PCR, Gel electrophoresis and DNA profiling OCR A A-Level Biology

Summary

In this biology revision session, Christine explores key concepts like PCR, gel electrophoresis, and DNA profiling. The session delves into understanding the genome, satellite DNA, and how specific DNA sequences are used in DNA profiling. Through PCR, minute DNA samples are amplified, and gel electrophoresis separates DNA fragments based on size. The process is further explained with a focus on restriction endonucleases, hybridization, and visualization of DNA fragments. Christine also highlights the application of DNA profiling in forensic science and disease risk analysis, stressing the importance of accuracy in these processes.

Takeaways

• 😀 The genome of an organism consists of all its genetic material, including nuclear DNA, mitochondrial DNA, and in plants, chloroplast DNA.
• 😀 The human genome is approximately 3.2 billion nucleotides long, with only 2% of it being genes (exons), which code for proteins.
• 😀 Satellite DNA refers to short, repetitive sequences found in introns and regions like telomeres, with micro and mini satellites being key examples.
• 😀 Microsatellites are short tandem repeats (2–4 base pairs), repeated 5 to 15 times, while minisatellites are longer repeats (20–40 base pairs), repeated 50–100s of times.
• 😀 DNA profiling involves extracting DNA, using restriction endonucleases to cut DNA into fragments, and then amplifying them through PCR.
• 😀 PCR (Polymerase Chain Reaction) amplifies DNA through cycles of denaturation, annealing, and extension, with Taq polymerase playing a crucial role in elongation.
• 😀 Each PCR cycle doubles the amount of DNA, leading to exponential amplification of the target DNA sample, with typically 30 cycles performed.
• 😀 Gel electrophoresis is used to separate DNA fragments based on size, with smaller fragments moving faster through agarose gel under an electric field.
• 😀 Southern blotting, a process of transferring DNA fragments to a membrane, allows hybridization with radioactive or fluorescent DNA probes to visualize specific sequences.
• 😀 DNA profiling is widely used in forensic science to compare DNA samples from crime scenes to databases, but errors in handling can impact the reliability of results.

Q & A

What is the human genome made up of?

The human genome consists of all the genetic material in a human, including nuclear DNA, mitochondrial DNA, and in plants, chloroplast DNA.

What percentage of the human genome is composed of genes that code for proteins?

Only 2% of the human genome consists of genes that code for proteins, known as exons.

What is satellite DNA and where is it found?

Satellite DNA consists of short, repetitive sequences found in introns of the DNA. These are non-coding regions that are often repeated many times.

How does DNA profiling work?

DNA profiling works by extracting DNA from a sample, cutting it into fragments using restriction endonucleases, amplifying the DNA with PCR, and separating the fragments using gel electrophoresis. The resulting patterns can then be analyzed for identification.

What role do restriction endonucleases play in DNA profiling?

Restriction endonucleases are enzymes that cut DNA at specific recognition sites, producing fragments of varying lengths that can be analyzed and compared during DNA profiling.

How does PCR (Polymerase Chain Reaction) amplify DNA?

PCR amplifies DNA by cycling through three steps: denaturation (separating DNA strands), annealing (primers bind to DNA), and extension (new DNA strands are synthesized). This process is repeated to generate billions of copies of the original DNA sample.

What is the function of Taq polymerase in PCR?

Taq polymerase is an enzyme used in PCR that synthesizes new DNA strands by extending the primers, enabling the amplification of the DNA sample.

What is the principle behind gel electrophoresis?

In gel electrophoresis, DNA fragments are separated by size as they move through a gel when exposed to an electric current. Smaller fragments move faster, while larger ones move slower, creating distinct bands based on their size.

What is hybridization in the context of DNA analysis?

Hybridization in DNA analysis refers to the process of using DNA or RNA probes that are complementary to specific DNA sequences. These probes bind to the target sequences, allowing them to be detected using radioactive or fluorescent markers.

How is DNA profiling used in forensics?

In forensics, DNA profiling is used to match DNA samples (such as blood, hair, or skin cells) to individuals by comparing the unique patterns of DNA fragments obtained through PCR, restriction digestion, and gel electrophoresis.

What are the limitations of DNA profiling in forensic science?

The limitations of DNA profiling in forensic science include potential human errors in sample collection, lab analysis, or data interpretation, which can affect the reliability of the results and either falsely convict or exonerate individuals.