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General Bio 1 for science majors > Chapter 16 > Flashcards

Chapter 16 Flashcards

1
Q

What posed a major challenge to biologists in the early 20th century?

A

The identification of the molecules of inheritance.

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2
Q

What became candidates for genetic material after T. H. Morgan’s group showed that genes are located on chromosomes?

A

The two components of chromosomes, DNA and protein.

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3
Q

How was the role of DNA in heredity first discovered?

A

By studying bacteria and the viruses that infect them.

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4
Q

When did the discovery of the genetic role of DNA begin?

A

With research by Frederick Griffith in 1928. Griffith worked with two strains of bacterium, one pathogenic and one harmless.

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5
Q

What was Griffith’s research?

A

He mixed heat killed remains of the pathogenic strain with living cells of the harmless strain, some living cells became pathogenic.

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6
Q

What did Griffith call the phenomenon he observed?

A

Transformation. Now defined as a change in genotype and phenotype due to assimilation of foreign DNA.

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7
Q

Who identified the transforming substance and as what?

A

Oswald Avery, Maclyn McCarty, and Colin MacLeod identified it as DNA. Many biologists remained skeptical, mainly because little was known about DNA.

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8
Q

What gave more evidence for DNA as the genetic material?

A

Studies from viruses that infect bacteria.

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9
Q

What are bacteriophages (phages)?

A

Viruses that infect bacteria.

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10
Q

What is a virus?

A

DNA (sometimes RNA) enclosed by a protective coat, often simply protein. Phages have been widely used as tools by researchers in molecular genetics.

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11
Q

What was shown in 1952?

A

Alfred Hershey and Martha Chase showed that DNA is the genetic material of a phage known as T2. They designed an experiment showing that only one of the two components of T2 (DNA or protein) enters an E. coli cell during infection. They concluded that the injected DNA of the phage provides the genetic information.

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12
Q

What is DNA?

A

A polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate group.

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13
Q

What are the nitrogenous bases?

A

Can be adenine (A), thymine (T), guanine (G), or cytosine (C).

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14
Q

What did Erwin Chargaff report in 1950?

A

That DNA composition varies from one species to the next. This evidence of molecular diversity among organisms made DNA a more credible candidate for the genetic material.

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15
Q

What two findings became known as Chargaff’s rules?

A

The base composition of DNA varies between species, and in any species the number of A and T bases is equal and the number of G and C bases is equal.
The basis for these rules was not understood until the discovery of the double helix.

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16
Q

What did Maurice Wilkins and Rosalind Franklin do?

A

Used a technique called X-ray crystallography to study molecular structure. Franklin produced a picture of the DNA molecule using this technique.

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17
Q

What did Franklin’s X-ray crystallography images allow?

A

James Watson was able to deduce that DNA was helical. The X-ray images also enabled Watson to deduce the width of the helix and the spacing of the nitrogenous bases.

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18
Q

What did Watson and Crick do?

A

Built models of a double helix to conform to the X-rays and chemistry of DNA. Franklin had concluded that there were two outer sugar-phosphate backbones, with the nitrogenous bases paired in the molecule’s interior.

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19
Q

What was special about the model?

A

The backbones were antiparallel, meaning their subunits run in opposite directions.

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20
Q

What did Watson and Crick do with the base pairs?

A

At first, they thought like paired with like ( A with A, etc), but such pairings did not result in uniform width. Instead, pairing a purine (A or G) with a pyrimidine (C or T) resulted in a uniform width consistent with the X-ray data.

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21
Q

What were the problems with each trial of base pairings?

A

Purine to purine were too wide, pyrimidine to pyrimidine were too narrow. Purine to pyrimidine width were consistent with the X-ray data.

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22
Q

What did Watson and Crick determine after the pairing widths?

A

That the pairing was more specific, dictated by the base structures. They determined that adenine (A) paired only with thymine (T), and guanine (G) paired only with cytosine (C).

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23
Q

What does the Watson Crick model explain?

A

Chargaff’s rules: in any organism, the amount of A=T and the amount of G=C.

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24
Q

How are the nitrogenous base pairs held together?

A

By hydrogen bonds.

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25
What do the two strands of DNA allow?
Since the two strands are complementary, each strand acts as a template for building a new strand in replication. This yields two exact replicas of the parental molecule.
26
What does Watson and Crick's semiconservative of replication model predict?
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.
27
What were competing models?
Competing models were the conservative model (the two parent strands rejoin) and the dispersive model (each strand is a mix of old and new).
28
Whose experiments supported the semiconservative model?
Matthew Meselson and Franklin Stahl.
29
Where does replication begin?
Particular sites called origins of replication, where the two DNA strands are separated, opening up a replication "bubble." Replication proceeds in both directions from each origin, until the entire molecule is copied.
30
How many origins of replication can a eukaryotic chromosome have?
May have hundreds or even thousands.
31
What is a replication fork?
At the end of each replication bubble is a replication fork, a Y shaped region where parental DNA strands are being unwound.
32
What are helicases?
Enzymes that untwist the double helix at the replication forks.
33
What are single strand binding proteins?
They bind to and stabilize single stranded DNA.
34
What are topoisomerases?
They relieve the strain of twisting of the double helix by breaking, swiveling, and rejoining DNA strands.
35
What does DNA polymerase require?
A primer to which they can add nucleotides. The initial nucleotide chain is a short RNA primer that is synthesized by the enzyme primase. The completed primer is five to ten nucleotides long, and the new DNA strand will start from the 3' end of the RNA primer.
36
What do DNA polymerases do?
Catalyze the synthesis of new DNA at a replication fork. Most DNA polymerases require a primer and a DNA template strand.
37
What is the rate of elongation?
About 500 nucleotides per second in bacteria and 50 per second in human cells.
38
What nucleotides are added to growing DNA strands?
A nucleoside triphosphate. dATP supplies adenine to DNA and is similar to the ATP of energy metabolism.
39
What is the difference between dATP and ATP?
dATP has deoxyribose and ATP has ribose instead.
40
How do the monomers join the DNA strand?
A dehydration reaction. It loses two phosphate groups as a molecule of pyrophosphate.
41
Does the antiparallel structure affect replication?
Yes. 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.
42
What does the DNA polymerase synthesize while moving toward the replication fork?
A leading strand.
43
How does the other new strand elongate?
Called the lagging strand, DNA polymerase must work in the direction away from the replication fork. It is synthesized as a series of segments called Okazaki fragments, which are joined together by DNA ligase.
44
What is the function of the protein helicase?
Unwinds parental double helix at replication forks.
45
What is the function of single strand binding proteins?
Binds to and stabilizes single stranded DNA until it is used as a template.
46
What is the function of topoisomerase proteins?
Relieves overwinding strain ahead of replication forks by breaking, swiveling, and rejoining DNA strands.
47
What is the function of primase?
Synthesizes an RNA primer at 5' end of leading strand and at 5' end of each Okazaki fragment of lagging strand.
48
What is the function of DNA pol III?
Using parental DNA as a template, synthesizes new DNA strand by adding nucleotides to an RNA primer or a pre-existing DNA strand.
49
What is the function of DNA pol I?
Removes RNA nucleotides of primer from 5' end and replaces them with DNA nucleotides added to 3' end of adjacent fragment.
50
What is the function of DNA ligase?
Joins Okazaki fragments of lagging strand; on leading strand, joins 3' end of DNA that replaces primer to rest of leading strand DNA>
51
What is the "DNA replication machine?"
The proteins that participate in DNA replication that form a large complex.
52
What have recent studies supported about the replication machine?
The DNA replication machine may be stationary during the replication process, and that they "reel in" parental DNA and extrude newly made daughter DNA molecules. The exact mechanism is not yet resolved.
53
What is mismatch repair of DNA?
When DNA polymerases proofread newly made DNA and are replacing any incorrect nucleotides, the repair enzymes replace incorrectly paired nucleotides that have evaded the proofreading process.
54
What happens if DNA is damaged?
Nucleotide excision repair, a nuclease cuts out and replaces damaged stretches of DNA.
55
What is the error rate after proofreading and repair?
Low, but not zero.
56
What happens to an error in the DNA?
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 and are ultimately responsible for the appearance of a few species.
57
What happens in linear DNA replication?
The usual replication machinery cannot complete the 5' ends of daughter DNA strands. There is no 3' end of a preexisting polynucleotide for DAN polymerase to add on to. Thus, repeated rounds of replication produce shorter DNA molecules with uneven ends.
58
What are telomeres?
Eukaryotic chromosomal DNA molecules with special nucleotide seuqneces at their ends.
59
What do telomeres do?
They do not prevent the shortening of DNA molecles, but they do postpone the erosion of genes near the ends of DNA molecules. It has been proposed that the shortening of telomeres is connected to aging.
60
What would happen if chromosomes of germ cells became shorter in every cell cyce?
Essential genes would eventually be missing form the gametes they produce. An enzyme called telomerase catalyzes the lengthening of telomeres in germ cells.
61
What might the shortening of telomeres do?
Protect cells from cancerous growht by limiting the number of cell divisions. There is evidence of telomerase activity in cancer cells, which may allow cancer cells to persist.
62
What are the differences in DNA between organisms?
Bacterial chromosomes are double stranded, and circular with a small amoutnt of protein. Eukaryotic chromosomes have linear DNA molecules associated with a large amoutn of protein. Ina bacterium, the DNA is "supercoiled" and found in a region of the cell called the nucleoid.
63
What happens when DNA is combined with proteins?
Creates a combplex called chromatin. They fit into the nucleus through an elaborate, multilevel system of packing.
64
What are histones?
Proteins that are responsible for the main level of DNA packing in interphase chromatin.
65
What does unfolded chromatin look like?
Beads on a string, with each bead being a nucleosome.
66
What is a nucleosome?
It is composed of DNA wound twice around a core of eight histones, two each of the four main histone types. The amino end of each histone (the histone tail) extends outward from the nucleosome and is involved in regulation of gene expression.
67
What is euchromatin?
Loosely packed chromatin.
68
What is heterochromatin?
Highly condensed regions of chromatin (centromeres and telomeres) during interphase. Dense packing of the heterochromatin makes it difficult for the cell to express genetic information coded in these regions.
69
What happens to chromatin during the cell life?
Interphase chromosomes occupy specific restricted regions in the nucleus, and the fibers of different chromosomes do not become entangled. Chromatin undergoes changes in packing during the cell cycle. As the cell prepares for mitosis, the chromatin is organized into loops and coils, eventually condensing into short, thick metaphase chromosomes.
70
What can happen to histones?
Can undergo chemical modifications that result in changes in chromatin condensation, and these changes can also have multiple ewffects on gene expression.
71

General Bio 1 for science majors (11 decks)

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