Structure of DNA
People involved with discovering DNA’s structure
*Rosalind Franklin & Maurice Wilkins –1950, photographs of the
DNA molecule using X-ray crystallography which showed the shape
to be a helix
*Erwin Chargaff – 1951, proved that the % of A = T and % of G = C
*James Watson & Francis Crick – 1953, used data from the other
scientists and built models to finally figure out the exact structure of
DNA…. 1962 won the nobel prize in Medicine/Physiology
DNA Structure
*DNA is a double helix (twisted ladder) made of subunits called
nucleotides
4 different nucleotides Adenine, Thymine, Guanine,
*Nucleotides are made of 3 parts
5-carbon sugar (deoxyribose)
phosphate group
nitrogenous base
Cytosine
The sides of the double helix are made of alternating sugars
and phosphate groups, the rungs of the double helix are
made of nitrogenous bases
Complementary Base Pairs in making the double helix
A pairs with T
G pairs with C
TEST YOURSELF
1. What are the three parts of a nucleotide?
2. Which parts make up the backbone of a DNA
strand?
3. What are the two base pairs found in DNA?
4. If six bases on one strand of DNA are AGTCGG
what are the six bases on lthe complementary
section of the other strand of DNA?
DNA REPLICATION
DNA replication is the process of copying the DNA molecule.
During DNA copying, the two strands of the double helix separate.
Each single strand acts as a “negative”for producing a new,
complementary strand.
Easy steps for DNA Replication.
Enzymes are protein molecules that catalyze chemical reactions in a
cell – usually any protein ending in “ase” is an enzyme
1. DNA replication begins at specific sites called the
origins
2. DNA helicase unwinds and separates the two
strands of original DNA molecule
3. DNA polymerase 3 adds complementary
nucleotides to each separated strand
4. DNA polymerase 1 checks for correct pairing of
nucleotides (looks for mutations) and fixes any
mispaired nucleotides
Protein Synthesis
Transcription
DNA
Translation
mRNA
Nucleus
Protein
Cytoplasm
Transcription is the making of single stranded mRNA from a DNA
strand within the nucleus of a cell.
During transcription, RNA nucleotides base-pair one-by-one
with DNA nucleotides on one of the DNA strands (called the template
strand). RNA polymerase links the RNA nucleotides together.
Base pairing is the same as during DNA replication, except that
RNA has uracil instead of thymine: the U in RNA pairs with A in DNA
EASY STEPS OF TRANSCRIPTION
1. DNA double helix unwound and separated by RNA
polymerase
2. RNA polymerase adds RNA nucleotides together,
making a single strand of mRNA which is
complementary to 1 strand of the DNA
3. mRNA is processed (modified) before leaving the
nucleus
a. introns (non coding regions) are cut out of
the mRNA
b. exons (coding regions) are “spliced” together to
form the final mRNA product
Molecular Genetics Activity #1 page 1
DNA STRUCTURE AND REPLICATION
BUILDING BLOCKS OF DNA:
Nucleotides:
1. 5 carbon sugar (deoxyribose)
2. Nitrogenous base (A, T, C, or G)
3. Phosphate group
NITROGENOUS BASES PYRIMIDINES PURINES
Single ring structure C and T
Double ring structure G and A
Cytosine
Guanine
Thymine
Adenine
DNA STRUCTURE
§ Double helix
§ Double stranded
§ Twisted ladder
§ Sides of ladder consist of alternating sugar & phosphate groups
§ Rungs of ladder
- 2 bases- Purine – pyrimidine
- A–T
- C–G
- Hydrogen bonds hold bases together
- 2 H bonds between A and T
- 3 H bonds between C and G
ANTIPARALLEL STRANDS
One strand 5’ at top & 3’ at bottom
Other strand: 5’ at bottom & 3’ at top
5’ end 5th carbon in deoxyribose
3’ end 3rd carbon in deoxyribose
DNA REPLICATION