Chapter 10
packet: DNA
and Protein
Synthesis
Discovery of the structure of DNA
•DNA is in the shape of a double helix –
discovered by Franklin & Wilkins through X-ray
diffraction of DNA (a)
•1953 - Watson & Crick used above information
to construct 1st model of DNA (b)
Structure of DNA
• DNA is a made of nucleotides
• Nucleotides are composed of:
• Phosphate
• Sugar (deoxyribose)
•1 of 4 nitrogencontaining bases adenine (A),
thymine (T),
guanine (G), or
cytosine (C)
•Hydrogen bonds – holds
2 strands of nucleotides
together
•Shape: double helix
(Two strands twist around
each other)
•Base pairing:
•A & T
•C & G
(complementary base
pairs)
DNA Replication
• Purpose: DNA makes an exact copy of
itself prior to cell division; ensures that
each new cell gets a complete copy of the
DNA
DNA
DNA
Replication
DNA
DNA
Cell Division
DNA
• Steps:
1. An enzyme attaches to DNA and
breaks H2 bonds between bases –
DNA chain unzips.
2. DNA polymerase (an enzyme)
attaches to separated strand.
3. Complementary nucleotides bind to
each side, forming 2 complete
strands.
Overview of DNA replication
Ladder configuration and DNA
replication
• Each old strand (parent strand) of
nucleotides serves as a template for each
new strand (daughter strand).
• Proofreading and repair limits error rate
to less than 1 per billion nucleotides.
• Website showing animation:
http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/dnarna2.swf
Video clip from class:
http://www.youtube.com/watch?v=4jtmOZaIvS0
DNA
RNA
(Deoxyribonucleic acid)
(Ribonucleic acid)
Sugar
Deoxyribose
Ribose
Bases
Strands
Adenine, thymine,
guanine, cytosine
Double-stranded
Adenine, uracil,
guanine, cytosine
Single-stranded
Helix
Yes
No
Location
Nucleus
Types
XXXXXXXXX
Nucleus,
cytoplasm
Messenger,
transfer, ribosomal
RNA
vs.
DNA
•Messenger RNA (mRNA) - carries
genetic information to the ribosomes
•Ribosomes - a part of the cell
where proteins are made
•Ribosomal RNA (rRNA) - found in
the ribosomes
•Transfer RNA (tRNA) - transfers
amino acids to the ribosomes
Making Proteins
• A gene is a segment of DNA that specifies
the amino acid sequence of a protein.
• DNA is found in the nucleus of a cell
• Proteins are made outside the nucleus at the
ribosomes.
Overview of gene expression
• Two processes are involved in the
synthesis of proteins in the cell:
• Transcription – DNA is copied into
mRNA, which will take a copy of the
DNA code to the ribosome to direct the
making of protein; occurs in nucleus
• Translation - the process of building
proteins, the sequence of bases of
mRNA is “translated” into a sequence
of amino acids; occurs in ribosome
The Genetic Code
• DNA holds instructions to make a protein
• Instructions are copied into mRNA, which
will be used to make a protein
• Codon - each three-letter unit of an mRNA
molecule
• Each codon represents 1 amino acid
• There are 64 possible codons, and only 20
amino acids, so most amino acids have
more than one codon
Messenger RNA codons
•
•
•
•
•
Transcription
Purpose – Makes a copy of the DNA code
that can leave the nucleus and travel to
the ribosome to direct protein synthesis –
mRNA
Occurs in the nucleus
Occurs at only 1 gene at a time
Adenine in DNA pairs with uracil in
RNA, not thymine
Thymine in DNA pairs with adenine in
RNA
Steps:
1. RNA polymerase attaches to DNA at the start of
a gene
2. DNA unwinds and unzips
3. ½ of DNA will serve as a template
4. Complementary bases are added along DNA
5. Once “stop” signal is reached, process ends,
DNA closes back up, and mRNA is released
Website showing animation:
http://wwwclass.unl.edu/biochem/gp2/m_biology/animation/g
ene/gene_a2.html
Transcription and mRNA synthesis
Translation
• Protein constructed during this process
• Occurs at the ribosomes
• Key players in translation:
• mRNA (messenger RNA)
• Made during transcription, has codons
• Travels from nucleus to ribosome
• Contains copy of DNA code to make protein
• tRNA (transfer RNA)
• rRNA (ribosomal RNA)
Transfer RNA (tRNA)
• tRNA molecules bring amino acids to the
ribosomes
• Free-floating in the cytoplasm of the cell
•Anticodon – sequence
of 3 bases on tRNA – it
is this sequence that
determines which amino
acid each tRNA has
•Complementary base pairing occurs between
anticodons of tRNA and codons of mRNA –
determines the sequence of amino acids to
construct the polypeptide.
•If mRNA codon is
AUG, tRNA anticodon
would be UAC
Ribosomal
RNA (rRNA)
• rRNA is made
in the
nucleolus (a
cell structure
found inside
the nucleus)
• Makes
ribosomes
(location in
cell where
proteins will
be made)
Steps:
1. Ribosome
attaches to start
codon on mRNA
2. tRNA brings amino acids to ribosome –
codon on mRNA pairs with anticodon on
tRNA
3. Amino acids are joined by peptide bonds
4. Stop codon is reached, mRNA is
released, and protein is released
Website showing
animation:
http://wwwclass.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a3.
html
Gene Mutations
• Definition - a change in the sequence of
bases within a gene
• Causes –
• Mutations can be spontaneous or
caused by environmental influences
called mutagens.
• Mutagens include radiation (X-rays,
UV radiation), and organic chemicals
(in cigarette smoke and pesticides).
•Types of mutations –
•Insertion –
•One or more bases are inserted from a
sequence of DNA
•Ex: AUGGGACCU 
AUGGGGGACCU
•Can result in nonfunctional proteins
•Can result in no protein at all – stop
codon where there shouldn’t be one
•Deletion –
•One or more bases are deleted
from a sequence of DNA
•Ex: AUGGGUACC 
AUGGGACC
•Can result in nonfunctional proteins
•Can result in no protein at all – stop
codon where there shouldn’t be one
•Point mutations –
•One base is substituted for another
•May result in change of amino acid
sequence
•Ex: UGC (Cysteine)  UGG
(tryptophan)
•May not affect protein at all
•Ex: UAU  UAC both code for
tyrosine
•May causes stop codon to occur early
•Ex: UAC  UAG (a stop codon)