Molecular Tools Flashcards

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1
Q

Describe the complication with DNA replication associated with the fact that DNA polymerase can only add nucleotides in the 5’ to 3’ direction.

A
  • Lagging strand has to be copied in a discontinuous manner
  • Leading strand can be copied in a continuous manner
  • Lagging strand has a series of short segments called Okazaki fragments that must be joined together
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2
Q

What is needed to initiate synthesis of a new DNA strand?

A
  • An RNA primer (7 to 10 nucleotides) is needed to initiate synthesis of a new DNA strand
  • Synthesized by an enzyme called primase
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3
Q

What are DNA polymerases?

A

Enzymes that synthesize new polynucleotides complementary to an existing DNA or RNA template.

  • DNA-dependent DNA polymerase
  • RNA-dependent DNA polymerase
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4
Q

Describe the biological activities of DNA polymerases.

A
  • In the cell, DNA polymerases are essential for DNA replication
  • In addition to adding nucleotides, they have enzymatic activities that are important for removing incorrect nucleotides
  • 3’ to 5’ exonuclease activity removes incorrect nucleotides (Called ‘proofreading’ activity)
  • 5’ to 3’ exonuclease activity is needed for DNA polymerases that must remove part of a polynucleotide that is already attached to the template strand that the polymerase is copying
    • For example, in DNA replication, an RNA primer is used to start DNA replication but must be removed by DNA polymerase 5’ to 3’ exonuclease activity and replaced by DNA nucleotides so that newly replicated strands can be ligated together
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5
Q

What are DNA polymerases used for in Molecular Biology research?

A
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6
Q

What is DNA polymerase I used for in molecular biology research?

A

DNA labelling
Has both 3’ to 5’ and 5’ to 3’ exonuclease activities

Unmodified E. coli enzyme.

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7
Q

What is Klenow polymerase used for in molecular biology research?

A

5’ to 3’ exonuclease activity removed but the 5’ to 3’ polymerase and 3’ to 5’ exonuclease activity remains

Modified version of E. coli DNA polymerase I.

The removal of the 5’ to 3’ exonuclease activity from Klenow polymerase is useful because it prevents DNA degradation during key processes like DNA labeling, blunt-end synthesis, and high-fidelity DNA replication, while retaining the necessary 3’ to 5’ proofreading function to maintain accuracy.

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8
Q

What is Taq polymerase used for in molecular biology research?

A
  • PCR
  • Isolated from a bacteria (Thermus aquaticus) that lives in hot springs

Thermus aquaticus DNA polymerase I - useful in PCR because it remains active at high temperatures.

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9
Q

Describe the polymerase chain reaction process.

A
  • Denaturation (95C)
  • Primer annealing (50-60C)
  • Elongation by Taq polymerase I (72C)
The first cycle. The next cycle of denaturation-annealing-synthesis leads to four products, two of which are identical to the first-cycle products and two of which are made entirely of new DNA. During the third cycle, the latter give rise to short products that, in subsequent cycles, accumulate in exponential fashion.
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10
Q

What are applications of PCR in food science? [4]

A
  • Detection of foodborne pathogens (multiplex PCR to detect for multiple foodborne pathogens)
  • Testing for the presence of GMOs
  • Making recombinant proteins (e.g., insulin production; chymosin production)
  • Identifying and characterizing microbial populations in food products
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11
Q

What is reverse transcriptase used for in molecular biology research?

A
  • cDNA synthesis
  • Makes DNA copies from RNA templates

RNA-dependent DNA polymerase, obtained from various retroviruses. Retroviruses (e.g., HIV) use RT to replicate their genomes.

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12
Q

What is reverse transcriptase useful for in molecular biology?

A

Transcriptomics - analysis of total gene expression of an organism, tissue or cell

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13
Q

Describe a schematic representation of a transcriptomic evaluation approach.

A
  1. Tissue sample
  2. RNA extraction
  3. Library preparation
  4. High-throughput sequencing
  5. Transcriptome assembly
  6. Differential expression and functional analyses
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14
Q

Describe how reverse transcriptase is used in molecular biology. [7]

A
  • Eukaryotic mRNAs have a poly(A) tail at their 3’end
    • Add a primer (oligonucleotide) with ~20 nucleotides of Ts to bind to the poly A tail
  • Reverse transcriptase enzyme synthesizes DNA from the RNA strand
  • Add Ribonuclease H to degrade most of the RNA strand
  • Synthesize the next strand with DNA polymerase using leftover RNA as primers
    • DNA polymerase that still has a 5’ to 3’ exonuclease activity to remove RNA and replace with DNA
  • Now you have a double stranded complementary DNA (cDNA) that can be used for DNA sequencing

cDNA has always been made from an RNA template.

The polyA tail of eukaryotic mRNA allows selection of mRNA as opposed to any RNA (rRNA etc.)
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15
Q

What is a nuclease?

A

Enzymes that degrade DNA molecules by breaking the phosphodiester bonds that link one nucleotide to another nucleotide

Example: 3' to 5' exonuclease activity of DNA polymerase removes incorrect nucleotides.
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16
Q

What are restriction endonucleases?

A
  • Cut double stranded DNA at a specific recognition sequence
  • Called ‘restriction endonucleases’ because in bacterial cells they cut DNA from bacteriophage

Example: EcoRI

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17
Q

What is EcoRI?

A
  • A restriction enzyme that binds DNA as a dimer and cuts both strands
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18
Q

Describe DNA digestion with EcoRI.

A

Sticky ends, 5’ overhang

GAATTC is the sequence that it recognizes; EcoRI breaks both strands
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19
Q

Restriction enzymes produce […]

A

Either blunt or sticky ends

Compatible enzymes have the exact same overhang sequence and that means you can put them back together. They have different recognition sequences but give the same sticky ends (e.g., BamHI and SauAI)
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20
Q

How many restriction endonucleases are available for use in the lab?

A
  • > 600!
  • Many have hexanucleotide (6bp) recognition sequences
  • Some have degenerate recognition sequences such as Bgl I (some specificity, but many nucleotides can just be whatever)
N = any nucleotide
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21
Q

How can restriction digests be analyzed?

A

DNA Gel electrophoresis

  • Agarose gel - pores ranging from 100 to 300 nm in diameter
  • Agarose is a linear polymer extracted from red algae
  • DNA is negatively charged and moves towards positive electrode (anode)
  • Smaller DNA fragments migrate faster than larger DNA fragments through the gel
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22
Q

How can DNA be visualized after gel electrophoresis?

A
  • Ethidium bromide wash
  • DNA migration pattern depends on agarose %
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23
Q

DNA migration pattern depends on agarose %. What % would you use for larger vs smaller fragments?

A
  • to look at larger DNA fragments run 0.75% gel
  • to look at smaller DNA fragments run 1.25% gel
24
Q

Describe the role of DNA ligase in vivo.

A
  • Needed during DNA replication after RNA primers have been replaced by DNA
25
Q

Desribe DNA ligase in vitro.

A
  • Easier to ligate sticky ends
26
Q

Give an outline of gene cloning.

A
  • Need a plasmid - circular DNA fragment
  • Animal gene has BamHI restriction sites on either side
  • After ligation of animal gene into plasmid, transform ligation mixture into E. coli
  • Select E. coli that have the plasmid and original E. coli plasmid
Due to recombinant DNA, we no longer need calf stomach to make cheese.
27
Q

Describe pUC8.

A
  • A simple E. coli plasmid
  • Used for recombinant plasmid selection
28
Q

Describe recombinant plasmid selection.

A
  • Done with plasmid pUC8
29
Q

Describe the structure of the yeast chymosin expression plasmid.

A

Note from Dr. Measday:

“The chymosin expression plasmid and the pUC8 plasmid are two separate plasmids. I did not show the chymosin expression plasmid before cloning in the chymosin gene but it would have been a plasmid just carrying the ori, two micron, AMP and LEU2 gene with a multiple cloning site. The PGK1 promoter, Pro-chymosin gene and PGK terminator would all have to be cloned into the plasmid. The cloning would all be done in E. coli - this could be done by PCR amplification of the promoter, chymosin gene and PGK terminator. Once the chymosin expression plasmid is created, then it would be transformed into yeast for expression of the chymosin gene.”

30
Q

What is pPGK1?

A
  • Promoter region from the yeast PGK1 gene (3-phosphoglycerate kinase) - strong, constitutive promoter
31
Q

What is pPGK1term?

A
  • Terminator region from the yeast PGK1 gene - needed for efficient transcription termination
32
Q

What is pro-chymosin?

A

Bovine chymosin gene

33
Q

What is ori?

A

Bacterial origin of replication

34
Q

What is 2µ?

A

Yeast replication gene that allows for high copy numbers of the plasmid in yeast.

35
Q

What is LEU2?

A
  • Selection for plasmid in yeast.
  • Must transform plasmid into a leu2D strain and select on -leucine agar plates
  • Plasmid transformation into yeast can be selected for on agar plates that do not have leucine
36
Q

What is AMP^R?

A
  • Gene for ampicillin resistance - codes for beta-lactamase
  • Must select on ampicillin agar plates
37
Q

Describe a milk clotting assay with yeast expressed pro-chymosin.

A
  • Pre – signal peptide (16 amino acids, that is lost during secretion - not in yeast plasmid)
  • Pro-chymosin – inactive form of chymosin
  • At low pH, an autocatalytic cleavage removes 42 amino acids from the N-terminus of prochymosin to form Chymosin which is active
38
Q

What happens to mRNAs after translation?

Once mRNAs enter the cytoplasm, they are either (1) translated, (2) stored for later translation or (3) degraded.

A
  • They may be temporarily translationally repressed
  • mRNAs are actually associated with proteins during their lifetime which are called mRNA-protein (mRNP) complexes
  • P-bodies are cytoplasmic mRNP granules that have translationally repressed mRNAs and proteins involved in mRNA decay
39
Q

What are mRNP complexes?

A

mRNAs associate with proteins during their lifetime, and they are referred to as mRNA-protein (mRNP) complexes

40
Q

What are P-bodies ?

A
  • Cytoplasmic mRNP granules that have (1) translationally repressed mRNAs and (2) proteins involved in mRNA decay
41
Q

Are amino acids the same between all organisms?

A
  • Yes - there are 20 proteinogenic amino acids in all organisms
42
Q

What is selenocysteine?

A
  • Analogue of cysteine with selenium in place of sulfer
  • Not coded for directly in the genetic code
  • Not universal in all organisms
  • UGA codon (stop codon) can encode for selenocysteine if cells are grown in selenium
  • Requires presence of selenocysteine insertion sequence (SECIS) in mRNA
43
Q

Define non-proteinogenic amino acids and give 2 examples.

A
  • Not incorporated into proteins
    • Gamma-aminobutyric acid (GABA) is produced by decarboxylation of glutamic acid
      • GABA is found in the human brain where it acts as an inhibitory neurotransmitter
    • L-3,4-dihydroxyphenylalanine (L-DOPA) is synthesized from the amino acid tyrosine
      • Found in plants and humans
      • L-DOPA is the precursor to neurotransmitters including dopamine
44
Q

How is GABA produced?

A

Decarboxylation of glutamic acid

GABA is found in the human brain where it acts as an inhibitory neurotransmitter

45
Q

How is L-DOPA synthesized?

A

From tyrosine

L-DOPA is found in plants and humans, and is the precursor to neurotransmitters including dopamine

46
Q

What are nucleases?

A

Enzymes which degrade DNA molecules by breaking down phosphodiester bonds that link one nucleotide to another; two general categories (endo and exo)

  • Endonuclease make internal cuts
  • Exonuclease remove from ends
Example: 3' to 5' exonuclease activity; DNA polymerase removes incorrect nucleotides
47
Q

What is a restriction endonuclease?

A
  • Cut double stranded DNA at a specific recognition sequence
Note that EcoRI has a magnesium ion cofactor
48
Q

Compare blunt and sticky ends.

A
  • A simple double-stranded cut called a blunt end
  • Sticky ends - DNA strands are cut at different positions, 2 to 4 nucleotides apart
    • Resultant DNA fragments have short, single-stranded overhangs at each end - 3’ or 5’ overhangs
    • Called ‘sticky’ or ‘cohesive’ ends
49
Q

What would happen if you cut the lacZ gene in the pUC8 plasmid with two enzymes?

A
  • A fragment would be lost/degraded in the cloning step
  • You could then clone in a gene with an EcoRI site on the left side and a BamHI site on the right side
50
Q

The pUC8 plasmid expresses the lacZ’ gene which encodes for […]

A

Beta-galactosidase

The lacZ’ gene is also where the restriction enzymes are located for cloning.

51
Q

What is the cellular function of beta-galactosidase?

A
  • To split lactose into glucose and galactose
LacZ can also cleave a chromophore called X-gal to produce blue colour.

The lacZ’ gene in the pUC8 plasmid is functional and so bacteria containing pUC8 will be blue!

52
Q

How are recombinant plasmids selected with pUC8?

A
  • When the DNA insert (gene) is cloned into the lacZ’ gene, it disrupts lacZ’ function and therefore X-gal is not cleaved and the bacterial colony will be white.
Blue colonies are bacteria with the pUC8 plasmid whereas white colonies are bacteria with the recombinant pUC8 plasmid (gene cloned into lacZ).
53
Q

Cloning done in E. coli and cloning done in yeast both need […]

A

To select for and replicate plasmid

54
Q

Describe a milk-clotting assay using yeast-expressed pro-chymosin (precursor to chymosin), an enzyme essential for cheese production due to its ability to clot milk.

A

Pro-chymosin and Chymosin:

At a low pH (around 2.0), pro-chymosin undergoes an autocatalytic cleavage, meaning it activates itself by removing 42 amino acids from the N-terminus (the start of the protein). This process converts it into active chymosin.

Pre-signal peptide:

The pre-signal peptide is a sequence of 16 amino acids present before secretion. This part is lost during secretion, meaning it is cut off during the process of producing the pro-chymosin in the yeast. It’s not part of the final active enzyme.
This pre-signal peptide is not present in the yeast plasmid, which is the vector used to express the pro-chymosin in yeast.

Assay Process:

The assay involves adding pro-chymosin yeast extract to a milk solution.
To activate the pro-chymosin, the yeast extract is treated at pH 2.0, which triggers the conversion of pro-chymosin to chymosin. When added to milk, the active chymosin clots the milk by breaking down casein proteins.

This assay is a way to test the clotting ability of chymosin expressed in yeast by tracking how effectively it converts from its inactive form at low pH and then clots milk.

55
Q

Outline the process of cloning the chymosin gene using the pUC8 plasmid, E. coli, and yeast for expression. [8]

A

1. Gene Identification and Isolation

  • Source of Chymosin mRNA: Isolate mRNA encoding the chymosin gene from calf stomach cells, where chymosin is naturally produced.
  • cDNA Synthesis: Use reverse transcriptase to synthesize complementary DNA (cDNA) from the mRNA template. An oligo(dT) primer binds to the poly-A tail of the mRNA, ensuring reverse transcription starts at the correct site.
  • Amplification: The cDNA corresponding to the chymosin gene is then amplified using PCR, making it ready for subsequent cloning steps.

2. Vector Selection: pUC8 Plasmid

  • Plasmid Characteristics: The pUC8 plasmid is selected as the cloning vector. It contains:
    • An origin of replication (ori) for replication in E. coli.
    • An ampicillin resistance gene (AMP^R) for selection in E. coli.
    • A multiple cloning site (MCS) within the lacZ gene, allowing for blue/white screening of recombinant plasmids.

3. Inserting the Chymosin Gene into pUC8 (Ligation)

  • Restriction Enzyme Digestion: Both the amplified chymosin gene and the pUC8 plasmid are cut using the same restriction enzymes to ensure compatible ends.
  • Ligation: The chymosin gene is inserted into the MCS of the pUC8 plasmid using DNA ligase, creating a recombinant plasmid.

4. Transformation into E. coli

  • Heat Shock/Electroporation: The recombinant pUC8 plasmid, now containing the chymosin gene, is introduced into competent E. coli cells by heat shock or electroporation.
  • Plating for Selection: The transformed E. coli are plated on media containing ampicillin and X-gal.

5. Selection and Screening in E. coli

  • Antibiotic Selection: Only E. coli cells that have taken up the recombinant plasmid will survive on the ampicillin plates, as they possess the AMP^R gene.
  • Blue/White Screening: Colonies that successfully incorporated the chymosin gene will disrupt the lacZ gene, appearing white on X-gal plates, while non-recombinant colonies remain blue.

6. Transfer to Yeast Expression Vector

  • Cutting from pUC8: Once the chymosin gene has been amplified in E. coli, restriction enzymes are used to excise the chymosin gene from the pUC8 plasmid.
  • Insertion into Yeast Expression Vector: The chymosin gene is inserted into a yeast expression plasmid that contains the LEU2 gene (used for selection in yeast) along with the PGK1 promoter for proper gene expression.
  • Amplification in E. coli: This yeast expression plasmid is further amplified in E. coli if necessary.

7. Transformation into Yeast

  • Transforming Yeast: The recombinant yeast expression plasmid is introduced into yeast cells, allowing the yeast to express the chymosin gene.
  • Selection: The transformed yeast cells are grown on leucine-deficient media. Only yeast cells that have taken up the plasmid, which includes the LEU2 gene, will survive and grow on the leucine-deficient media.

8. Expression and Harvesting

  • Chymosin Production: The yeast cells express the chymosin gene under the control of the PGK1 promoter, producing pro-chymosin, which is harvested.
  • Protein Purification: The pro-chymosin protein is purified using techniques such as affinity chromatography to isolate the enzyme.
  • Testing Activity: The activity of the purified chymosin is tested using a milk-clotting assay, where acid induces autocatalytic cleavage of 42 amino acids from the
    N-terminus end of pro-chymosin to form the active enzyme chymosin.