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DNA and Protein Synthesis Summary
12–1 DNA
To understand genetics, biologists had to learn the chemical makeup of
the gene. Scientists discovered that genes are made of DNA. Scientists
also found that DNA stores and transmits the genetic information
from one generation of an organism to the next. Scientists began
studying DNA structure to find out how it carries information, decides
traits, and replicates itself.
• DNA is a long molecule called a nucleic acid, made up of units called
nucleotides. Each nucleotide is made up of a 5-carbon sugar, a
phosphate group, and a nitrogen-containing base.
Nucleotide
Nucleic acid
• There are four kinds of bases: adenine (A), guanine (G), cytosine (C),
and thymine (T).
Watson and Crick made a three-dimensional model of DNA.
Their model was a double helix, in which two strands were wound
around each other. A double helix is like a twisted ladder. Sugar and
phosphates make up the sides of the ladder. Hydrogen bonds between the
bases hold the strands together. Bonds form only between certain base
pairs: between adenine and thymine, and between guanine and cytosine.
This is called base pairing.
12–2 Chromosomes and DNA Replication
Most prokaryotes have one large DNA molecule in their cytoplasm.
Eukaryotes have DNA in chromosomes in their nuclei. Before a cell
divides, it copies its DNA in a process called replication.
During DNA replication,
• the DNA molecule separates into two strands. Each strand of the
DNA molecule serves as a model for the new strand. This form of
replication is called semiconservative replication.
• Following the rules of base pairing, new bases are added to each strand.
For example, if the base on the original strand is adenine, thymine is
added to the newly forming strand. Likewise, cytosine is always added
to guanine.
• The end result is two identical strands.
DNA Polymerase is the primary enzyme that reads the DNA chain and
attaches the complementary chain. Sometimes, the DNA polymerase
attaches the nucleotides in long continuous chains. Other times, they are
attached in short fragments. All of these chains are put together into one
complete chain by an enzyme called DNA Ligase
1. What are genes made of?
2. What does DNA do?
3. What is DNA made of?
4. What is the difference between a nucleotide and a nucleic acid?
5. What are the parts of a nucleotide?
6. What are the four bases on DNA?
7. Who made a model of DNA?
8. What is a double helix?
9. What are the sides of the DNA ladder?
10. What holds the two sides together?
11. Which bases pair together?
12. How is the DNA different in prokaryotes and eukaryotes?
13. What happens during DNA replication?
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How is DNA replication semiconservative?
What are the steps of DNA replication?
What does DNA polymerase do?
What does DNA ligase do?
19. Sketch a picture that shows what Helicase does.
20. Sketch a picture that shows what DNA Polymerase does.
18.
12–3 RNA and Protein Synthesis
For a gene to work, the genetic instructions in the DNA molecule must
be decoded. The first step is to copy the DNA sequence into RNA. RNA
is a molecule which contains instructions for making proteins. RNA is
similar to DNA, except for three differences:
• The sugar in RNA is ribose instead of deoxyribose.
• RNA is single-stranded.
• RNA has uracil in place of thymine.
Most RNA molecules are involved in making proteins. There are three
main kinds of RNA:
• Messenger RNA has the instructions for joining amino acids to make a
protein.
• Proteins are assembled on ribosomes. Ribosomes are made up of
proteins and ribosomal RNA.
• Transfer RNA carries each amino acid to the ribosome according to
the coded message in messenger RNA. RNA is copied from DNA in a
process called transcription.
During transcription:
• The enzyme RNA polymerase binds to DNA and separates the two
DNA strands.
• RNA polymerase builds a strand of RNA using one strand of DNA
as the template.
• The DNA is transcribed into RNA following base-pairing rules except
that uracil binds to adenine. The directions for making proteins are in the
order of the four nitrogenous bases. This code is read three letters at a
time. Each codon, or group of three nucleotides, stands for an amino
acid. Some amino acids are specified by more than one codon. One
codon (AUG) is a start signal for translation. Three codons signal the end
of a protein. For example, the codon CUU codes for the amino acid
Leucine, the codon AGC codes for the amino acid Serine.
Translation is the process in which the cell uses information from
messenger RNA to make proteins. Translation takes place on
ribosomes.
• Before translation can begin, messenger RNA is transcribed from DNA.
• The messenger RNA moves into the cytoplasm and attaches to a
ribosome.
• As each codon of the messenger RNA moves through the ribosome, the
proper amino acid is brought into the ribosome by transfer RNA. Each
tRNA has an anticodon that pairs to the codon on the mRNA chain. The
ribosome joins together each amino acid. In this way, the protein chain
grows.
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• When the ribosome reaches a stop codon, it releases the newly formed
polypeptide and the process of translation is complete.
12–4 Mutations
Mutations are mistakes made when cells copy their own DNA.
Mutations are changes in the genetic material of a cell.
• Gene mutations are changes in a single gene. A point mutation occurs
at a single point in the DNA sequence of a gene. When a point mutation
causes one base to replace another, only one amino acid is affected. If a
nucleotide is added or removed, it causes a frameshift mutation. All the
groupings of codons are changed. This can cause the gene to make a
completely different protein.
• In a chromosome mutation, there is a change in the number or the
structure of chromosomes. There are four kinds of chromosomal
mutations: deletions, duplications, inversions, and translocations.
What is the first step in decoding DNA?
What is RNA for?
What are the three differences between DNA and RNA?
What are the three types of RNA and what does each do?
What is transcription?
What are the two main steps in transcription?
How does base pairing differ with RNA than DNA?
What is a codon?
What does a codon stand for?
What is the start codon?
Using the table, what amino acid would be produced from
a. CAC
b. ACU AAG GUA?
What is Translation?
What attaches to the mRNA for translation to occur?
What is an anticodon and where is it found?
What does the tRNA provide?
What is a mutation?
What is a point mutation?
What is a frameshift mutation?
What could be the result of a frameshift mutation?
In the box above, draw a diagram of a mutation occurring on DNA
Use the word bank provided the label the letters in the diagram above.
Transcription, Translation, mRNA, DNA, tRNA, Amino Acid,
Polypeptide, Ribosome, Nucleus
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Point Mutation
Frameshift Mutation
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13–2 Manipulating DNA
To increase variation, scientists can make changes directly to DNA.
Genetic engineering is the intentional changing of an organism’s DNA.
Scientist use their knowledge of the structure of DNA and its
chemical properties to study and change DNA molecules.
• Extracting DNA. Scientists can extract, or separate, DNA from the
other cell parts using a chemical procedure.
• Cutting DNA. Scientists can cut DNA into smaller pieces using
restriction enzymes. The DNA is cut to produce sticky ends, areas with
exposed nucleotides that can reattach to new DNA chains that have the
same sticky ends.
• Separating DNA. Scientists use gel electrophoresis, a method in
which DNA fragments are put at one end of a porous gel. When an
electric current is applied to the gel, DNA molecules move toward the
positive end of the gel. The shorter the length of the DNA fragment, the
faster it is able travel through the gel. The longer the DNA fragment, the
slower the DNA can travel though the gel. This technique allows
scientists to compare the gene composition of different organisms or
different individuals.
Scientists also use different techniques to read, change, and copy the
DNA sequence.
• Scientists can read the order of nucleotide bases in a DNA fragment.
They make a copy of a single strand of DNA with colored nucleotides
inserted at random places. Reading the order of colored bands in a gel
gives the nucleotide sequence of the DNA fragment.
• Scientists can change DNA sequences. Short sequences of DNA made
in the laboratory can be joined to the DNA molecule of an organism.
DNA from one organism can be attached to the DNA of another
organism. These DNA molecules are called recombinant DNA because
combining DNA from different sources makes them.
• Scientists often need many copies of a certain gene to study it. A
technique called polymerase chain reaction (PCR) allows scientists to
do that. PCR is a chain reaction in which DNA copies become templates
to make more DNA copies.
13–3 Cell Transformation
DNA fragments do not work by themselves. They must be part of the
DNA molecule in an organism. During transformation, a cell takes in
DNA from outside the cell. This external DNA becomes a component
of the cell’s DNA. Bacterial, plant, and animal cells can be transformed.
To add DNA fragments to bacteria, the fragment is placed in a plasmid.
A plasmid is a small, circular DNA molecule that occurs naturally in
some bacteria. These plasmids with added DNA fragments are
recombinant DNA. The plasmids are mixed in a solution with other
bacteria. Some of the bacteria take up the plasmids. These bacteria are
transformed.
Plant cells can be transformed in several ways.
• Some plant cells in culture can take up DNA on their own. These plant
cells have had their cell walls removed.
• Scientists can also insert a DNA fragment into a plasmid. This plasmid
is transformed into a bacterium that infects plants.
• Scientists can also inject DNA directly into some plant cells.
If transformation is successful, the recombinant DNA is integrated
into one of the chromosomes of the cell.
Animal cells can be transformed in ways similar to plant cells. An egg
cell may be large enough to inject DNA directly into its nucleus. Once
inside, the repair enzymes may help insert the DNA fragment into the
chromosomes of the egg.
13–4 Applications of Genetic Engineering
Scientists wondered if genes from one organism could work in a different
organism. Some scientists isolated the gene from fireflies that allows
them to glow. Then they inserted this gene into the DNA of plants. The
plants glowed in the dark. This showed that both plants and animals use
the same process to translate DNA into proteins. The glowing plant is
transgenic because it has a gene from another species.
Genetic engineering has spurred the growth of biotechnology, which
is changing the way we interact with the living world.
Some examples of genetic engineering include:
• Human genes have been added to bacteria. These transgenic bacteria
are used to make human proteins such as insulin, human growth
hormone, and clotting factor.
• Scientists have made transgenic animals to study the role of genes and
to improve the food supply. Transgenic animals may be used to supply us
with human proteins that can be collected in the animal’s milk.
• Transgenic plants that can make their own insecticide have been
formed. Others are resistant to weed killers. Some have been engineered
to contain vitamins needed for human health.
A clone is a member of a population of genetically identical cells that
were produced from a single cell. Clones are useful in making copies of
transgenic organisms. It is easy to produce cloned bacteria and plants.
Animals are difficult to clone.
However, in the 1990s, scientists in Scotland successfully cloned a sheep.
Animal cloning has risks. Studies suggest that cloned animals may have
genetic defects and other health problems. The use of cloning also raises
serious ethical and moral issues.
Questions:
1. What is genetic engineering?
2. What is used to cut DNA?
3. What are sticky ends on DNA for?
4. How does gel electrophoresis separate DNA?
5. What is recombinant DNA?
6. How is Polymerase Chain Reaction used by scientists?
7. What happens when a cell goes through transformation?
8. What is a plasmid?
9. How are they used?
10. What does transgenic mean?
11. Why are transgenic animals produced?
12. Why are transgenic plants produced?
13. What is a clone?