DNA Replication
and Protein
Synthesis
I. DNA –
Deoxyribonucleic Acid
Genetic Code – Way that
cells store info (in nucleus)
to be passed to the next
generation.
II. DNA Structure
• DNA is a macromolecule
made up of nucleotides.
Each nucleotide molecule
has 3 subunits:
Phosphate
Group
5-carbon sugar
Nitrogen base
Four Possible Nitrogen
Bases:
1. Adenine
2. Guanine
3. Thymine
4. Cytosine
A & G: purines
T & C: pyrimidines
Nucleotides are joined to
form DNA molecules.
NOTE: The sugars and
phosphates make up the
backbone of DNA.
(“siderails” of a ladder)
The Discovery of DNA
http://www.youtube.com/watch?v=sf0YXnAFBs
8&feature=autoplay&list=PL4900A106005340D
0&lf=results_video&playnext=2
Rosalind Franklin (1952)
Took an X-ray of the
DNA molecule that led to
the discovery of its
shape.
Watson & Crick (early
1950’s) = Construct the
DNA Double Helix Model.
Chargaff
Discovered “Base Pairing
Rules.”
Adenine always pairs with
Thymine (2 H bonds)
Cytosine and Guanine always
pair (3 H bonds)
ALWAYS pyrimidine + purine
III. DNA Replication
The 2 strands of DNA are
complementary.(fit together
like puzzle pieces)
Each strand can serve as
a pattern, to put together
the sequence of bases on
the other half.
Why must a cell duplicate
its DNA???
DNA replication occurs before the
cell divides so that each cell has
a complete copy of DNA.
ST
1 :
DNA helicase unzips and
unwinds the 2 strands by
breaking H-bonds.
*Where DNA separates, it makes a
Y-shape (replication fork)
nd
2 :
DNA
polymerases start at
replicaton forks,
adding new
nucleotides and
bases to the
exposed strands
DNA polymerases also
“proofread” the bases to
make sure they are paired
correctly.
(Usually only 1 mutation per 1
billion bases occurs.)
rd
3 :
2 identical molecules
of DNA are produced.
Each is made up of:
ONE ORIGINAL
STRAND
ONE NEW STRAND
Original DNA Strand:
A-T-T-C-G-C-T-A-G-T-A
Complementary strand:
___________________________
IV. Introduction to RNA
Gene expression: turning
genes into specific traits, done
by RNA.
st
1 step: copy part of the DNA
sequence into RNA
(Ribonucleic Acid). 
TRANSCRIPTION
nd
2 :
information in RNA is
used to make a specific
protein 
TRANSLATION
Structure of RNA
 RNA molecules are chains of
nucleotides.
 RNA vs. DNA
1. Sugar in RNA is ribose, DNA is
deoxyribose.
2. It is single-stranded.
3. 4 Nitrogen bases are:
Adenine
Guanine
Cytosine
Uracil replaces Thymine
NOTE! All base-pair rules
are followed:
C=G
A = U, since T is gone
3 Types of RNA
• Messenger RNA (mRNA) – made
when DNA is transcribed into RNA
• Transfer RNA (tRNA) – reads the
mRNA during translation, translates
it into amino acids.
• Ribosomal RNA (rRNA) – RNA
found in ribosomes
Transcription: RNA Synthesis
A. Copies DNA into mRNA
B. The RNA molecule will be
COMPLEMENTARY to the
DNA molecule, NOT
identical –b/c it is in “RNA”
language.
C. Carried out by RNA
polymerase
Steps of Trans.
• RNA polymerase binds to
“promoter” sequence (starting
point on DNA)
• RNA poly unwinds & separates
DNA.
• Complementary RNA bases are
added to DNA bases, makes
single mRNA strand
• mRNA carries genetic info
from DNA out to the
ribosomes, where proteins
are made
Transcription Practice!
DNA = A-T-C-T-G-T-T-A
RNA = ?
The DNA code is determined
by how the 4 nitrogen bases
are arranged.
Why does DNA code for
proteins and NOT other
molecules??
•
Because proteins control the
biochemical pathways in cells.
II. Nature of the Genetic
Code
Genes are “directions”
that help make proteins
(made up of a.a.)
Each combination of 3
nucleotides on a strand of
mRNA is called a CODON,
or 3-letter code word.
Ex. C-U-G, A-C-U are
both codons.
Each codon specifies an
amino acid that is to be
placed in the protein chain.
See page 307 in your
book.
Translation – Process of
using the info in mRNA to
put together amino acids
and make proteins.
- Occurs in a series of
steps, involves, 3 kinds of
RNA, and results in a
polypeptide.
In Translation:
1. Ribosome, mRNA and
tRNA which is carrying the
amino acid, methionine,
bind together.
2. New tRNA arrives and
binds to next codon on
mRNA.
3. First tRNA detaches and leaves its
amino acid,
4. Process ends when the “stop”
codon is reached.
5. Amino acid chain is released and
ribosome complex falls apart.
NOTE!!! tRNA knows which
amino acids to bring b/c its
anticodon will be
complementary to the mRNA
codon.