Topic 2.3: DNA/RNA Structure and Function

Question 1

Summarize the significance of the Griffith and Avery experiments.

Answer 1

Griffith concluded that some substance was passed from the dead S strain bacteria to the living R strain, and this substance has the capacity to transform the R strain. This was indicated to be genetic material. Later, Avery and others identified the genetic material as DNA.

Question 2

Explain how, at the completion of the Hershey-Chase experiment, the results suggested that DNA was genetic material.

Answer 2

They discovered that the radioactive tracers for DNA, but not protein, ended up inside the bacteria cells, causing them to become transformed. As only the genetic material could have caused this transformation, Hershey and Chase determined that DNA must be the genetic material.

Question 3

Describe the structure of a DNA molecule.

Answer 3

DNA is a double helix. Unwinding the helix reveals a ladder configuration in which the uprights are composed of sugar and phosphate molecules, and the rungs are complementary bases. The bases in DNA pair in such a way that the sugar-phosphate backbones are oriented in different directions. 3’ and 5’ are part of the system for numbering the carbon atoms that make up the sugar.

Question 4

What does DNA stand for? Please define DNA.

Answer 4

Deoxyribonucleic acid.

Nucleic acid polymer produced from covalent bonding of nucleotide monomers that contain the sugar deoxyribose; the genetic material of living organisms.

Question 5

Define pyrimidines

Answer 5

Nucleotides with a single ring in their structure; examples are thymine, cytosine, and uracil.

Question 6

Define purines

Answer 6

Nucleotides with a double-ring structure; examples are adenine and guanine.

Question 7

Define double helix

Answer 7

Double spiral; describes the three-dimensional shape of DNA.

Question 8

Define complementary base pairing

Answer 8

Hydrogen bonding between particular purines and pyrimidines; responsible for the structure of DNA, and some RNA, molecules.

Question 9

Explain why DNA replication is said to be semiconservative

Answer 9

A new double strand of DNA is composed of one old strand and one newly synthesized strand.

Question 10

Summarize the sequence of events that occur during DNA replication.

Answer 10

1. The DNA helicase unzips by the breakage of hydrogen bonds between the paired bases
2. New complimentary nucleotides attach to their partners (AT/CG). They are put together by the enzyme DNA polymerase. The DNA polymerase uses each individual strand as a template to form the new strand.
3. Because the strands of DNA are oriented in an antiparallel configuration, and the DNA polymerase may add new nucleotides only to one end of the chain, DNA synthesis occurs in opposite directions. The leading strand follows the helicase enzyme, while synthesis on the the lagging strand results in the formation of short segments of DNA called Okazaki fragments.
4. To complete replication, the enzyme DNA ligase connects all the fragments and seals any breaks in the sugar-phosphate backbone.
5. The two double-helix molecules are identical to each other and to the original DNA molecule.

Question 11

Define DNA replication

Answer 11

Synthesis of a new DNA double helix prior to mitosis and meiosis in eukaryotic cells and during prokaryotic fission in prokaryotic cells.

Question 12

Explain the role of mRNA, tRNA and rRNA in gene expression.

Answer 12

mRNA carries the genetic information encoded in DNA from the nucleus to the ribosomes in the cytoplasm. At the ribosome composed of rRNA and proteins, mRNA assembles amino acids carried by tRNA into chains, which are eventually released to form functional proteins.

Question 13

Describe the movement of information from the nucleus to the formation of a functional protein.

Answer 13

The information stored in DNA is transcribed into mRNA which then, with the help of tRNA at the ribosomes made of rRNA, translates the information into protein structure.

Question 14

Discuss why the genetic code is said to be degenerate.

Answer 14

The genetic code contains more than one tripled codon for each amino acid.

Question 15

Define RNA (ribonucleic acid)

Answer 15

Nucleic acid produced from covalent bonding of nucleotide monomers that contain the sugar ribose; occurs in many forms, including messenger RNA, ribosomal RNA, and transfer RNA.

Question 16

Define transcription

Answer 16

First stage of gene expression; process whereby a DNA strand serves as a template for the formation of mRNA.

Question 17

Define translation

Answer 17

During gene expression, the process whereby ribosomes use the sequence of codons in mRNA to produce a polypeptide with a particular sequence of amino acids.

Question 18

Define gene

Answer 18

Unit of heredity existing as alleles on the chromosomes; in diploid organisms, typically two alleles are inherited—one from each parent.

Question 19

Define messenger RNA (mRNA)

Answer 19

Type of RNA formed from a DNA template and bearing coded information for the amino acid sequence of a polypeptide.

Question 20

Define RNA polymerase

Answer 20

During transcription, an enzyme that creates an mRNA transcript by joining nucleotides complementary to a DNA template.

Question 21

Define promoter

Answer 21

In an operon, a sequence of DNA where RNA polymerase binds prior to transcription.

Question 22

Define exons

Answer 22

Segment of mRNA containing the protein-coding portion of a gene that remains within the mRNA after splicing has occurred.

Question 23

Define introns

Answer 23

Intervening sequence found between exons in mRNA; removed by RNA processing before translation.

Question 24

Define codon

Answer 24

Three-base sequence in messenger RNA that during translation directs the addition of a particular amino acid into a protein or directs termination of the process.

Question 25

Define transfer RNA (tRNA)

Answer 25

Type of RNA that transfers a particular amino acid to a ribosome during protein synthesis. At one end, it binds to the amino acid, and at the other end, it has an anticodon that binds to an mRNA codon.

Question 26

Define anticodon

Answer 26

Three-base sequence in a transfer RNA molecule base that pairs with a complementary codon in mRNA.

Question 27

Define ribosomal RNA (rRNA)

Answer 27

Structural form of RNA found in the ribosomes.

Question 28

Define initiation

Answer 28

First stage of translation in which the translational machinery binds an mRNA and assembles.

Question 29

Define elongation

Answer 29

Middle stage of translation in which additional amino acids specified by the mRNA are added to the growing polypeptide.

Question 30

Define termination

Answer 30

End of translation that occurs when a ribosome reaches a stop codon on the mRNA that it is translating, causing release of the completed protein.

Question 31

The double-helix model of DNA resembles a twisted ladder in which the rungs of the ladder are
a) a purine paired with a pyrimidine
b) A paired with G, and C paired with T
c) sugar-phosphate paired with sugar-phosphate
d) a 5’ end paired with a 3’end
e) both a and b are correct

Answer 31

a) a purine paired with a pyrimidine

Question 32

If 30% of an organism’s DNA is thymine, then
a) 70% is purine
b) 20% is guanine
c) 30% is adenine
d) 70% is pyrimidine
e) both b and c are correct

Answer 32

e) both b and c are correct

Question 33

If the sequence of bases in one strand of DNA is 5’ TAGCCT 3’, then the sequences of bases in the other strand is
a) 3’ TCCGAT 5’
b) 3’ TAGCCT 5’
c) 3’ ATCGGA 5’
d) 3’ AACGGUA 5’
e) none of the above

Answer 33

c) 3’ ATCGGA 5’

Question 34

DNA replication is said to be semiconservative because
a) one of the new molecules conserves both of the original DNA strands
b) the new DNA molecule contains two new DNA strands
c) both of the new molecules contain one new strand and one old strand
d) DNA polymerase conserves both of the old strands

Answer 34

c) both of the new molecules contain one new strand and one old strand

Question 35

The enzyme responsible for separating double-stranded DNA into single-stranded DNA is
a) DNA helicase
b) DNA primase
c) DNA polymerase
d) DNA ligase

Answer 35

a) DNA helicase

Question 36

The enzyme responsible for adding new nucleotides to a growing DNA chain during DNA replication is
a) helicase
b) RNA polymerase
c) DNA polymerase
d) DNA ligase

Answer 36

c) DNA polymerase

Question 37

Which of the following processes occurs in the nucleus and forms a complementary copy of one strand of the DNA molecule for gene expression?
a) DNA replication
b) translation
c) transcription
d) RNA processing
e) none of the above

Answer 37

c) transcription

Question 38

RNA processing
a) removes the exons, leaving the introns
b) is the same as transcription
c) is an event that occurs after RNA is transcribed
d) is the rejection of old, worn-out RNA
e) all of the above

Answer 38

e) all of the above

Question 39

If the sequence of bases in the coding strand of a DNA molecule is TAGC, then the sequence of bases in the mRNA will be
a) AUCG
b) TAGC
c) UAGC
d) ATCG

Answer 39

a) AUCG