H. Bio DNA Unit 9

Linus Pauling

Proposed that DNA had a triple helix with the bases on the outside of the molecule (wrong)

Rosalind Franklin

Determined that DNA molecule has a helical shape.

Used x-ray crystallography to take a picture of DNA

Erwin Chargaff

Determined that the % of guanine and cytosine are almost equal in any DNA sample

Determined that the % of adenine and thymine are almost equal in any sample of DNA

James Watson & Francis Crick

Constructed a 3-D model of a DNA molecule.

Won the Nobel prize for their work (1962).

Work helped to explain how DNA carries the genetic info and how it can be copied.

Proved that DNA is a double helix shape

Nucleotides— related to DNA

DNA is an extremely long polymer made of individual monomers called this

Parts of a nucleotide

Phosphate group

5-Carbon sugar (deoxyribose)

Nitrogenous base

Purine

Double ring bases

“Double structure”

Pyrimidine

Single ring bases

4 nitrogenous bases of DNA

Pyrimidines:

1 Thymine

2 Cytosine

Purines:

3 Adenine

4 Guanine

How nitrogenous bases bond together to form base pairs

A purine always bonds with a pyrimidine (Chargaff rule)

Cytosine always bonds with Guanine

Adenine always bonds with Thymine

How a DNA molecule is like a ladder

Sugar phosphate backbone = sides of the ladder

Nitrogenous bases = rungs

How are the strands of DNA arranged?

Antiparallel (opposite directions) to one another so each strand can be read and copied during replication.

Each side ends in a phosphate (5’) and a sugar (3’).

Antiparallel = one strand of DNA goes 3’ to 5’ and the other strand goes 5’ to 3’

What type of bond forms between the nitrogenous base pairs?

Hydrogen bonds

So DNA strands can be easily separated when it comes time for the cell to make a copies of the DNA.

Strong enough to hold molecules together but can still be pulled apart to make protein and DNA replication.

Which part of the nucleotide does NOT bond to the nitrogenous base?

The phosphate portion of the nucleotide only bonds bonds to the sugar in the backbone of the molecule

It DOES NOT bond with the nitrogenous base

What shape does DNA take? Who discovered this?

Each strand of DNA is helical in shape (spiral); when combined, DNA as a molecule is a double helix.

Rosalind Franklin→ thanks to her work with x-ray crystallography. Famous “photo 51”