Bio 2: Chapter 5

Several experiments were required to demonstrate how traits are inherited. Which scientist or team of scientists first demonstrated that cells contain some component that can be transferred to a new population of cells and permanently cause changes in the new cells?

Hershey and Chase

Several experiments were required to demonstrate how traits are inherited. Which scientist or team of scientists obtained definitive results demonstrating that DNA is the genetic molecule?

Hershey and Chase

Which of the following sequences can fully base-pair with itself?

5'-AAGCGCTT-3'

Given the sequence of one strand of a DNA helix as 5'-GCATTCGTGGGTAG-3',

CGTAAGCACCCATC

The manner in which a gene sequence is related to its respective protein sequence is referred to as the _________ code

genetic

The complete set of information found in a given organism's DNA is called its ____________.

genome

The images of chromosomes we typically see are isolated from mitotic cells. These mitotic chromosomes are in the most highly condensed form. Interphase cells contain chromosomes that are less densely packed and __________________________.

occupy discrete territories in the nucleus

The DNA from two different species can often be distinguished by a difference in the ______________________.

ratio of A + T to G + C

The process of sorting human chromosomes pairs by size and morphology is called karyotyping. A modern method employed for karyotyping is called chromosome painting. How are individual chromosomes "painted"?

using fluorescent DNA molecules

Mitotic chromosomes are _____ times more compact than a DNA molecule in its extended form

10,000

Many of the breakthroughs in modern biology came after Watson and Crick published their model of DNA in 1953. In what decade did scientists first identify chromosomes?

1880s

Interphase chromosomes are about______ times less compact than mitotic chromosomes, but still are about______ times more compact than a DNA molecule in its extended form

20; 500

The human genome has enough DNA to stretch more than 2 m. However, this DNA is not contained in a single molecule; it is divided into linear segments and packaged into structures called chromosomes. What is the total number of chromosomes found in each of the somatic cells in your body?

46

Mitotic chromosomes were first visualized in the 1880s with the use of very simple tools: a basic light microscope and some dyes. Which of the following characteristics of mitotic chromosomes reflects how they were named?

color

Which of the following chemical groups is not used to construct a DNA molecule

six-carbon sugar

In a DNA double helix, _____________________.

the two DNA strands run anitparrallel

Define a gene

A gene is a segment of DNA that stores the information required to specify the particular sequence found in a protein (or, in some cases, the sequence of a structural or catalytic RNA).

Consider two different species of yeast that have similar genome sizes. Is it likely that they contain the same number of genes? A similar number of chromosomes?

A similar genome size indicates relatively little about the number of genes and virtually nothing about the number of chromosomes. For example, the commonly studied yeasts Saccharomyces cerevisiae (Sc) and Schizosaccharomyces pombe (Sp) are separated by roughly 400 million years of evolution, and both have a genome of 14 million base pairs. Yet Sc has 6500 genes packaged into 16 chromosomes and Sp has 4800 genes in 3 chromosomes.

The number of cells in an average-sized adult human is on the order of 1014. Use this information, and the estimate that the length of DNA contained in each cell is 2 m, to do the following calculations (look up the necessary distances and show your working): A. Over how many miles would the total DNA from the average human stretch? B. How many times would the total DNA from the average human wrap around the planet Earth at the Equator? C. How many times would the total DNA from the average human stretch from Earth to the Sun and back? D. How many times would the total DNA from the average human stretch from the Earth to Pluto and back?

A. 2 × 1014 m = 124,274,238,447 miles. B. The Earth's circumference at the Equator is 7,926 miles. The length of DNA from the average human body could wrap around the Earth 15,678,759 times. C. The average distance from the Earth to the Sun is 93,000,000 miles. So, the round-trip distance is 186,000,000 miles. The length of DNA from the average human body could stretch from the Earth to the Sun and back 668 times. D. The distance from the Earth to Pluto is, on average, about 39 × 93,000,000 miles. The length of DNA from the average human body could stretch from the Earth to Pluto and back 36 times.

A. In principle, what would be the minimum number of consecutive nucleotides necessary to correspond to a single amino acid to produce a workable genetic code? Assume that each amino acid is encoded by the same number of nucleotides. Explain your reasoning. B. On average, how often would the nucleotide sequence CGATTG be expected to occur in a DNA strand 4000 bases long? Explain your reasoning.

A. Because there are 20 amino acids used in proteins, each amino acid would have to be encoded by a minimum of three nucleotides. For example, a code of two consecutive nucleotides could specify a maximum of 16 (42) different amino acids, excluding stop and start signals. A code of three consecutive nucleotides has 64 (43) different members and thus can easily accommodate the 20 amino acids plus a signal to stop protein synthesis. B. Because 46 (= 4096) different sequences of six nucleotides can occur in DNA, any given sequence of six nucleotides would be expected to occur on average once in a DNA strand 4000 bases long, assuming a random distribution of sequences.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. A. DNA molecules, like proteins, consist of a single, long polymeric chain that is assembled from small monomeric subunits. B. The polarity of a DNA strand results from the polarity of the nucleotide subunits. C. There are five different nucleotides that become incorporated into a DNA strand. D. Hydrogen bonds between each nucleotide hold individual DNA strands together.

A. False. DNA is double-stranded. It is actually is made of two polymers that are complementary in sequence. B. True. C. False. There are four different nucleotides that are used to make a DNA polymer: adenine, thymine, guanine, and cytosine. A fifth nucleotide, uracil, is found exclusively in RNA molecules, replacing thymine nucleotides in the DNA sequence. D. False. Nucleotides are linked covalently through phosphodiester bonds. Hydrogen-bonding between nucleotides from opposite strands holds the DNA molecule together.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. A. Comparing the relative number of chromosome pairs is a good way to determine whether two species are closely related. B. Chromosomes exist at different levels of condensation, depending on the stage of the cell cycle. C. Eucaryotic chromosomes contain many different sites where DNA replication can be initiated. D. The telomere is a specialized DNA sequence where microtubules from the mitotic spindle attach to the chromosome so that duplicate copies move to opposite ends of the dividing cell

A. False. There are several examples of closely related species that have a drastically different number of chromosome pairs. Two related species of deer—Chinese and Indian muntjac—have 23 and 3, respectively. B. True. C. True. D. False. The telomere is a specialized DNA sequence, but not for the attachment of spindle microtubules. Telomeres form special caps that stabilize the ends of linear chromosomes.

Consider the structure of the DNA double helix. A. You and a friend want to split a double-stranded DNA molecule so you each have half. Is it better to cut the length of DNA in half so each person has a shorter length, or to separate the strands and each take one strand? Explain. B. In the original 1953 publication describing the discovery of the structure of DNA, Watson and Crick wrote, "It has not escaped our notice that the specific pairings we have postulated immediately suggest a possible copying mechanism for the genetic material." What did they mean?

A. It is better to separate the strands and each take a single strand, because all of the information found in the original molecule is preserved in a full-length single strand but not in a half-length double-stranded molecule. B. Watson and Crick meant that the complementary base pairing of the strands allows a single strand to contain all of the information necessary to direct the synthesis of a new complementary strand.

The human genome comprises 23 pairs of chromosomes found in nearly every cell in the body. Answer the quantitative questions below by choosing one of the numbers in the following list: 23 69 >200 46 92 >10^9 A. How many centromeres are in each cell? What is the main function of the centromere? B. How many telomeres are in each cell? What is their main function? C. How many replication origins are in each cell? What is their main function?

A. There are 46 centromeres per cell, one on each chromosome. The centromeres have a key role in the distribution of chromosomes to daughter cells during mitosis. B. There are 92 telomeres per cell, two on each chromosome. Telomeres serve to protect the ends of chromosomes and to enable complete replication of the DNA of each chromosome all the way to its tips. C. There are far more than 200 replication origins in a human cell, probably about 10,000. These DNA sequences direct the initiation of DNA synthesis needed to replicate chromosomes.

When double-stranded DNA is heated, the two strands separate into single strands in a process called melting or denaturation. The temperature at which half of the duplex DNA molecules are intact and half have melted is defined as the Tm. A. Do you think Tm is a constant, or can it depend on other small molecules in the solution? Do you think high salt concentrations increase, decrease, or have no effect on Tm? B. Under standard conditions, the expected melting temperature in degrees Celsius can be calculated from the equation Tm = 59.9 + 0.41 [%(G + C)] - [675/length of duplex]. Does the Tm increase or decrease if there are more G + C (and thus fewer A + T) base pairs? Does the Tm increase or decrease as the length of DNA increases? Why? C. Calculate the predicted Tm for a stretch of double helix that is 100 nucleotides long and contains 50% G + C content.

A. Tm depends on the identity and concentration of other molecules in the solution. High salt concentrations are more effective at shielding the two negatively charged phosphate-sugar backbones in the double helix from each other, so the two strands repel each other less strongly. Thus, a high salt concentration stabilizes the duplex and increases the melting temperature. B. The Tm increases as the proportion of G + C bases increases and as the length increases. The thermal energy required for melting depends on how many hydrogen bonds between the strands must be broken. Each G-C base pair contributes three hydrogen bonds, whereas an A-T base pair contributes only two. C. Inserting values into the equation in part B gives Tm = 59.9 + (0.41 × 50) - (675/100) = 73.65°C, which is about twice the normal temperature of the human body and nearly too hot to touch.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. A. Each strand of DNA contains all the information needed to create a new doublestranded DNA molecule with the same sequence information. B. All functional DNA sequences inside a cell code for protein products. C. Gene expression is the process of duplicating genes during DNA replication. D. Gene sequences correspond exactly to the respective protein sequences produced from them.

A. True. B. False. Some sequences encode only RNA molecules, some bind to specific regulatory proteins, and others are sites where specific chrosomosomal protein structures are built (for example, centromeric and telomeric DNA). C. False. Gene expression is the process of going from gene sequence to RNA sequence, to protein sequence. D. False. This statement is false for two reasons. First, genes often contain intron sequences. Second, genes always contain nucleotides flanking the protein-coding sequences that are required for the regulation of transcription and translation

Using terms from the list below, fill in the blanks in the following brief description of the experiment with Streptococcus pneumoniae that identified which biological molecule carries heritable genetic information. Some terms may be used more than once

Cell-free extracts from S-strain cells of S. pneumoniae were fractionated to PURIFY DNA, RNA, protein, and other cell components. Each fraction was then mixed with R-STRAIN cells of S. pneumoniae. Its ability to change these into cells with PATHOGENIC properties resembling the S- STRAIN cells was tested by injecting the mixture into mice. Only the fraction containing DNA was able to transform the R- STRAIN cells to PATHOGENIC (or S- STRAIN) cells that could kill mice

Explain the differences between chromosome painting and the older, more traditional method of staining chromosomes being prepared for karyotyping. Highlight the way in which each method identifies chromosomes by the unique sequences they contain.

Chromosome painting relies on the specificity of DNA complementarity. Because unique sequences for each chromosome are known, short DNA molecules matching a set of these sites can be designed for each chromosome. Each set is labeled with a specific combination of fluorescent dyes and then allowed to hybridize (form base pairs) with the two homologous chromosomes that contain the unique sequences being targeted. Giemsa stain is a nonfluorescent dye that has a high affinity for DNA, and specifically accumulates in regions that are rich in A-T nucleotide pairs. This dye produces a pattern of dark and light bands, which differ for each chromosome on the basis of the distribution of A-T-rich regions.

For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below. Not all words or phrases will be used; each word or phrase should be used only once

Each chromosome is a single molecule of DNA whose extraordinarily long length can be compacted by as much as 1000-fold during INTERPHASE and tenfold more during MITOSIS. This is accomplished by binding to PROTEINS that help package the DNA in an orderly manner so it can fit in the small space delimited by the NUCLEAR ENVELOPE. The structure of the DNA-protein complex, called CHROMATIN, is highly DYNAMIC over time.

For each of the following sentences, choose one of the options enclosed in square brackets to make a correct statement about nucleosomes

For each of the following sentences, choose one of the options enclosed in square brackets to make a correct statement about nucleosomes. A. Nucleosomes are present in [procaryotic/eucaryotic] chromosomes, but not in [procaryotic/eucaryotic] chromosomes. B. A nucleosome contains two molecules each of histones [H1 and H2A/H2A and H2B] as well as of histones H3 and H4. C. A nucleosome core particle contains a core of histone with DNA wrapped around it approximately [twice/three times/four times]. D. Nucleosomes are aided in their formation by the high proportion of [acidic/basic/polar] amino acids in histone proteins. E. Nucleosome formation compacts the DNA into approximately [one-third/onehundredth/one-thousandth] of its original length.

For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below. Not all words or phrases will be used; each word or phrase should be used only once.

In eucaryotic CHROMOSOMES, DNA is complexed with proteins to form CHROMATIN. The paternal and maternal copies of human Chromosome 1 are HOMOLOGOUS, whereas the paternal copy of Chromosome 1 and the maternal copy of Chromosome 3 are NONHOMOLOGOUS. Cytogeneticists can determine large-scale chromosomal abnormalities by looking at a patient's KARYOTYPE. Fluorescent molecules can be used to paint a chromosome by a technique that employs DNA HYBRIDIZATION, and thereby to identify each chromosome by microscopy.

Because hydrogen bonds hold the two strands of a DNA molecule together, the strands can be separated without breaking any covalent bonds. Every unique DNA molecule "melts" at a different temperature. In this context, Tm, melting temperature, is the point at which two strands separate, or become denatured. Order the DNA sequences listed below according to relative melting temperatures (from lowest Tm to highest Tm). Assume that they all begin as stable doublestranded DNA molecules. Explain your answer. A. GGCGCACC B. TATTGTCT C. GACTCCTG D. CTAACTGG

The order in which the DNA molecules would denature as the temperature increases is: 1—B; 2—D; 3—C; 4—A All the DNA molecules are the same length, so only the A + T and G + C content determine their relative Tm. Molecules with higher G + C content will be more stable than molecules with a high A + T content. This is because there are three hydrogen bonds between each G-C base pair but only two between each A-T base pair. More energy (heat) is required to disrupt a larger number of hydrogen bonds.