Download Chapter 10 DNA: Replication, Transcription, Translation

Chapter 10 DNA: Replication, Transcription, Translation
1. What are genes made of?
A. Mendel’s work answered the question of why you look like your parents but his work
led to questions about what genes are made of
B. Griffith’s Experiments
a. Frederick Griffith, a bacteriologist, was trying to prepare a vaccine against S.
pneumoniae
b. He injected mice with one of 2 strains of the bacteria, either the virulent strain or
the strain that did not cause disease
i. Mice who got the virulent strain died - Mice who got the second strain were
healthy - Griffith thought this was due to the protein coat that protects the
virulent strain from the body’s immune system
ii. He then injected the mice that got the second strain with the virulent strain They lived!
iii. He then heat-killed the virulent strain so that it didn’t produce its protective
coat and injected mice with this vaccine - They lived!
iv. He mixed the heat-killed strain with the non-virulent strain and injected it
into mice - They died!
v. What happened is that the non-virulent strain picked up some of the heatkilled virulent strain’s DNA and incorporated it into their own DNA - This
incorporation allowed the non-virulent strain to make the capsule, which
made the strain virulent!
1. This is called transformation
C. Avery’s experiments
a. Avery’s experiments showed that it was the DNA that allowed the S. pneumoniae
to make the protein coat that protected the virus
D. Hershey and Chase show that Virus genes are made of DNA
a. Viruses are made of DNA and sometimes RNA
b. A bacteriophage is a virus that infects bacteria
i. When this happens, the phage takes over the bacteria’s DNA so that more
viruses can be made - The viruses then break out of the cell (lyse)
ii. Hershey and Chase wanted to know if it was the DNA that was responsible
for the change, the protein coat, or both
c. Their experiment:
i. They worked with bacteriophage T2
ii. They grew T2 with E. coli that had radioactive sulfur in the agar -The
radioactive sulfur would incorporate into the virus’s protein coat
iii. They also grew T2 with E. coli that had radioactive phosphorus in the agar This phosphorus would incorporate into the viruses DNA
iv. These 2 viruses were then used to infect more E. coli - Because the viruses
were now laced with radioactivity, the scientists could see whether it was the
protein coat or the DNA that was transforming the E. coli’s DNA
v. The scientist found that the protein coats stayed outside of the cells and that
the DNA was injected into the cells
E. These important experiments showed that DNA is the molecule that stores genetic
information in living cells
2. Explain the principal function of DNA.
The primary function of DNA is to store and transmit the genetic information that tells cells
which proteins to make and when to make them.
3. Describe the structure of DNA.
A. Structure of DNA
1. Double Helix – looks like a twisted ladder with two strands twisting around each other
2. Made up of repeating subunits called nucleotides
3. Each DNA molecule consists of two long chains of nucleotides
4. A nucleotide has three parts:
a. A deoxyribose sugar molecule
b. A phosphate group
c. And a molecule called a nitrogen-containing base
5. The deoxyribose sugar and the phosphate group are identical in all DNA nucleotides
and make up the backbone of the DNA molecule
6. The nitrogen-containing base may be any one of four different kinds
a. Adenine = A - have 2 rings of carbon and nitrogen atoms - purines
b. Guanine = G - have 2 rings of carbon and nitrogen atoms - purines
c. Cytosine = C - have only 1 ring of carbon and nitrogen atoms – pyrimidines
d. Thymine = T - have only one ring of carbon and nitrogen atoms - pyrimidines
4. Scientists who worked on DNA
A. Erwin Chargaff: showed that the amount of adenine always equaled the amount of
thymine and that the amount of cytosine always equaled the amount of guanine
B. Maurice Wilkins and Rosalind Franklin: used X-ray diffraction to take pictures of DNA These pictures suggested that DNA was a double helix
C. Watson and Crick: used Chargaff’s rule and Franklin’s picture of DNA to make a 3-D
model of what DNA looked like - They came up with “spiral staircase” model
5. Define the term complementary base pairing.
A. The DNA nucleotides normally pair in combinations called complementary base pairs
which together make up the steps of the DNA molecule
1. Cytosine pairs with guanine with a triple H bond
2. Adenine pairs with thymine with a double H bond
B. Complementary base pairs are connected to each other by hydrogen bonds
C. The nucleotide sequence in one nucleotide chain of the DNA molecule is an exact
complement of the nucleotide sequence in the other chain
a. Base-pairing rule = Chargaff’s rule
6. Summarize the main features of DNA replication.
A. The complementary strands of DNA serve as templates for building new DNA - The
process of making a copy of DNA is called DNA replication
B. The first step is the separation of the two nucleotide chains
1) The point at which the two chains separate is called the replication fork
2) The chains are separated by enzymes called helicases
3) As the helicase enzymes move along the DNA molecule, they break hydrogen
bonds between the complementary bases, and the chains separate
4) Enzymes called DNA polymerases bind to the separated chains of DNA and each
chain serves as a template for a new nucleotide chain
5) As DNA polymerases move along the separated chains, new chains of DNA are
assembled using nucleotides in the surrounding medium that are complementary to
the existing DNA chains
6) Nucleotides are joined to the new chains by covalent bonds between deoxyribose
sugars and phosphate groups
7) They are joined to the original nucleotide chain by hydrogen bonds
C. The complementary nature of the two chains of DNA is the foundation for accurate
DNA replication
D. DNA replication does not begin at one end of the molecule and proceed to the other
1) Replication occurring simultaneously at different sites permits faster DNA
replication
2) Example: Bacterial DNA usually has 2 because it is a circle
3) Example 2: Human DNA is replicated in about 100 sections that contain 100,000
nucleotides - This means an entire chromosome can be copied in about 8 hours
E. Checking for Errors
1) DNA replication occurs with a high degree of accuracy – about 1 error in every
10,000 paired nucleotides
2) DNA is proofread in case there are any errors in the code - This is also done by
DNA polymerase
3) If there is an error, DNA polymerase backtracks, removes the incorrect nucleotide,
and replaces it with the correct one
4) A change in a nucleotide sequence can result in a mutation
a. Mutation – a change in DNA
b. Some mutations occur – usually due to outside agents, such as UV light,
chemicals, etc.
F. Mutations can result in Nonfunctional protein
1) Mutations in the body cells affect only the person in which they occur
2) Mutations in gametes will affect the offspring and possibly be passed onto their
children
3) Mutations that change one or just a few nucleotides in a gene on a chromosome are
called point mutations
a. Substitution: one nucleotide is replaced with a different nucleotide
 This can cause a different protein to be made, not enough of a protein
being made, no protein being made, or nothing happens at all.
b. Insertion or deletion: one more nucleotide is added or deleted from a gene
 This can cause the gene to be misread (frame shift mutation).
G. When replication is completed, two new exact copies of the original DNA molecule
are produced
a. Each new strand contains one of the original DNA’s nucleotide chains and
one new complementary strand – semi-conservative replication
7. Explain the primary functions of RNA.
A. Proteins are made by decoding the information in DNA
B. A genes instructions for making proteins are coded in the sequence of
nucleotides in the gene
C. Ribonucleic acid (RNA) is responsible for the movement of genetic
information from the DNA in the nucleus to the site of protein
synthesis in the cytosol
8. Compare the structure of RNA with that of DNA.
A. DNA is a double stranded helix with the nitrogen bases adenine, thymine, guanine,
and cytosine and deoxyribose as the sugar – found only in the nucleus of a nondividing eukaryotic cell
B. RNA is single stranded with uracil instead of thymine and ribose as the sugar – can be
found in the nucleus and cytoplasm
9. Describe the structure and function of each type of RNA.
A. Three types
1) Messenger RNA (mRNA) consists of RNA nucleotides in the form of a single
uncoiled chain
a) mRNA carries genetic information from the DNA in the nucleus to the cytosol
of a eukaryotic cell
2) Transfer RNA (tRNA) consists of a single chain of about 80 RNA nucleotides
folded into a hairpin shape that binds to specific amino acids
a) There are about 45 varieties of tRNA involved in protein synthesis
3) Ribosomal RNA (rRNA) is the most abundant form of RNA
a) rRNA consists of RNA nucleotides in a globular form
b) Joined by proteins, rRNA makes up the ribosomes where proteins are made
10. Summarize the process of transcription.
A. The instructions for making a protein are transferred from a gene to the RNA
molecule during the process of transcription
1) Transcription: the process by which genetic information is copied from DNA
to RNA
2) The entire process by which proteins are made based on the information stored
in DNA is called gene expression
B. Steps of Transcription
1) RNA polymerase, the primary transcription enzyme, synthesizes RNA copies of
specific sequences of DNA
a) RNA polymerase initiates RNA transcription by binding to specific regions
of DNA called promoters which mark the beginning of the DNA chain that
will be transcribed
b) Only one of the separated DNA chains, called the template, is used for
transcription
2) RNA polymerase attaches to the first DNA nucleotide of the template chain
a) Then it begins adding complementary RNA nucleotides to the newly
forming RNA molecule
b) Complementary base pairing determines the nucleotide sequence of the
RNA chain in transcription, just as it does in DNA replication, except that
uracil pairs with adenine
3) Transcription continues one nucleotide at a time until the RNA polymerase
reaches a DNA region called the termination signal
a) The termination signal is a specific sequence of nucleotides that marks the
end of a gene
b) At the termination signal, RNA polymerase releases both the DNA molecule
and the newly formed RNA molecule
4) All three types of RNA molecules are transcribed in this process
5) Following transcription, mRNA moves through the pores of the nuclear
membrane into the cytosol of the eukaryotic cell
a) Transcription in prokaryotes happens in the cytoplasm
11. Summarize the process of translation.
A. The process of assembling polypeptides from information encoded in mRNA is called
translation
1) The process of translation begins when mRNA leaves the nucleus through pores in
the nuclear membrane
2) The mRNA then migrates to a ribosome in the cytosol, the site of protein synthesis
B. The Genetic Code
1) The genetic code is written in 3-nucleotide "words"
a. Messenger RNA is a form of RNA that carries instructions for making proteins
from a gene and delivers it to the site of translation
b. RNA instructions are written in 3-nucleotide sequences called codons - There
are 64
o Anticodon – the opposite sequence of a codon
c. Each codon on the mRNA corresponds to a specific amino acid, or signifies a
start or stop signal for translation
o Start codon – AUG = methionine
o Stop codons – UAA, UAG, UGA
d. Genetic code = the amino acids and start and stop signals that are coded for by
each of the possible 64 mRNA codons
B. tRNA and Anticodons
1) Amino acids floating freely in the cytosol are transported to the ribosomes by
tRNA molecules
2) The loop opposite the site of amino-acid attachment bears a sequence of three
nucleotides called an anticodon
a) The tRNA anticodon is complementary to and pairs with its corresponding
mRNA codon
b) Thus, the pairing of an anticodon with a codon ensures that the amino acids are
added to the growing polypeptide in the order prescribed by the mRNA
transcript
C. Ribosomes
1) Ribosomes are composed of two rRNA subunits (pieces) and proteins and are
usually both free in the cytosol or attached to the endoplasmic reticulum
2) Ribosomes have three binding sites that are key to translation
a) One binding site holds a mRNA transcript so that its codons are accessible to
rRNA molecules = A site
b) The other two binding sites hold tRNAs whose anticodons pair with the mRNA
codons = P site
D. Protein Assembly
1) The assembly of a polypeptide begins when a ribosome attaches to the start codon
(AUG) on a mRNA transcript
a) The start codon pairs with the anticodon UAC on a tRNA
b) Because the tRNA that bears the UAC anticodon also carries the amino acid
methionine, the first amino acid in every polypeptide is initially methionine,
however, it may be removed later
2) As a ribosome moves along a mRNA transcript, each mRNA codon is sequentially
paired with its tRNA anticodon
a) The pairing of an anticodon with a codon causes the specified amino acid to
attach to the previously translated amino acid with a covalent bond called a
peptide bond
b) In this way, amino acids are joined to a growing polypeptide chain in the order
specified by a mRNA transcript
c) As each amino acid is added to the polypeptide chain, the ribosome moves three
nucleotides - one codon - ahead on the mRNA transcript, where the next amino
acid will be translated
3) Eventually, the ribosome reaches a stop codon, bringing translation to an end
a) At this point the mRNA is released from the ribosome and the polypeptide is
complete
4) Several ribosomes may simultaneously translate the same mRNA transcript
5) As the polypeptide folds and associates with other polypeptides that make up the
protein, it assumes the functional structure of the completed protein
F. The genetic code is common throughout most species - It is thought
that there is a common ancestor of all organisms that had a single genetic code
Chapter 10 Nucleic Acid and Protein Synthesis
 DNA stores the information that tells cells which proteins to make and when to make them.
 DNA is made up of two chains of nucleotides.
 A DNA nucleotide is composed of a deoxyribose sugar molecule, a phosphate group, and a
nitrogen-containing base. The four nitrogen-containing bases found in DNA nucleotides are
adenine (A), guanine (G), cytosine (C), and thymine (T).
 Adenine and guanine are called purines. Cytosine and thymine are pyrimidines.
 Cytosine pairs with guanine, and adenine pairs with thymine. Complementary base pairs are
connected to each other by hydrogen bonds.
 Before a cell divides, it copies its DNA by a process called replication. Replication results in
two exact copies of the cell’s DNA.
 Replication begins with the separation of the DNA chains by helicase enzymes. Then as
DNA polymerases move along the separated chains, new chains of DNA are assembled
using nucleotides in the surrounding medium.
 Nucleotides in DNA are grouped into genes, which contain the information required for the
production of specific proteins.
 Three forms of RNA are involved in protein synthesis: mRNA, tRNA, and rRNA.
 mRNA carries genetic information from the DNA in the nucleus to the cytosol of a
eukaryotic cell. The process of copying genetic information from DNA to mRNA is called
transcription.
 tRNA binds to specific amino acids, helping to form polypeptide chains.
 rRNA makes up the ribosomes where proteins are made.
 The production of proteins is also called protein synthesis.
 The genetic code shown in Table 10-1 is used by most organisms to translate mRNA
transcripts into proteins.
 The genetic information necessary for making proteins is encoded in mRNA codons. Each
codon codes for a specific amino acid.
 The process of assembling polypeptides from information encoded in mRNA is called
translation. During translation, tRNA anticodons pair with corresponding mRNA codons,
and amino acids are joined together to form a polypeptide.
 As a polypeptide folds and associates with other polypeptides, it assumes the functional
structure of a completed protein.
Vocabulary List
Anticodon
Base-pairing rule
Codon
Complementary base pair
Deoxyribose
DNA polymerase
Double helix
Genetic code
Helicase
Messenger RNA (mRNA)
Mutation
Nitrogen-containing base
Promoter
Protein synthesis
Purine
Pyrimidine
Replication
Replication fork
Ribose
Ribosomal RNA (rRNA)
RNA polymerase
Start codon
Stop codon
Termination signal
Transcription
Transfer RNA (tRNA)
Translation