DNA Structure

IB Biology Core Topic 3.3

Outcomes

� 3.3.1 Outline DNA nucleotide structure in terms of sugar (deoxyribose), base and phosphate.

� 3.3.2 State the names of the four bases in DNA.

� 3.3.3 Outline how DNA nucleotides are linked together by covalent bonds into a single strand.

� 3.3.4 Explain how a DNA double helix is formed using complementary base pairing and hydrogen bonds.

� 3.3.5 Draw and label a simple diagram of the molecular structure of DNA.

The DNA Song!

� http://www.youtube.com/watch?v=FUA6_Ucw3i4

History of the discovery of DNA

� http://www.youtube.com/watch?v=sf0YXnAFBs8

What is DNA?

� DNA (deoxyribonucleic acid) is the genetic material of living organisms.

� Nucleic acids are biological molecules composed of building blocks called nucleotides.

� There are two types

of nucleic acids, DNA

and RNA

(ribonucleic acid).

� Nucleotides are complex molecules composed of three different molecules linked together.

� The three components are a sugar molecule, phosphate molecule, and a nitrogen base.

� Of the three components of nucleotides there is some variety in the possibilities.

� The sugar may be one of two types of pentose sugars; ribose or deoxyribose.

� The nitrogen base can be one of five bases; Adenine, Thymine, Cytosine, Guanine, and Uracil.

� The phosphate is H3PO4 and it forms 5 bonds with other atoms and is known as a phosphate group.

The 4 bases of DNA

� DNA contains four different nitrogen bases. Adenine and Guanine are known as purines due to their double ring structure while Thymine and Cytosine are known as pyrimidines due to their single ring structure.

� In DNA adenine always pairs with thymine and guanine always pairs with cytosine. (A with T and G with C)

Nucleotide Link

� Covalent bonds form between nucleotides via dehydration synthesis.

� The phosphate group of one nucleotide bonds with the sugar molecule of another nucleotide forming the phosphate sugar “backbone” of the DNA strand.

� The nitrogen bases are attached to each ribose (sugar) and stick out from the phosphate-sugar backbone.

The DNA double helix

� DNA is a double stranded molecule. The structure of DNA is known as a double helix; two strands of complementary coiled DNA.

� Hydrogen bonds between the complementary base pairs keep the DNA molecule bonded together.

� There are two hydrogen bonds between A and T and there are three hydrogen bonds between G and C.

� The sugar-phosphate backbone of the strand of DNA runs antiparallel, forming the sides of the ladder with the bases bonding to form the rungs of the ladder in the middle.

� Complementary base pairing simply means that in DNA molecules, A will only pair with T and G will only pair with C. (a purine bonds with a pyrimidine)

Antiparallel Strands

� Imagine you are a nucleotide – I’ll give you a minute!

� Your body is the Deoxyribose Sugar

� Your right arm is a nitrogenous base

� Your left hand is a phosphate group.

� Line up and pair up!!!

� What’s wrong????

Can you draw and label this structure? Tour of the basics from Learn Genetics

� http://learn.genetics.utah.edu/content/begin/tour/

Genetics and the Structure of DNA

� http://www.youtube.com/watch?v=-1ZvsYDTxc8&feature=player_embedded#!