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DNA
Definition
Deoxyribonucleic acid (DNA) – molecule
that contains genetic information that
directs the activities of cells.
DNA contains the instructions cells use to
make proteins.
Helps
make bones, muscle, new cells, etc.
History
Rosalind Franklin - took x-ray pictures
of DNA molecules which helped to
determine its shape.
History
James Watson and Francis Crick built the first DNA model and are usually
given credit for determining its shape.
Structure
Monomers called
NUCLEOTIDES.
3 parts:
sugar (deoxyribose)
phosphate
nitrogen base
Structure
There are four different DNA nitrogen
bases:
Purines
- adenine and guanine
(double rings)
Pyrimidines
- cytosine and thymine
(single rings)
Structure
The nitrogen bases always bond to the
deoxyribose molecule (sugar) of the
nucleotide.
Purines
Adenine
Guanine
Phosphate group
Pyrimidines
Cytosine
Thymine
Deoxyribose
Structure
Nucleotides bond together to form a
double stranded DNA molecule.
DNA’s structure looks like a twisted ladder
This
structure is called the double helix.
The deoxyribose and phosphate form the
“rails,” of the ladder.
Nitrogen bases bond together to form the
“rungs,” of the ladder.
Structure
Nucleotide
Sugarphosphate
backbone
Key
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
Chargaff’s Rule
Chargaff's Rule:
Adenine
(A) = Thymine (T)
Guanine (G) = Cytosine (C)
Nitrogen Base Pairing Rules
ADENINE always bonds to THYMINE
CYTOSINE always bonds to GUANINE
Chemical Structure
The nitrogen bases are
bonded with weak
hydrogen bonds to
hold the two DNA
strands together.
The structure and
shape of the nitrogen
bases allows them to
always bond to their
complement
Scientist Check Point
The picture to the right shows an x-ray
diffraction of DNA. The x-ray diffraction
of DNA led to the idea that DNA —
What Scientist Took this picture?
Why are Watson and Crick famous?
Draw 2 strands, four
nucleotides each:
Remember, each
nucleotide consists of
sugar, phosphate,
base!
Label the sugars “S”,
and the phosphates “P”
… notice a pattern
Notice: opposite directions!
?
?
?
?
The Genetic Code
The order of the nucleotides in creates a
code used to make proteins.
The arrangement of nitrogen bases (A-TG-C) determines the amino acids used to
construct proteins
Like
the alphabet makes words
How Does It All Fit???
All your DNA (end-to-end,
from one cell) is about 2
meters long!
One cell’s nucleus is about
2 micrometers in diameter!
So, how can 2m of DNA fit
into 2µm of space?
Supercoiling!
1. DNA wraps
around
histone DNA + histone
= nucleosome
proteins
2. Nucleosomes
form a coil
Supercoiling!
3. Coils coil again
… This is a
SUPERCOIL!
Supercoiling!
A Chromosome Is Supercoiled DNA
Supercoiling!
Got It?
So, Again,
What is A Chromosome?
A
chromosome
is supercoiled
DNA
Making Connections