Topic 16

Cytosine makes up 38% of the nucleotides in a sample of DNA from an organism. Approximately what percentage of the nucleotides in this sample will be thymine?

12

If a DNA sample were composed of 10% thymine, what would be the percentage of guanine?

40%

At a specific area of a chromosome, the sequence of nucleotides below is present where the chain opens to form a replication fork: 3' C C T A G G C T G C A A T C C 5' An RNA primer is formed starting at the underlined T (T) of the template. Which of the following represents the primer sequence?

5' A C G U U A G G 3'

What would be the sequence of the DNA molecule made by replication of the DNA molecule shown below? 5' GCGAATA 3'

5' TATTCGC 3'

Cytosine makes up 42% of the nucleotides in a sample of DNA from an organism. Approximately what percentage of the nucleotides in this sample will be thymine?

8%

In an analysis of the nucleotide composition of DNA, which of the following will be found?

A + C = G + T

Which of the following shows the correct base pairing of deoxyribonucleotides that occurs in nature?

A with T

A space probe returns with a culture of a microorganism found on a distant planet. Analysis shows that it is a carbon-based life-form that has DNA. You grow the cells in 15N medium for several generations and then transfer them to 14N medium and grown them for another generation. Which pattern in the figure shown below would you expect if the DNA was replicated in a conservative manner?

B

Why do our chromosomes become shorter as we age?

Because DNA polymerases can only add nucleotides to an existing nucleotide chain, an RNA primer is used to initiate DNA synthesis. Once that RNA primer is removed from the end of a linear chromosome, there is no mechanism to completely fill in the missing nucleotides at the end of a linear chromosome

Which of the following shows the correct base pairing of deoxyribonucleotides that occurs in nature?

C with G

In the late 1950s, Meselson and Stahl grew bacteria in a medium containing "heavy" nitrogen (15N) and then transferred them to a medium containing 14N. Which of the results in the figure shown below would be expected after one round of DNA replication in the presence of 14N?

D

Which of the following enzymes removes the RNA nucleotides from the primer and adds equivalent DNA nucleotides to the 3' end of DNA fragments resulting from DNA replication of one of the two DNA strands?

DNA polymerase I

Which enzyme catalyzes the elongation of a DNA strand in the 5' → 3' direction?

DNA polymerase III

What is the basis for the difference in how the leading and lagging strands of DNA molecules are synthesized?

DNA polymerase can join new nucleotides only to the 3' end of a growing strand

Why are primases necessary in order for DNA replication to occur?

DNA polymerases can not initiate synthesis of new nucleotide chains

Eukaryotic telomeres replicate differently than the rest of the chromosome. This is a consequence of which of the following?

DNA polymerases can only add deoxyribonucleotides to the 3' end of an existing nucleotide chain

Which of the following statements regarding DNA polymerases is TRUE?

DNA polymerases can only add nucleotides to the 3' end of a nucleotide chain DNA polymerases can add nucleotides to the 3' end of an RNA molecule DNA polymerases can NOT initiate synthesis of a new deoxyribonucleotide chain

E. coli cells grown on 15N medium are transferred to 14N medium and allowed to grow for two more generations (two rounds of DNA replication). DNA extracted from these cells is centrifuged. Which of the samples shown in the figure below shows the density distribution of DNA you would expect in this experiment?

E

Will the chromosomes you pass onto your children be shorter than your chromosomes?

Hopefully not, because telomerase will restore the telomeres in your germline cells (the cells that produce egg or sperm cells) to approximately their original length

How do some scientists feel that telomerase might be used to slow the aging process?

If telomerase activity was increased in our cells, the rate at which our chromosomes shorten as we age might be decreased

What happens to the short RNA "primers" that primase synthesizes that are extended by DNA polymerase III during DNA replication?

In E. coli, DNA polymerase I removes the RNA primers and fills in the resulting gaps with deoxyribonucleotides

Why do some scientists feel that telomerase might be used to slow the aging process?

Telomere extension in mice and worms has reversed some signs of aging

What is meant by the description "antiparallel" regarding the strands that make up DNA?

The 5' to 3' direction of one strand runs counter to the 5' to 3' direction of the other strand

The order in which chromatin packing occurs is as follows:

The DNA wraps around histones, forming nucleosomes; the histones within each nucleosome interact with adjacent nucleosomes and with the DNA between nucleosomes to form a 30-nm fiber; the chromatin forms loops by attaching to scaffold proteins, forming a 300-nm fiber

A chromosome is

a single long DNA molecule plus the proteins with which that DNA molecule is complexed

According to the "semi-conservative" model of DNA replication,

after DNA replication occurs, each of the two double-stranded DNA molecules will consist of one "daughter" and one "parental" DNA strand

If an organism uses the "conservative" model of DNA replication, rather than the normal "semi-conservative" model of DNA replication,

after DNA replication occurs, one of the two double-stranded DNA molecules will consist of two "daughter" DNA strands and the other will consist of two "parental" DNA strands

Which of the following are NOT components of a nucleotide?

an "R" group

The formation of hydrogen bonds between complementary nucleotides is known as

base pairing

The pairs of nucleotides that interact to join together two DNA strands are known as

complementary nucleotides

According to the "semi-conservative" model of DNA replication, after a double-stranded DNA molecule is replicated

each of the two resulting double-stranded DNA molecules will consist of one "daughter" and one "parental" DNA strand

In a nucleosome, the DNA is wrapped around

histones

Which of the following statements about histones is FALSE?

histones help transport mRNA molecules out of the nucleus

What kind of chemical bond is found between paired bases of the DNA double helix?

hydrogen

The two strands of a double-stranded DNA molecule are held together by

hydrogen bonds

In a double-stranded DNA molecule, the two DNA strands base pair in such a way that the 5' end of one DNA strand base pairs with the 3' end of the other DNA strand. The two DNA strands are said to be

in an antiparallel arrangement

Which of the following enzymes covalently connects segments of DNA?

ligase

Which of the following represents the order of increasingly higher levels of organization of chromatin?

nucelosome, 30-nm chromatin fiber, looped domain

DNA polymerases can add deoxyribonucleotides to

only the 3' end of an existing nucleotide chain

DNA replication begins are special sites on the parental DNA molecule known as

origins of replication

The covalent linkages joining together nucleotides in a DNA molecule are known as

phosphodiester linkages

Which of the following enzymes initiates the synthesis of a new nucleotide chain during DNA replication?

primase

The ends of linear chromosomes are known as

telomeres

Telomeres are

the ends of linear chromosomes

The leading and lagging strands differ in that

the leading strand is synthesized in the same direction as the movement of the replication fork, and the lagging strand is synthesized in the opposite direction

How do the nucleotides used to make RNA molecules differ from those used to make DNA molecules?

the nucleotides used to make RNA molecules have ribose instead of deoxyribose as their sugar and uracil instead of thymine as one of their nitrogenous bases

What is the function of the DNA polymerase III?

to add nucleotides to the 3' end of a growing DNA strand

The process by which DNA molecules are copied so that when cells divide each cell can have a complete copy of the DNA is known as

DNA replication

Which of the following will happen to your chromosomes as you become older?

Your chromosomes will become shorter

The reaction used to join together nucleotides to form a nucleotide chain is known as a

dehydration reaction

The two strands of a double-stranded DNA molecule are held together by

hydrogen bonds between complementary nitrogenous bases

Which of the following statements describes the eukaryotic chromosome?

it consists of a single linear molecule of double-stranded DNA plus proteins

What is the role of DNA ligase during DNA replication?

it joins DNA fragments together

You briefly expose bacteria undergoing DNA replication to radioactively labeled nucleotides. When you centrifuge the DNA isolated from the bacteria, the DNA separates into two classes. One class of labeled DNA includes very large molecules (thousands or even millions of nucleotides long), and the other includes short stretches of DNA (several hundred to a few thousand nucleotides in length). These two classes of DNA probably represent

leading strands and Okazaki fragments

Why are there concerns about using increased telomerase activity to slow down the aging process?

one concern is that increasing telomerase activity could increase cancer rates

Which of the following statements regarding primases is TRUE?

primases catalyze the formation of an RNA molecule primases add nucleotides to the 3' end of a nucleotide or nucleotide chain primases can initiate synthesis of a new nucleotide chain

Which of the following describes the sequence of events that occurs during DNA replication of a double-stranded DNA molecule?

separation of the DNA strands at DNA replication origins, unwinding of the DNA double helix, synthesis of RNA primers, synthesis of DNA, ligation of DNA fragments

The DNA molecule is able to carry a vast amount of hereditary information in which of the following?

sequence of bases

When a DNA molecule is replicated,

the DNA strands that are made by copying the parental DNA strands are known as the "daughter" strands replication generally occurs by a "semi-conservative" mechanism the DNA strands in the original molecule are known as the "parental" DNA strands

What is "chromatin"?

the combination of DNA and proteins that forms chromosomes

The difference between ATP and the nucleoside triphosphates used during DNA synthesis is that

the nucleoside triphosphates have the sugar deoxyribose; ATP has the sugar ribose