Biology · 17 Inheritance

17.2 Mitosis

Before you start

A chromosome is a thread of DNA in the nucleus, and a gene is a length of DNA that codes for a protein.
A human body cell has 46 chromosomes, arranged as 23 pairs.
Cells can be specialised — a red blood cell, a nerve cell and a root hair cell all look different because each is built for one job.
Most body cells of one organism contain the same genes; they differ because different genes are expressed (17.1).

Syllabus objectives covered (5)

Describe mitosis as nuclear division giving rise to genetically identical cells (details of the stages of mitosis are not required)
State the role of mitosis in growth, repair of damaged tissues, replacement of cells and asexual reproduction
State that the exact replication of chromosomes occurs before mitosis
State that during mitosis, the copies of chromosomes separate, maintaining the chromosome number in each daughter cell
Describe stem cells as unspecialised cells that divide by mitosis to produce daughter cells that can become specialised for specific functions

Part 1 of 5

Mitosis is a division of the nucleus

Every one of the cells in your body came from one fertilised egg. To get from one cell to about thirty trillion, cells had to divide again and again — and each time, the hard part was not splitting the cytoplasm, it was sorting out the nucleus. That is where the chromosomes are, and every new cell needs a full set.
Mitosis is the name for that sorting-out. The syllabus wording is that mitosis is nuclear division giving rise to genetically identical cells. Both halves of that sentence are worth a mark, so learn it as a sentence, not as a vibe.
"Nuclear division" is the part people drop. Mitosis is the division of the nucleus. The cell then splits around it, but the word mitosis refers to the nucleus. Write "the cell splits in two" and you have described the wrong event.
"Genetically identical" means the two daughter cells have the same alleles as each other and the same alleles as the parent cell they came from. Nothing is shuffled and nothing is left behind.
One piece of good news: the syllabus says explicitly that details of the stages are not required. Prophase, metaphase, anaphase, telophase — you will not be asked to name them and there is no mark for reciting them. What you are asked for is what mitosis achieves.

Part 2 of 5

Copy first, then separate

Start with the problem. A human cell has 46 chromosomes. If a nucleus simply handed 23 chromosomes to one daughter and 23 to the other, both daughters would be missing half their genetic information, and each division would halve the number again. That cannot be how growth works.
So something has to happen first. Before mitosis begins, the chromosomes are exactly replicated — every chromosome is copied, base for base. The word exact is doing real work here: an exact copy is why the daughter cells end up genetically identical rather than merely similar.
Now there are two identical copies of each chromosome sitting together, still inside the one nucleus. Nothing has been shared out yet — you have doubled, not divided.
During mitosis, the copies of each chromosome separate, one copy going to each end of the cell. Each new nucleus therefore receives one complete copy of every chromosome.
$46 \overset{\overset{}{replication}}{\to} 46 chromosomes, each as two copies \overset{\overset{}{mitosis}}{\to} 46 and 46$
That is the chain of reasoning behind the phrase "the chromosome number is maintained". A cell with 46 chromosomes gives two cells with 46 chromosomes each — not 23, and not 92. If an exam asks you to explain the number, this is the explanation: exact replication before, separation of the copies during.
Notice that the same chain also explains "genetically identical". Exact copies, split so that each daughter gets one of each. Identical content, identical number. One mechanism, two marks.

Part 3 of 5

The four jobs mitosis does

The syllabus lists four roles, and questions usually want them named separately, so hold them as four items rather than one blur.
Growth. An organism gets bigger mainly by making more cells, not bigger cells. A growing root tip or a growing child is mitosis happening repeatedly.
Repair of damaged tissues. Cut your finger and the cells at the edge of the wound divide by mitosis to close the gap with cells that match the ones already there.
Replacement of cells. Some cells wear out on a schedule — red blood cells and gut lining cells among them — and are replaced continuously by mitosis, even when nothing is damaged and nothing is growing.
Asexual reproduction. A strawberry plant sends out a runner and a whole new plant grows at the end of it. Only one parent is involved, the new plant is produced by mitosis, and so it is genetically identical to the parent — a clone.
The exam trick here is small: repair and replacement sound like the same idea, but they are listed separately. Repair follows damage; replacement is routine turnover. In a "State two roles" question they count as two.

Part 4 of 5

Stem cells: cells that have not chosen a job yet

Nearly every cell in you is specialised: its shape and contents are adapted to one function, and it stays that way. A red blood cell has no nucleus and is packed with haemoglobin; it can never become a nerve cell.
A stem cell is the opposite: it is unspecialised. It has the genes for everything but has not yet been committed to any one function.
A stem cell does two things, and the definition needs both. First, it divides by mitosis — so the cells it produces carry exactly the same genetic information as it does.
Second, the daughter cells it produces can become specialised for specific functions. A stem cell in bone marrow can give rise to red blood cells and to white blood cells.
Do not let that sound like a contradiction. The daughter cells are genetically identical — they all have the same genes — but they specialise because different genes are expressed in different cells, which is the idea you met in 17.1. Same instruction book, different pages open.

Part 5 of 5

Writing about mitosis so that it scores

Four phrases carry almost every mark in this subtopic: nuclear division, genetically identical, exact replication of chromosomes before mitosis, and chromosome number maintained. If your answer contains the relevant ones, it scores; if it paraphrases them loosely, it often does not.
"The cell copies itself and splits" contains no marking point at all. "The chromosomes are exactly replicated, then the copies separate so each daughter cell has the same number of chromosomes as the parent cell" contains three.
Be careful with the word same. Say what is the same: the same number of chromosomes, and the same alleles. An unqualified "the cells are the same" is too vague to be given credit.
One more, ready for the next subtopic. Mitosis produces cells that are genetically identical; meiosis produces cells that are genetically different. Whenever a question mentions variation or gametes, mitosis is not the answer.

Worked examples

Example 1. A cell in the growing tip of an onion root has 16 chromosomes. The cell divides by mitosis. State the number of chromosomes in each daughter cell, and explain how that number is achieved.

Show the working

Deal with what happens before the division. Each of the 16 chromosomes is exactly replicated, so the nucleus now holds two identical copies of every chromosome.
Now the division itself. During mitosis the copies of each chromosome separate, one copy going into each of the two new nuclei.
Count what each daughter receives: one copy of chromosome 1, one copy of chromosome 2, and so on for all 16. So the number is maintained, not halved.
$16 \overset{\overset{}{replication}}{\to} 16 pairs of copies \overset{\overset{}{separation}}{\to} 16 and 16$
Write the explanation in the order the events happen — replication, then separation. An answer that only says "16 because mitosis keeps the number the same" restates the question instead of explaining it.

Answer: 16 chromosomes in each daughter cell. Each chromosome is exactly replicated before mitosis, and during mitosis the copies separate so that each daughter cell receives one complete copy of every chromosome, maintaining the chromosome number.

Example 2. A gardener grows new strawberry plants from runners produced by one parent plant. Explain why the new plants have exactly the same characteristics as the parent plant.

Show the working

Identify the type of reproduction first. One parent, no gametes, no fertilisation — this is asexual reproduction, which is one of the four roles of mitosis.
Name the division. The new plant grows from cells produced by mitosis.
Say what mitosis gives you. Mitosis produces genetically identical cells, because the chromosomes are exactly replicated and the copies are shared out so that every new cell has the same alleles as the parent cell.
Close the loop back to the question, which asked about characteristics, not chromosomes. Same alleles means the same genetic information, so the same features develop.

Answer: The runners are asexual reproduction, and the new plants grow by mitosis. Mitosis is nuclear division that gives genetically identical cells, so the new plants have the same alleles as the parent plant — they are clones — and therefore develop the same characteristics.

Example 3. Stem cells taken from a person's bone marrow can produce both red blood cells and white blood cells. Describe what a stem cell is, and suggest why cells produced from the same stem cell can end up so different from each other.

Show the working

Start with the definition, which has two halves: a stem cell is unspecialised, and it divides by mitosis to produce daughter cells that can become specialised for specific functions.
Now handle the apparent contradiction in the question. Because the division is mitosis, all the cells produced are genetically identical — they all carry the same genes.
So the difference between them cannot be in which genes they have. It has to be in which genes are used. Different genes are expressed in different cells, so each cell makes only the proteins it needs for its own function.
Give the concrete ending: one line of cells expresses the genes for haemoglobin and becomes red blood cells, another does not and becomes white blood cells.

Answer: A stem cell is an unspecialised cell that divides by mitosis to produce daughter cells that can become specialised for specific functions. All the cells produced are genetically identical because they come from mitosis, so they carry the same genes; they differ because different genes are expressed in each specialised cell type.

Where people go wrong

Mistake: Writing that mitosis halves the chromosome number, or that the daughter cells have half a set.

Instead: Mitosis maintains the chromosome number: a cell with 46 chromosomes gives two cells with 46 chromosomes each.

Halving belongs to meiosis. The halving idea sticks because the cell divides in two, but the chromosomes were exactly replicated first, so there is a full set to hand to each daughter.

Mistake: Describing mitosis as "the cell dividing into two".

Instead: Mitosis is nuclear division — the division of the nucleus — giving genetically identical cells.

The mark is for the words "nuclear division". Describing the cytoplasm splitting describes the wrong structure, and an examiner cannot award a marking point that is not there.

Mistake: Saying the daughter cells are "the same" without saying same in what respect.

Instead: Say genetically identical: the same number of chromosomes and the same alleles as the parent cell.

"The same" could mean the same size or the same shape, neither of which is guaranteed. Marking points in genetics are almost always about alleles or chromosome number, so name one.

Mistake: Treating a stem cell as a special kind of specialised cell, or writing only that it "can turn into anything".

Instead: A stem cell is unspecialised, divides by mitosis, and its daughter cells can then become specialised for specific functions.

The definition is worth two or three marks and each element is a separate marking point. "Turns into anything" gives an examiner nothing precise to credit.

Mistake: Using mitosis to explain variation between offspring, or to explain how gametes are made.

Instead: Mitosis gives genetically identical cells, so it produces no variation. Gametes are produced by meiosis (17.3).

Any question mentioning gametes, sexual reproduction or variation between offspring is signalling meiosis. Mitosis is for growth, repair, replacement and asexual reproduction.

Check yourself

State what mitosis is, in the two parts the syllabus asks for.
Answer
Mitosis is nuclear division that gives rise to genetically identical cells.
State what happens to the chromosomes before mitosis begins.
Answer
Each chromosome is exactly replicated — an exact copy of every chromosome is made.
A cell with 24 chromosomes divides by mitosis. Give the number of chromosomes in each daughter cell and state why.
Answer
24 in each. The copies of the chromosomes separate during mitosis, so each daughter cell receives one complete copy of every chromosome and the chromosome number is maintained.
State the four roles of mitosis given in the syllabus.
Answer
Growth; repair of damaged tissues; replacement of cells; asexual reproduction.
Describe what is meant by a stem cell.
Answer
An unspecialised cell that divides by mitosis to produce daughter cells that can become specialised for specific functions.
Explain why mitosis produces no genetic variation between the cells it makes.
Answer
The chromosomes are exactly replicated before mitosis, so the copies carry identical alleles; the copies then separate so that each daughter cell receives an identical set. The daughter cells are therefore genetically identical to each other and to the parent cell.
A lizard regrows a lost tail. Identify which role of mitosis this is, and state one other role mitosis carries out in the same lizard.
Answer
Repair of damaged tissues. Another role: growth (or replacement of worn-out cells such as skin or blood cells).

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. Cells at the tip of a growing plant root divide repeatedly by mitosis. One of these cells contains 16 chromosomes.

(a) State what is meant by mitosis. 2 marks

Mark scheme

B1 nuclear division / division of the nucleus
B1 gives (two) genetically identical cells / cells with the same alleles as the parent cell

(b) State what happens to the chromosomes in this cell before mitosis begins. 1 mark

Mark scheme

B1 each chromosome is exactly replicated / an exact copy of each chromosome is made; ignore references to the stages of mitosis

(c) The root tip cell contains 16 chromosomes. Explain how many chromosomes each of the two daughter cells contains. 3 marks

Mark scheme

B1 16 (chromosomes) in each daughter cell
B1 the copies of the chromosomes separate (during mitosis)
B1 each daughter cell receives one copy of every chromosome, so the chromosome number is maintained

(d) Apart from growth, state two other roles of mitosis in a plant. 2 marks

Mark scheme

B1 repair of damaged tissues
B1 replacement of (worn-out) cells or asexual reproduction, e.g. producing new plants from runners

Question 2. Bone marrow contains stem cells. In a laboratory, one stem cell was allowed to divide many times, and some of the cells produced went on to become red blood cells while others became white blood cells.

(a) Describe what is meant by a stem cell. 2 marks

Mark scheme

B1 an unspecialised cell
B1 divides by mitosis to produce daughter cells that can become specialised (for specific functions)

(b) Explain why all of the cells produced from this one stem cell contain the same genetic information. 2 marks

Mark scheme

B1 the cells are produced by mitosis, which gives genetically identical cells
B1 the chromosomes are exactly replicated (before division) and the copies separate, so each cell receives an identical set of chromosomes/alleles

(c) Suggest why the red blood cells and the white blood cells look different from each other, even though they came from the same stem cell. 2 marks

Mark scheme

B1 the daughter cells become specialised for different functions
B1 different genes are expressed / each cell makes only the proteins it needs, although the genes present are the same

How this is examined

Why each daughter cell ends up with the same chromosome number as the parent cell: a three-mark Explain, often attached to a stated chromosome number such as "a cell with 16 chromosomes divides by mitosis" The number stated correctly (unchanged), plus "the chromosomes are exactly replicated before mitosis" and "the copies of the chromosomes separate, so each daughter cell receives one copy of each chromosome". The word exact and the before/during order are what separate a full-mark answer from a two-mark one.
Mitosis gives genetically identical cells, which is why clones are identical: a two- or three-mark Explain about cuttings, runners, tissue culture or a wound healing with matching cells "Mitosis is nuclear division" plus "produces genetically identical cells" plus the link that identical alleles mean identical characteristics. Saying only "they are the same" without naming alleles or chromosome number does not score.
What a stem cell is and what makes it different from an ordinary body cell: a two- or three-mark Describe, often as the first part of a question about bone marrow, a plant meristem or tissue repair Three separable points: unspecialised; divides by mitosis; daughter cells can become specialised for specific functions. Examiners credit these independently, so write all three even if the question is worth two marks.
Naming which role of mitosis is happening in a described situation: a one- or two-mark State or Identify — a healing cut, a growing root tip, worn-out red blood cells being replaced, a plant grown from a runner The syllabus term itself: growth, repair of damaged tissues, replacement of cells, or asexual reproduction. Repair and replacement count as two different roles, so both can be given in a "State two" answer.

Words to know

Mitosis: Nuclear division giving rise to genetically identical cells.
Genetically identical: Having the same alleles, and the same number of chromosomes, as the cell divided from.
Replication (of chromosomes): The exact copying of each chromosome, which happens before mitosis begins.
Daughter cell: One of the cells produced by a cell division.
Stem cell: An unspecialised cell that divides by mitosis to produce daughter cells that can become specialised for specific functions.
Specialised cell: A cell whose structure is adapted to carry out one particular function.
Asexual reproduction: Reproduction from one parent, producing offspring that are genetically identical to that parent.

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