Chapter 16- Lecture Outline

Question 1

In 1953, James Watson and Francis Crick introduced an elegant double-helical model for the structure of deoxyribonucleic acid, or DNA

Answer 1

Hereditary information is encoded in DNA and reproduced in all cells of the body
This DNA program directs the development of biochemical, anatomical, physiological, and
(to some extent) behavioral traits

Question 2

DNA is copied____________________during and cells can repair their DNA

Answer 2

DNA replication,

Question 3

Early in the 20th century, the identification of the molecules of inheritance loomed as a major challenge to biologists

Answer 3

DNA is the genetic material

Question 4

When T. H. Morgan’s group showed that genes are located on chromosomes, the two components of chromosomes—DNA and protein—became candidates for the genetic material

Answer 4

The role of DNA in heredity was first discovered
by studying bacteria and the viruses that
infect them

Question 5

The discovery of the genetic role of DNA began with research by Frederick Griffith in 1928

Answer 5

Griffith worked with two strains of a bacterium, one pathogenic and one harmless

Question 6

When he mixed heat-killed remains of the pathogenic strain with living cells of the harmless strain, some living cells became pathogenic

Answer 6

He called this phenomenon transformation, now defined as a change in genotype and phenotype due to assimilation of foreign DNA

Question 7

In 1944, Oswald Avery, Maclyn McCarty, and Colin MacLeod announced that the transforming substance was DNA

Answer 7

Many biologists remained skeptical, mainly because little was known about DNA

Question 8

More evidence for DNA as the genetic material came from studies of viruses that infect bacteria

Answer 8

Such viruses, called bacteriophages (or phages), are widely used in molecular genetics research
A virus is DNA (sometimes RNA) enclosed by a protective coat, often simply protein

Question 9

In 1952, Alfred Hershey and Martha Chase showed that DNA is the genetic material of a phage known as T2

Answer 9

They designed an experiment showing that only one of the two components of T2 (DNA or protein) enters an E. coli cell during infection

Question 10

They concluded that the injected DNA of the phage provides

Answer 10

the genetic information

Question 11

Two findings became known as Chargaff’s rules

Answer 11

The base composition of DNA varies between species

In any species the number of A and T bases are equal and the number of G and C bases are equal

Question 12

rules was not understood until the discovery of

Answer 12

the double helix

Question 13

After DNA was accepted as the genetic material, the challenge was to determine how its structure accounts for its role in heredity

Answer 13

Maurice Wilkins and Rosalind Franklin were using a technique called X-ray crystallography to study molecular structure
Franklin produced a picture of the DNA molecule using this technique

Question 14

Watson and Crick built models of a double helix to conform to the X-rays and chemistry of DNA

Answer 14

Franklin had concluded that there were two outer sugar-phosphate backbones, with the nitrogenous bases paired in the molecule’s interior

Question 15

Watson built a model in which the backbones were

Answer 15

antiparallel (their subunits run in opposite directions)

Question 16

At first, Watson and Crick thought the bases paired like with like (A with A, and so on), but such pairings did not result in a uniform width

Answer 16

Instead, pairing a purine with a pyrimidine resulted in a uniform width consistent with the X-ray data

Question 17

The relationship between structure and function is manifest in the double helix

Answer 17

Watson and Crick noted that the specific base pairing suggested a possible copying mechanism for genetic material

Question 18

Since the two strands of DNA are complementary, each strand acts as a template for building a new strand in replication

Answer 18

In DNA replication, the parent molecule unwinds, and two new daughter strands are built based on base-pairing rules

Question 19

Watson and Crick’s _____________ predicts that when a double helix replicates, each daughter molecule will have one old strand (derived or “conserved” from the parent molecule) and one newly made strand

Answer 19

semiconservative model of replication

Question 20

Competing models were the conservative model

Answer 20

(the two parent strands rejoin) and the dispersive model (each strand is a mix of old and new)

Question 21

Experiments by Matthew Meselson and Franklin Stahl supported the semiconservative model

Answer 21

They labeled the nucleotides of the old strands with a heavy isotope of nitrogen, while any new nucleotides were labeled with a lighter isotope

Question 22

The first replication produced a band of hybrid DNA, eliminating the conservative model

Answer 22

A second replication produced both light and hybrid DNA, eliminating the dispersive model and supporting the semiconservative model

Question 23

The copying of DNA is remarkable in its speed and accuracy

Answer 23

More than a dozen enzymes and other proteins participate in DNA replication

Question 24

Replication begins at particular sites called

—————————, where the two DNA strands are separated, opening up a replication “bubble”

Answer 24

origins of replication

Question 25

A eukaryotic chromosome may have hundreds or even thousands of origins of replication

Answer 25

Replication proceeds in both directions from each origin, until the entire molecule is copied

Question 26

At the end of each replication bubble is a replication fork, a Y-shaped region where
new DNA strands are elongating
Helicases are enzymes that untwist the double helix at the replication forks

Answer 26

Single-strand binding proteins bind to and stabilize single-stranded DNA
Topoisomerase corrects “overwinding” ahead of replication forks by breaking, swiveling, and rejoining DNA strands

Question 27

DNA polymerases cannot initiate synthesis of a polynucleotide; they can only add nucleotides to an existing 3′ end

Answer 27

The initial nucleotide strand is a short RNA primer

Question 28

An enzyme called
————– can start an RNA chain from scratch and adds RNA nucleotides one at a time using the parental DNA as a template

Answer 28

primase

Question 29

The primer is short (5–10 nucleotides long), and

Answer 29

the 3′ end serves as the starting point for the new DNA strand

Question 30

Enzymes called ____________ catalyze the elongation of new DNA at a replication fork

Answer 30

DNA polymerases

Question 31

Most DNA polymerases require a primer and a DNA template strand

Answer 31

The rate of elongation is about 500 nucleotides per second in bacteria and 50 per second in human cells

Question 32

Each nucleotide that is added to a growing DNA strand is a nucleoside triphosphate
dATP supplies adenine to DNA and is similar to the ATP of energy metabolism

Answer 32

The difference is in their sugars: dATP has deoxyribose while ATP has ribose
As each monomer of dATP joins the DNA strand, it loses two phosphate groups as a molecule of pyrophosphate

Question 33

The antiparallel structure of the double helix affects replication

Answer 33

DNA polymerases add nucleotides only to the free 3′ end of a growing strand; therefore, a new DNA strand can elongate only in the 5′ to 3′ direction

Question 34

Along one template strand of DNA, the DNA polymerase synthesizes a
————– continuously, moving toward the replication fork

Answer 34

leading strand

Question 35

To elongate the other new strand, called

———————-, DNA polymerase must work in the direction away from the replication fork

Answer 35

the lagging strand

Question 36

The lagging strand is

Answer 36

synthesized as a series of segments called Okazaki fragments, which are joined together by DNA ligase

Question 37

DNA polymerases proofread newly made DNA, replacing any incorrect nucleotides
In mismatch repair of DNA, repair enzymes correct errors in base pairing

Answer 37

DNA can be damaged by exposure to harmful chemical or physical agents such as cigarette smoke and X-rays; it can also undergo spontaneous changes
In nucleotide excision repair, a nuclease cuts out and replaces damaged stretches of DNA

Question 38

Error rate after proofreading repair is low but

not zero

Answer 38

Sequence changes may become permanent and can be passed on to the next generation
These changes (mutations) are the source of the genetic variation upon which natural selection operates

Question 39

Limitations of DNA polymerase create problems for the linear DNA of eukaryotic chromosomes

Answer 39

The usual replication machinery provides no way to complete the 5′ ends, so repeated rounds of replication produce shorter DNA molecules with uneven ends
This is not a problem for prokaryotes, most of which have circular chromosomes

Question 40

Eukaryotic chromosomal DNA molecules have special nucleotide sequences at their ends called

Answer 40

telomeres

Question 41

Telomeres do not prevent the shortening of DNA molecules, but they do postpone the erosion of genes near the ends of DNA molecules

Answer 41

It has been proposed that the shortening of telomeres is connected to aging

Question 42

If chromosomes of germ cells became shorter in every cell cycle, essential genes would eventually be missing from the gametes they produce

Answer 42

An enzyme called telomerase catalyzes the lengthening of telomeres in germ cells

Question 43

The shortening of telomeres might protect cells from cancerous growth by limiting the number of cell divisions

Answer 43

There is evidence of telomerase activity in cancer cells, which may allow cancer cells to persist

Question 44

The bacterial chromosome is a double-stranded, circular DNA molecule associated with a small amount of protein
Eukaryotic chromosomes have linear DNA molecules associated with a large amount
of protein

Answer 44

In a bacterium, the DNA is “supercoiled” and found in a region of the cell called the nucleoid

Question 45

In the eukaryotic cell, DNA is precisely combined with proteins in a complex called chromatin

Answer 45

Chromosomes fit into the nucleus through an elaborate, multilevel system of packing

Question 46

Most chromatin is loosely packed in the nucleus during interphase and condenses prior to mitosis
Loosely packed chromatin is called euchromatin

Answer 46

During interphase a few regions of chromatin (centromeres and telomeres) are highly condensed into heterochromatin
Dense packing of the heterochromatin makes it difficult for the cell to express genetic information coded in these regions

Question 47

__________can undergo chemical modifications that result in changes in chromatin organization

Answer 47

Histones