Untitled LessonLesson 12.2

Makes YOU
want to isolate
DNA…doesn’t it?

� What is a nitrogenous base?

⚫ Bases that have nitrogen in them!

⚫ DNA has 4 kinds of nitrogenous bases

� Adenine

Thymine Cytosine

Guanine

� Often referred to as A, T, C, and G

� Can be strung together in many different sequences

� Great at absorbing UV light (why is this interesting?)

Purines are as pure
AsGold (adenine,
guanine)

Pyramidines are CUT
(cytosine, uracil, thymine) – if
you get a paper cut, you only
really want 1 (single ringed)

HOWDO I REMEMBER THAT?

� Apple in a tree. Car in the garage.

� All talk, can’t get it.

Thanks
Destiny Serrata
Class of ‘23

DNA Nucleotides

� Erwin Chargaff’s Rule

⚫ %A = %T and %C = %G

� Rosalind Franklin – X-ray Evidence

⚫ X-ray diffraction shows DNA structure

� DNA is twisted like coils (helix)

� Angles suggest two strands

� Nitrogenous bases near the center of the molecule

�Click here for a summary on Rosalind Franklin and DNA

�Another short on the double helix discovery (Thanks Gabriella Benetatos '22)

� James Watson & Francis Crick

⚫ Built a model explaining DNA structure &

properties

⚫ DNA - double helix, 2 strands of nucleotide

sequences were wound around each other

� Twisted ladder or spiral staircase

� Accounted for many of Franklin’s x-ray patterns

� The model explained Chargaff’s rule of base pairing

Click here for Mr Anderson What is DNA video

Crick’s original
sketch of DNA

Let’s go to the video tape: Photo 51 approx 1 hour

Thanks
Anay
Tillu
Class
of ‘23

� Antiparallel Strands

⚫ The two strands run in opposite directions

� Allows nitrogenous bases on both strands to meet in the

center

� Allows each strand in the helix to carry a sequence of

nucleotides

� Hydrogen Bonding

⚫ Forms between the nitrogenous bases and holds

the strands together

⚫ Relatively weak chemical forces, but that’s good

because the strands need to be pulled apart,
which is critical to the DNA’s functions

Antiparall
el

� Base Pairing

⚫ Hydrogen bonds

create a perfect fit
between nitrogenous
bases near the center
of the molecule

� A can only pair w/ T

� G can only pair w/ C

⚫ Explains Chargaff’s

rules

⚫ Explains Franklin’s

x-ray patterns

CHROMOSOMES NJCTL

� DNA is configured

into structures called
chromosomes

� Recall that

prokayrotes have 1
c’some that is
double-stranded and
circular.