كيف يحسب الرقم الهيدروجيني pH للمحاليل؟ How to calculate pH and pOH?

يحسب الرقم الهيدروجيني بالعلاقة: pH = -log[H⁺]، وللمحاليل المائية عند 25°C: pH + pOH = 14. إذا كان pH 7 فالمحلول قاعدي. [الصيغة الرمزية الموزونة: pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴]

D1.1Theme D — Continuity and change

DNA replication — IB Biology HL Revision Notes & Topical Questions

5 min read 11 parts Lesson 2 of 18 2 quizzes

📚 IB Biology HL Hub / Topical Revision

Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution

pH = -log[H⁺]  |  pOH = -log[OH⁻]  |  pH + pOH = 14  |  [H⁺][OH⁻] = 10⁻¹⁴

Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14

Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.

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Lesson summary

D1.1 DNA replication — IB BIOLOGY HL notes: Explain semi-conservative DNA replication, its enzymes and directionality, and the uses of PCR, gel electrophoresis and DNA profiling.Purpose — before a parent cell divides, its DNA is copied so both daughter cells receive a full copy.R…

Complementary base pairing
Importance
DNA polymerase
Antiparallel strands
Preparing DNA
Thermal cycler

Part 1 of 9Semi-conservative replication

Semi-conservative replication

Purpose

Purpose before a parent cell divides, its DNA is copied so both daughter cells receive a full copy.
Replication is important for growth, replacement of old or damaged cells and tissues, and reproduction.
A parent cell is any cell that divides, not the parent organism.

Semi-conservative

Semi-conservative each new DNA molecule keeps one original strand (the template) and has one newly made strand.

Complementary base pairing

Complementary base pairing free nucleotides in the nucleus are added one by one to the template.
Adenine pairs with thymine and cytosine pairs with guanine.
Hydrogen bonds form only between correctly paired bases.

Importance

Importance keeping one original strand ensures genetic continuity with high accuracy, so new cells inherit all genes with the correct base sequence.

Evidence

Watson and Crick proposed the semi-conservative model, and Meselson and Stahl later provided the supporting evidence.

Part 2 of 9Helicase and DNA polymerase

Timing

Timing replication occurs before mitosis so that two genetically identical daughter cells can form.

Helicase unwinds the helix like untwisting a rope ladder and breaks the hydrogen bonds between bases like unzipping a zipper, exposing bases on each strand.

Each strand acts as a template, and free nucleotides are attracted to the exposed bases by base pairing.

DNA polymerase

DNA polymerase it catalyses condensation reactions between the deoxyribose sugar and phosphate of adjacent nucleotides,
forming the sugar-phosphate backbone of the new strand.
It always builds in the 5' to 3' direction, reading the template in the 3' to 5' direction.
Hydrogen bonds then form between the template and new strand bases.

Part 3 of 9Directionality, leading and lagging strands

Directionality

DNA polymerase adds nucleotides only to the free 3' –OH of a growing strand, and the incoming nucleotide's 5' phosphate bonds to it.

Antiparallel strands

Antiparallel strands the two strands run in opposite directions, so each new strand forms differently as the fork opens one way.

Feature	Leading strand	Lagging strand
Synthesis	Continuous, following the fork	Discontinuous, in short Okazaki fragments away from the fork
RNA primer	Needed once	Needed for each fragment
Joining	Not needed	DNA ligase joins the fragments

Primer
- an RNA primer gives DNA polymerase III a binding point before nucleotides are added.

Part 4 of 9Enzymes of replication

Enzyme	Role
Helicase	Unwinds the double helix at the replication fork and breaks hydrogen bonds
Single-stranded binding proteins	Keep the separated strands apart while the template is copied
DNA primase	Makes a short RNA primer to start synthesis
DNA polymerase III	Starts next to the primer and adds nucleotides in the 5' to 3' direction
DNA polymerase I	Removes RNA primers on both strands and replaces them with DNA
DNA ligase	Joins Okazaki fragments by forming sugar-phosphate bonds

Part 5 of 9Proofreading

Scale

Scale each human cell replication makes 3 billion new base pairs.

Errors

Errors copying mistakes are mutations, which can harm the new cell and cause diseases such as cancer.

Proofreading in prokaryotes

Proofreading

DNA polymerase III recognises an incorrect nucleotide in the daughter strand.

It reverses direction and removes the incorrect nucleotide from the 3' end.

The correct nucleotide is inserted and replication continues.

Part 6 of 9Gel electrophoresis

Gel electrophoresis

Gel electrophoresis separating DNA, RNA or proteins by size and net charge using an electric current.

DNA is negatively charged because of its phosphate groups, so it moves towards the anode (positive pole).
Smaller molecules move faster through the tiny pores of the gel, and larger molecules move slowly.
Agarose gel is used for DNA and polyacrylamide for proteins, and pore size affects speed.

Preparing DNA

DNA is collected from sources such as a hair root or saliva, amplified by PCR and cut into fragments with restriction enzymes.

Method

Make an agarose gel plate in a tank with wells cut at one end.

Submerge the gel in electrolyte solution.

Load DNA fragments into the wells with a micropipette.

Apply an electric current with the negative electrode at the well end.

Shorter fragments move faster and further than larger fragments.

Transfer the invisible fragments to absorbent paper or nitrocellulose and heat to separate the strands.

Add probes, either radioactive (X-ray film darkens in bands) or fluorescent such as ethidium bromide (bands fluoresce under UV light).

Part 7 of 9Polymerase chain reaction (PCR)

PCR

PCR in vitro amplification of DNA or RNA, producing billions of identical copies of a specific fragment from very small amounts in hours.

Uses are DNA profiling, genetic engineering and diagnostic tests that amplify small amounts of viral RNA, as in COVID-19.

Requirements

Target DNA or RNA, with only specific sections copied, identified by primers that bind to them.
DNA polymerase, usually Taq polymerase from the thermophilic bacterium Thermus aquaticus, which does not denature at high temperature.
Free nucleotides.
Buffer solution for the optimum pH.

Thermal cycler

Thermal cycler it provides the temperature and time for each stage automatically.

Polymerase chain reaction (PCR)

Denaturation: heat to 95 °C to break the hydrogen bonds between the strands.

Annealing: cool to 50–60 °C so primers bind to the ends of the single strands.

Elongation: raise to 72 °C for at least a minute so Taq polymerase builds the complementary strands.

Amplification
- DNA doubles each cycle, so 20 cycles give about a million molecules and 30 cycles give about 2³⁰ (over 1 billion) copies from one molecule.

Part 8 of 9DNA profiling

DNA profiling

DNA profiling (genetic fingerprinting)

DNA profiling (genetic fingerprinting) identifying individuals from the pattern of VNTRs, used to identify suspects, victims and relatives.

VNTRs (variable number tandem repeats)

VNTRs (variable number tandem repeats) short, repeating, non-coding DNA regions 20–50 bases long.

Every person except identical twins has a unique combination.

Method

Extract DNA from a hair root, blood, semen or saliva.

Amplify it by PCR.

Cut it into fragments with restriction endonucleases.

Separate fragments by size with gel electrophoresis.

Add radioactive or fluorescent probes complementary to the VNTRs.

Visualise the probes by X-ray film or UV light.

Analyse and compare the band patterns.

Part 9 of 9Applications of DNA profiling

Ancestry and paternity

Everyone inherits VNTRs from both parents, so band patterns are compared with those of possible parents.
A parent shares about 50% of the child's bands.
Every band in the child's profile must be present in the mother's or the father's profile, otherwise that man is not the father.

Forensics

Samples from the crime scene or victim are profiled and compared with samples from the suspect (or database), victim and controls.
Contamination must be avoided.
Profiling can identify victims and eliminate innocent people.
The suspect with the most fragments in common with the crime scene DNA is the likely match.

Nature of science

Nature of science reliability is raised by more measurements, which matters in court.
Using more VNTR markers lowers the chance of a false match.

Key terms

Term	Meaning
Semi-conservative replication	replication in which each new molecule has one original and one new strand.
Okazaki fragment	a short segment of the lagging strand made discontinuously.
Helicase	the enzyme that unwinds DNA and breaks hydrogen bonds between strands.
Primer	a short RNA sequence that starts DNA synthesis.
PCR	polymerase chain reaction, the in vitro amplification of DNA.
Gel electrophoresis	separation of molecules by size and charge in a gel.
VNTR	a variable number tandem repeat, a repeated non-coding DNA region used in profiling.
Taq polymerase	a heat-stable DNA polymerase from Thermus aquaticus.

Avoid these mistakes

Semi-conservative means half the DNA is copied — the whole molecule is copied and each new molecule keeps one old strand.

DNA polymerase builds in the 3' to 5' direction — it builds new strands 5' to 3'.

DNA moves to the cathode in electrophoresis — it is negatively charged and moves to the anode.

PCR needs the whole genome — only the target section marked by primers is copied.

Last updated: 25 October 2026

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RNA: 5′ → 3′ 3′ → 5′.
: AUG UAC .
: UAA, UAG, UGA tRNA (Release Factor) .