Grade 11-12 (age 16-18)

high school Biotechnology

Biotechnology uses restriction enzymes to cut DNA at chosen sites, DNA ligase to join the pieces, and PCR to copy a chosen DNA fragment over and over. A restriction enzyme recognizes a palindromic base sequence, one that reads the same on both strands, and cuts the two strands at staggered points to leave sticky ends, while each PCR cycle of denaturation, primer annealing and extension doubles the DNA. Putting a gene into a vector such as a plasmid and expressing it in a host such as E. coli yields proteins such as insulin in large amounts. In the diagram you can watch EcoRI cut GAATTC so that each piece is left with a single-stranded AATT, and see the number of DNA molecules rise as a straight line on a log scale as PCR cycles add up.

Cutting, Joining and Copying Genes

💡 The Basic Tools of Biotechnology

①Restriction enzyme: molecular scissors that recognize a particular base sequence in DNA and cut exactly there

②DNA ligase: molecular glue that joins the sugar–phosphate backbones of cut DNA pieces

③PCR (polymerase chain reaction): a technique that picks out a chosen DNA fragment and copies it millions of times or more

④Tests for viral infection use PCR to multiply the virus's genetic material in a sample until it can be detected

⑤Recombinant DNA made with these tools is used to mass-produce proteins such as insulin and growth hormone

Restriction Enzymes: Molecular Scissors

Cutting progress0

🔍 How a Restriction Enzyme Cuts

①EcoRI recognizes 5'-GAATTC-3', and the complementary strand also reads GAATTC from 5' to 3', so the site is a palindrome

②EcoRI cuts both strands between G and A, so the cuts on the two strands are staggered

③As a result, each piece ends with four single-stranded bases, AATT, sticking out: a sticky end

④Any other DNA cut with the same enzyme has the same sticky ends, so the pieces pair up and join easily

⑤Some restriction enzymes cut both strands at the same point and leave blunt ends with no single-stranded overhang

PCR: Multiplying Just the DNA You Want

PCR cycles10

DNA Made by PCR

number of DNA molecules = starting number × 2ⁿ (n = number of cycles)

in theory, 30 cycles turn 1 molecule into about a billion (2³⁰ ≈ 1.07 × 10⁹)

💡 The Three Steps of PCR

①Denaturation (about 94 °C): high heat breaks the hydrogen bonds of double-stranded DNA and separates the two strands

②Primer annealing (about 55 °C): when the temperature drops, primers bind to each end of the region to be copied

③Extension (about 72 °C): a heat-stable DNA polymerase extends the primers and builds new strands

④These three steps make one cycle, each cycle doubles the DNA, and the cycle is usually repeated 25 to 35 times

⑤To test for an RNA virus, reverse transcriptase first turns the RNA into cDNA, which is then multiplied by PCR

Recombinant DNA and Its Uses

Making Recombinant DNA

cut the gene and the plasmid with the same restriction enzyme → join them with DNA ligase → put them into E. coli (transformation) → select bacteria carrying the recombinant plasmid

a chosen gene is put into another organism so that the organism makes the gene's product

📐 Key Techniques and Uses

①Vector: a carrier that moves a gene into a host cell; plasmids, which copy themselves inside bacteria, are the classic example

②Transformation: bacteria that take in a foreign gene express it and show a new trait

③E. coli carrying the human insulin gene makes human insulin in large amounts for treating diabetes

④Gel electrophoresis: negatively charged DNA fragments move through a gel toward the positive pole, and smaller fragments travel farther

⑤DNA fingerprinting: comparing short tandem repeats (STRs), whose number of repeats differs from person to person, tells individuals apart

Worked Examples and Exam Practice

Example 1

In theory, how many molecules do you get from one DNA molecule after 10 cycles of PCR?

Each PCR cycle doubles the DNA, so after n cycles there is 2ⁿ times as much.

After 10 cycles, 2¹⁰ = 1024, so one molecule becomes 1024.

▸ 2¹⁰ = 1024 molecules

The vertical axis of the graph is a log scale, so 2ⁿ appears as a straight line. Every 10 more cycles multiplies the DNA by about 1000, giving about a million after 20 cycles and about a billion after 30.

Example 2

Two different DNA molecules are cut with the same restriction enzyme to give sticky ends. Which enzyme joins the two pieces into one?

Cut with the same enzyme, the sticky ends of the two pieces have complementary base sequences.

Once the ends pair up, DNA ligase joins the sugar–phosphate backbones to complete one recombinant DNA molecule.

▸ DNA ligase

If the restriction enzyme is the scissors, DNA ligase is the glue. The two are used together to insert a chosen gene into a plasmid.

School-exam style

Which statement about biotechnology is correct?

①The extension step of PCR takes place at about 94 °C.

②Restriction enzymes cut DNA at random positions.

③In theory, n cycles of PCR multiply the DNA 2ⁿ times.

④In gel electrophoresis, larger DNA fragments move faster.

⑤DNA ligase is an enzyme that cuts DNA.

▸ ③ In theory, n cycles of PCR multiply the DNA 2ⁿ times.

PCR repeats denaturation, primer annealing and extension as one cycle, doubling the DNA each cycle, so after n cycles there is 2ⁿ times as much.

Extension takes place at about 72 °C (①), restriction enzymes cut only particular palindromic sequences (②), smaller fragments move faster in electrophoresis (④), and DNA ligase joins DNA (⑤).

Summary

PCR Amplification

n cycles → 2ⁿ times as much (n = number of cycles)

in theory about a million times after 20 cycles and about a billion times after 30

🎯 Exam Points

①PCR repeats denaturation (about 94 °C) → primer annealing (about 55 °C) → extension (about 72 °C) as one cycle

②In theory, after n cycles there is 2^n times as much DNA

③Restriction enzymes recognize particular palindromic sequences and cut them, leaving sticky or blunt ends

④Making recombinant DNA uses restriction enzymes, DNA ligase and a vector (plasmid)

⑤In gel electrophoresis smaller DNA fragments travel farther, and DNA fingerprinting tells individuals apart by differences in STR repeat numbers

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Last updated 2026-10-17

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