DNA Replication

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शिक्षकों के लिए: DNA Replication (Biology, SL) के लिए इस्तेमाल के लिए तैयार लेसन स्लाइड्स, रिवीज़न नोट्स — इन्हें अपने लेसन में इस्तेमाल करें, या टॉपिक को एक इंटरैक्टिव क्लास एक्टिविटी की तरह चलाएं जिसे आपके शिक्षार्थी लाइव गेम की तरह खेलें।

लेसन नोट्स

Why DNA Replication Matters

DNA replication is semi-conservative and depends on complementary base pairing.
It occurs in the nucleus and takes place before a parent cell divides.
It ensures each daughter cell receives a full copy of the parental DNA.
In multicellular organisms it is needed for growth, replacement of old or damaged cells and tissues, and reproduction.

Semi-Conservative Replication

Semi-conservative means one strand of the parent DNA is kept in each daughter molecule — this is the template strand.
The other strand is built up from free nucleotides in the nuclear space around the chromosomes.
Each new DNA molecule therefore contains half of the original DNA and one newly made strand.
Keeping one original strand ensures genetic continuity with a high degree of accuracy between cell generations.
New cells inherit all their genes with the correct sequence of DNA bases from their parent cells.

Complementary Base Pairing

If an adenine is the next exposed base on the template strand, a thymine nucleotide is added, and vice versa.
If a cytosine is the next exposed base, a guanine nucleotide is added, and vice versa.
Nucleotides are added one by one to the new strand according to the rules of complementary base pairing.
Hydrogen bonds can only form between the template strand and the new strand if the correct bases are paired up.

The DNA double helix

Crick, Watson, Meselson and Stahl

As part of their discovery of the double-helix structure of DNA, Crick and Watson hypothesised that DNA copies semi-conservatively.
They proposed the semi-conservative model but had not provided the evidence.
Meselson and Stahl later provided the evidence supporting this hypothesis.
Analysis of Meselson and Stahl's results gave the necessary support for semi-conservative replication of DNA.

Helicase and DNA Polymerase

Helicase first unwinds the DNA, flattening out its helical structure, then breaks the hydrogen bonds between pairs of bases.
This exposes bases on either strand, and each single polynucleotide strand acts as a template.
DNA polymerase links nucleotides together to form a new strand, using the pre-existing strand as a template.
DNA polymerase catalyses condensation reactions between the deoxyribose sugar and phosphate groups of adjacent nucleotides.
This creates the sugar-phosphate backbone of the new DNA strands.
Hydrogen bonds then form between the complementary base pairs of the template and new DNA strands.

Direction of Strand Building

DNA polymerase always builds a new DNA strand in the same direction: the 5' to 3' direction.
It attaches to the 3' end of the original template strand and reads the strand in a 3' to 5' direction.
Because the two DNA strands are antiparallel, the new strand is built in the 5' to 3' direction.

Gel Electrophoresis

Gel electrophoresis separates molecules with an electric current according to their size or mass and their net charge.
Positively charged molecules move towards the cathode (negative pole); negatively charged molecules move towards the anode (positive pole).
DNA is negatively charged due to its phosphate groups, so it moves towards the anode.
Smaller molecules move quickly through the tiny pores in the gel, whereas larger molecules move slowly.
Agarose gel is used for DNA and polyacrylamide for proteins; different gels have different sized pores.

Steps of Gel Electrophoresis

An agarose gel plate is created in a tank, with wells cut into the gel at one end.
The gel is submerged in an electrolyte solution (a salt solution that conducts electricity).
DNA fragments are loaded into the wells using a micropipette.
An electrical current is applied; the negative electrode must be connected to the end with the wells.
Smaller (shorter) fragments move faster and further from the wells than larger fragments.
Fragments are transferred onto absorbent paper or nitrocellulose and heated to separate the two DNA strands.
Probes are added to develop a visual output: a radioactive label (e.g. a phosphorus isotope) or a fluorescent stain or dye (e.g. ethidium bromide).

Polymerase Chain Reaction (PCR)

PCR is the in vitro method of DNA amplification, producing large quantities of specific DNA or RNA fragments from very small quantities.
Each reaction requires the target DNA or RNA, DNA polymerase, free nucleotides, and a buffer solution for optimum pH.
Taq polymerase comes from the thermophilic bacterium×Thermus aquaticus×and does not denature at the high temperatures used.
Stage 1 – Denaturation: DNA is heated to 95°C, breaking the hydrogen bonds between the two strands.
Stage 2 – Annealing: temperature decreases to 50–60°C so primers can anneal to the ends of the single strands.
Stage 3 – Elongation/Extension: temperature increases to 72°C for at least a minute, the optimum for Taq polymerase to build complementary strands.
Each cycle doubles the DNA, so amplification is exponential — 30 cycles can generate over a billion copies in a few hours.
PCR occurs in a thermal cycler, which automatically provides the optimal temperature for each stage.

DNA Profiling

DNA profiling (genetic fingerprinting) can identify suspects of a crime and identify victims.
The regions analysed are VNTRs (Variable Number Tandem Repeats): repeating, short, non-coding regions between 20 and 50 bases long.
Every person apart from identical twins has a unique combination of VNTRs.
DNA profiling uses gel electrophoresis to separate VNTR fragments according to length, creating a unique pattern of bands.
The sequence is: DNA extraction, PCR amplification, cutting with restriction endonucleases, separation by gel electrophoresis, adding probes complementary to the VNTRs, then visualising by X-ray or UV light.
Every person inherits VNTRs from both mother and father, so profiling can identify parents or ancestors — a parent shares about 50% of the offspring's bands.
Reliability can be increased by increasing the number of VNTR markers compared, lowering the chance of a false match.

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साइन अप करें — फ्री लॉग इन

प्रैक्टिस सवाल

फ्री प्रीव्यू — 62 में से 8 सवाल। सभी देखने के लिए साइन अप करें।

1.What is meant by semi-conservative replication?
Easy
- AOne strand of the parent DNA is kept in each daughter molecule
- BBoth strands of the parent DNA are kept in one daughter molecule
- CNeither strand of the parent DNA is kept in the daughter molecules
- DHalf of the bases in each strand are conserved
2.Which enzyme is responsible for unwinding the DNA double helix and breaking the hydrogen bonds between base pairs during DNA replication?
Easy
- ADNA polymerase
- BHelicase
- CLigase
- DRestriction endonuclease
3.During DNA replication, DNA polymerase catalyses condensation reactions between which two components of adjacent nucleotides?
Medium
- AThe deoxyribose sugar of one nucleotide and the phosphate group of the next
- BThe phosphate group of one nucleotide and the phosphate group of the next
- CThe nitrogenous base of one nucleotide and the deoxyribose sugar of the next
- DThe nitrogenous base of one nucleotide and the phosphate group of the next
4.DNA polymerase always builds a new DNA strand in the 5' to 3' direction.
Easy
True or false?
5.In the polymerase chain reaction (PCR), what is the role of the primer?
Medium
- AIt provides a starting point for DNA polymerase to begin synthesis
- BIt unwinds the double-stranded DNA
- CIt joins together the Okazaki fragments
- DIt provides free nucleotides for the new strand
6.Which of the following is a property of Taq polymerase that makes it suitable for use in PCR?
Medium
- AIt is resistant to denaturation at high temperatures
- BIt can synthesise RNA from a DNA template
- CIt can cut DNA at specific recognition sites
- DIt can join DNA fragments together
7.Which of the following are required for a polymerase chain reaction (PCR)? (select all that apply)
Medium
- ATarget DNA or RNA
- BDNA polymerase
- CFree nucleotides
- DBuffer solution
- ERestriction endonucleases
8.What is the function of DNA polymerase during DNA replication?
Easy
- AIt links nucleotides together to form a new strand using a template strand
- BIt breaks hydrogen bonds between complementary bases
- CIt unwinds the DNA double helix
- DIt adds a poly-A tail to the new strand

और सवाल

9.Which of the following correctly describes the direction in which DNA polymerase reads the template strand and builds the new strand?
Medium
- AReads 3' to 5', builds 5' to 3'
- BReads 5' to 3', builds 3' to 5'
- CReads 3' to 5', builds 3' to 5'
- DReads 5' to 3', builds 5' to 3'
10.Match each molecule with its function in PCR.
Medium
- Taq polymerase
- Primer
- Free nucleotides
- Buffer solution
- Builds the new DNA strand
- Provides a starting point for synthesis
- Used in the construction of the new DNA strands
- Provides the optimum pH for the reaction
11.Place the stages of one PCR cycle in the correct order.
Medium
- Denaturation
- Annealing
- Elongation
12.Which of the following are reasons why DNA replication is important in multicellular organisms? (select all that apply)
Easy
- AGrowth
- BReplacement of old or damaged cells and tissues
- CReproduction
- DDigestion of food
- EProduction of ATP
13.Which statements about semi-conservative replication are correct? (select all that apply)
Medium
- AOne strand of the parent DNA is kept in each daughter molecule
- BThe template strand determines the sequence of the new strand by complementary base pairing
- CBoth strands of the daughter DNA are newly synthesised
- DHydrogen bonds form between complementary bases on the template and new strands
- EThe parent DNA molecule is completely destroyed during replication
14.Which of the following are required for the polymerase chain reaction (PCR)? (select all that apply)
Medium
- ATaq polymerase
- BPrimers
- CFree nucleotides
- DBuffer solution
- EHelicase
15.Which of the following are properties of DNA that allow it to be separated by gel electrophoresis? (select all that apply)
Medium
- ADNA is negatively charged
- BDNA fragments of different sizes move at different rates through the gel
- CDNA is positively charged
- DDNA molecules are attracted to the anode
- EDNA is neutral and unaffected by an electric current
16.Which of the following are applications of PCR? (select all that apply)
Easy
- ADNA profiling in forensic investigations
- BDiagnostic testing for viral RNA, such as during the COVID-19 pandemic
- CPaternity testing
- DSeparating proteins by size
- EDetermining the amino acid sequence of a protein directly
17.Which of the following are correct about the polymerase chain reaction? (select all that apply)
Medium
- AIt is an in vitro method of DNA amplification
- BIt can produce billions of identical copies within a few hours
- CIt requires a thermal cycler to control temperature
- DIt uses helicase to unwind the DNA
- EIt occurs at a constant temperature throughout
18.Match each molecule with its function in DNA replication.
Medium
- Helicase
- DNA polymerase
- Free nucleotides
- Template strand
- Unwinds the DNA double helix and breaks hydrogen bonds between base pairs
- Links nucleotides together by catalysing condensation reactions
- Provide the building blocks for the new DNA strand
- Provides the sequence that determines the order of bases in the new strand
19.Match each stage of PCR with its correct temperature.
Medium
- Denaturation
- Annealing
- Elongation / Extension
- 95°C
- 50–60°C
- 72°C
20.Match each component used in gel electrophoresis with its role.
Medium
- Agarose gel
- Micropipette
- Electrical current
- Fluorescent dye
- Provides a porous medium through which DNA fragments move at different rates
- Used to load DNA fragments into the wells
- Causes negatively charged DNA to move towards the anode
- Makes DNA fragments visible as coloured bands under UV light
21.Place the following steps of gel electrophoresis in the correct order.
Medium
- Create an agarose gel plate and cut wells into it
- Submerge the gel in an electrolyte solution
- Load DNA fragments into the wells using a micropipette
- Apply an electrical current to the tank
- Transfer DNA onto absorbent paper or nitrocellulose and heat to separate strands
- Add probes and visualise using X-ray film or UV light
22.Place the steps of DNA profiling in the correct sequence.
Medium
- DNA is extracted from a sample such as blood or hair root
- DNA is amplified using PCR
- Restriction endonucleases cut the DNA into fragments
- Fragments are separated by mass using gel electrophoresis
- Radioactive or fluorescent probes are added
- X-ray imaging or UV light is used to visualise the bands
23.DNA replication is semi-conservative because one strand of the parent DNA is kept in each daughter molecule.
Easy
True or false?
24.During DNA replication, DNA polymerase builds the new strand in the 3' to 5' direction.
Easy
True or false?
25.In gel electrophoresis, larger DNA fragments move faster and further through the gel than smaller fragments.
Easy
True or false?
26.Taq polymerase is used in PCR because it does not denature at the high temperatures involved.
Easy
True or false?
27.VNTRs are coding regions of DNA that are identical in all individuals except identical twins.
Easy
True or false?
28.During PCR, the annealing stage occurs at a lower temperature than the denaturation stage.
Easy
True or false?
29.Molecule X in the diagram is a DNA primer. What is the role of a DNA primer in PCR?
Medium
- AIt binds to the single-stranded DNA and provides a starting point for DNA polymerase
- BIt unwinds the double helix by breaking hydrogen bonds
- CIt joins nucleotides together to form the new strand
- DIt provides free nucleotides for the new strand
30.Stage 3 in PCR involves an enzyme. Why is this enzyme suitable for its role in PCR?
Medium
- AIt is heat-stable and does not denature at high temperatures
- BIt works best at very low temperatures
- CIt can only function at a pH of 2
- DIt is destroyed by heating, which stops the reaction
31.Which of the following are reasons why the researchers used PCR in their investigation? (select all that apply)
Medium
- ATo amplify small amounts of DNA
- BTo produce many copies of the DNA for analysis
- CTo separate DNA fragments by size
- DTo cut DNA into fragments
- ETo determine the sequence of proteins
32.The DNA profiles show that the three children shared the same biological parents, and their mother is Adult B. Which adult is most likely the children's father?
Medium
- AThe adult whose profile shares the most bands with the children that are not present in the mother's profile
- BThe adult whose profile shares no bands with the children
- CThe adult whose profile is identical to the mother's
- DThe adult whose profile has the fewest bands overall
33.Which suspect most likely committed the crime?
Medium
- AThe suspect whose DNA profile has the most bands in common with the crime scene DNA
- BThe suspect whose DNA profile has no bands in common with the crime scene DNA
- CThe suspect whose DNA profile has the fewest bands overall
- DThe suspect whose DNA profile is identical to the victim's
34.A sample of DNA is analysed and found to contain 30% adenine. What percentage of the bases in this DNA sample would be guanine?
Medium
- A20%
- B30%
- C40%
- D70%
35.In a gel electrophoresis experiment, a DNA sample is cut with a restriction enzyme and loaded into a well. After the current is applied, the fragments separate. Which of the following correctly describes the movement of the DNA fragments?
Medium
- AAll fragments move towards the anode, with smaller fragments moving further.
- BAll fragments move towards the cathode, with larger fragments moving further.
- CAll fragments move towards the anode, with larger fragments moving further.
- DAll fragments move towards the cathode, with smaller fragments moving further.
36.Which of the following statements about the polymerase chain reaction (PCR) are correct? (select all that apply)
Medium
- AThe denaturation step breaks the hydrogen bonds between complementary bases.
- BThe annealing step allows primers to bind to the single-stranded DNA.
- CThe elongation step is carried out at 72°C, the optimum temperature for Taq polymerase.
- DTaq polymerase is used because it is denatured at high temperatures.
- EThe number of DNA molecules doubles with each complete cycle.
37.In gel electrophoresis, DNA fragments are separated according to their size, with smaller fragments moving further through the gel than larger fragments.
Easy
True or false?
38.Match each stage of the polymerase chain reaction (PCR) with the correct temperature.
Medium
- Denaturation
- Annealing
- Elongation
- 95°C
- 50–60°C
- 72°C
39.A researcher wants to increase the reliability of a DNA profile used as evidence in a court case. Which action would best achieve this?
Medium
- AIncrease the number of VNTR markers that are observed and compared
- BIncrease the voltage used during gel electrophoresis
- CUse a larger micropipette to load more DNA into each well
- DUse a single VNTR marker but run the gel for longer
40.In gel electrophoresis, why must the negative electrode be connected to the end of the gel plate containing the wells?
Medium
- ADNA is negatively charged due to its phosphate groups, so it is repelled from the negative electrode and moves towards the positive anode
- BDNA is positively charged due to its phosphate groups, so it is attracted to the negative electrode
- CThe negative electrode heats the gel, which allows the DNA fragments to move faster
- DThe negative electrode attracts the DNA because DNA has a net positive charge from its bases
41.In a length of DNA 1000 nucleotides long, there are 382 guanine nucleotides in one strand. Explain why it is not possible to calculate the number of guanine nucleotides in the opposite strand from the information provided.
Medium
42.Complete the sentence about the purpose of DNA replication.
Easy
The purpose of DNA replication is to ensure that each daughter cell receives a complete copy of the ____ material.
43.In the absence of mutagens, the rate of mutations during DNA replication is approximately 160 bases per cell cycle. Given that the human genome contains 3.2 billion base pairs, calculate the percentage copying error rate of each cell cycle.
Medium
44.Name the enzyme that unwinds the DNA double helix and breaks the hydrogen bonds between base pairs during DNA replication.
Easy
45.Name the enzyme used in PCR that is heat-stable and builds the new DNA strands.
Easy
46.What is the name given to the short, repeating, non-coding regions of DNA used in DNA profiling?
Easy
47.A PCR reaction starts with a single DNA molecule. After 20 cycles, how many DNA molecules are produced? Give your answer as a whole number.
Medium
48.One round of PCR takes approximately 75 seconds. Calculate the time in minutes for 30 cycles. Give your answer to the nearest whole minute.
Medium
49.In the absence of mutagens, the rate of mutations during DNA replication is approximately 160 bases per cell cycle. The human genome contains 3.2 billion base pairs. Calculate the percentage copying error rate per cell cycle. Give your answer in scientific notation to 2 significant figures.
Hard
50.Complete the sentence about the semi-conservative model of DNA replication.
Medium
In semi-conservative replication, each new DNA molecule contains one ____ strand from the original DNA molecule and one ____ strand.
51.Name the three stages of a PCR cycle in order.
Medium
The three stages of PCR are ____, ____, and ____.
52.Complete the sentence about the direction of DNA synthesis.
Medium
DNA polymerase reads the template strand in the ____ to ____ direction and builds the new strand in the ____ to ____ direction.
53.Describe the structural features of a DNA molecule.
Hard
54.Explain why only bases that are complementary to the bases on the template strand can be added to the new DNA strand during DNA replication.
Hard
55.Outline the events that occur during the annealing stage of PCR.
Medium
56.Explain the use of VNTRs in DNA profiling.
Hard
57.Outline the events that are taking place during Stage 1 of the PCR cycle.
Medium
58.In a length of DNA 1000 nucleotides long, there are 382 guanine nucleotides in one strand. Explain why it is not possible to calculate the number of guanine nucleotides in the opposite strand from the information provided.
Medium
- A0
- B1
- C2
- D-1
59.In the absence of mutagens, the rate of mutations during DNA replication is approximately 160 bases per cell cycle. Given that the human genome contains 3.2 billion base pairs, calculate the percentage copying error rate of each cell cycle.
Medium
- A-1 %
- B1 %
- C2 %
- D0.000005 %
60.A PCR reaction starts with a single DNA molecule. After 20 cycles, how many DNA molecules are produced? Give your answer as a whole number.
Medium
- A1153434 molecules
- B943718 molecules
- C1258292 molecules
- D1048576 molecules
61.One round of PCR takes approximately 75 seconds. Calculate the time in minutes for 30 cycles. Give your answer to the nearest whole minute.
Medium
- A34 minutes
- B46 minutes
- C42 minutes
- D38 minutes
62.In the absence of mutagens, the rate of mutations during DNA replication is approximately 160 bases per cell cycle. The human genome contains 3.2 billion base pairs. Calculate the percentage copying error rate per cell cycle. Give your answer in scientific notation to 2 significant figures.
Hard
- A1 %
- B2 %
- C-1 %
- D0.000005 %

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