Comparing mitosis and meiosis – Mitosis and Meiosis (Senior 3 Biology) | Ganzaa

Mitosis and Meiosis

Lessons

Senior 3 · Biology

Mitosis and Meiosis

4 lessons

Lesson 4 of 4

Comparing mitosis and meiosis

25 min

Two graphs, two divisions

A scientist measured the amount of DNA in cells from two different tissues of the same animal, over the time it took each cell to divide. One graph comes from a cell in the lining of the gut; the other from a cell in a testis. Before reading on, decide which is which, and why.

Figure 1: DNA per cell in two dividing cells from the same animal (illustrative data).

Worked example 1: reading the graphs

Both graphs start at 8 units and rise to 16: in both cells the DNA is copied once, in the S phase of interphase.
Graph A falls once, back to 8 units: one division, and each daughter cell has as much DNA as the parent cell started with. This is mitosis, in the gut lining.
Graph B falls twice: to 8 units after the first division and to 4 after the second, half of what the cell started with. Two divisions after one copy, ending with half the DNA: this is meiosis, in the testis.

Rule: a final DNA level equal to the starting level means mitosis; a final level of half the start means meiosis.

Side by side

The two divisions use the same machinery: a copy of the DNA in interphase, a spindle, the same four stage names, and cytokinesis at the end. They differ in almost everything else.

Feature	Mitosis	Meiosis
Where	Body cells: skin, gut lining, bone marrow, plant meristems	Reproductive organs only: testes, ovaries, anthers, ovules
Divisions	One	Two (meiosis I and II)
Cells made	Two	Four
Chromosome number	Stays the same: $2n \to 2n$	Halved: $2n \to n$
Homologous chromosomes	Do not pair up	Pair up in prophase I
Crossing over	Does not happen	Happens in prophase I
At metaphase	Chromosomes line up singly	Pairs line up (metaphase I); single chromosomes (metaphase II)
At anaphase	Sister chromatids separate	Homologous chromosomes separate (anaphase I); sister chromatids separate (anaphase II)
Daughter cells	Genetically identical to the parent cell and to each other	Genetically different from the parent cell and from each other
Used for	Growth, repair, replacement, asexual reproduction	Making gametes for sexual reproduction; variation

Telling the divisions apart in a drawing

Exam questions often show a cell and ask which division it is in. Look first at the equator: single chromosomes or pairs? Then count the chromosomes and compare with the body-cell number.

Figure 2: Three cells from one animal whose body cells have 2n = 6, each in a metaphase. Chromosomes from the father are blue and from the mother orange.

Worked example 2: which division?

Identify the division and stage of each cell in Figure 2. Give your reasons.

Cell A: metaphase of mitosis. It holds all 6 chromosomes (the diploid number), and they line up singly, not in pairs.
Cell B: metaphase I of meiosis. It holds 6 chromosomes, but they are lined up as 3 homologous pairs, one of each pair on each side of the equator. Pairing happens only in meiosis I.
Cell C: metaphase II of meiosis. It holds only 3 chromosomes, one of each pair (the haploid number), lined up singly. A cell with the haploid number whose chromosomes still have two chromatids must be between the end of meiosis I and anaphase II.

Worked example 3: filling in a count table

Maize has $2 n = 20$ . Complete the table for one cell that divides by mitosis and one that divides by meiosis.

Quantity	Mitosis	Meiosis
Chromosomes in the parent cell	20	20
Chromatids at the first metaphase	$20 \times 2 = 40$	$20 \times 2 = 40$
Cells made	2	4
Chromosomes in each cell made	20	$\frac{20}{2} = 10$

Check: total chromosomes out $= 2 \times 20 = 40$ for mitosis and $4 \times 10 = 40$ for meiosis. Both equal the 40 chromatids present at the first metaphase, because in the end every chromatid becomes the chromosome of one new cell.

Both divisions in one life cycle

In every animal that reproduces sexually, including you, the two divisions take turns. Meiosis makes the haploid gametes; fertilisation restores the diploid number; mitosis then builds a whole body from the zygote.

Figure 3: The human life cycle. Meiosis happens once in each generation, in the testes and ovaries; every other cell division is mitosis.

When meiosis goes wrong

Sometimes a homologous pair fails to separate in anaphase I, or a pair of sister chromatids fails to separate in anaphase II. One gamete then carries an extra chromosome and another is missing one. If a human egg with 24 chromosomes is fertilised by a normal sperm with 23, the zygote has $24 + 23 = 47$ . A person with three copies of chromosome 21 instead of two has Down syndrome. This shows how exact the separation in meiosis normally has to be.

Warning

Common mistakes. (1) Writing "mitosis makes sex cells": in animals, gametes come from meiosis. (2) Writing that meiosis makes "two haploid cells": it makes four. (3) Saying mitosis makes cells "similar" to the parent: they are genetically identical. (4) Forgetting that both divisions start with one copy of the DNA in the S phase.

Mitosis vs. Meiosis: Side by Side Comparison, Amoeba Sisters. Watch the two divisions run in parallel, and compare what lines up at metaphase in each.

Tip

Memory hook: "miTOsis makes TWO, meiosis makes FOUR, and mEiosis makes Eggs." For a drawing, ask two questions: pairs or singles on the equator? Diploid or haploid number?

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Practice: mitosis and meiosis compared

State two similarities between mitosis and meiosis.
Copy and complete: mitosis makes ___ cells that are genetically ___; meiosis makes ___ cells that are genetically ___.
A cell from a goat ( $2 n = 60$ ) is seen with 30 chromosomes lined up singly on the equator. Which division and stage is it in? Explain.
A cell with 8 units of DNA before S divides and gives four cells with 4 units each. Name the division.
Give the chromosome number in (a) a human skin cell, (b) a human egg, (c) a human zygote.
Explain why the cells of a banana plant grown from a sucker are identical to the parent, while the plants grown from a bean pod's seeds are not.
In a cell with $2 n = 6$ , how many cells are made, and how many chromosomes are in each, by (a) mitosis, (b) meiosis?
A human egg with 22 chromosomes is fertilised by a normal sperm. How many chromosomes does the zygote have? Explain how the egg could have come to have 22.

Show answersHide answers

For example: both follow one copy of the DNA; both use a spindle; both have prophase, metaphase, anaphase and telophase; both end with cytokinesis.
Two, identical (to the parent and each other); four, different.
Metaphase II of meiosis: 30 is the haploid number, so meiosis I has already happened, and the chromosomes are still lined up singly, not in pairs.
Meiosis: two divisions, and each cell ends with half the starting DNA.
(a) 46; (b) 23; (c) 46.
The sucker grows from the parent by mitosis, so it has exactly the same genes. Seeds come from sexual reproduction: gametes made by meiosis (with crossing over and independent assortment) fuse at fertilisation, so each seed has a new mix of genes.
(a) 2 cells, 6 chromosomes each; (b) 4 cells, 3 chromosomes each.
$22 + 23 = 45$ . During meiosis in the ovary, one pair of homologous chromosomes (or a pair of sister chromatids) failed to separate, so this egg missed a chromosome.

Key points

Mitosis: one division, two identical diploid cells. Meiosis: two divisions, four different haploid cells.
Only meiosis has pairing of homologous chromosomes, crossing over and the separation of pairs.
In a life cycle, meiosis makes gametes, fertilisation restores 2n, and mitosis builds the body.

1Two graphs, two divisions

2Side by side

3Telling the divisions apart in a drawing

4Both divisions in one life cycle

5When meiosis goes wrong

Two graphs, two divisions

Side by side

Telling the divisions apart in a drawing

Both divisions in one life cycle

When meiosis goes wrong