DNA replication and recombination

model building by putting the chemical and physical date together

A and T could hydrogen bond G and C could hydrogen bond. the diameter was 20 angstrom between purine-pyrimidine

what are the purines?

Adenine and Guanine (2 rings)

leading strand synthesis

made continuously

how it is possible for DNA to carry complex genetic information when the structure is so simple?

the information in DNA is contained in the order of its building blocks. there are 3 billion nucleotides in the complete haploid set of 23 human chromosomes. this amount of sequence can provide a huge amount of info. although there are only 4 building blocks, they can be combined in a lot of combinations. when considering a short 10 nucleotide long piece of DNA, 4^10 or 1,048,576 different possible sequences exist.

in what way might telomerase be a useful target for cancer therapy?

you would want to inhibit the enzyme to stop the cancel cells from dividing

how many hydrogen bonds are between C and G?

3

who discovered the structure of DNA?

James Watson and Francis Crick (double helix)

what does DNA pol I do?

removes RNA primer from okazaki fragments and replaces with DNA. only adds nucleotides to 3'

conservative model

(1 band) replication of 2 strands 15N results in 2 strands 15N and 2 strands 14N (2 bands). replication of these strands results in 2 strands 15N and 3 sets of 2 strands of 14N (2 bands with 3x more DNA molecules at top)

dispersive model

(1 band) starts off with 2 strands of 15N. after division, each strand of both daughter molecules contains a mixture of old and new DNA. 50% 15N 50% 14N (1 band in the middle). after second replication, there are 4 sets of 2 strands with 25% 15N and 75% 14N (1 band higher than middle)

what are bacteriophages?

(bacteria eaters) viruses that infect bacteria, are made of DNA and proteins

(instapoll) how many divisions did it take to rule out the conservative mode and dispersive mode of replication respectively?

1 for conservative and 2 for dispersive

(instapoll) if DNA helix undergoes one turn every 34 angstrom and the vertical distance between 2 nucleotides is 3.4 angstrom, how many base pairs are there per one turn of helix?

10 (34/3.4)

had hershey and chase performed their experiment designed to determine the molecule of heredity using the 16N radioisotope, what would have been their results?

16N will label both DNA and protein, so radioactivity would be found within the bacterial cells and on phage ghosts

what did watson and crick know to be able to discover the structure of DNA?

1951: watson learns about x-ray diffraction pattern projected by DNA knowledge of the chemical structure of nucleotides (deoxyribose sugar, phosphate, and nitrogenous base) erwin chargaff's experiments demonstrate that ratio of A and T is 1:1 and G and C is 1:1 1953: watson and crick propose the double helix DNA model

(instapoll) how many bands would you see after 3 rounds of semiconservative replication in 14N?

2

how many hydrogen bonds are between A and T?

2

products of recombination

2 chromatids have heteroduplex after crossover. 2 chromatids are recombined after crossover

Hershey and Chase's waring blender experiment (1952)

2 different culture media contained different radioisotopes. T2 phage, infect e. coli and grow in 32P containing medium, phages with 32P labeled DNA. introduce phages into bacteria culture, (blend briefly) radioactivity recovered in host and passed onto phage progeny. T2 phage, infect e. coli and grow in 35S containing medium, phages with 35S labeled protein. introduce phages into bacteria culture, (blend briefly) radioactivity recovered in phage ghosts.

what is a heteroduplex region made up of?

2 strands, 1 from each nonsister chromatid. it may contain mismatch (G-A and T-C instead of G-C and T-A)

protein is made of what?

20 building blocks

Suppose a protein contains 100 amino acids. How many unique sequences could it potentially represent?

20^100

nucleotides can only be added to the

3' OH end of an existing strand

in what direction does DNA pol move on the template strand?

3' to 5' (moves 5' to 3' on daughter strand)

if 20% of the bases in a region of the mouse genome are cytosine, what percentage in that region are adenine?

30%

(instapoll) which radioactivity would have remained in bacterial cells if phages injected protein into their host cell?

35S

DNA is made of what?

4 subunits/building blocks known as nucelotides building blocks are linked in a chain by covalent bonds called phosphodiester bonds 2 strands/chains associate by hydrogen bonds

what are the pyrimidines?

Cytosine, Thymine, and Uracil (1 ring)

does DNA or protein carry genetic information?

DNA

is it DNA or protein that allows production of more phages?

DNA

is it DNA or protein that enters the host cell?

DNA

what is the difference between RNA and DNA?

DNA (deoxyribose) is double-stranded, RNA (ribose) is single-stranded. DNA has H attached, RNA has OH attached (both at 2' carbon) DNA has Thymine as a nitrogenous base and RNA has Uracil. DNA has uniform structure while RNA has diverse structure

what were the 2 radioisotopes hershey and chase used?

DNA 32P phosphorus protein 35S sulfur

what do chromosomes contain?

DNA and protein

the action of the enzyme DNA ligase is joining okazaki fragments together. these fragments are connected only after the RNA primers at their ends have been removed. infer the type of chemical bond whose formation is catalyzed by DNA ligase and whether or not a source of energy will be required to promote this reaction. explain why DNA ligase and not DNA polymerase is required to join okazaki fragments.

DNA ligase also catalyzes the formation of a phosphodiester bond between the nucleotides located at the 3' end of one okazaki fragment and the 5' end of the next okazaki fragment. because only a single phosphate group is attached to the 5' carbon atom of the nucleotide at the 5' end of an okazaki fragment after the primer is removed, DNA ligase needs an external source of energy to catalyze the formation of a phosphodiester bond. ligase obtains this energy by hydrolyzing ATP. this lack of a triphosphate group at the 5' end of an okazaku fragment explains why DNA polymerase cannot be used to join okazaki fragments- it does not hydrolyze ATP and no high energy bond would be available at the junction of okazaki fragments, so the DNA polymerase enzyme would have no available energy source to catalyze the formation of a phosphodiester bond.

which enzymes form phosphodiester bonds in RNA or DNA?

DNA ligase, primase, DNA poll III, DNA pol I, telomerase

mechanism of DNA replication: elongation (polymerization) by DNA pol III

DNA pol III simultaneously polymerizes a strand from the top and bottom. top strand moves toward fork (leading) while bottom strand moves away fork (lagging) in fragments. pol III only adds nucleotides to 3'

DNA polymerase cleaves the high-energy bonds between phosphate groups in nucleotide triphosphates (3 phosphate groups are attached to the 5' carbon atom of the deoxyribose sugar). the enzyme uses this energy to catalyze the formation of a phosphodiester bond when incorporating new nucleotides into the growing chain. how does this info explain why DNA chains grow during replication in the 5' to 3' direction?

DNA polymerase needs to interact with the OH group at the 3' end of the previously incorporated nucleotide and with the phosphate groups at the 5' end of the incoming nucleotide triphosphate. DNA polymerase allows the OH at the 3' end of the growing strand to attack the bond between the phosphate closest to the 5' end of the incoming nucleotide triphosphate and the next phosphate. this reaction adds the incoming nucleotide to the 3' end of the growing strand via a new phosphodiester bond. this action of DNA polymerase dictates the 5' to 3' growth of the DNA chain during replication.

If a chemical inhibitor of DNA topoisomerase or gyrase was added to bacterial cells after initiation of DNA replication, the outcome would be

DNA would be supercoiled ahead of the replication fork and replication would not proceed to completion.

some viruses do not contain DNA but have RNA inside the phage particle. an example is the tobacco mosaic virus (TMV) that infects tobacco plants, causing lesions in the leaves. 2 different variants of TMV exist that have different forms of a protein in the virus particle that can be distinguished. design an experiment to show that RNA, rather than protein, acts as the hereditary material in TMV.

Mix RNA from virus type 1 with protein from virus type 2 to reconstitute a hybrid virus. In a parallel experiment, mix RNA from virus type 2 with protein from virus type 1. Infect cells with each of these reconstituted hybrid viruses separately and analyze the protein in the progeny viruses. You will find that the progeny viruses in each case have the protein that corresponds to the type of RNA in the parent hybrid virus. The protein in the progeny did not correspond to the protein in the parent hybrid virus.Thus, RNA is the hereditary material in TMV virus while protein is not.

replication at the ends of linear chromosomes

RNA primers used during replication are removed by ribonucleases, leaving a gap at the 5' ends. without intervention, chromosomes keep getting shorter with each replication by the primer's length

nitrogen and carbon are more abundant in proteins than sulfur. why did hershey and chase use radioactive sulfur instead of nitrogen and carbon to label the protein portion of their bacteriophages in their experiments to determine whether parental protein or parental DNA is necessary for progeny phage production?

Sulfur is found only in proteins, never in DNA, while phosphorus is a major constituent of the backbone of the DNA molecule and is found rarely in proteins. Nitrogen and carbon are found in both proteins and DNA. Hershey and Chase needed to differentiate between protein and DNA, so they needed to be able to specifically label the proteins and not the DNA and vice versa. If they had used labeled nitrogen or carbon, there would be no way to differentiate protein and nucleic acid.

when a double stranded DNA molecule is exposed to high temperature, the 2 strands separate, and the molecule loses its helical form. the DNA has been denatured. regions of the DNA that contain many A:T base pairs are the first to become denatured as the temperature of a DNA solution is raised. why?

The A:T base pairs have only 2 hydrogen bonds, so it takes less heat energy to denature these base pairs. G⋮C base pairs have 3 hydrogen bonds holding them together. It thus takes more energy to break the bonds between Cs and Gs. In most organisms, the regions between genes have a higher proportion of A:T base pairs than the genes themselves.

Griffith worked with two types of S. pneumoniae bacteria - the virulent smooth forms (S) and the non-virulent rough (R) forms. Neither the heat-killed S strain nor the live R strain produced infection when injected into laboratory mice, but a mixture of the two killed the animals. What was his conclusion that was followed up by three other scientists?

The R strain was transformed into the S strain due to genetic material from the heat-killed S.

Griffith demonstrated that bacterial strains could be genetically transformed. what was the key experiment that Avery, MacCleod, and McCarty performed to prove that DNA was responsible for the genetic change from rough cells into smooth cells?

The proof that DNA was the transforming principle was the treatment of the transforming extract with an enzyme (DNase) that degrades DNA. The extract was no longer able to transform rough, nonvirulent strains of Streptococcus pneumonia bacteria into smooth, virulent cells that could kill mice. Avery, MacCleod, and McCarty also showed that treatments with RNase and proteinase did not abolish the transforming activity of their extracts, indicating that the transforming principle was neither RNA nor protein. These experiments were important because it could be argued that the purified transforming principle these investigators isolated as DNA might have contained proteins or other molecules as contaminants.

Meselson and Stahl relied on equilibrium density gradient centrifugation to resolve the DNA containing 14N from the DNA containing 15N. They started off with DNA in media containing the heavy isotope, then switched to media containing the lighter isotope and allowed DNA replication to take place. Under conservative model of DNA replication, after THREE rounds of replication, how many bands of DNA would you see and which band would contain more DNA molecules?

Two bands (one light, one heavy). the light band contains more DNA molecules

DNA direction

always 5' to 3', left to right

the 2 DNA strands are?

antiparallel. example: 5' ATCGTTGACC 3' 3' TAGCAACTGG 5'

how many different DNA molecules composed of 100 bp could possibly exist?

at each 1 of the 100 base pairs, 4 different possibilities exist A:T, T:A, G:C, C:G. therefore 4^100= 1.6 x 10^60 different 100 bp DNA molecules could exist.

mendelson and stahl method

bacteria grown in 15N medium. then transferred to 14N medium. DNA was isolated and centrifuged after each step

what does initiator protein do?

binds to the origin of replication

polynucleotide chain

carbons in the sugar are numbered 1' - 5'. phosphodiester bond is between 5' phosphate and 3' hydroxyl

what does DNA polymerase do?

catalyzes covalent bond formation with energy from newly paired nucleotide triphosphate

semiconservative model

correct model (1 band) 2 strands of the parental 15N molecule separate, and each is a template for the new 14N complementary strand. 2 sets of 2 strands. (1 band in the middle) after another replication, there are 4 sets of 2 strands. 2 of the sets are the same as previous, other 2 sets are 14N (2 bands, one on top and one in middle)

The temperature of Melting (Tm) is defined as the temperature at which 50% of double-stranded DNA is changed to single-stranded DNA. Tm depends on both the length and the nucleotide sequence composition of the molecule (GC content as opposed to AT). Tm will drop with

decrease in length of DNA, increase in AT content (because A T have 2 hydrogen bonds and are easy to break)

what are DNA nucleotides composed of?

deoxyribose (5 carbon sugar), phosphate, and 4 nitrogenous bases: purines (adenine, guanine), pyrimidines (thymine, cytosine)

deoxyribonucleic acid

deoxyribose sugar found in the nucleus acidic

lagging strand synthesis

discontinuously in fragments

mechanism of recombination for single crossover event

double strand break in 1 DNA molecule. exonuclease chews back 5' ends to create single stranded 3' ends. 1 invading strand forms a 1st heteroduplex. displaced strand forms a 2nd heteroduplex. heteroduplex region of both DNA molecules is lengthened during branch migration. after cleavage, the red DNA strand connects to the blue DNA strand at each junction. resolution of Holliday junction by resolvase and ligase cutting all 4 DNA strands once results in a crossover.

what difficulty is encountered in producing copies of both DNA strands at the end of a chromosome?

during DNA replication, after an RNA primer is removed from the 5' end of an okazaki fragment, the lost information in the strand being synthesized can be replaced. this occurs when DNA polymerase extends the 3' end of the preceding okazaki fragment by copying the template strand exposed by primer removal. however, this replacement of RNA primers cannot occur at the 5' ends of the new DNA strands because DNA polymerase can only add nucleotides to an OH at the 3' end of a primer. when the primer is removed from the very 5' end of a newly synthesize strand, there is no way DNA polymerase can replace the primer with DNA. this process would leave to a successive shortening of chromosomes in each replication. telomeres help overcome this obstacle.

what are the 4 building blocks that make up DNA? can each block be subdivided into smaller units, and if so, what are they? what kinds of chemical bonds link the building blocks?

each of the 4 building blocks is a nucleotide. each is made of the sugar deoxyribose base and a phosphate group. deoxyribose + a base makes a nucleoside. when phosphate groups are added, these becomes nucleotides. in a strand of DNA, adjacent nucleotides are connected by phosphodiester bonds that link a phosphate group to both the 3' carbon atom of the deoxyribose of one nucleotides and the 5' carbon atom of the deoxyribose of the next nucleotide in the chain.

how can eukaryotic cells overcome losing DNA at the 5' ends after each replication?

extending telomeres can counterbalance loss of DNA. telomeres in humans consist of 5' TTAGGG 3' repeats. telomerase uses its complementary RNA (3' AAUCCC 5') to extend telomeres. so even when chromosomes shorten with replication, no essential sequence is lost

Avery-MacLeod-McCarty experiment (1944)

found what transforms non virulent rough strain into the virulent smooth strain. they purified the "substance X" to test its activity. protease: protein destroyed, introduce into R cells, S cells (transformation) RNase: RNA destroyed, introduce into R cells, S cells (transformation) DNase, DNA destroyed, introduce into R cells, R cells (no transformation) ultracentrifugation: fats eliminated, introduce into R cells, S cells (transformation)

who experimented with the transformation of bacteria?

griffith and avery, macleod, mccarty. deduced that DNA is the genetic material

what is equilibrium density gradient centrifugation?

heavy salt solution that has dissolved DNA in it. after centrifuged, there is a concentration gradient with the lowest density at the top and highest density at the bottom of the tube. (light DNA at the top, heavier DNA at the bottom)

who experimented with transduction via phages?

hershey and chase. deduced that DNA is the genetic material

how can you generate new combinations of existing alleles during meiosis?

homologous recombination in crossing-over. physically breaks nucleotide sequences

how stable is DNA?

homosapiens mated with neanderthals 30,000 years ago and most europeans and asians have 1-4% neanderthal DNA. (indigenous sub-saharan africans have no neanderthal DNA) however, no intact DNA from fossils dated even less than a million years

if 30% of the bases in human DNA are A, what percentage are C? what percentage are T? what percentage are G?

human DNA is double stranded. if 30% of the bases are A, A must pair with 30% of T. that leaves 40% for C + G. C must be 20% and G must be 20%.

when and how was DNA first described?

in 1869, it was isolated from the nuclei of human white blood cells.

how does the underlying structure of RNA differ from that of DNA?

in RNA, the sugar is ribose instead of deoxyribose. RNA contains the base U instead of T. most DNA molecules found in nature are double stranded while most RNA molecules are single stranded. DNA strands can be very long (100,000,00 nucleotides in a human chromosome) and the longest naturally occurring RNA molecules are shorter, about 20,000 nucleotides at most

replication happens

in a bi-directional mode. 2 forks move in opposite direction from the origin

a particular virus with DNA as its genetic material has the following proportions of nucleotides: 20% A, 35% T, 25% G, and 20% C. how can you explain this?

in double-stranded DNA, A pairs with T and C pairs with G so their percentages must be equal. so the chromosome of this virus must be single stranded.

how does DNA have an advantage over protein?

it is much more stable. more resistant to heat, pressure, and humidity.

what do single strand binding proteins do?

keep the DNA helix open

rosalind franklin's x-ray diffraction data

long, thin helical structure, space between repeating units along the helix is 3.4 angstrom, helix undergoes a turn every 34 angstrom, helix is 20 angstrom wide

4 requirement for the genetic material

must copy info (DNA replication), must allow for info to change (mutation), must carry info (cracking genetic code), must govern the expression of phenotypes (gene function)

what are the outcomes of mismatch repair?

no conversion (for ex, if originally Bb it will remain Bb) or gene conversion (becomes homozygous) each equally likely

in the process of homologous recombination,

nonsister chromatids are able to base pair with each other through sequence homology, phosphodiester bonds break and reform as the Holliday junctions are resolved, heteroduplex, consisting of one strand for each nonsister chromatid, forms in each recombinant chromatid

what is the difference between a nucleoside and a nucleotide?

nucleoside= sugar + base nucleotide= nucleoside + phosphate

bacteriaphage (T2) life cycle

phage attaches to bacterium (host). phage injects its genes into host cell. phage DNA replicates; new phage proteins are made. phage particles assemble. cells bursts, releasing new phage.

why are DNA molecules acidic?

phosphate group. protonated form: phosphoric acid. it can lose the H+ to become acidic

what are nucleotides held together by?

phosphodiester bonds.

why did hershey and chase choose those elements to label the phages?

phosphorus is only present in DNA and sulfur is contained in only a few protein building blocks and is never found in DNA. they serve as markers for the location of each material

Avery-MacLeod-McCarty experiment conclusion

physical and chemical analysis indicates predominance of DNA. enzymes that degrade proteins, RNA, or polysaccharide had no effect on the transforming activity. but enzyme that degrades DNA destroyed the transforming activity

what do phosphodiester bonds give DNA?

polarity/direction. 5 ' phosphate -> 3' hydroxyl

the bases of one of the strands of DNA in a region where DNA replication begins are shown: 5' AATTCGTATA 3'. what is the sequence complementary to the bases?

primers for DNA synthesis are RNA molecules made by primase, therefore 3' UUAAGCAUAU 5'

(instapoll) which enzyme would have destroyed transformation if protein was the transforming substance instead?

protease

chromosomes contain more?

protein by weight than DNA

why did they think protein was originally the components that created our genetics materials?

protein has greater potential for generating diversity than DNA. different 10-residue molecules for DNA: 4^10= 1,048,576 different 10-residue molecules for protein: 20^10= 10,240,000,000,000. protein has almost 10,000,000 times more combinations

Hershey and Chase's experiment conclusion

radioactivity found in bacterial cells= 32P radioactivity recovered in phage ghosts = 35S injected viral material that programs replication of more phages is DNA, not protein

which of the following statements are true about double stranded DNA?

remember that in double stranded DNA, the amount of A=T and the amount of G=C. but these 2 sets of amounts do not have to equal each other. the following are true: A+C= T+G A+G= C+T A/G = T/c (frequencies of pairs are equal)

mechanism of DNA replication: initiation and formation of a replication bubble

replication bubble grows, both replication forks move away from the origin. involves initiator protein, DNA helicase, single strand binding proteins, and primase.

what does ligase do?

seals the gap between okazaki fragments by making a phosphodiester bond

what are Okazaki fragments?

short fragments on the lagging stand

Griffith's experiment (1928)

showed transformation using streptococcus pneumoniae. smooth form (virulent): produces a polysaccharide coat that helps defeat the host's immune system. usually fatal rough form (non-virulent): lacks an enzyme required for production of the polysaccharide coat. fails to cause infection. griffith injected smooth form into mice and they died. rough form did not kill injected mice. when he heated the smooth form and injected them, the mice lived. when he mixed the heated smooth form with the rough form, the mice died and living smooth form was recovered after. something from the heat-killed smooth form transformed the rough form into smooth. most likely the genetic material

(instapoll) after DNA double helix is heated to 100C, what chemical structures would be present? (what is the weakest chemical bond that is easily disrupted by heat?)

single stranded DNA molecules

variations of genetic information

some dsDNA molecules are circular: prokaryotes, mitochondria, chloroplasts, viruses some viruses carry ssDNA: either linear or circular some viruses carry RNA: ssRNA or dsRNA, HIV, poliovirus, coronavirus

what is transformation?

something transforms one form of bacteria to the other, the phenotype changes and is heritable

what does primase do?

synthesizes RNA primers to prime DNA synthesis

Meselson and Stahl experiment (1958)

tested 3 possible modes of DNA replication to find out how DNA replicates. used equilibrium density gradient centrifugation.

describe the results you would expect from dispersive replication after 3 rounds of DNA synthesis on 14N.

the 15N strand is becoming less prominent as it is divided upon 8 molecules. 12.5% 15N and 87.5% 14N. The single density line is above the middle and closer to the top of the tube because there are more light strands than heavy strands.

describe the results you would expect from semiconservative replication after 3 rounds of DNA synthesis on 14N.

there are 2 molecules that are made up of one 15N strand and one 14N strand, along with 6 molecules of 2 14N strands. The density line for the 14N is at the top of the tube (with 3 times more DNA molecules than 15N) while the density line for both the 15N and 14N strands combined is in the middle.

describe the results you would expect from conservative replication after 2 rounds of DNA synthesis on 14N.

there are still 2 intact 15N strands, but there are now 3 molecules of 2 intact 14N strands. The density line for the 14N is at the top of the tube (with 3 times more DNA molecules than 15N) while the density line for the 15N strands is at the bottom.

describe the results you would expect from conservative replication after 3 rounds of DNA synthesis on 14N.

there are still 2 intact 15N strands, but there are now 7 molecules of 2 intact 14N strands. The density line for the 14N is at the top of the tube (with 7 times more DNA molecules than 15N) while the density line for the 15N strands is at the bottom.

unlike somatic cells, telomerase is expressed in germ cells, stem cells, and cancer cells. what is the consequence for these cells expressing telomerase?

they are able to divide

most somatic cells do not express the genes that encode telomerase. what is the consequence for these somatic cells?

they lose all telomere. the gene may be lost and unable to divide

bacteria origin of replication

they only have 1 origin and they are circular. when you try to pull the strand/hydrogen bonds apart, the unwinding causes supercoiling which impedes replication and distorts molecule

if the temperature is lowered, the original DNA strands can renature. in addition to the double-stranded molecules, some molecules as the type shown (1 strand with 2 hanging orbs) are seen. how can you explain these structures?

this denatured single-stranded DNA must contain stretches of nucleotides that are complementary to a nearby sequence but in an inverted orientation. These stem-loop structures are regions where the single strand of DNA formed a double-stranded region (the stem) separated by a single-stranded region (the loop). It is important to keep in mind that the loops, as well as the stems, are all made from the same single molecule that is a single strand of DNA.

how are supercoils removed?

topoisomerase, a class of enzymes that relax supercoils by cutting one or both strands, rotating and resealing DNA. the chemical bond being broken is phosphodiester bond

imagine you have 3 test tubes containing identical solutions of purified, double stranded DNA. you expose the DNA in tube 1 to an agent that breaks the (sugar-phosphate) phosphodiester bonds. you expose the DNA in tube 2 to an agent that breaks the bonds that attach the bases to the sugars. you expose the DNA in tube 3 to an agent that breaks the hydrogen bonds. after treatment, how would the structure of the molecules in the 3 tubes differ?

tube 1: in theory, you would see individual pairs of complementary nucleotides held together by hydrogen bonds, but in practice the nucleotides of each pair are likely to separate. this is because hydrogen bonds that hold together individual complementary nucleotides are not very strong. the phosphate groups would be free in solution. in practice most agents that break phosphodiester bonds break only one sugar-phosphate linkage so each nucleotide would have a phosphate connected to 5' carbon atom of its deoxyribose or connected to 3' carbon atom of its deoxyribose. tube 2: you'd see nitrogenous base pairs and sugar phosphate chains without the bases and no intact nucleotides would be present. tube 3: would contain single strands of DNA. note: high temperature can melt double stranded DNA into single strands

what does DNA helicase do?

unwinds the helix

what is transduction?

viruses infect host cells and take over, more phages are produced from the host that bursts.