GCSE · Biology · Edexcel · Spec 1BI0

Growth in animals and plants

A seedling grows roots and leaves. A broken bone knits back together. Different jobs, same trick, and soon you'll be able to explain it.

Rebuild it

The cell cycle: copy first, then divide

3 stages are missing. Use the rest of the loop to work out which goes where.

Stage 2: Interphase: sub-cellular structures are copied
Still interphase, so the cell is copying more than its chromosomes.
Stage 4: Mitosis: the nucleus divides
Mitosis is a type of cell division that produces genetically identical cells.
Stage 5: The cell divides in two
One cell becomes two.

…and stage 6 leads back to stage 1.

Check your gut

What happens to the chromosomes?

A cell in a growing seedling is about to divide by mitosis. Before it does, it has a set of chromosomes.

Which of these is closest to what you think happens to those chromosomes?

They get halved, so each new cell gets half of them.They get shared out at random, so the two new cells end up with different mixes.They only get copied once the cell has already split in two.Each chromosome is copied first, so both new cells get a copy of the same chromosomes.

How sure are you?

I'm sureI think soI'm guessing

They get halved, so each new cell gets half of them.

This is a common misconception

It's an easy trap: one cell turning into two sounds like a sharing-out. But chromosomes are copied, not halved. The copying already happened in interphase, so both new cells end up with a copy of the same chromosomes.

They get shared out at random, so the two new cells end up with different mixes.

This is a common misconception

Mitosis isn't a lucky dip. The chromosome copies line up along the middle of the cell, the nucleus divides, and both new cells have a copy of the same chromosomes. That is why they are genetically identical.

They only get copied once the cell has already split in two.

This is a common misconception

That's the wrong way round. Each chromosome is copied during interphase, before mitosis. It is the copies that line up along the middle of the cell, so they have to exist already.

Each chromosome is copied first, so both new cells get a copy of the same chromosomes.

This matches the evidence

Yes. Copying happens in interphase. Then the copies line up along the middle in mitosis and the nucleus divides. Both new cells have a copy of the same chromosomes, which is what makes them genetically identical.

Chromosomes are copied, not halved: copy in interphase, then divide in mitosis.

Zoom out: predict, then check

Back to the seedling. Commit to a gut answer first.

A seedling grows from a tiny shoot into a plant with roots, a stem and leaves. How did it get bigger?

Same process, different job

Multicellular organismvsUnicellular organism

Look at the first row. It's the one that matters.

Focus

What cell division is used for

Multicellular organism

Usually growth and repair, e.g. new cells to heal a broken bone

Unicellular organism

Reproduction

The insight

The cell does the same thing; the job it does for the organism changes. In a many-celled organism it adds cells to the organism you already have. In a one-celled organism, the cell dividing is the organism reproducing.

How the new cells are made

Multicellular organism

An existing cell copies its genetic material, then divides in two

Unicellular organism

An existing cell copies its genetic material, then divides in two

What the two new cells are like

Multicellular organism

Genetically identical

Unicellular organism

Genetically identical

Where the new cells end up

A seedling: cells organised into a plant

Showing 5 layers: Ground, Root system, Stem, Leaves, Vascular bundle

Layers

Explore

Inside the stem: xylem and phloem cells.

Schematic seedling, not to scale. Tap a part to see what it is made of.

Your turn

Write it in your own words

Explain why the two cells made by mitosis are genetically identical, and how cell division allows a multicellular organism to grow and to repair damaged tissue. [5 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Each chromosome is copied during interphase, before the cell divides (1 mark)
In mitosis the chromosome copies line up along the middle of the cell and the nucleus divides (1 mark)
So both new cells have a copy of the same chromosomes, which makes them genetically identical (1 mark)
For growth: cell division increases the number of cells the organism has (1 mark)
For repair: new cells made by cell division replace damaged tissue, e.g. to heal a broken bone (1 mark)

Model answer

Before the cell divides, interphase copies each chromosome. In mitosis the chromosome copies line up along the middle of the cell and the nucleus divides, so both new cells have a copy of the same chromosomes. That makes the two cells genetically identical. A multicellular organism grows by increasing its number of cells, and cell division makes those extra cells. It also makes new cells to replace damaged tissue, for example to heal a broken bone.

Say the order (copy, then divide) and then say the two jobs (growth and repair). That's the whole answer.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

More cells, made by copying first and dividing second.

What you need to know

Growth is part of every organism's life cycle, and multicellular organisms grow by making more cells.
New cells are made when an existing cell copies its genetic material, then divides in two, giving two genetically identical cells.
Have a goJo insists a new cell can just appear whenever the body needs one, like a vending machine for cells. Where do new cells actually come from?
From existing cells, which copy their genetic material and then divide in two.
Every new cell is made by an existing cell dividing, which is why the two cells it makes are genetically identical.
Most cells in multicellular organisms follow a cell cycle; its main phases are interphase and mitosis.
In interphase, each chromosome is copied, and so are the cell's sub-cellular structures.
In mitosis, the chromosome copies line up along the middle and the nucleus divides, so both new cells get a copy of the same chromosomes.
Have a goIn mitosis the chromosome copies line up along the middle. If a cell skipped interphase, what would be missing at that point?
The chromosome copies, because there would be nothing to line up.
The copies are made in interphase, so copying has to come before dividing.
Chromosomes are tightly coiled strands of DNA that carry genetic material, and mitosis copies them, never halves them.
Cell division also repairs damaged tissue, for example to heal a broken bone.
In multicellular organisms cell division is usually for growth and repair, but in unicellular organisms it is for reproduction.
Have a goA single-celled organism divides in two. Is that growth, repair or reproduction?
Reproduction.
In unicellular organisms cell division is used for reproduction; in multicellular organisms it is usually used for growth and repair.
Cells form tissues, organs and organ systems in animals and plants, and a leaf is an organ made of different cells.
Animal and plant cells both have a nucleus (it holds the DNA); plant cells also have chloroplasts, a permanent vacuole and a cell wall.

The big picture

Animals and plants grow by making more cells, not by stretching the ones they already have. Each new cell comes from an existing cell that copies its chromosomes in interphase and then divides in mitosis, giving two genetically identical cells. The same process repairs damaged tissue, and in unicellular organisms it is used for reproduction.

Key points

1Multicellular organisms grow by increasing the number of cells they have, and that involves cell division.

2Cell division copies a cell's genetic material and then divides the cell in two, making two genetically identical cells.

3The cell cycle has two main phases: interphase copies each chromosome and the sub-cellular structures, then mitosis divides the nucleus.

4Chromosomes are copied, not halved, so both new cells have a copy of the same chromosomes.

5Cell division is usually used for growth and repair in multicellular organisms, but for reproduction in unicellular organisms.

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Worked example

Problem

Imagine a cell with just 4 chromosomes (a made-up small number, to keep the counting easy). It goes once round the cell cycle and divides by mitosis. How many chromosome copies does interphase make, and how many chromosomes does each new cell end up with?

⚠ Watch out

Saying that chromosomes are halved when a cell divides by mitosis. They are copied during interphase first, so both new cells get a copy of the same chromosomes.

🧠

Memory hook

Think of photocopying a set of notes before handing one set to each of two friends. Copy first, then split. If you split the originals instead, each friend gets half the notes. (The analogy breaks if you take it too literally: cells copy chromosomes, not paper.)

✓

Check yourself

Cover the screen. In one breath, say the order of the cell cycle from interphase to two new cells, then say why copying has to come before dividing.

Flashcards

(15)

How do multicellular organisms grow?

By increasing the number of cells they have, which involves cell division.

What is cell division?

The process by which cells replicate to produce two genetically identical cells.

Where do all new cells come from?

Existing cells copy their genetic material and then divide in two.

What is the cell cycle?

The process by which cells divide. Most cells in multicellular organisms go through its phases; the main ones are interphase and mitosis.

What happens to chromosomes and sub-cellular structures in interphase?

Each chromosome is copied, and sub-cellular structures are copied.

In mitosis, which two things happen before the cell divides in two?

The chromosome copies line up along the middle of the cell, and the nucleus divides.

What is a chromosome?

A tightly coiled strand of DNA that carries the genetic material.

Are chromosomes halved or copied when a cell divides by mitosis?

Copied, not halved.

Why do multicellular organisms need cell division apart from growth?

To repair damaged tissue: new cells are made by cell division, for example to heal a broken bone.

What is cell division usually used for in multicellular and in unicellular organisms?

Multicellular: usually growth and repair. Unicellular: reproduction.

How are multicellular organisms organised?

Cells form tissues, organs and organ systems, which together form the whole organism, in animals and plants.

What is a leaf?

An organ made of different cells working together.

Which parts of a plant contain root hair cells, and xylem and phloem cells?

Root hair cells are in the root system. Xylem and phloem cells are in the vascular bundle in the stem, transporting substances up and down the plant.

What does the nucleus do?

It contains the genetic material (DNA) and controls the cell's activities.

What do plant cells have in addition to the structures they share with animal cells?

Chloroplasts, a permanent vacuole containing cell sap, and a cell wall.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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The cell cycle: copy first, then divide

What happens to the chromosomes?

Write it in your own words

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