Major Function of DNA is to code for and regulate the process of making
proteins. The proteins produced can be either structural or functional. Structural
proteins like keratin form our hair and nails; they also create the muscles that
help us move, create our organs and give us life. Functional proteins are small
and cannot be seen by the naked eye—they perform critical life creating
functions in our bodies—antibodies give us an immune system, hemoglobin
transports oxygen, and enzymes enable metabolic reactions that digest our food
for energy and regulate homeostasis.
DNA is short for DeoxyriboNucleic Acid. A nucleic acid is built from a
repeating unit of nucleotides. Nucleotides are made from three parts: a sugar,
phosphate and nitrogen base. The sugar is called deoxyribose, on one side of the
sugar you will find a phosphate and on the other side you can find one of four
nitrogen bases.
In 1953, James Watson and Francis Crick established the structure of DNA. The
structure is a double helix, which is like a twisted ladder. The sides of the ladder
are made of alternating sugar and phosphate molecules. The rungs of the ladder
are pairs of 4 types of nitrogen bases: Adenine, Guanine, Thymine and
Cytosine. The bases are known by their coded letters A, G, T, C. These bases
always bond in a certain way. Adenine will only bond to thymine. Guanine will
only bond with cytosine. This is known as the Base-Pair Rule or Chargaff’s
Rule. The order of these bases is the code that contains the instructions for making proteins. For
instance ATGCACATA would code for a different gene than AATTACGGA. A strand of DNA
contains millions of bases. (For simplicity, the image only contains a few.) Note that the bases
attach to the sides of the ladder at the sugars and not the phosphate.
Erwin Chargaff found that a peculiar regularity in the ratios of nucleotide bases. In the DNA of
each species he studies, the number of adenines approximately equaled the number of thymine,
and the number of guanines approximately equaled the number of cytosine.
Example: If one strand of DNA has the following percentages:
Adenine 18% Thymine 25% Cytosine 30% Guanine 27%
What percentages will be found in the complementary strand?
______% A
______% T
______% C
______% G
Practice
Strand #1: A—10% T—30% C—40% G—20%
Strand #1: A—15% T—55% C—25% G—5%
Strand #2: A ____% T____% C ____% G ____%
Strand #2: A ____% T____% C ____% G ____%
Strand #1: A—26% T—13% C—24% G—27%
Strand #1: A—30% T—20% C—30% G—20%
Strand #2: A ____% T____% C ____% G ____%
Strand #2: A ____% T____% C ____% G ____%
Directions: Practice Chargaff’s Rule/Base Pairing—for each strand of DNA, make its complementary strand.
1.
2.
GGA
TCT
GAA
TGT
ATG
ATT
TGT
ATG
GCC
GAG
TCC
AAT
CAT
GCG
TGT
ATA
GAG
AGA
ATT
TAC
TCT
TAA
GGT
TGT
CGT
3.
ATG
GAA
CCC
CTT
ATA
TGT
GCG
ACC
GAA
GCT
ACT
TAG
GCA
4.
ATT
TGA
GCC
ATG
GAT
CGA
ATC
TAG
ATT
AAT
GGG
TAG
5.
GAT
AAT
GCT
ATG
ATC
TCG
CTT
ATC
TTT
GAA
TAG
TAG
6
ATG
GAA
TCA
ATA
GGT
AAC
TGA
GCT
TAA
CAC
AAC
GAC
TAG
7.
TGG
GCT
ATG
GAG
AGA
ATA
TCG
TAA
TGT
TTG
GAG
GCT
CGC
8.
ATG
TCT
GCC
GTT
GAG
ACC
CAC
GAA
ACA
AGA
GAA
GAG
TCA
GCC
GAG
TGC
TAG
AAC
GCT
CAT
GCT
AGG
TAT
ACT
CCT
CGT
GAG
CGG
TAG
TAG
Answer Key for Chargaff’s Rules:
Strand #1: A—10% T—30% C—40% G—20%
Strand #1: A—15% T—55% C—25% G—5%
Strand #2: A 30% T 10% C 20% G 40%
Strand #2: A 55% T 15% C 5% G 25%
Strand #1: A—26% T—13% C—24% G—27%
Strand #1: A—30% T—20% C—30% G—20%
Strand #2: A 13% T 26% C 27% G 24%
Strand #2: A 20% T 30% C 20% G 30%
Answer Key for Base Pairing
1. DNA  CCT CTT TAC ACA CGG AGG GTA CGC TAT TCT ATG ATT ACA CGG TTG CGA TCC ATA ATC
2. DNA  AGA ACA TAA TAC CTC TTA ACA CTC TAA AGA CCA GCA CTC CGA TGA ACT GGA GCA
3. DNA  TAC CTT GGG GAA TAT ACA CGC TGG CTT CGA TGA ATC CGT ACG GTA CTC GCC ATC
4. DNA  TAA ACT CGG TAC CTA GCT TAG ATC TAA TTA CCC ATC
5. DNA  CTA TTA CGA TAC TAG AGC GAA TAG AAA CTT ATC ATC
6. DNA  TAC CTT AGT TAT CCA TTG ACT CGA ATT GTG CGC TTG CTG ATC
7. DNA  ACC CGA TAC CTC TCT TAT AGC ATT ACA AAC CTC CGA GCG
8. DNA  TAC AGA CGG CAA CTC TGG GTG CTT TGT TCT CTT CTC AGT ATC