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DNA and Protein
Synthesis
California Standards
4 and 5
The Components and
Structure of DNA
• The Components and
Structure of DNA
•DNA is made up of
nucleotides.
The Components and
Structure of DNA
• A nucleotide is a monomer of
nucleic acids made up of
– a five-carbon sugar called
deoxyribose
–a phosphate group
–a nitrogenous base
• There are four kinds of bases
in in DNA:
•adenine
•guanine
•cytosine
•thymine
• The backbone of a DNA chain
is formed by sugar and
phosphate groups of each
nucleotide.
• The nucleotides can be joined
together in any order.
–X-Ray Evidence
•Rosalind Franklin used X-ray
diffraction to get information
about the structure of DNA.
•She aimed an X-ray beam at
concentrated DNA samples
and recorded the scattering
pattern of the X-rays on film.
• The Double Helix
–Using clues from Franklin’s
photograph and research
from other scientists, James
Watson and Francis Crick
were able to build a physical
model that explained how
DNA carried information and
could be copied.
• The Double Helix
Watson and Crick's
model of DNA was a
double helix, in
which two strands
were wound around
each other, like a
twisted ladder.
• DNA Double Helix
The Components and Structure
of DNA
• Watson and Crick discovered
that hydrogen bonds can form
only between certain base
pairs—adenine and thymine,
and guanine and cytosine.
A-T, G-C
• This principle is called base
pairing.
A
T
G
C
Review Questions
A. What are the three parts of a
nucleotide?
B. What are the four bases and
how to they attach together
C. What is the shape of DNA
12–1
– DNA is a long molecule
made of monomers called
A. nucleotides.
B. purines.
C. pyrimidines.
D. sugars.
12–1
–
In DNA, the following base
pairs occur:
A. A with C, and G with T.
B. A with T, and C with G.
C. A with G, and C with T.
D. A with T, and C with T.
DNA Replication
• DNA Replication
•For a cell to reproduce, it must
make a copy of its DNA so each
cell has a complete copy of the
information
•The process of making a copy
of the DNA is called DNA
Replication
DNA Replication
• During DNA replication, the
DNA molecule separates into
two strands. Each strand,
side, of the DNA chain has
all the same information as
the other strand, side.
A-T-T-G-A-C-C-G-G-T-C-A-A-T
T-A-A-C-T-G-G-C-C-A-G-T-T-A
A-T-T-G-A-C-C-G-G-T-C-A-A-T
TA A C TGG C CA G TTA
AT T G AC C G GT C AAT
T-A-A-C-T-G-G-C-C-A-G-T-T-A
• Each strand of the double helix
of DNA serves as a template for
the new strand.
• This form of replication is
called semiconservative
replication.
–How Replication Occurs
•DNA replication is carried out
by enzymes, called Helicase,
that “unzip” a molecule of
DNA.
•Hydrogen bonds between base
pairs are broken and the two
strands of DNA unwind.
• The principal enzyme involved
in DNA replication is DNA
polymerase.
• DNA polymerase joins
individual nucleotides to
produce a DNA molecule and
then “proofreads” each new
DNA strand.
DNA Replication
• DNA Replication
•In most prokaryotes, DNA
replication begins at a single
point and continues in two
directions.
• In eukaryotic chromosomes,
DNA replication occurs at
hundreds of places. Replication
proceeds in both directions
until each chromosome is
completely copied.
• The sites where separation and
replication occur are called
replication forks
• Replication can begin in the
middle of the DNA chain.
• The two chains unzip to form a
replication bubble.
• The two chains unzip until
both chains have be separated
and copied.
DNA Replication
New Strand
Original strand
Nitrogen Bases
Growth
Growth
Replication Fork
Replication Fork
DNA Polymerase
• As the replication bubble
expands, one of the new DNA
chains is replicated in a single
continuous strand.
• This is the “leading” strand.
• The opposite strand, being
formed in the opposite
direction is the “lagging”
strand.
• It is formed in many small
fragments that are ultimately
attached together by an
enzyme called DNA Ligase
• DNA ligase attaches all DNA
fragments together into a
single complete chain.
Review Questions
a. What is semiconservative
replication?
b. What does DNA Polymerase,
DNA Ligase and Helicase do?
12–2
–
The first step in DNA
replication is
A. producing two new strands.
B. separating the strands.
C. producing DNA polymerase.
D. correctly pairing bases.
–
A DNA molecule separates,
and the sequence
GCGAATTCG occurs in one
strand. What is the base
sequence on the other strand?
A. GCGAATTCG
B. CGCTTAAGC
C. TATCCGGAT
D. GATGGCCAG
–
In addition to carrying out the
replication of DNA, the enzyme
DNA polymerase also functions
to
A. unzip the DNA molecule.
B. regulate the time copying
occurs in the cell cycle.
C. “proofread” the new copies to
minimize the number of
mistakes.
D. wrap the new strands onto
histone proteins.
12-3 RNA and Protein
Synthesis
RNA Function
• Genes are coded DNA instructions
that control the production of
proteins.
• Genetic messages can be decoded
by copying part of the nucleotide
sequence from DNA into RNA.
• RNA contains coded information
for making proteins.
The Structure of RNA
• The Structure of RNA
•RNA consists of a long
chain of nucleotides.
•Each nucleotide is made up
of a 5-carbon sugar, a
phosphate group, and a
nitrogenous base.
• There are three main
differences between RNA and
DNA:
•The sugar in RNA is ribose
instead of deoxyribose.
•RNA is generally singlestranded.
•RNA contains uracil in place
of thymine.
Deoxyribose
Ribose
HOCH2 HO HOCH2 HO
OH
H
OH OH
DNA
RNA
–What are the three main
types of RNA?
Types of RNA
• Types of RNA
–There are three main types of
RNA:
•messenger RNA
•ribosomal RNA
•transfer RNA
• Messenger RNA (mRNA)
carries copies of instructions
for assembling amino acids
into proteins.
Ribosome
Ribosomal RNA
• Ribosomes are made up of
proteins and ribosomal RNA
(rRNA).
• During protein
construction,
transfer RNA
(tRNA) transfers
each amino acid to
the ribosome.
Amino acid
Transfer RNA
Transcription
–What is transcription?
Transcription
RNA
RNA polymerase
DNA
•Transcription is when a
section of DNA is copied by
an enzyme to make a
duplicate copy. The copy is
made out of RNA.
•Transcription requires the
enzyme RNA polymerase.
–During transcription, RNA
polymerase binds to DNA and
separates the DNA strands.
–RNA polymerase then uses
one strand of DNA as a
template from which
nucleotides are assembled
into a strand of RNA.
• RNA polymerase binds only to
regions of DNA known as
promoters.
• Promoters are signals in DNA
that indicate to the enzyme
where to attach to make RNA.
Transcription
RNA
RNA polymerase
DNA
RNA Editing
•The DNA of eukaryotic genes
contains sequences of
nucleotides, called introns,
that are not involved in coding
for proteins.
•The DNA sequences that code
for proteins are called exons.
•When RNA molecules are
formed, introns and exons are
copied from DNA.
RNA Editing
• The introns
are cut out of
RNA
molecules.
• The exons are
the spliced
together to
form mRNA.
Exon Intron
DNA
Pre-mRNA
mRNA
Cap
Tail
The Genetic Code
•The genetic code is the
“language” of mRNA
instructions.
•The code is written using
four “letters” (the bases: A,
U, C, and G).
• A codon consists of three
consecutive nucleotides on
mRNA that specify a particular
amino acid.
•Each codon specifies a
particular amino acid that
is to be placed on the
polypeptide chain.
•Some amino acids can be
specified by more than one
codon.
•There is one codon AUG
that can either specify the
amino acid methionine or
serve as a “start” codon for
protein synthesis.
•There are three “stop”
codons that do not code for
any amino acid. These
“stop” codons signify the
end of a polypeptide.
Translation
–What is translation?
–Translation is the decoding of
an mRNA message into a
polypeptide chain (protein).
–Translation takes place on
ribosomes.
–During translation, the cell
uses information from
messenger RNA to produce
proteins.
•Messenger RNA is
transcribed in the nucleus,
and then enters the
cytoplasm where it attaches
to a ribosome.
Nucleus
mRNA
• Translation begins when an
mRNA molecule attaches to a
ribosome.
• As each codon of the mRNA
molecule moves through the
ribosome, the proper amino acid
is brought into the ribosome by
tRNA.
• In the ribosome, the amino acid is
transferred to the growing
polypeptide chain.
• Each tRNA molecule carries
only one kind of amino acid.
• In addition to an amino acid,
each tRNA molecule has three
unpaired bases.
• These bases, called the
anticodon, are complementary
to one mRNA codon.
•The ribosome binds new tRNA
molecules and amino acids as it
moves along the mRNA.
Phenylalanine
Methionine
Ribosome
mRNA
Start codon
Lysine
tRNA
• Peptide bonds form between the
amino acids creating a polypeptide
chain
Peptide Bond
Lysine
tRNA
Translation direction
mRNA
Ribosome
The process continues until
the ribosome reaches a stop
codon.
Polypeptide
Ribosome
tRNA
mRNA
The Roles of RNA and DNA
•The cell uses the DNA
“Recipe Book” (genes) to
prepare mRNA “Recipe”
(copy). The DNA stays in the
nucleus.
•The mRNA molecules go to
the protein building sites in
the cytoplasm—the
ribosomes.
• The sequence of
bases in DNA (gene)
is used as a
template for mRNA.
• The codons of
mRNA specify the
sequence of amino
acids in a protein
that is assembled at
the ribosome.
Codon
Codon Codon
Single strand of DNA
Codon Codon Codon
mRNA
Alanine Arginine Leucine
Amino acids within
a polypeptide
Protein Synthesis
1. DNA is copied to make mRNA in
the nucleus
2. Ribosomes attach to the mRNA in
the cytoplasm
3. tRNA brings Amino Acids to the
ribosomes to make Proteins
DNA→mRNA→Ribosome→Protein
Genes and Proteins
• Genes contain instructions for
assembling proteins.
• The different genes in DNA
cause different proteins to be
produced.
• Each type of cell in the body
produces different proteins that
allow the cell to do specific jobs.
12–3
–
The sequence that protein
synthesis occurs?
A. mRNA, DNA, Ribosome,
protein
B. DNA, Ribosome, mRNA,
Protein
C. Ribosome, DNA, protein,
mRNA
D. DNA, mRNA, Ribosome,
Protein
12–3
–
A base that is present in
RNA but NOT in DNA is
A. thymine.
B. uracil.
C. cytosine.
D. adenine.
12–3
– The nucleic acid
responsible for bringing
individual amino acids to
the ribosome is
A. transfer RNA.
B. DNA.
C. messenger RNA.
D. ribosomal RNA.
12–3
– A regions of a DNA
molecule that codes for a
polypeptide (protein)
A. introns.
B. exons.
C. promoters.
D. codons.
12–3
– A codon typically carries
sufficient information to
specify a(an)
A. single base pair in RNA.
B. single amino acid.
C. entire protein.
D. single base pair in DNA.
Mutations
–Mutations are changes in the
genetic material.
Kinds of Mutations
•Mutations that produce
changes in a single gene are
known as gene mutations.
•Mutations that produce
changes in whole
chromosomes are known as
chromosomal mutations.
•Gene mutations involving a
change in one or a few
nucleotides are known as
point mutations because
they occur at a single point
in the DNA sequence.
•Point mutations include
substitutions, insertions,
and deletions.
• Substitutions
usually affect
no more than
a single
amino acid.
• The effects of insertions or
deletions are more dramatic.
• The addition or deletion of a
nucleotide causes a shift in the
grouping of codons.
• Changes like these are called
frameshift mutations.
• Frameshift mutations may
change every amino acid that
follows the point of the
mutation.
• Frameshift mutations can alter
a protein so much that it is
unable to perform its normal
functions.
• In an
insertion, an
extra base is
inserted into a
base
sequence.
• In a deletion, the loss of a
single base is deleted and the
reading frame is shifted.
–
A mutation that affects every
amino acid following an
insertion or deletion is called
a(an)
A. frameshift mutation.
B. point mutation.
C. chromosomal mutation.
D. inversion.
– The type of point mutation
that usually affects only a
single amino acid is called
A. a deletion.
B. a frameshift mutation.
C. an insertion.
D. a substitution.
Manipulating DNA
The Tools of Molecular
Biology
How do scientists make
changes to DNA?
–Scientists use their
knowledge of the structure
of DNA and its chemical
properties to study and
change DNA molecules.
–Scientists use different
techniques to:
•extract DNA from cells
•cut DNA into smaller pieces
•identify the sequence of
bases in a DNA molecule
•make unlimited copies of
DNA
•In genetic engineering,
biologists make changes in
the DNA code of a living
organism.
–DNA Extraction
•DNA can be extracted from
most cells by a simple
chemical procedure.
•The cells are opened and
the DNA is separated from
the other cell parts.
–Cutting DNA
•Most DNA molecules are too
large to be analyzed, so
biologists cut them into
smaller fragments using
restriction enzymes.
–Each restriction enzyme cuts
DNA at a specific sequence of
nucleotides.
Recognition sequences
DNA sequence
Restriction enzyme EcoR I cuts
the DNA into fragments
Sticky end
• A restriction enzyme will cut a
DNA sequence only if it
matches the sequence
precisely.
• The locations where the
enzyme opens the DNA are
called sticky ends.
Using the DNA Sequence
–Cutting and Pasting
•Short sequences of DNA can
be assembled using DNA
synthesizers.
•“Synthetic” sequences can
be joined to “natural”
sequences using enzymes
that splice DNA together.
Using the DNA Sequence
• These enzymes also make it
possible to take a gene from
one organism and attach it to
the DNA of another organism.
• Such DNA molecules are
sometimes called recombinant
DNA.
Separating DNA
• In gel electrophoresis, DNA
fragments travel through a gel,
a porous material.
• The shorter lengths travel
faster than the longer lengths.
• Gel electrophoresis can be
used to compare the DNA of
different organisms or different
individuals.
Longer
fragments
Shorter
fragments
Gel Electrophoresis
Making Copies
• Polymerase chain reaction
(PCR) is a technique that
allows biologists to make
copies of genes.
• A biologist adds short pieces of
DNA that are complementary
to portions of the sequence.
Using the DNA Sequence
• DNA is heated to separate its
two strands
• DNA polymerase starts making
copies of the region between
the primers two separated
strands
Polymerase Chain Reaction (PCR)
DNA heated to
separate strands
DNA polymerase adds
complementary strand
DNA fragment
to be copied
PCR cycles 1
DNA copies 1
2
2
3
4
4
8
5 etc.
16 etc.
Transforming Bacteria
• Bacteria can be given human
DNA so the bacteria will make
human proteins.
• Human insulin is common
example.
Transforming Bacteria
• Foreign DNA is first joined to a
small, circular DNA molecule
found in bacteria.
• These rings are known as
plasmids.
• Using restriction enzymes,
human DNA is cut, and
inserted in the bacteria
plasmid.
Transforming Bacteria
Recombinant
DNA
Gene for human
growth hormone
Gene for human
growth hormone
Human Cell
Bacterial
chromosome
Sticky
ends
DNA
recombination
DNA
insertion
Bacteria cell
Plasmid
Bacteria cell
containing gene
for human growth
hormone
Transforming Plant Cells
• To help improve the quality of
many crops, plants receive
recombinant DNA for a variety
of genes.
• Plants receive genes that make
them resistant to herbicides
and pesticides.
13-3
– Which types of cells have
plasmids?
A. bacteria only.
B. plant cells only.
C. plant, animal, and
bacterial cells.
D. animal cells only.
13-2
– Restriction enzymes are
used to
A. extract DNA.
B. cut DNA.
C. separate DNA.
D. replicate DNA.
•
13-2
A particular restriction enzyme
is used to
A. cut up DNA in random
locations.
B. cut DNA at a specific
nucleotide sequence.
C. extract DNA from cells.
D. separate negatively charged
DNA molecules.
13-2
–
During gel electrophoresis, DNA
fragments become separated
because
A. multiple copies of DNA are
made.
B. recombinant DNA is formed.
C. DNA molecules are negatively
charged.
D. smaller DNA molecules move
faster than larger fragments.
13-2
–
Which technique is used to make
many copies of DNA in a short
period of time?
A. Restriction enzymes.
B. Gel electrophorysis.
C. Recombinant DNA
D. Polymerase Chain Reaction
(PCR)