2.6 Structure of DNA and RNA

IB BiologyY2

Thanks to Ms. Stuckey; modified by Ms. Azurée 2015

Nucleic Acids and Nucleotides2.6U1 The nucleic acids DNA and RNA are polymers of nucleotides.

●Nucleic Acids○Two types!

■DNA and RNA○Monomer

■Nucleotide●5-carbon (pentose) sugar●Phosphate group (acidic, negatively-charged)●Base - 1 or 2 ring, contains N

○All linked by covalent bonds

Nucleic Acids and Nucleotides2.6U1 The nucleic acids DNA and RNA are polymers of nucleotides.

NucleotidesBook says 4 different nucleotides because

there are 4 bases - is that right?

Nucleic Acids and Nucleotides2.6U1 The nucleic acids DNA and RNA are polymers of nucleotides.

PolymerizationCovalent bonds

form between phosphate group on one nucleotide and pentose sugar on another

Nucleic Acids and Nucleotides2.6U1 The nucleic acids DNA and RNA are polymers of nucleotides.

Differences between DNA and RNA2.6U2 DNA differs from RNA in the number of strands normally present, the base composition, and the type of pentose sugar.

Difference DNA RNA

pentose sugar deoxyribose (1 less O) ribose

# of strands/polymers

2 (DOUBLE helix!) 1

basesadenine, cytosine, guanine, thymine

(A, C, G, T)

adenine, cytosine, guanine, uracil

(A, C, G, U)

Let’s Draw!2.6 S1 Drawing simple diagrams of the structure of single nucleotides and of DNA and RNA, using circles, pentagons, and rectangles to represent phosphates, pentoses, and bases

Phosphate =Sugar =

Base =

1

3

5

4

2

NucleotidesLinked by covalent bonds

ANTIPARALLEL5’-3’ and 3’-5’ (one is “upsidedown”)

Double helixHydrogen bondsComplementary base pairs

A-T, G-C

Structure of DNA2.6 U3 DNA is a double helix made of two antiparallel strands of nucleotides linked by hydrogen bonding between complementary base pairs.

How did they know what it was going to look like?

They played with MODELS! (yes, really!)

Molecular Models2.6 NOS1 Using models as representation of the real world: Crick and Watson used model-making to discover the structure of DNA.

Such handsome science-y devils

Used evidence from X-rays (thanks, Franklin!)

Triple helix? Not enough Mg (thanks, Franklin!)

Made it hard to fit togetherIgnored Chargaff’s discovery

Using cardboard to make model molecules

A-T, C-G both are the same size and fit insideAntiparallel

“It just looked right!”

Watson and Crick’s Model2.6 A1 Crick and Watson’s discovery of the structure of DNA using model-making.