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Interest Grabber
Section 12-3
 DNA contains the information that a cell needs to carry
out all of its functions. In a way, DNA is like the cell’s
encyclopedia. Suppose that you go to the library to do
research for a science project. You find the information
in an encyclopedia. You go to the desk to sign out the
book, but the librarian informs you that this book is for
reference only and may not be taken out.
1. Why do you think the library holds some books for
reference only?
2. If you can’t borrow a book, how can you take home the
information in it?
3. All of the parts of a cell are controlled by the
information in DNA, yet DNA does not leave the
nucleus. How do you think the information in DNA
Figure 12-10 Chromosome Structure
of Eukaryotes
Section 12-2
Chromosome
Nucleosome
DNA
double
helix
Coils
Supercoils
Histones
1) DNA Replication
 Before cellular division the cell must double its DNA
 Prokaryotes – DNA replication begins at a single point in
the chromosome and proceeds in two directions until
replicated.
 Eukaryotes – DNA has two complementary strands
wound up in the shape of double helix. The DNA
polymerase unwinds the DNA and a new strand is
copied. The new strand is a complementary copy of the
original.
 Complementary means that if the original DNA base is
adenine the complementary base is thymine.
Figure 12–7 Structure of
DNA
Section 12-1
Nucleotide
Hydrogen
bonds
Sugar-phosphate
backbone
Key
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
Figure 12–11 DNA
Replication
Section 12-2
New strand
Original
strand
DNA
polymerase
Growth
DNA
polymerase
Growth
Replication
fork
Replication
fork
New strand
Original
strand
Nitrogenous
bases
How do DNA genes work?
 DNA is a specific set of instructions for what?
 DNA (genes) code for the production of protein
 How does DNA get out of the nucleus?
 The first step in decoding genes is to copy a
sequence of DNA onto RNA.
What is RNA?
RNA - Ribonucleic acid is usually
single stranded and contains
Uracil instead of thymine and it
has ribose as it’s sugar.
There are three types of RNA
Concept Map
Section 12-3
RNA
can be
Messenger RNA
also called
Ribosomal RNA
which functions to
mRNA
Carry instructions
also called
which functions to
rRNA
Combine
with proteins
from
to
to make up
DNA
Ribosome
Ribosomes
Transfer RNA
also called
which functions to
tRNA
Bring
amino acids to
ribosome
STEP ONE DNA to mRNA
 DNA needs to be transcribed into m
(messenger) RNA
 How is this done?
 RNA polymerase (enzyme) unzips the DNA
molecule and RNA uses one strand of DNA to
make an exact copy. A pairs with U and C pairs
with G. Nucleotides (Phosphate+ base+ sugar)
of mRNA are assembled into a strand of RNA.
The mRNA leave the nucleus.
CODONS
RNA will bind only to specific sites
along the DNA called promoters –
they have specific base
sequences that appear in triplets.
Figure 12–14 Transcription
Section 12-3
Adenine (DNA and RNA)
Cystosine (DNA and RNA)
Guanine(DNA and RNA)
Thymine (DNA only)
Uracil (RNA only)
RNA
polymerase
DNA
RNA
Questions
How is DNA replicated?
 DNA is replicated with DNA polymerase in
the nucleus
What enzyme unzips DNA for RNA
transcription?
RNA polymerase unzips DNA for
Step Two - Translation
 mRNA has made a copy of DNA – a set of instructions to
make protein.
 mRNA leaves the nucleus and looks for a ribosome to
attach to so translation of the mRNA can begin.
 Each codon (three letter code of RNA) is read by the
ribosome. The ribosome uses the codon as a set of
instructions to make a protein. Each three unit codon is
matched to its anitcodon – also called tRNA:

AUG is matched with UAC
 And the anti codon is attached to a specific amino acid
Figure 12–18 Translation
Section 12-3
Translation Continued
 The ribosome also bonds each amino acid to the
next amino acid. This is how polypeptide chains
of amino acids are formed.
 The ribosome also breaks the bonds between
the amino acid and its anticodon.
 The polypeptide chin continues to grow until the
ribosome reaches a stop codon.
Figure 12–18 Translation (continued)
Section 12-3
http://www.pbs.org/wgbh/aso/tryit/dna/#
Figure 12–17 The Genetic Code
Section 12-3
Interest Grabber
Section 12-4
Determining the Sequence of a Gene
DNA contains the code of instructions for
cells. Sometimes, an error occurs when
the code is copied. Such errors are called
mutations.
Section 12-4
Gene Mutations:
Substitution, Insertion,
and Deletion
Deletion
Substitution
Insertion
Figure 12–20 Chromosomal
Mutations
Section 12-4
Deletion
Duplication
Inversion
Translocation
Interest Grabber
Section 12-5
Regulation of Protein Synthesis
 Every cell in your body, with the exception of gametes, or
sex cells, contains a complete copy of your DNA. Why,
then, are some cells nerve cells with dendrites and
axons, while others are red blood cells that have lost
their nuclei and are packed with hemoglobin? Why are
cells so different in structure and function? If the
characteristics of a cell depend upon the proteins that
are synthesized, what does this tell you about protein
synthesis? Work with a partner to discuss and answer
the questions that follow.
Interest Grabber continued
Section 12-5
1. Do you think that cells produce all the proteins for which the DNA
(genes) code? Why or why not? How do the proteins made affect the
type and function of cells?
2. Consider what you now know about genes and protein synthesis. What
might be some ways that a cell has control over the proteins it
produces?
3. What type(s) of organic compounds are most likely the ones that help
to regulate protein synthesis? Justify your answer.