guanine always pairs with cytosine.

We call these two groupings complementary base-pairs

DNA’s winding staircase

In charts showing DNA’s form, these complementary base-pairings always show the groupings of adenine-thymine and cytosine-guanine.

These will often be displayed as base-pairs abbreviated as A-T T-A C-G G-C

DNA’s winding staircaseComputer Lab: After completing your

worksheet: today’s research assignment is to use the internet to find, research and write a short report describing the following IN YOUR OWN WORDS.

1: Name and describe the three parts of the DNA nucleotide

2: Demonstrate (by drawing a chart of the DNA model) the complementary base-pairing rules

3: Explain why the two strands of the double-helix are called complementary