DNA Structure/Replication & Protein
Synthesis
DNA Structure and how it is arranged was determined
through the efforts of a number of scientists. 3 of the most
important of these scientists and their contributions are:
1. Erwin Chargaff:
a. CHARGAFF’S RULE: DNA strands pair with their COMPLIMENTARY (matching)
STRAND; A=T and G=C
b. Adenine always pairs with Thymine, and Guanine always pairs with Cytosine
c. The percentage of Adenine therefore must equal the percentage of Thymine and
the percentage of Guanine must equal the percentage of Cytosine
i. %A = %T------%G = %C  %A + %T + %G + %C = 100%
2. James Watson and Francis Crick:
a. Determined that DNA was shaped as a DOUBLE HELIX and created a model of
the DNA molecule
b. Helix= a 3D twisting/curved shape
i. DNA has two twisting strands much like a twisted ladder, hence,
DOUBLE HELIX
3. Rosalind Franklin:
a. Known for Photo 51 taken using X-Ray Crystallography
b. Photo 51 The scientific community knew that DNA resembled a ladder, but
PHOTO 51 provided the last clue that DNA was actually a TWISTED LADDER
(Double Helix)
DNA Structure
DNA stands for Deoxyribonucleic acid and it is a NUCLEIC ACID composed of NUCLEOTIDES
DNA NUCLEOTIDE: 3 Parts
1. Phosphate Group: represented by a circle in the “rails” of the ladder
2. Deoxyribose (sugar in DNA): represented by a pentagon
3. Base/Nitrogen Base/Nitrogenous Base: pairs form the “rungs” of the ladder,
either A, T, C, or G in DNA!
Purine: 2-ring
base A & G
Pyrimidine: 1-ring
base T & C
******In DNA, The Sugar is DEOXYRIBOSE******
1. The
BACKBONE:
composed of
alternating
Phosphates
and
Deoxyriboses
2. The
RUNGS:
composed of
nitrogenous
bases joined
by
HYDROGEN
BONDS
DNA Replication
***DNA perfectly replicates itself in a semi-conservative way***
“Semi”= Half + “Conservative”= Saving “Half-Saving”
DNA replicates in such a manner that from one “parent” molecule of
DNA, two IDENTICAL “daughter” molecules are producedeach
containing one of the parent strands and one new strand of DNA
Protein Synthesis
CENTRAL DOGMA: describes how the information
stored in DNA is accessed and used by an organism
DNARNAPROTEIN
Replication….Transcription&Translation....Gene Expression
Protein Synthesis relies on another nucleic acid very similar to DNA…RNA!!
 RNA, RIBONUCLEIC ACID, is similar to DNA but with a few key differences:
o RNA contains the pyrimidine URACIL (U) instead of THYMINE (T)
o RNA contains the sugar RIBOSE instead of DEOXYRIBOSE
o RNA is SINGLE-STRANDED while DNA is DOUBLE-STRANDED
o RNA has 3 TYPES while DNA has ONLY ONE
 mRNA- messenger RNAcopy of DNA segment created in
transcription and “read” by ribosome to make protein in
translationfound in nucleus and cytoplasm
 tRNA- transfer RNAcarries amino acids to add to protein
chain at the ribosomefound in cytoplasm
 rRNA- ribosomal RNAsite of translationfound in
cytoplasm
TRANSCRIPTION: Genetic information is copied from
DNA onto a COMPLIMENTARY RNA strand in the Nucleus
 mRNA is complimentary to the target DNA sequence
o EX. DNA: TACGGGACT
RNA: AUGCCCUGT
o NOTE!! U replaces T in the complimentary base pairing
because it RNA not DNA
TRANSLATION: mRNA moves from the NUCLEUS into the
CYTOPLASM and heads to a RIBOSOME where the
genetic information copied from the DNA is “read” to
manufacture a specific PROTEIN
 rRNA “reads” the mRNA in groups of 3 bases known as CODONS
o CODON: sequence of 3 bases that codes for a specific amino
acid
 tRNA molecules bring amino acids to the ribosome one-at-a-time
and in a specific order and add to a growing protein chain
o each tRNA carries ONE amino acid and matches up with the
right spot of the mRNA sequence using ANTICODONS
o ANTICODON: group of 3 bases that is complimentary to a
codon on an mRNA strand…so the ORDER OF CODONS ON
THE mRNA determines the order that tRNAs add amino
acids
Let’s take a look at an example that follows a gene being
expressed for the DNA to the Protein:
DNA Sequence: TACAACGACTTAGCTTAGATT
Step 1: Let’s separate the sequence into codons (groups of 3)
TAC AAC GAC TTA GCT TAG ATT
Step 2: Now let’s create the complimentary mRNA sequence
AUG UUG CUG AAU CGA AUC UAA
Step 3: Now, so I don’t make a mistake, I am going to use my codon
chart and see what amino acid sequence these mRNA CODONS code
for (remember the amino acid sequence is determined by the
CODONS on the mRNA…NOT the ANTICODONS on the tRNA)
Methionine-Leucine-Leucine-Asparagine-Arginine-Isoleucine-Stop
Step 4: Now that I know I have translated my mRNA codon sequence
correctly, I will determine the tRNA anticodon sequence that would
shuttle these amino acids to the ribosome
UAC AAC GAC UUA GCU UAG AUU
**Notice the anticodon sequence is the same as the original DNA
sequence except there are U’s instead of T’s (because it is RNA!)**
IMPORTANT!! Remember the mRNA codon AUG codes for the amino
acid methionine and is the START codon…UAA, UAG, and UGA code
for NO amino acid and are the STOP codons
Mutations: A change in the nucleotide sequence, and,
thus, the genetic code of an organism.
 Can be caused by radiation (UV, X-Ray, Gamma, etc.), chemicals,
random replication errors, and other environmental factors
 Can benefit, harm, or have no impact on an organism and/or its
offspring/species
Types to know:
1. Insertion: a nucleotide is added to the existing nucleotide
sequence EX. ATTT become ATTTC
2. Deletion: a nucleotide is taken out of the existing nucleotide
sequence EX. GGCT become GCT
3. Substitution: a nucleotide is switched in the existing nucleotide
sequence EX. AATG becomes TATG
4. Inversion: a mutation in which a segment of DNA is reversed endto-end EX AGTT flips to form TTGA
**When trying to determine the type of mutation, it is helpful to
start with comparing the number of bases, and then reading the
sequence left to right and comparing**
Substitution
Deletion
Insertion
Inversion