Ch 4: Molecular Biology (Princeton Review)

Question 1

DNA and RNA are called nucleic acids because they are found in the nucleus and possess many ______ _________ groups.

Answer 1

acidic phosphate

p. 65

Question 2

A mnemonic to remember which nitrogenous bases are the pyrimidines is….

Answer 2

….CUT the Py.

p. 65

Question 3

Which kind of nitrogenous bases are guanine and adenine?

Answer 3

purines

p. 65

Question 4

What is a nucleoside?

Answer 4

a ribose or deoxyribose with a nitrogenous base linked to the 1’ carbon

p. 65

Question 5

When nucleotides contain three phosphate residues, they are also referred to as…

Answer 5

….nucleoside triphosphates (NTP), or dNTP if the sugar is deoxyribose.

Question 6

Nucleotides in the DNA chain are covalently linked by ______________ bonds between the 3’ ______ group of one deoxyribose and the 5’ phosphate group of the next deoxyribose.

Answer 6

phosphodiester

hydroxy

p. 66

Question 7

In double-stranded DNA, a hydrogen-bonded pair of nitrogenous bases always consists of…

Answer 7

….a purine and a pyrimidine.

p. 68

Question 8

What does Chargoff’s rule state?

Answer 8

that [A] = [T] and [C]=[G]

and

[A] + [C] = [G] + [T]

p. 68

Question 9

The guanine-cytosine pair is held together by _ hydrogen bonds and the adenine-thymine pair by _ hydrogen bonds.

Answer 9

3

2

(remember thymine starts with a T and so does “Two”)

p. 68

Question 10

The binding of two complementary strands of DNA into a double-stranded structure is termed _________, or ______________.

Answer 10

annealing or hybridization

p. 68

Question 11

The separation of two strands of DNA is termed _______ or ____________.

Answer 11

melting or denaturation

p. 68

Question 12

True or false: if the fraction of purine nucleotides and the total molecular weight of a double helix are known, the amount of cytosine can be calculated.

Answer 12

False. The ratio of purines to pyrimidines is always the same (50:50) since each purine is paired with a pyrimidine. In order to calculate the amount of any one base, you have to know the ratio of AT to GC pairs.

p. 68

Question 13

True or false: The two chains in a piece of ds-DNA containing mostly purines will be bonded together more tightly than the two chains in a piece of ds-DNA containing mostly pyrimidines

Answer 13

False, again, the ratio or purines to pyrimidines is always 50:50. However, two chains containing mostly GC pairs will bond more tightly than 2 chains containing mostly AT pairs, since GC pairs are held together by 3 H-bonds while AT pairs have only 2.

p. 68

Question 14

In eukaryotic DNA, the double helix is stabilized by __________ interactions between the bases.

Answer 14

van der Waals

(p. 69)

Question 15

What is a genome?

Answer 15

The sum total of an organism’s genetic information.

p. 69

Question 16

How is prokaryotic DNA shaped?

Answer 16

Bacterial genomes are composed of a single circular chromosome.

p. 69

Question 17

Is there any direct correlation between genome size and evolutionary sophistication?

Answer 17

No. While humans have 10⁹ base pairs compared to bacteria containing 10⁶ base pairs, the organisms with the largest known genomes are amphibians. Much of the size difference in higher eukaryotic genomes is the result of repetitive DNA that has no known function.

p. 69

Question 18

Prokaryotes have an enzyme called ___ ______ which twists the DNA, resulting in a twisted circle structure composed of double-stranded DNA. The twists created by this enzyme are called __________.

Answer 18

DNA gyrase

supercoils (bc they are coils of a structure that is already coiled)

p. 69

Question 19

DNA has the microscopic appearance of beads on a string. What are those beads really?

Answer 19

The “beads” are called nucleosomes, and they are composed of DNA wrapped around an octamer of histones (group of 8).

p. 70

Question 20

When you see a length of “string” between “beads”, this is a length of double-helical DNA called ______ ___, and is bound by a single ______ histone.

Answer 20

linker DNA

linker

p. 70

Question 21

Fully packed DNA is called _________. It is composed of closely stacked nucleosomes.

Answer 21

chromatin

p. 70

Question 22

When chromosomes are treated, distinct light and dark regions become visible. The darker regions are denser and are called _______________. The lighter regions are less dense and are called _______________.

Answer 22

heterochromatin

euchromatin

p. 70

Question 23

What is significant about the lighter regions of stained chromosomes?

Answer 23

The lighter regions have higher transcription rates and therefore higher gene activity. Looser packing makes DNA accessible to enzymes and proteins.

p. 70

Question 24

A centromere is the region of the chromosome to which spindle fibers attach during cell division. The fibers attach via __________, multiprotein complexes that act as anchor attachment sites for spindle fibers.

What are centromeres made of?

Answer 24

kinetochores

heterochromatin

p. 71

Question 25

Chromosomes have p and q arms. Which are long and which are short?

Answer 25

p arms are short

q arms are long

p. 71

Question 26

The ends of linear chromosomes are called telomeres. At the DNA level, these regions are distinguished by repeated nucleotide sequences. The repeated unit is usually 6-8 base pairs long and rich in _______. Many vertebrates have the same repeat: 5’-TTAGGG-3’. Telomeres are composed of both single- and double-stranded DNA. Where is the single-stranded DNA found?

Answer 26

guanine

At the very end of the chromosome, and is about 300 base pairs in length. It loops around to form a knot, held together by many telomere-associated proteins.

p. 71

Question 27

Why don’t prokaryotes have telomeres in their DNA?

Answer 27

Bc most of them have circular genomes.

p. 71

Question 28

Why do we say that the human genome contains 24 different chromosomes?

Answer 28

Because we have 22 autosomes that are all different, plus 2 sex chromosomes which are different from each other.

p. 72

Question 29

What is a gene? What else does it include?

Answer 29

A gene is a DNA sequence that encodes a gene product.

It includes both regulatory regions (such as promoters and transcription stop sites), and a region that codes for either a protein or a non-coding RNA.

p. 72

Question 30

What are the single nucleotide changes called which are found about once in every 1000 base pairs in the human genome?

Answer 30

single nucleotide polymorphisms

p. 72

Question 31

Just like the Krebs cycle and glycolysis, DNA replication is an _________ process. It occurs during S phase (_________ phase) in interphase of the cell cycle.

Answer 31

enzymatic

synthesis

p. 75

Question 32

Stop codons are also called _________ codons, since they don’t code for any amino acid.

Answer 32

nonsense

p. 75

Question 33

What do we call two or more codons which code for the same amino acid?

Because the genetic code has such synonyms, it is said to be __________.

Answer 33

synonyms

degenerate

p. 75

Question 34

It is very important to realize that though an amino acid may be specified by several codons, each codon specifies only a ______ amino acid. This is why we say that the genetic code has no _________ – because it can only be interpreted in one way.

Answer 34

single

ambiguity

p. 75

Question 35

The genetic code shown is used by most organisms. Some protists use an alternate genetic code, and the __________ genome of many organisms (including humans and many other vertebrates) uses a slightly different code.

Answer 35

mitochondrial

p. 75

Question 36

Meselson and Stahl showed that DNA replication is ___________; after replication, one strand of the new double helix is parental (old) and one strand is newly synthesized daughter DNA.

Answer 36

semiconservative

p. 76

Question 37

What is the name of the enzyme that unwinds the DNA double helix and separates the strands?

Answer 37

helicase

p. 76

Question 38

When helicase unwinds the helix at the origin of replication, the helix gets wound more tightly upstream and downstream from this point. The chromosome would get tangled and eventually break, except that enzymes called __________ cut one or both of the strands and unwrap the helix, releasing the excess tension created by the helicases.

Answer 38

topoisomerases

p. 77

Question 39

After unwinding, this single stranded DNA can become must less stable. What do single-strand binding proteins (SSBPs) do?

Answer 39

They protect the DNA that has been unpackaged in preparation for replication and help keep the strands separated.

p. 77

Question 40

An RNA primer must be synthesized for each DNA template strand. This is accomplished by a set of proteins called the _________, of which the central component is an RNA polymerase called ______. Primer synthesis is important because DNA polymerase cannot start a new DNA chain from scratch. It can only add nucleotides to an existing nucleotide chain.

Answer 40

primosome

primase

p. 77

Question 41

Both prokaryotes and eukaryotes have mobile genetic elements in their genomes, called transposable elements or __________. It is thought that many eukaryotic transposons are old, defective retroviruses. These short segments can jump around the genome, causing mutations.

Answer 41

transposons

p. 87

Question 42

mRNA is the only type of ______ RNA. This molecule carries genetic information to the ________, where it can be translated into protein.

Answer 42

coding

ribosome

p. 95

Question 43

Each mRNA has several regions. The 5’ region is not translated into protein (so it is called the 5’ untranslated region, or 5’UTR), but is important in initiation and regulation. Following this is the region that actually codes for a protein. This starts at a start codon and ends at a stop codon, and is called the ____ _______ _____. The 3’ end of the mRNA (after the stop codon) isn’t translated into protein, but often contains regulatory regions that influence post-transcriptional gene expression.

Answer 43

open reading frame

p. 95

Question 44

Eukaryotic mRNA is usually __________ and obeys the “one gene, one protein” principle. This means that each piece of mRNA encodes only one polypeptide (and so contains one open reading frame). Hence, there are as many different mRNAs as their are proteins. That said, each mRNA can be read many times, so a single transcript can be used to make many copies of its polypeptide.

Only recently, some polycistronic eukaryotic mRNAs have been discovered.

Answer 44

monocistronic

p. 95

Question 45

What is the part of the post-transcriptional modification of RNA which cuts out introns? What gets joined together?

Answer 45

Splicing. Before the RNA can be translated, introns must be removed and exons joined together.

p. 99

Question 46

An important difference between prokaryotic and eukaryotic gene expression is that the primary transcript in prokaryotes is mRNA, and is ready to be __________. Thus, bacteria lack a mechanism for splicing out introns.

In contrast, the eukaryotic primary transcript is extensively ________ prior to translation.

Answer 46

translated

modified

p. 99

[Also referenced on Q 14, Bio/Biochem section of free AAMC practice exam]

Question 47

When does X-inactivation occur in female mammals that have 2 X chromosomes?

Answer 47

early in development, at the blastocyst stage

p. 114

Question 48

The active X chromosome is called Xa, and the inactive chromosome is called __. Each cell in the inner cell mass randomly inactivates an X chromosome, and this decision is ___________.

Answer 48

Xi

irreversible

p. 114

Question 49

[Regarding X-chromosome inactivation]

Every cell derived from each cell in the inner cell mass will have the same X chromosome inactivated, however, because each cell “makes its own decision”, an adult can have different X chromosomes inactivated in different tissues and cells. Because of X-inactivation, all humans have the same number of ____ _______ for the X chromosome.

Answer 49

gene products

p. 114

Question 50

The Xi chromosome is very condensed, and packaged in ___________. It has high levels of DNA ________.

Answer 50

heterochromatin

methylation

p. 114

Question 51

In regard to the prokaryotic lac operon, if the repressor cannot bind to the ________, nothing will prevent RNA polymerase from transcribing all the genes on the operon in an unregulated, constitutively (continuous) fashion.

Answer 51

operator

p. 118

Question 52

An operon containing multiple genes in prokaryotic cells is transcribed from a ______ promoter found upstream from the first gene on the operon.

Answer 52

single

p. 117

(definition taken from glossary)

Question 53

Operons may be inducible or repressible. What is the difference?

Answer 53

The default setting for a repressible system is “ON”, while the default for an inducible system is “OFF”.

p. 115