Download DNA

Homework
Read Section 17.1, pages 568­572.
Complete Questions #1­4 on page 572.
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Homework
1)
What is the relationship between nuclein and a chromosome ?
Nuclein was the term Friedrich Miescher gave to a substance he
isolated from white blood cell nuclei. He found that it was composed
of both nucleic acid and protein.
A chromosome is formed when a strand of DNA joins in a complex with proteins, called histones, which provide a base for the DNA
to coil on. So, a chromosome is a complex strand formed by a combination of the two components which make up nuclein*.
*The term nuclein is no longer used; the combination of DNA and
protein which make up a chromosone is now called chromatin .
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Homework
2)
Identify the five different nucleotides. Which one is found only
in RNA?
The five nucleotides are Adenine, Thymine, Cytosine, Guanine,
and Uracil. The first four are found in DNA. Uracil replaces
Thymine in RNA.
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Homework
3)
Draw a general structure of a DNA nucleotide, and label
each of its components.
Phosphate group
P
O
Pentose (5­carbon) sugar
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S
Nitrogenous base
B
Homework
4)
Define Chargaff's rule, and explain its significance.
Chargaff's Rule: Adenine and Thymine always occur in the same ratio within a DNA molecule, and so do Guanine and Cytosine.
This is because Adenine always forms a link
with Thymine in the DNA double helix, and Guanine links with Cytosine, so there are
equal numbers of each base in any DNA
molecule.
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Homework
The diagram below illustrates the structure of a chain of nucleotides.
Copy this diagram and label each of the components.
(b)
Prepare a short caption that explains how the chain of nucleotides is held together.
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6)
Individual nucleotides are held together by bonds between the sugar of one nucleotide and the phosphate group of the next nucleotide. The nitrogenous bases
stick out like the teeth on a zipper.
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Structure of a Nucleotide
P
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T
The 3' (said "three prime") carbon of one nucleotide attaches to the phosphate of the next nucleotide.
P
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S
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The carbons in each pentose sugar are numbered beginning with the carbon attaches to the nitrogenous base.
G
The 5' sugar is attached to the phosphate of the same nucleotide.
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Structure of a Nucleotide
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Adenine and guanine are known as purines. They are nitrogen­
containing compounds composed of a double ring structure.
G
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S
S
Cytosine and thymine are known as pyrimidines. They are nitrogen­
containing compounds composed of a single ring structure.
P
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C
T
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U
Uracil is also a pyrimidine, similar in structure to thymine
Structure of a DNA
When forming a double helix, the nucleotide Adenine will join with Thymine. Guanine will join with Cytosine. These are called complementary base pairings. The structure of the bases determines which bases are complementary.
Since Thymine and Adenine both form a double bond, they are able to link across the middle of the DNA molecule.
Guanine and Cytosine form a triple bond, and will link together.
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The sugars and phosphate groups form the sides of the double helix, while the bases form the "rungs".
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Each individual bond between the bases is in reality very weak. However, despite the weak bonds, DNA itself is an extremely strong molecule, and does not come apart easily.
Can you explain this?
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The two strands which make up the DNA molecule, aside from being complementary, are also antiparallel.
This means that the molecules are oriented in opposite directions...one strand begins with a phosphate and a 5' carbon, while the complementary strand begins at the same end with a 3' carbon.
Structure of a DNA
What rule is a result of the complementary pairings of adenine/thymine and cytosine/guanine?
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Thinking Lab Page 575
Read and complete the thinking lab questions.
Adenine
Cytosine
Thymine
Guanine
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31
18
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Homework
Read pages 573­578, taking notes on these pages.
Stop at the section titled "Genes and the Genome"
In your own words, describe the meaning of the following terms:
Nucleoid; plasmid; nucleosome; histone; chromatin
Complete Questions #2­6 on page 581
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Homework
2)
Describe the evidence that led Watson and Crick to their particular model of DNA.
The shadow patterns on X­ray images of DNA taken by Rosalind Franklin, and Erwin Chargaff's discovery that Adenine ­ Thymine and Cytosine ­ Guanine occur in equal ratios led to their model. They also worked with physical models of various shapes, and found that the double helix was the only one which could account for all of the experimental evidence.
Title: Oct 16­9:43 AM (19 of 115)
Homework
3)
The DNA molecule is a double helix, resembling a twisted ladder.
Describe the component molecules that make up the ladder
uprights, and the pattern of their arrangment.
The sugar­phosphate backbone of the DNA strand forms the uprights, and the nitrogenous bases form the rungs.
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5'
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3'
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Homework
4)
Create a chart that compares and contrasts the similarities and
differences in the structure of DNA and RNA.
DNA
Similarities
Differences
Polymer of repeating nucleotide units
Polymer of repeating nucleotide units
Contains the 5­carbon sugar deoxyribose
Contains the 5­carbon sugar ribose
Contains the bases adenine, Contains the bases adenine, thymine, cytosine, and uracil, cytosine, and guanine
guanine
Forms a double­helix (twisted ladder) shape
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RNA
Forms a single­helix (one strand, which may curve back on itself
Homework
5)
Describe the organization of genetic material in prokaryotic
organisms.
In prokaryotes, the DNA forms a single, circular molecule bound to protein. Since prokaryotes do not have a nucleus, the DNA is not separated from the rest of the cell contents within a membrane. Instead, it is located in a region called the nucleoid. The proteins which bind to DNA help it coil and remain in this region.
Prokaryotes may also contain plasmids, small circular DNA molecules floating freely in the cytoplasm. Although not considered a part of the prokaryote's genome, the information found in the plasmids is functional. Plasmids can be copied and transferred between living cells, as well as passed on during cell division. They may also join with the cell's nucleoid DNA, and become incorporated as part of the genome.
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Homework
6)
Describe the organization of DNA in eukaryotic organisms.
What is the reason for the compact nature of chromosomes?
In eukaryotes, including humans, DNA exists as distinct chromosomes, each composed of a single strand of DNA combined with proteins (histones). Together, the DNA and proteins are called chromatin.
The DNA molecule wraps tightly around groups of eight histone proteins, creating small bead­like structures called nucleosomes. These are held together by the attraction between the acidic portion (DNA) and alkaline portion (histone) of the chromatin.
A short segment of DNA between each nucleosome is attached to another histone, called an H1 molecule. This provides a flexible region, allowing the nucleosomes to wind around each other further, forming a compact, tightly folded structure that we see as chromosomes during cell division.
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Homework
Loops
H1 molecule
DNA molecule
Histone complex
Condensed chromosome
Supporting protein scaffold
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In Class Work
Review pages 573­578, and on your own complete
Questions #9­12 on page 581
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In Class Work
9.
Explain what is meant by the phrase "complementary base pairing."
What are the complementary base pairs in DNA?
Base pairing means that each nitrogenous base forms hydrogen bonds with another specific base, forming the rungs of the DNA molecule. Each base will only form bonds with its complement ; Adenine with Thymine and Cytosine with Guanine.
A set of complementary base pairs will always include a double­
ringed purine (adenine or guanine) and a single­ringed pyrimidine (thymine or cytosine).
Title: Oct 16­9:43 AM (27 of 115)
In Class Work
10. Explain what is meant by the phrase "the two strands of a DNA
molecule are antiparallel".
Each of the strands which form a DNA molecule have a 5'
end and a 3' end. The 5' end of one strand lines up with the
3' end of the other strand.
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In Class Work
11. One strand of a DNA molecule contains the following sequence of nucleotides:
5'­­AGTTGCA­­3'
How would the sequence of its complementary strand be written, according to convention?
5'­­AGTTGCA­­3'
3'­­TCAACGT­­5'
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In Class Work
12. The best DNA extractions come from cells that are dividing
rapidly. If you were planning to extract DNA from cells other
than fish muscle cells (which is described in Investigation 17­A
on pages 576­577), what cells might you use?
Explain your answer.
Some suggestions: cells at the ends of growing roots; cells around a healing injury;
skin cells; embryonic cells; bacterial cells
Embryonic cells would be particularly good, since as they divide by cleavage, the total cell volume decreases. This means the nucleus would be a larger part of the total
cell volume, making it easier to extract the DNA.
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Genes and the Genome
What is meant by an organism's genome?
Genome:
The total of all hereditary DNA carried in the organism's cells.*
*Why are plasmids not considered part of a prokaryote's genome?
*Because they are often not the result of inheritance, but rather are passed between living cells.
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Genes and the Genome
What is meant a gene?
Gene:
A segment of DNA molecule which controls the expression of
a trait, usually by coding for the production of a protein.
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Introns and Exons
Not all of the DNA making up the genome serve to control traits, particularly in complex organisms. Much of the DNA strand does not actually code for protein production. These sections do serve useful functions; for example, non­coding segments of DNA are found in the centromere, and play a role in attachment of the spindle during cell division.
In bacterial cells, almost all of the DNA serves to code for expression of traits. In humans, however, many genes contain one or more stretches of non­coding DNA. These segments are called introns. The coding segments of a gene are called exons, since their code is expressed. Introns may make up over half of the total length of DNA in a human gene.
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Introns and Exons
Introns
Exons
The number of introns in a typical gene seems to be related to organism complexity. Only about 5% of yeast genes contain introns, while vertebrates have introns in about 95% of their genes. More complex organisms also tend to have more introns per gene; a single human gene may contain dozens of introns.
*Note that a gene always starts and ends with an exon
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Other Non­coding DNA
In addition to introns, the genome may contain other segments of non­coding DNA. These can include:
*Repetitive sequences , in which the same set of nucleotides are
repeated anywhere from hundreds to thousands of times
*Pseudogenes , which is a non­functional copy of a functional gene
from another location on the genome.
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Homework
Read pages 578­582
• Take notes on these pages.
• Explain what is meant by the term "semiconservative"
as it relates to DNA replication. Include a diagram.
• Complete Questions #8 and 13­14 on page 581
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Homework
8.
P
Make a sketch showing the structure of a
double­stranded DNA molecule.
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Homework
8.
Make a sketch showing the structure
of a double­stranded DNA molecule.
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Homework
9.
Explain what is meant by the phrase "complementary base pairing".
What are the complementary base pairs in DNA?
Base pairing means that each nitrogenous base forms hydrogen bonds with another base pair, forming the rungs of the DNA molecule. Each base will only form bonds with its complement ; Adenine with Thymine and Cytosine with Uracil.
A set of complementary base pairs will always include a double­
ringed purine (adenine or guanine) and a single­ringed pyrimidine (thymine or cytosine).
Title: Oct 16­9:43 AM (39 of 115)
Homework
10. Explain what is meant by the phrase "the two strands of a
DNA molecule are anitparallel".
When a strand of DNA is forming, the phosphate group that is on the 5' carbon of the sugar on one nucleotide forms a chemical bond with the 3' carbon on the next nucleotide. So, each side of a DNA has a 5' carbon at one end and a 3' carbon at the other end.
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5'
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The two sides forming the double helix run in opposite directions, so at the end of a double­stranded DNA molecule, one sie begins with the phosphate attached to a 5' carbon, the other begins with 3'
a 3' carbon.
Homework
11. One strand of a DNA molecule contains the following sequence
of nucleotides:
5'­­AGTTGCA­­3'
How would the sequence of its complementary strand be written,
according to convention?
5'­­AGTTGCA­­3'
3'­­TCAACGT­­5'
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Homework
12. The best DNA extractions come from cells that are dividing rapidly.
If you were planning to extract DNA from cells other than fish
muscle cells (similar to what is described in Investigation 17­A on page 576) what cells might you use? Explain your answer.
Embryonic cells would provide a good source of DNA, since the cells are growing rapidly. Also, as the embryo divides, the cells get smaller (cleavage) so the nucleus and DNA would be a large part of the cell volume.
Other examples might be skin cells, root tip cells, or bacterial cells.
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Homework
13. Discuss how the definition of a gene has changed since Mendel conducted his experiments. What evidence contributed to this
change?
Originally, the definition for a gene was a portion of inherited information which controlled expression of a trait. With the discovery of the DNA molecule as the material of heredity, the definition changed. And, as our understanding of how DNA controls expression of characteristics has been modified and expanded, the definition has changed. A current definition of a gene might be a section of DNA which codes for the production of a polypeptide, although even this definition is limited, and is subject to further refinement as research continues.
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Homework
14. If a gene has three introns, how many exons does it contain?
Draw a labelled diagram showing how the introns and exons
are arranged along a stretch of DNA.
If a gene has three introns, it must have four exons, since the gene will always start and stop with an exon.
exons
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introns
DNA Animation
http://207.207.4.198/pub/flash/24/menu.swf
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Section 17.3 ­ DNA Replication
Return to Contents
Title: 17.3 (46 of 115)
DNA Replication
During replication, a single molecule of DNA creates two new molecules. The replication is semi­conservative , in that each new molecule consists of one new strand of nucleotides, and one original strand of nucleotides.
new DNA
original DNA
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DNA Replication
There are three major stages to DNA replication.
Initiation:
A part of the double helix unwinds, and the two
strands separate, preparing to be copied
Elongation:
Two new strands of DNA are assembled, each using
one of the original strands as a template
Termination: Replication is complete, and the two new strands
return to the double­helix shape
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DNA Replication
Although each stage is distinct, all three may be occurring on the same DNA strand simultaneously .
Initiation
An enzyme called a helicase recognizes a sequence of nucleotides, and begins to unravel the helix, separating the two strands. An enzyme called a polymerase then attaches to one of the strands, and begins adding the complementary nucleotides. A replication bubble is the area where the two strands are separated. The replication fork is the region where the two strands meet.
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DNA Replication
There may be many replication forks at any one time. In other words, the DNA doesn't simply get copied end to end, but in many smaller "chunks".
replication origins
replication bubble
replication forks
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DNA Replication
Elongation
The polymerase needs the presence of an RNA primer to begin adding nucleotides. The polymerase attaches to the primer, then begins reading the DNA code, and adds the complementary nucleotides to the strand as it progresses.
DNA polymerase
RNA primer
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DNA Replication
The 5' end of new nucelotides can only be added to the 3' end of the new strand.
In other words, elongation occurs in the 3' to 5' direction on the original strand of DNA, and in the 5' to 3' direction on the new strand.
DNA polymerase
5'
RNA primer
3'
3'
new nucleotide strand
5'
3'
3'
RNA primer
5'
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5'
DNA Replication
Elongation
Because elongation occurs on both strands at the same time, and in the same direction, this creates a problem. The DNA has to be copied as it unzips.
DNA polymerase
RNA primer
3'
new nucleotide strand
3'
Direction of elongation
5'
?
RNA primer
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5'
DNA Replication
The solution is to copy the opposite strand in small sections, called Okazaki fragments . A helicase will create an RNA primer a short way down the strand, and polymerase will attach the 5' end of a new complementary nucleotide to the 3' end of an existing nucleotide until reaching the previously copied section.
DNA polymerase
RNA primer
3'
new nucleotide strand
5'
Direction of elongation
5'
3'
RNA primer
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DNA Replication
As the DNA continues to unwind and the strands separate, one strand within a replication fork uses only a single DNA polymerase. The opposite strand uses several, one for each Okazaki fragment.
DNA polymerase
5'
3'
Direction of elongation
5'
3'
RNA primer
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DNA Replication
When the polymerase reaches the end of the Okazaki fragement, a new polymerase attaches, removing the RNA primer, and adding in the remaining complementary nucleotides.
DNA polymerase
5'
3'
5'
RNA primer
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3'
DNA Replication
Finally, a DNA ligase connects the nucleotide at the end of the Okazaki fragment to the nucleotide at the beginning of the next segment. Farther along the strand, helicase has created a new RNA primer, and a polymerase has begun attaching the next section of complementary nucleotides.
DNA polymerase
5'
3'
5'
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ligase
3'
DNA Replication
Eventually, the entire DNA strand has been copied. 3'
5'
3'
5'
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5'
3'
5'
3'
DNA Replication
Animation 1
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Homework
Read pages 582­587 in Section 17.3
Complete Questions #2­4 on page 588.
Question 1 should be completed in detail!
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Homework
2.
Explain the role of the following enzymes in DNA replication:
(a) Helicase:
Unwinds segments of DNA, and separates
the strands
(b) DNA Ploymerase: Adds nucleotides to the 3' end of existing
nucleotides. Also, replaces RNA primers with
segments of DNA
(c) DNA Ligase:
Attaches two existing nucleotides ona DNA
molecule after the RNA primer is removed
(d) DNA Primase: Creates and attaches an RNA primer on an
existing strand of DNA, to act as a starting point for polymerase to add nucleotides
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Homework
3.
What is the purpose of the Okazaki fragments?
What happens to them during replication?
The Okazaki fragments allow the lagging strand to keep up with the leading strand during DNA replication. Because polymerase can only add nucleotides to the 3' end of an existing nucleotide, the lagging strand cannot be built continuously. Instead, it is built in short segments, called Okazaki fragments. An RNA primer is attached to the lagging strand near the replication fork; polymerase builds the new strand back from the primer, until it reaches the previous primer. Another polymerase then removes the primer, and ligase attaches the final two DNA nucleotides.
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Homework
4.
Explain how replication errors are corrected.
DNA polymerase is able to detect whether hydrogen bonding has occurred after adding a nucleotide; if the interstrand bond does not form, polymerase removes the nucleotide, and adds a different one. Since hydrogen bonding only occurs between complementary nucleotides, this prevents errors in copying. Some errors do occur, but this double­checking process during polymerase action reduces copying errors to a rate of about 1/1 000 000 000
, or about one in a billion.
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DNA Replication ­ Simulation
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DNA Replication ­ Simulation
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DNA Replication ­ Simulation
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DNA Replication ­ Simulation
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Termination
Once the new DNA strands are complete, the double helix rewinds. This does not require the action of enzymes, since the molecule will automatically return to the most stable shape.
However, the method of replication does create one further difficulty.
At one end of each original DNA strand, there will be an RNA primer which cannot be removed and replaced with DNA. This is because, as emphasized earlier, polymerase can only add nucleotides the the 3' end of an existing nucleotide.
Title: Oct 16­9:43 AM (72 of 115)
Termination
At the 5' each strand, there will be an initial RNA primer. Once this primer is removed, there is no existing 3' end beyond it for a polymerase to use for attaching new nucleotides.
3'
5'
3'
5'
Title: Oct 16­9:43 AM (73 of 115)
5'
3'
5'
3'
Termination
These gaps at each end cannot be filled, and the complementary 3' ends eventually break off.
3'
5'
3'
5'
3'
5'
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5'
3'
Termination
As a result, the DNA molecule becomes progressively shorter during each replication.
3'
5'
3'
5'
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5'
3'
5'
3'
Termination
In humans, the DNA molecule loses approximately 100 base pairs from the 3' end of each original strand with every replication. Even with the several billion base pairs of DNA contained within the human genome, eventually this would result in crucial genetic information being lost.
This problem is solved by the telomeres at the end of each chromosome. These are areas of repeating, non­coding DNA.
In human cells, the sequence TTAGGG may be repeated several thousand times.
There is also an enzyme (called telomerase) , which acts to lengthen telomeres, further protecting the coding segments of DNA within the chromosomes.
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Title: Jan 13­2:58 PM (77 of 115)
Termination
Why wouldn't this loss of DNA at the ends of each chromosome be a problem for bacterial cells?
A bacterial cell's DNA is a circular molecule, with no ends, so they do not end up losing segments of DNA during replication.
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Homework
Review the material in Section 17.3 (pages 582­588)
and complete Questions #5, 8, 9, and 11 on pages 601­602
Title: Oct 16­9:43 AM (79 of 115)
Homework
5.
What is the base sequence of the DNA strand that is complementary
to a strand with the sequence:
A C G T T G C T A
T G C A A C G A T
Title: Oct 16­9:43 AM (80 of 115)
Homework
8.
What is the function of primase in DNA replication?
Primase builds the RNA primer which provides a starting point
for the addition of complementary nucleotides by the enzyme
polymerase.
Title: Oct 16­9:43 AM (81 of 115)
Homework
9.
Briefly compare the roles played by the following:
(a) Leading strand and lagging strand
(b) Pyrimidine and purine
(c) DNA polymerase and DNA ligase
Title: Oct 16­9:43 AM (82 of 115)
Homework
11. Fill in the following table to show the molecules and enzymes
involved in DNA replication:
Function
Molecule or Enzyme
Primer
DNA Ligase
DNA Polymerase
Okazaki fragments
Helicase
Title: Oct 16­9:43 AM (83 of 115)
Involved in leading strand / lagging strand synthesis, or both
DNA Replication
Animation 2
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Section 17.4 ­ Protein Synthesis
Return to Contents
Title: 17.4 (85 of 115)
Homework
Read pages 589­590 in your text.
Complete Question #1 on page 600.
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The Genetic Code
Cells store information about how they operate in DNA, using the genetic code. The code is made up of the base pairs on the nucleotides of the DNA molecule.
What information does the genetic code actually store? *Instructions on how to use a sequence of amino acids to create a protien molecule
What are some of the uses of proteins in cells?
*structural elements (such as keratin)
*transport (hemoglobin)
*information transfer (protein hormones)
*perhaps most importantly...ENZYMES
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Role of Enzymes
Enzymes are used to catalyze chemical reactions in cells.
This could involve different roles, depending on the reaction:
*initiating a reaction
*increasing a reaction rate
*decreasing a reaction rate
*stopping or inhibiting a reaction
By controlling almost all of the chemical reactions that occur in the cells, enzymes control virtually all cell (and therefor body) functions...
...and since DNA controls the production of proteins, DNA is really in ultimate control of all the organism's systems.
Title: Oct 16­9:43 AM (88 of 115)
Gene Expression
Like any code, the genetic code stored in DNA is only useful if it can be interpreted. DNA itself does not create proteins...instead, the genetic code is read and used to instruct other parts of the cell in the creation of the necessary proteins. DNA is used to create a strand of RNA, which is then read by a ribosome. The ribosome strings together a polymer of amino acids based on the information contained on the RNA, creating a protein.
Title: Oct 16­9:43 AM (89 of 115)
Gene Expression
There are two main phases of protein synthesis:
*Transcription
This involves making an exact copy of the genetic
information contained on a single gene...the copy
is in the form of a strand of RNA
*Translation
This stage involves reading the RNA strand, and using the information to create a different
molecule...going from a nucleic acid to a protein
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Reading the Genetic Code
Proteins are polymers of amino acids. The number and order of the amino acids determines the protein. Each protein will have a characteristic shape, due to the arrangement of amino acids.
There are 20 amino acids, used to make all known proteins.
However, there are only 4 bases used in the DNA code (there are actually five bases in total, but in RNA the base thymine is replaced with the base uracil, so that there are only 4 bases on any individual nucleic acid.)
So, how is it possible for only 4 bases to code for 20 amino acids?
Title: Oct 16­9:43 AM (91 of 115)
Reading the Genetic Code
The bases that make up the genetic code are not read individually, but rather are arranged into CODONS. Each codon is a sequence of three bases, resulting in a total of 64 possibilities .
When creating a protein, the RNA will interpret the codons using a reading frame , just a sequence of three bases. Each codon indicates one of the 20 amino acids; however, since there are 64 possible codons, most amino acids are indicated by more than a single codon.
Title: Oct 16­9:43 AM (92 of 115)
Reading the Genetic Code
During transcription, it is essential that the ribosome know exactly where to begin reading the code. Consider the sequence:
AACACTTAGCTACGTCCTAGCCTC
Beginning at the start, this sequence would be broken into the codons:
AAC ACT TAG CTA CGT CCT AGC CTC
However, if you begin a single base from the start, the sequence changes to:
A ACA CTT AGC TAC GTC CTA GCC TC
This series of codons would produce a completely different set of amino acids.
Title: Oct 16­9:43 AM (93 of 115)
Reading the Genetic Code
To overcome this problem, a specific codon, called a start codon, is used to tell the ribosome where to begin reading.
This codon is the sequence AUG, and indicates the amino acid methionine.
In the same way, there are three codons called stop codons, which tell the ribosome that it has reached the end of the amino acid chain required for that protein. When an RNA reaches a stop codon, it releases the RNA, and the protein is complete.
The stop codons are UAA, UAG, and UGA.
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Table of Codons
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Homework
Page 600, #1
a) What amino acids are coded for by each of the following codons?
(i) UUC
(ii) ACU
(iii) GCG
(iv) UAA
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Homework
Page 600, #1
b) What codons could code for the amino acid serine?
The amino acid aspartate ?
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Homework
Page 600, #1
c)
Write all the possible codon sequences that code for the polypeptide sequence:
serine­methionine­glutamate
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Translation Practice
Use the table on page 590 of your text to write the polypeptide sequence indicated by the following segment of an RNA strand. Be sure to remember the Start and Stop codons!
CGUCGAAGAUGUCAGCUACCUGGCAGCCAACAUUUCGGUAACGCCUGGUA
CGUCGAAG AUG UCA GCU ACC UGG CAG CCA ACA UUU CGG UAA CGCCUGGUA
Start ­ Ser ­ Ala ­ Thr ­ Tryp ­ Glu ­ Pro ­ Thr ­ Phen ­ Arg ­ Stop
(Meth)
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Homework
Read pages 591­596 in your text, stopping at the section titled "Mutations".
Take notes on these pages.
Complete Questions #3­8 on page 600.
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Homework
Questions #9­12 on page 600, and 5 & 14 on page 601­602
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Stages in Protein Synthesis
Protein synthesis, like DNA replication, can be broken into stages.
We have already looked at the two major stages. What is involved in each?
Transcription:
Translation:
Title: Oct 16­9:43 AM (102 of 115)
Transcription
During transcription, the length of DNA which makes up a gene is copied onto a specific type of RNA called messenger RNA, or mRNA. In eukaryotic cells, transcription occurs inside the nucleus.
The enzyme responsible for translation is RNA polymerase. The RNA polymerase molecule will attach to the DNA at a specific location, open a section of the double helix, and begin adding RNA nucleotides. This is elongation , similar to what occurs during DNA synthesis.
The RNA polymerase will add nucleotides in the 3' to 5' direction (adding each nucleotide onto the 3' end of the previous nucleotide. As the RNA polymerase continues, the double helix reforms behind it.
Title: Oct 16­9:43 AM (103 of 115)
Transcription
Once the mRNA has been completely copied, it separates from the RNA polymerase and the DNA. The double helix completely reforms.
In prokaryotes, the mRNA is now ready to begin translation.
However, in eukaryotic cells, another step has to occur first: mRNA processing.
This step involves a processing enzyme which adds code to each end of the mRNA, and removes any non­coding sequences from the strand.
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Transcription
Why is it NOT necessary for this processing to occur in prokaryotic cells?
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Translation
There are several elements involved in translation to create a protein strand.
In addition to the mRNA created during transcription, the process requires a ribosome containing rRNA, and several molecules of tRNA.
Ribosome:
a ribosome is composed of proteins bound together with a
molecule of ribosomal RNA, or rRNA. Each ribosome has
two subunits (a large and a small) which fit together to form a single ribosome. The ribosome is responsible for reading
the mRNA strand, and adding the appropriate amino acids.
Transfer RNA, or tRNA:
tRNA molecules are designed to carry and link
amino acid units onto the growing polypeptide by binding to
the appropriate codon on an mRNA. tRNA have a distinct
cloverleaf shape. The "stem" binds to and carries a specific
amino acid, while the middle "leaf" displays the nucleotide sequence which is complementary to the codon for that amino
acid. This sequence is called the anticodon.
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Translation
Translation begins when an mRNA binds to an active ribosome. The ribosome begins reading the mRNA code until it reaches the start codon (AUG).
CCG AUG GGC ACC AAA GUC CAU Title: Oct 16­9:43 AM (107 of 115)
Translation
The start codon and the next in line are exposed in two active regions on the ribosome. The first amino acid (methionine) is then added. From here, translation follows a three­step sequence.
Met
CCG AUG GGC ACC AAA GUC CAU UAG Title: Oct 16­9:43 AM (108 of 115)
Translation
1.
A tRNA molecule carrying the appropriate amino acid attaches to the
second exposed codon. Met
Cys
CCG AUG GGC ACC AAA GUC CAU UAG Title: Oct 16­9:43 AM (109 of 115)
Translation
2.
Enzymes form a peptide bond between the amino acids. The first tRNA
releases its amino acid, so that the next tRNA now holds the entire chain.
Me
t
Cys
CCG AUG GGC ACC AAA GUC CAU UA
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Translation
3.
The ribosome moves to the next codon in the mRNA strand, and the first tRNA molecule is released. Another codon is now exposed, and ready to
accept the next tRNA. M
Thr
et
Cys
CCG AUG GGC ACC AAA GUC CAU UAG UCC
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Translation
This cyle continues until the ribosome reaches one of the stop codons.
al
V
Met
Lys
Cy
s
Thr
His
Stop codon
G AUG GGC ACC AAA GUC CAU UAG UCC
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Animation
Protein Synthesis in Detail
Title: Oct 16­9:43 AM (113 of 115)
Homework
Review pages 596­600 in your text, and complete the following:
1.
Compare and contrast the following sets of terms:
a) Germ cell and Somatic cell mutations
b) Substitution mutation, insertion mutation, and deletion mutation
c) Silent mutations, mis­sense mutations, and nonsense mutations
d) Spontaneous and induced mutations
2.
Explain the meaning of the term transposon. How are mutations caused
by transposons different from point mutations?
3.
What is a mutagen? Explain what is meant by physical and chemical
mutagents, and give some examples of each.
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Assignment
Create a graphic booklet explaining all steps in protein synthesis, similar to what you made for DNA replication.
Include all structures involved, and the action of all enzymes.
You do NOT have to include mutations in this assignment.
You will be given some class time to complete the work, but some will have to be done on your own.
This will be due on Friday, January 25.
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Attachments
Endrocrine Gland Function.wpd
codontable2[1]