Molecular Tools Flashcards
Describe the complication with DNA replication associated with the fact that DNA polymerase can only add nucleotides in the 5’ to 3’ direction.
- 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
What is needed to initiate synthesis of a new DNA strand?
- An RNA primer (7 to 10 nucleotides) is needed to initiate synthesis of a new DNA strand
- Synthesized by an enzyme called primase
What are DNA polymerases?
Enzymes that synthesize new polynucleotides complementary to an existing DNA or RNA template.
- DNA-dependent DNA polymerase
- RNA-dependent DNA polymerase
Describe the biological activities of DNA polymerases.
- 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
What are DNA polymerases used for in Molecular Biology research?
What is DNA polymerase I used for in molecular biology research?
DNA labelling
Has both 3’ to 5’ and 5’ to 3’ exonuclease activities
Unmodified E. coli enzyme.
What is Klenow polymerase used for in molecular biology research?
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.
What is Taq polymerase used for in molecular biology research?
- 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.
Describe the polymerase chain reaction process.
- Denaturation (95C)
- Primer annealing (50-60C)
- Elongation by Taq polymerase I (72C)
What are applications of PCR in food science? [4]
- 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
What is reverse transcriptase used for in molecular biology research?
- 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.
What is reverse transcriptase useful for in molecular biology?
Transcriptomics - analysis of total gene expression of an organism, tissue or cell
Describe a schematic representation of a transcriptomic evaluation approach.
- Tissue sample
- RNA extraction
- Library preparation
- High-throughput sequencing
- Transcriptome assembly
- Differential expression and functional analyses
Describe how reverse transcriptase is used in molecular biology. [7]
- 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.
What is a nuclease?
Enzymes that degrade DNA molecules by breaking the phosphodiester bonds that link one nucleotide to another nucleotide
What are restriction endonucleases?
- Cut double stranded DNA at a specific recognition sequence
- Called ‘restriction endonucleases’ because in bacterial cells they cut DNA from bacteriophage
Example: EcoRI
What is EcoRI?
- A restriction enzyme that binds DNA as a dimer and cuts both strands
Describe DNA digestion with EcoRI.
Sticky ends, 5’ overhang
Restriction enzymes produce […]
Either blunt or sticky ends
How many restriction endonucleases are available for use in the lab?
- > 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)
How can restriction digests be analyzed?
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
How can DNA be visualized after gel electrophoresis?
- Ethidium bromide wash
- DNA migration pattern depends on agarose %