What you need to know
- 8.11phytochromeA plant pigment that exists in two forms, Pr and Pfr, which change into each other in red and far-red light., auxinA plant growth substance made in shoot tips that promotes cell elongation. and gibberellinA plant growth substance that causes stem elongation and switches on amylase genes in germinating seeds. affect transcriptionMaking an mRNA copy of a gene, using one strand of DNA as a template, in the nucleus..
No named flowering-gene codes are required. Second-messenger cascades are not required. The starch-agar assay is Core Practical 18.
By the end, you should be able to…
- State that Pfr can alter which geneA sequence of DNA bases that codes for a polypeptide or a functional RNA. are transcribed.
- Connect gibberellin to transcription of amylase.
- Separate the hormone or pigment from the enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. that is produced.
Build your understanding
Pfr reaches the genes
Pfr does not only sit in the cytoplasmEverything inside the cell surface membrane except the nucleus: the fluid cytosol and the organelles. as a light switch. It can move into the nucleusThe organelle that contains the cell’s chromosomes, surrounded by a nuclear envelope with pores. and change the activity of transcription factorA protein that binds to DNA near a gene and switches its transcription on or off.. geneA sequence of DNA bases that codes for a polypeptide or a functional RNA. that were quiet can be transcribed, and others can be switched down. Flowering genes are the response you have already timed with night length. Greening, the production of chlorophyll-related proteins when a seedling sees light, is the same kind of story. You do not need the gene symbols.
If Pfr falls during a long night, those transcriptionMaking an mRNA copy of a gene, using one strand of DNA as a template, in the nucleus. changes reverse or fail to happen. A red flash restores Pfr and can restore the gene activity. Far-red removes Pfr again. The pigment and the genes are one pathway.
Gibberellin’s gene is easier to see
In the aleurone, gibberellinA plant growth substance that causes stem elongation and switches on amylase genes in germinating seeds. leads to transcription of the gene for amylase. More messenger RNAA single-stranded RNA copy of a gene that carries the code from the nucleus to a ribosome. means more enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction., which is secreted, which digests starchThe storage polysaccharide in plants, made of amylose and amylopectin. It is compact and insoluble, so it does not affect water potential.. A clear zone on starch agar after iodine is evidence that the starch has gone, not a photograph of the mRNA. If a question asks for the molecular step, say transcription. If it asks for the visible step, say starch digestion.
auxinA plant growth substance made in shoot tips that promotes cell elongation. likewise changes transcription of proteins involved in cell-wall loosening. The bending shoot is the visible result. The new proteins are the 8.11 result.
Same idea as other signals
A signal binds or activates a transcription factor, and the factor binds DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code.. That general account is how gene switching is described in human cells as well. In this lesson the signals are Pfr, auxin and gibberellin. Do not import a human steroid into the aleurone.
Core Practical 18 tests the gibberellin step with half-grains. It does not test flowering. Do not explain a clear zone with night length.
NOT: phytochrome does not cut DNA, and a clear zone is not a photograph of mRNA.
- Pfr acts through transcription factors
- The DNA sequence stays the same
- Gibberellin switches on the amylase gene
- The enzyme, not the hormone, digests starch
Two traps this diagram blocks
- Do not say phytochrome cuts the flowering genes.
- Do not explain a starch-plate clear zone using night length.
From the diagram alone
Question
How can Pfr change a plant’s proteins without changing its DNA sequence? [3]
Show a model answer
Pfr affects transcription factors, so some genes are transcribed more or less. The set of mRNA and proteins changes. The DNA sequence itself stays the same.
Make your reasoning visible
Pigment, factor, gene
Pfr changes transcription factorA protein that binds to DNA near a gene and switches its transcription on or off.. The factors change transcriptionMaking an mRNA copy of a gene, using one strand of DNA as a template, in the nucleus.. Name that order if the question says “how”.
Enzyme is the product
Amylase is made because the geneA sequence of DNA bases that codes for a polypeptide or a functional RNA. was switched on. gibberellinA plant growth substance that causes stem elongation and switches on amylase genes in germinating seeds. is not the enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction..
Key explanations
- The DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. sequence is not cut or rewritten.
- A clear zone after iodine shows that starchThe storage polysaccharide in plants, made of amylose and amylopectin. It is compact and insoluble, so it does not affect water potential. has gone. It is indirect evidence of transcription, not the messenger RNAA single-stranded RNA copy of a gene that carries the code from the nucleus to a ribosome. itself.
Avoid this misconception
The claim: phytochromeA plant pigment that exists in two forms, Pr and Pfr, which change into each other in red and far-red light. cuts DNA, and that is how flowering genes are switched on.
The correction: Pfr influences transcription factors. The DNA sequence is not cut. Transcription of chosen genes increases or decreases.
Order phytochrome to transcription
Phytochrome lets a plant respond to light by changing which genes are transcribed. Order the chain.
Order the steps
What to do
- Reorder the cards below: drag a card (on a touch screen, drag its number), tap a card then tap where it goes, or use the ↑ and ↓ arrows at its right.
- Press Check under the cards to mark each one green (right place) or red; the count shows how many are right.
- Stuck? Hint first outlines the first wrong place, then gives a clue, then places that step for you.
- Shuffle mixes the cards again; Start again returns them to the starting order. Keyboard: Space picks up, arrow keys move, Space drops.
A long-day plant is exposed to daylight. Put the chain in order.
0 of 5 in the right place
Photoperiod control is modelled in 8B 6. Other phytochrome responses (for example de-etiolation) follow the same chain with different genes.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
How can Pfr change a plant’s proteins without changing its DNA sequence?
Show suggested answer
It affects transcription factors, so some genes are transcribed more or less. The sequence stays the same. The set of mRNA and proteins changes.
Practice question
Retrieval and explanation · suggested answer
What gene is switched on in the aleurone by gibberellin, and what does the product do?
Show suggested answer
The amylase gene. Amylase hydrolyses starch in the endosperm.
Practice question
Retrieval and explanation · suggested answer
Why is a clear zone on a starch plate indirect evidence of transcription?
Show suggested answer
The zone shows that starch was digested, which means amylase was present. Amylase is produced after the gene is transcribed. The zone is not the mRNA itself.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A half-grain without an embryo is given gibberellin, and a clear zone appears. A second half-grain is given gibberellin plus a substance that blocks transcription, and no clear zone appears. What does the pair show? Assume the starch and the iodine worked.
Show a hint
The hormone is present in both. Only transcription differs.
Explore the reasoning
Gibberellin alone is enough to cause starch digestion, so the embryo is not needed if the hormone is supplied. Blocking transcription stops the clear zone, so the hormone acts by allowing transcription, not by digesting starch itself. The assumption is that the blocker did not simply kill the iodine test. A plate with starch and iodine and no grain would confirm that the stain still works.
Recall this lesson
Lesson recall questions
Question 1 of 5: Describe, in order, how daylight and darkness convert phytochrome.
Plant growth regulators
Practice question
Retrieval and explanation · suggested answer
Describe, in order, how daylight and darkness convert phytochrome.
Lesson: 8B 7: Phytochrome and Transcription
Build the sequence from memory first. Then reveal the model order.
Tap the steps in the order you recall. Tap a placed step to send it back.
- 01Next step
- 02Waiting
- 03Waiting
Tap to place
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
The page is human genetics. The logic still fits a seedling.
8B · Phytochrome and TranscriptionMedlinePlus Genetics says signals activate transcription factors that bind regulatory DNA and change how much a gene is transcribed. That is the mechanism you should write for Pfr,…Specification 8.11Read & use the scienceClose article
Why this matters
MedlinePlus Genetics says signals activate transcription factors that bind regulatory DNA and change how much a gene is transcribed. That is the mechanism you should write for Pfr, auxin and gibberellin. The page does not name those plant molecules. Do not pretend it is a botany article. 8.11: phytochrome and the two hormones bring about responses by effects on transcription. A scan or growth response is indirect evidence: it cannot alone identify every signalling step or prove the proposed cause.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 8.11 Phytochrome, auxin and gibberellin effects on plant responses and transcription.
Use the science
Why is “the seed’s DNA changed to include amylase” the wrong description of germination?
Explore a possible response
The amylase gene is already in the DNA. Gibberellin changes its transcription. The sequence is not edited.
Explore the source
- MedlinePlus Genetics: can genes be turned on and off?US National Library of Medicine
Malting begins with a gene-expression decision
8B · Phytochrome and TranscriptionBarley researchers measured gibberellin-related transcripts and enzyme activity in different tissues of germinating grain. The study connects a crop process familiar to food production with 8.11: hormone signalling…Specification 8.11Read & use the scienceClose article
Why this matters
Barley researchers measured gibberellin-related transcripts and enzyme activity in different tissues of germinating grain. The study connects a crop process familiar to food production with 8.11: hormone signalling can promote transcription that leads to amylase production, allowing starch reserves to be mobilised for the young plant. The timing and tissue matter; a whole grain is not one uniform cell. This is also why the Unit 5 core practical on amylase in germinating cereals needs a control for germination stage. A high amylase reading alone does not identify the precise hormone concentration or gene-regulatory step that produced it.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 8.11 Phytochrome, auxin and gibberellin effects on plant responses and transcription.
Use the science
Why should an investigation of grain amylase compare seeds at a controlled stage of germination?
Explore a possible response
Gibberellin signalling and amylase production change as germination proceeds. Mixing stages could change enzyme activity independently of the variable the experiment aims to test.
Explore the source
- Gibberellin and gene activity in germinating barleyPeer-reviewed research via NIH PubMed Central