Explore PCR, DNA fragments, plasmids, and the possibilities and limits of genetic tools.
Part of the free Knowledge Mountain biology library. Read an explanation, try a question, and use a memory cue to check what you can recall.
Restriction enzymes cut DNA at particular sequences. DNA ligase joins DNA pieces. PCR copies a chosen DNA region. Gel electrophoresis separates fragments. Sequencing determines base order.
Different tools answer different questions. A gel can compare fragment sizes, but it does not directly read the complete nucleotide sequence of every band.
What does PCR amplify, and what happens during its three repeated temperature steps?
A selected DNA region. Heating separates strands, cooling lets primers bind, and a suitable polymerase extends new strands.
The primers define the target region. Repeating the cycle produces many copies, ideally approximately doubling the target each cycle in early stages.
Two DNA fragments run through the same gel for the same time. One is 200 base pairs and the other is 1,000. Which usually travels farther?
The 200-base-pair fragment.
DNA is negatively charged and moves toward the positive electrode. Smaller fragments usually pass through the gel’s pores more easily and move farther.
Plasmids are small DNA molecules that can serve as vectors. Scientists can insert DNA into a vector and introduce it into cells that copy or express the added sequence.
Cloning a gene means making copies of that DNA; it does not necessarily mean making an entire cloned organism. Regulatory sequences and host compatibility affect expression.
Does a match at several DNA markers, by itself, prove exactly when or how a person’s DNA reached a scene?
No.
DNA profiles compare selected markers and can strongly support identity-related conclusions, but timing and circumstances require additional evidence. Contamination and sample handling also matter.