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Protein Synthesis (Chapter 3 – Part 4 )
Eukaryotic vs. Prokaryotic DNA
 Eukaryotic DNA
•
DNA is located in the
nucleus in the form of a
number of chromosomes.
Eukaryotic vs. Prokaryotic DNA
 Prokaryotic DNA
•
•
•
Lack nucleus and many
of the organelles found
in eukaryotes.
DNA is located in the
cytoplasm.
Have a single circular
DNA molecule (usually
referred to as the cell's
chromosome).
Some Terms
 Chromosome - Very
long, continuous piece of
DNA (a single DNA
molecule), which
contains many genes.
 Genes - Sequence of
DNA that codes for a
protein and thus
determines a trait.
Chromosome Structure
 DNA molecules are surprisingly long.
•
•
The chromosome of the prokaryote E. coli, which
can live in the human colon, contains 4,639,221
base pairs!
Analogy: Imagine trying to pack a 300-meter
length of rope into your school backpack!
 DNA must be dramatically folded and tightly
packed to fit within the cell.
More Terms
 Chromatin - Granular material visible within the
nucleus; consists of DNA tightly coiled around
proteins
Proteins
 Proteins are made up of a chain of amino acids.
 Proteins are enzymes, which catalyze and
regulate chemical reactions.
2 Steps to Make a Protein
1. Transcription
DNA → RNA
2. Translation
RNA → Protein
(Chain of amino
acids)
Step 1: Transcription
 Transcription - Process in
which part of the nucleotide
sequence of DNA is copied
into a complementary
sequence in RNA.


The DNA strands are separated.
One strand of DNA is used as a
template from which nucleotides
are assembled into a strand of
RNA.
Transcription: DNA → RNA
Transcription Animation
 http://www.ncc.gmu.edu/dna/mRNAanim.htm
 http://www.johnkyrk.com/DNAtranscription.html
 http://www-
class.unl.edu/biochem/gp2/m_biology/animation/
gene/gene_a2.html
 http://207.207.4.198/pub/flash/26/transmenu_s.s
wf (very good but need to skip some parts)
After Transcription
 The mRNA
leaves the
nucleus and
travels to the
ribosomes in
the cytoplasm.
Codons
 Codon - Three-nucleotide sequence
on messenger RNA that codes for a
single amino acid.
The Genetic Code
 The “language” of mRNA instructions
is called the genetic code.
 The genetic code is read three letters
at a time, so that each “word” of the
coded message is three bases long.
Step 2: Translation
 Translation - Decoding of a mRNA
message into a protein (amino acid chain)
 Takes
place on Ribosomes
Steps of Translation
1. Begins when an mRNA molecule in
the cytoplasm attaches to a
ribosome.
2. As each codon of the mRNA
molecule moves through the
ribosome, the proper amino acid is
brought into the ribosome by tRNA.
Anticodon
 Each tRNA molecule has three unpaired
bases called the anticodon, which are
complementary to one mRNA codon.
Anticodon Example
 In the case of the
tRNA molecule for
methionine:
 The
anticodon UAC
codes for the amino
acid methionine
 The anticodon bases
You Try It: List the
are UAC
codon, anticodon, and
 The codon bases are the amino acid coded
nd codon.
for
the
2
AUG
Translation Animations
 http://207.207.4.198/pub/flash/26/transmenu_
s.swf (very good animation!)
Mutations
 Mutation - Change in a DNA
sequence that affects genetic
information; error or mistake in
copying DNA.
Types of Mutations
 Point Mutations - Mutation that affects
a single nucleotide, usually by
substituting one nucleotide for
another.

Substitution – one base is changed to
another
Original: AUGUAC → Met – Tyr
Mutated: AUGUAG → Met – Stop
(causes the amino acid chain to stop protein production early)
Types of Mutations
 Frameshift Mutations - Mutation that shifts
the “reading” frame of the genetic message
by inserting or deleting a nucleotide.
–
–
Insertions – A base is inserted into the DNA
sequence.
Deletions - A base is removed from the
DNA sequence.
Original: The fat cat ate the wee rat.
Frame Shift: The fat caa tet hew eer at.
(Frame shift mutations affect all subsequent amino acids!)
Significance of Mutations
 Many mutations have little or no effect on the
expression of genes.
 Mutations may be harmful and may be the
cause of many genetic disorders and cancer.
 Source of genetic variability in a species (may
be highly beneficial).
What Varies in Our DNA
 Mutations - Occur by changing the
sequence of the nitrogen bases or
the order of nucleotides.
 Differences Between Organisms Occur because the sequence of the
nitrogen bases or the order of
nucleotides are different.