Lecture Chapter 16: The molecular basis of inheritance

Question 1

The ratios of A, T, G, and C were roughly ___ across al organisms. This supports the

Answer 1

1:1

helical model of DNA

Question 2

X ray diffraction revealed the __ of DNA

Answer 2

alpha helical shape

Question 3

What is the basic principle?

Answer 3

The idea that the two separate strands of DNA can act as a template

Question 4

A semiconservative replicated strand of DNA contains

Answer 4

one parent strand and one new strand

Question 5

How did we know that the semiconservative model of DNA replication was correct

Answer 5

Meselson and Stahl

Bacteria cultured in a Nitrogen isotope and transferred to a lighter isotope

the DNA sample was centrifuged after the first replication, then the second

The DNA separated based on density. First replication completely in middle. Second replication one in middle one at top

Question 6

How might Meselson and Stahl’s results be different if DNA replication followed a conservative model? A dispersive?

Answer 6

conservative: one at bottom and one top after first, same for second

dispersive: one band throughout

Question 7

How did Taylor, Woods, and Hughes demonstrate semiconservative replication in eukaryotes?

Answer 7

autoradiography causes appearance of black spots where radioactivity is present

chromatids were only half radioactive after second replication

Question 8

Compare the origin of replication in eukaryotes vs prokaryotes

Answer 8

eukaryotes: multiple origin sites because more complex and long

prokaryotes: only one origin site/replication bubble

Question 9

RNA primers ____ with the help of the enzyme ___

Answer 9

initiate DNA replication

primase

Question 10

Primase generates ___ because

Answer 10

short RNA molecules which will later be replaced with DNA

polymerase needs something to attach to, can’t start from scratch

Question 11

Helicase is involved in

Answer 11

separating the two strands of DNA

Question 12

Topoisomerase is responsible for

Answer 12

relieves the strain on DNA strands during replication

Question 13

Single-strand binding proteins allow

Answer 13

the two strands of DNA to remain separate and not form hydrogen bonds after helicase separates them

Question 14

DNA polymerase requires

Answer 14

a small amount of DNA to be present before it creates the longer strand

Question 15

Elongation occurs in what direction?

Answer 15

5’ to 3’ direction

Question 16

DNA is replicated __ per each cycle

Answer 16

once

Question 17

Okazaki fragments exist because

Answer 17

DNA replciation needs to occur at every section of DNA, even if it is not in the 5’ to 3’ direction

Question 18

After the first okazaki fragment is done, DNA polymerase 3

Answer 18

detaches and makes a second okazaki fragment. Then DNA pol 1 removes the RNA primers and replaces with DNA

Question 19

Ligase is responsible for

Answer 19

attaching okazaki fragments and making a continuous strand

Question 20

The lagging strand bends and loops in order to

Answer 20

ensure the leading strand stays in one direction

Question 21

Describe the end replication problem

Answer 21

when the final primer is removed at the 3’ end, DNA polymerase should replace it. But DNA can’t, and it creates an overhang. The second round of replication cuts it off, and the DNA gets shorter and shorter until it gets an essential part and the cell dies.

Question 22

Telomeres are

Answer 22

special nucleotide sequences at the end of chromosomes

Question 23

Telomerase is responsible for

Answer 23

catalyzing the end of telomeres in some cells to prevent shortening. Not active in most eukaryotic somatic cells

Question 24

Describe the bacterial chromosome

Answer 24

double stranded
circular
small amount of protein
the DNA is supercoiled and found in the nucleoid

Question 25

Describe the eukaryotic chromosome

Answer 25

linear DNA associated with a large amount of protein

Question 26

Histones are responsible for

Answer 26

packing chromatin (first level)

Question 27

Histone methylation leads to

Answer 27

tighter packing

Question 28

Histone acetylation leads to

Answer 28

looser packing

Question 29

Heterochromatin genes are __

Answer 29

not expressed

Question 30

euchromatin genes are ___

Answer 30

expressed

Question 31

Given a polynucleotide sequence such as GAATTC, explain what further information you would need in order to identify which is the 5′ end.

Answer 31

n order to tell which end is the 5′ end, you need to know which end has a phosphate group on the 5′ carbon (the 5′ end) and/or which end has an —OH group on the 3′ carbon (the 3′ end)

Question 32

What role does complementary base pairing play in the replication of DNA?

Answer 32

Complementary base pairing ensures that the two daughter molecules are exact copies of the parental molecule. When the two strands of the parental molecule separate, each serves as a template on which nucleotides are arranged by the base- pairing rules; they will then be polymerized by enzymes into new complementary strands.

Question 33

Identify two major functions of DNA pol III in DNA replication

Answer 33

DNA pol III covalently adds nucleotides to new DNA strands and proofreads each added nucleotide for correct base pairing.

Question 34

What is the relationship between DNA replication and the S phase of the cell cycle?

Answer 34

In the cell cycle, DNA synthesis occurs during the S phase, between the G1 and G2 phases of inter- phase. DNA replication is therefore complete before the mitotic phase begins.

Question 35

Describe the structure of a nucleosome, the basic unit of DNA packing in eukaryotic cells.

Answer 35

A nucleosome is made up of eight histone proteins, two each of four different types, around which DNA is wound. Linker DNA runs from one nucleosome to the next.

Question 36

How does euchromatin differ from heterochromatin in structure and function?

Answer 36

The 10-nm fiber of euchromatin is less compacted during inter- phase than in mitosis and is accessible to the cellular proteins responsible for gene expression. In contrast, the 10-nm fiber of heterochromatin is relatively compacted (densely arranged) during interphase, and genes in heterochroma- tin are largely inaccessible to proteins necessary for gene expression

Question 37

Interphase chromosomes appear to be attached to the nuclear lamina and perhaps also the nuclear matrix. Describe these two structures.

Answer 37

The nuclear lamina is a netlike array of protein filaments that provides mechanical support just inside the nuclear envelope and thus maintains the shape of the nucleus. Considerable evidence also supports the existence of a nuclear matrix, a framework of protein fibers extending throughout the nuclear interior.

Question 38

In his work with pneumonia-causing bacteria and mice, Griffith found that (A) the protein coat from pathogenic cells was able to trans- form nonpathogenic cells. (B) heat-killed pathogenic cells caused pneumonia. (C) some substance from pathogenic cells was transferred to nonpathogenic cells, making them pathogenic. (D) the polysaccharide coat of bacteria caused pneumonia

Answer 38

C

Question 39

What is the basis for the difference in how the leading and lagging strands of DNA molecules are synthesized? (A) The origins of replication occur only at the 5′ end. (B) Helicases and single-strand binding proteins work at the 5′ end. (C) DNA polymerase can join new nucleotides only to the 3′ end of a pre-existing strand, and the strands are antiparallel. (D) DNA ligase works only in the 3′ --> 5′direction.

Answer 39

C

Question 40

In analyzing the number of different bases in a DNA sample, which result would be consistent with the base-pairing rules? (A) A = G (B) A + G = C +T (C) A +T = G + C (D) A = C

Answer 40

B

Question 41

The elongation of the leading strand during DNA synthesis (A) progresses away from the replication fork. (B) occurs in the 3′ S 5′ direction. (C) produces Okazaki fragments. (D) depends on the action of DNA polymerase

Answer 41

D

Question 42

In a nucleosome, the DNA is wrapped around (A) histones. (B) ribosomes. (C) polymerase molecules. (D) a thymine dimer.

Answer 42

A

Question 43

E. coli cells grown on 15 N medium are transferred to 14 N medium and allowed to grow for two more generations (two rounds of DNA replication). DNA extracted from these cells is centrifuged. What density distribution of DNA would you expect in this experiment? (A) one high-density and one low-density band (B) one intermediate-density band (C) one high-density and one intermediate-density band (D) one low-density and one intermediate-density band

Answer 43

D

Question 44

A student isolates, purifies, and combines in a test tube a variety of molecules needed for DNA replication. When she adds some DNA to the mixture, replication occurs, but each DNA molecule consists of a normal strand paired with numerous segments of DNA a few hundred nucleotides long. What has she probably left out of the mixture? (A) DNA polymerase (B) DNA ligase (C) Okazaki fragments (D) primase

Answer 44

B

Question 45

The spontaneous loss of amino groups from adenine in DNA results in hypoxanthine, an uncommon base, opposite thymine. What combination of proteins could repair such damage? (A) nuclease, DNA polymerase, DNA ligase (B) telomerase, primase, DNA polymerase (C) telomerase, helicase, single-strand binding protein (D) DNA ligase, replication fork proteins, adenylyl cyclase

Answer 45

A

Question 46

Although the proteins that cause the E. coli chromosome to coil are not histones, what property would you expect them to share with histones, given their ability to bind to DNA

Answer 46

Like histones, the E. coli proteins would be expected to contain many basic (positively charged) amino acids, such as lysine and arginine, which can form weak bonds with the negatively charged phos- phate groups on the sugar-phosphate backbone of the DNA molecule.