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Structure and chemical composition of DNA

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This quiz is based on the physical structure and chemical composition of DNA. It deals with basic concepts related to DNA chemistry and DNA methods.
 

1. Which base is found in RNA but not DNA ?
• Uracil
• Adenine
• Cytosine
• Thymine
Answer: Uracil

Adenine, Cytosine and Guanine are found in both RNA and DNA. Thymine is found only in DNA; Uracil takes its (Thymine) place in RNA molecules.



2. In the DNA Double Helix, complementary base pairs are held together by :
• N-glycosidic bonds
• hydrogen bonds
• ionic bonds
• peptide bonds
Answer: hydrogen bonds

The complementary base pairs : A - T and G - C are held together by hydrogen bonds.



3. The three pyrimidine bases in DNA are :
• Adenine, Thymine and Guanine
• Thymine, Guanine and Cytosine
• Cytosine,Thymine and Uracil
• Adenine, Uracil and Guanine
Answer: Cytosine,Thymine and Uracil

In all, there are five nitrogeneous bases : Cytosine, Thymine, Uracil, Adenine and Guanine. The purine bases (Adenine and Guanine), have a structure which consists of two rings of atoms. The other three bases are called pyrimidine bases.
The correct answer is : Cytosine, Thymine and Uracil.



4. A DNA strand has the sequence A-C-A-G-C-C-G-T-A. What would be its complementary strand ?
• A-C-A-G-C-C-G-T-A
• U-G-U-C-G-G-C-A-U
• T-G-T-C-G-G-C-A-T
• G-T-G-A-T-T-A-C-G
Answer: T-G-T-C-G-G-C-A-T

Note that complementary base pairs are : A - T and G - C. So, to find the complementary strand, replace A by T, G by C and vice versa.
U replaces T in an RNA strand and is absent in DNA. Here, the letters A, T, G, C and U represent the base pairs that form both DNA and RNA molcules.
The correct answer is T-G-T-C-G-G-C-A-T.



5. The DNA molecules of different species differ in their :
• phosphate backbone
• sequence of bases
• type of nucleotides
• All of the above
Answer: sequence of bases

The DNA molecules of different species have different sequences of base-pairs.



6. The difference between DNA and RNA is :
• The RNA sugar phosphate backbone contains ribose rather than deoxyribose.
• DNA molecules are double stranded while RNA molecules are single stranded for the most part.
• Thymine in DNA is replaced by Uracil in RNA.
• All of the above.
Answer: All of the above.

The three options provided accurately list the differences between DNA and RNA molecules. Note that RNA molecules are single stranded with a few double stranded regions of complementary base-pairing. These regions are called hairpin loops. Also, the 5-carbon sugar in RNA has more oxygen than the corresponding sugar in DNA.
The correct answer is : All of the above.



7. Where would you expect to find genetic information describing the characteristics of a Dodo ?
• In the ribosomes.
• In virtually every cell in its body.
• In the primary spermatocytes.
• In its feathers !
Answer: In virtually every cell in its body.

Genetic information would be present in almost all cells in its body.



8. The number of hydrogen bonds that hold the Adenine - Thymine base pair together is :
• 3
• 4
• 5
• 2
Answer: 2

The Adenine - Thymine base pair is held together by 2 hydrogen bonds while the Guanine - Cytosine base pair is held together by 3 hydrogen bonds. That is also the reason why the two strands of a DNA molecule can be separated in more easily at sections that are densely populated by A - T base pairs.



9. The DNA molecule is a polymer. Its monomer units are :
• nucleic acids
• amino acids
• nucleotides
• nucleosides
Answer: nucleotides

The DNA molecule is a polymer whose monomer units are nucleotides and the polymer is known as a "polynucleotide". Each nucleotide consists of a 5-carbon sugar (deoxyribose), a nitrogen containing base attached to the sugar, and a phosphate group. There are four different types of nucleotides found in DNA. They are : Adenine, Guanine, Cytosine and Thymine.



10. The total length of a DNA molecule in the human genome when fully outstretched is approximately :
• 1.8 centimeters.
• 1.8 millimeters.
• Varies from person to person.
• 1.8 meters.
Answer: 1.8 meters.

The correct answer is 1.8 meters. The coiled molecule is contained within the cell nucleus which has a diameter of about 6 micrometers !!!



11. Because one original strand of the double stranded DNA helix is found in each daughter cell (after cell division), the DNA replication process is :
• conservative
• semiconservative
• derivative
• dispersive
Answer: semiconservative

The DNA replication process is semiconservative because one original strand of the double stranded DNA helix is found in each daughter cell after replication.



12. The technique Rosalind Franklin used to determine the structure of DNA was :
• centrifuge analysis
• X-Ray crystallography
• bacteriophages
• None of the above
Answer: X-Ray crystallography

Rosalind Franklin used X-Ray crystallography to determine the structure of DNA. She was in fact the first to state that the the sugar-phosphate backbone of DNA lies on the outside of the molecule. She also elucidated the basic helical structure of the molecule.



13. Each nucleotide in a DNA molecule consists of :
• a phosphate group,a pentose sugar and a nitrogeneous heterocyclic base.
• a phosphate group, a pentose sugar, and a oxygen base.
• a phosphate group, a hexose sugar and a nitrogeneous heterocyclic base.
• a sulfonyl group, a pentose sugar, and a nitrogeneous heterocyclic base.
Answer: a phosphate group,a pentose sugar and a nitrogeneous heterocyclic base.

A nucleotide consists of a nitrogenous heterocyclic base (a purine or a pyrimidine), a pentose sugar (deoxyribose in DNA or ribose in RNA), and a phosphate or polyphosphate group. A DNA molecule (a polynucleotide) is made up of a large number of these nucleotide (monomer) units.
Note : A polynucleotide is a nucleic acid.



14. A nucleoside consists of :
• a pentose sugar and a oxygen base.
• a hexose sugar and a nitrogeneous heterocyclic base.
• a pentose sugar and a nitrogeneous heterocyclic base.
• a phosphate group, a pentose sugar and a nitrogeneous heterocyclic base.
Answer: a pentose sugar and a nitrogeneous heterocyclic base.

A nucleoside is similar to a nucleotide, except that it contains only the sugar and base, without the phosphate group. Examples include guanosine, thymidine and inosine.



15. How does the sequence of a strand of DNA correspond to the amino acid sequence of a protein ? This concept is explained by the central dogma of molecular biology which states that :
• DNA is replicated and sections of the replica are used to make protein.
• DNA is used to make RNA which is used to make protein.
• protein is manufactured directly from DNA without any intermediate.
• None of the above.
Answer: DNA is used to make RNA which is used to make protein.

The correct answer is : DNA is used to make RNA which is used to make protein.
The RNA molecule is an intermediate between the DNA molecule and the protein. This lets the DNA stay pristine and protected, away from the caustic chemistry of the cytoplasm outside the nucleus. Also, many copies of an RNA molecule can be made which enables the amplification of genetic information.



16. The single stranded molecule that is transcribed from a DNA template (i.e a photocopy of a specific section of the DNA molecule) and is subsequently used to manufacture proteins is :
• mRNA
• tRNA
• rRNA
• snRNA
Answer: mRNA

mRNA is a photocopy of a gene with a sequence complementary to one section of the DNA strand (gene) and identical to the other. The mRNA carries this genetic information from the cell nucleus to the ribosomes in the cytoplasm where proteins are manufactured.
Note : mRNA - messenger RNA ; tRNA - transfer RNA ; rRNA - ribosomal RNA ; snRNA - small nuclear RNA



17. The process of protein manufacture within ribosomes using mRNA is called :
• transcription
• protein derivation
• translation
• None of the above
Answer: translation

The correct answer is translation. In short, a gene is transcribed to give mRNA which is then used to manufacture proteins in ribosomes.



18. A significant difference between prokaryotes and eukaryotes is :
• The DNA of prokaryotes floats freely around the cell while the DNA of eukaryotes is held within its nucleus.
• Prokaryotes do not have DNA as their genetic material while eukaryotes do.
• Prokaryotes do not have ribosomes while eukaryotes do.
• Prokaryote cells are not membrane-bound while eukaryote cells are.
Answer: The DNA of prokaryotes floats freely around the cell while the DNA of eukaryotes is held within its nucleus.

The DNA of prokaryotes floats freely around the cell while the DNA of eukaryotes is held within its nucleus. Also, the DNA of eukaryotes is much more complex and therefore much more extnsive than the DNA of prokaryotes.



19. Melting of DNA is a standard method for preparing "single stranded DNA". A 4000 kb (kilobase- pair) DNA molecule melts at 79oC. What percent of the base pairs are GC pairs?
• 2.44 %
• 30.4 %
• 24.4 %
• 42.4 %
Answer: 24.4 %

The two strands of a DNA molecule can be melted into single strands by heat, which breaks the hydrogen bonds between complementary bases. Since G - C pairs consist of three hydrogen bonds, while A - T pairs only have two hydrogen bonds, the temperature at which different DNA molecules "melt" varies depending on their basepair sequences. A DNA molecule consisting of only A - T pairs will melt at approximately 69oC, while a DNA molecule consisting of only GC pairs will melt at approximately 110oC. The relationship between GC content in the molecule and the melting temperature (Tm) of the molecule is as follows :
%(G - C content) = 2.44(Tm - 69)
Substitute Tm = 79oC
=> %(G - C content) = 24.4
The correct answer is 24.4 %.



20. The unit within the nucleus that contains a protein complex (located at the center of the unit) of two H2A, H2B, H3 and H4 histone proteins with DNA wrapped around the complex in two turns, each turn consisting of about 80 base pairs is called a :
• nucleosome
• ribosome
• centrosome
• histosome
Answer: nucleosome

The unit described above is called a nucleosome. The main purpose of nucleosomes is the organization and packing of DNA. Under an electron microscope, the DNA looks like a bead on a string. The beads are nucleosomes, and the string is the DNA.




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