The Structure of DNA

What is DNA?

• It is a nucleic acid that contains deoxyribose sugar and holds an organisms genetic information

• DNA achieves its control by producing proteins called enzymes

Structure of DNA• In the 1950’s, Rosalind Franklin used a

technique called X-ray diffraction to get information about the structure of DNA

• The x-ray pattern revealed the X-shaped pattern and gave clues that the DNA strands are twisted

• The twisted shape is known as a helix• James Watson and Francis Crick used

Franklin’s x-ray to develop a model of the DNA structure

Description of the DNA molecule

• DNA is a polymer of repeating subunits called nucleotides arranged in a double helix

• Nucleotides have three basic parts:– 5 carbon sugar called deoxyribose– Phosphate group– Nitrogenous base

• The deoxyribose sugar and phosphate groups form the backbone of a DNA chain in an alternating pattern

• The nitrogenous bases are attached to the deoxyribose sugar which stick out sideways from the chain

Description of the DNA molecule

• Four nitrogenous bases in DNA– Adenine– Guanine– Cytosine– Thymine

• Adenine bonds to Thymine by forming hydrogen bonds

• Cytosine pairs with Guanine in the same way

Purines (double rings)

Pyrimidines (single ring)

Solving the Structure of DNA• When trying to understand the

structure of DNA, one scientist Erwin Chargaff discovered that the percentages of adenine (A) and thymine (T) were almost equal and that the same was true of guanine (G) and cytosine (C)

• The observation that A = T and G = C became known as Chargaff’s rule

Chromosomes

and

DNA

Where is DNA found?

• In prokaryotic cells

• In eukaryotic cells

In the cytoplasm of the cell

It is also referred to as the chromosome

In the nucleus

In the chromosomes

one

more than one

Chromosome Structure

• What is a chromosome?

• What does it look like?

A threadlike structure within the nucleus containing genetic information

DNA and proteins (histones) tightly coiled into beadlike structures called nucleosomes that loop around

Chromosome structure