Meiosis
Mitosis makes copies; meiosis makes variety. To produce sperm and egg cells, a special division halves the chromosome number and, in the process, shuffles the genes so that no two gametes are ever quite the same. This lesson describes the three sources of that shuffling, crossing over, independent assortment and random segregation, and models the named phases of meiosis so you can see exactly where the variation comes from.
Get oriented
Make a prediction, read the goals and the key terms, then clear the trap before you start.
Before You Read
warm-upHumans have 46 chromosomes in body cells. When a sperm and an egg join at fertilisation, the new cell must also have 46.
Before reading: how many chromosomes must a sperm and an egg each carry so the fertilised cell has exactly 46? What must meiosis do to the chromosome number, and why?
Learning Intentions
goalsKnow
- That meiosis halves the chromosome number and makes four cells
- What crossing over, independent assortment and random segregation are
- The named phases across meiosis I and meiosis II
Understand
- Why each process increases genetic variation
- Why halving the chromosome number is essential for sexual reproduction
- How meiosis differs from mitosis in outcome
Can Do
- Describe crossing over, independent assortment and random segregation
- Model the named phases of meiosis and cytokinesis
- Explain where in meiosis each source of variation occurs
Scan these before reading
vocabMisconceptions To Fix
watch outBuild the first idea
The core of the dot point, then two checks you can answer straight from the card.
Core Content
Halving the Chromosome Number
+5 XPTwo divisions, four cells, half the chromosomes
Meiosis exists to make gametes, and to do that it must halve the chromosome number so fertilisation can restore the full set.
Text description
- Parent cell: diploid, 2n = 4 (two homologous pairs; one chromosome of each pair shown coral, the other plum). The DNA has been replicated once, so each chromosome is two sister chromatids.
- Meiosis I: the homologous pairs separate, giving two cells, each with one chromosome from each pair.
- Meiosis II: the sister chromatids separate, giving four cells.
- Result: four haploid cells (n = 2), each with half the parent's chromosome number.
Meiosis is a special cell division that occurs in the formation of gametes (sex cells). It involves two rounds of division, meiosis I and meiosis II, and produces four daughter cells, each with half the chromosome number of the parent cell (haploid). The DNA is replicated once (in interphase before meiosis I), but the cell divides twice, which is why the chromosome number is halved. Halving is essential so that when two gametes fuse at fertilisation, the full chromosome number is restored rather than doubling every generation.
Pause, copy the highlighted definition and the reason for halving into your book before moving on.
Meiosis produces _____ daughter cells, each with half the chromosome number of the parent.
Fill the gaps. Meiosis replicates the DNA but divides , producing daughter cells, each with the chromosome number of the parent.
Take it further
The next move, with the diagram to work from and two more checks.
Three Sources of Variation
+5 XPCrossing over, independent assortment and random segregation
We just saw that meiosis makes four haploid cells. That raises a question: why is every one of them genetically different? This card answers it → three shuffling processes mix the genes before the cells separate.
The genetic variety of gametes comes from three distinct processes that rearrange the chromosomes during meiosis.
Text description
- Crossing over (prophase I): a homologous pair (one chromosome coral, one plum, each two chromatids) crosses at a chiasma; the inner chromatids exchange equivalent segments.
- Independent assortment (metaphase I): the homologous pairs line up at the equator independently. In arrangement 1 both coral chromosomes face the same pole; in arrangement 2 they face opposite poles.
- Because pairs line up randomly, each gamete gets a random mix.
Crossing over occurs in prophase I: homologous chromosomes pair up and exchange equivalent segments with each other, producing new combinations of alleles on each chromosome. Independent assortment occurs in metaphase I: each pair of homologous chromosomes lines up at the equator independently of the other pairs, so the maternal and paternal chromosomes are mixed randomly between the gametes. Random segregation is the random separation of the homologous chromosomes (in anaphase I) and then the chromatids (in anaphase II) into different gametes.
Pause, copy each of the three processes and the meiosis phase in which it occurs into your book.
During which event do homologous chromosomes exchange equivalent segments to create new allele combinations?
Match each source of variation to what it actually does.
- Crossing over
- Independent assortment
- Random segregation
- Fertilisation
- A random sperm meets a random egg, multiplying the possibilities again
- Paired homologous chromosomes swap equivalent segments, mixing alleles within a chromosome
- Which chromosome, then which chromatid, ends up in which gamete is left to chance
- Each homologous pair lines up at the equator independently of the others
Work it yourself
Put the idea on paper in your own book before you read on.
Activity 1, Practical
Model the Named Phases of Meiosis and Cytokinesis
Pattern D, Hands-on model
Using two colours of pipe cleaners (or modelling clay) to represent a maternal and a paternal homologous pair, model meiosis in order: prophase I (pair the homologues and model crossing over by swapping a segment), metaphase I (line the pairs up, independent assortment), anaphase I & telophase I + cytokinesis (separate homologues into two cells), then meiosis II (prophase II → metaphase II → anaphase II → telophase II + cytokinesis) to give four cells. Record what each process does to the genetic make-up of your four gametes.
Finish the picture
The last piece of the dot point, then a true or false check across all three cards.
Why It All Adds Up to Variation
+5 XPThree processes, combined with fertilisation, make every offspring unique
We just described the three processes individually. That raises a question: how much variation do they create together, and why does it matter? This card answers it → they multiply together to make a vast number of possible gametes.
The three processes do not act alone, they combine, so the number of genetically different gametes an individual can make is enormous.
Crossing over, independent assortment and random segregation work together so that each gamete carries a unique combination of alleles, and an individual can produce a huge number of genetically different gametes. When a random sperm fertilises a random egg, the genetic possibilities multiply again. This genetic variation is the raw material for natural selection and is why sexually reproducing species can adapt to changing environments, a key reason meiosis is so important, which you will evaluate fully in the next lesson.
Add the point that the three processes combine to create unique gametes, and why this variation matters, to your notes before the check.
Meiosis produces four genetically different gametes, each with half the chromosome number.
Independent assortment occurs because homologous chromosomes always line up in the same fixed order.
Crossing over takes place in prophase I when homologous chromosomes exchange segments.
See where it goes
Where this shows up outside the classroom, and why the dot point is worth knowing.
Why no two people (except twins) are alike
From just 23 pairs of chromosomes, independent assortment alone can produce over 8 million different gametes, and crossing over multiplies that figure enormously. This is the biological reason siblings differ, why plant and animal breeders can select new combinations of traits, and why genetic variation lets populations survive new diseases or climates. When meiosis goes wrong and a chromosome fails to segregate, the result can be a condition such as Down syndrome, a direct link between the processes you have just modelled and human health.
Describe the processes of crossing over, independent assortment and random segregation during meiosis, and their effect on genetic variation.
Describe the three processes means saying when each occurs, what physically happens, and what it produces. The three are distinct and are easily blurred together, so name the stage for each.
6 marks. Roughly two per process: the stage, the physical event, and the contribution to variation. Set up the homologous pair first or none of the three can be described precisely.
A homologous pair is two chromosomes of the same size and gene sequence, one inherited from each parent, carrying the same genes at the same positions but possibly different versions, or alleles, of them. In meiosis the DNA is replicated once but the cell divides twice, in meiosis I and meiosis II, producing four haploid cells; all three sources of variation act on these homologous pairs during the first division.
Crossing over occurs during prophase I. Homologous chromosomes pair up closely along their length in a process called synapsis, and at one or more points, called chiasmata, the chromatids of the two chromosomes break and rejoin so that corresponding segments are exchanged. Each chromosome afterwards carries a combination of maternal and paternal alleles that neither parent chromosome had, so crossing over produces new combinations of alleles on a single chromosome.
Independent assortment occurs during metaphase I. The homologous pairs line up along the equator of the cell, and the orientation of each pair — which member faces which pole — is random and independent of every other pair. With twenty-three pairs in a human cell there are two to the power of twenty-three, over eight million, possible arrangements, so each gamete receives a random mixture of maternally and paternally derived chromosomes.
Random segregation occurs during anaphase I, and follows directly from that random alignment: the homologous pairs are separated and one member of each pair is drawn to each pole, so which of the two homologues any daughter cell receives is a matter of chance for every pair independently. This is the physical event that converts the random arrangement at metaphase I into four genetically different cells.
Together these three processes make it effectively certain that every gamete an individual produces is genetically unique, since the millions of possible chromosome combinations are further multiplied by the countless possible crossover points. Combined with the random fusion of two such gametes at fertilisation, they are the principal source of the genetic variation within a sexually reproducing population, which is the raw material on which natural selection acts.
A homologous pair is two chromosomes of the same size and gene sequence, one from each parent, carrying the same genes at the same positions but possibly different alleles of them; in meiosis the DNA is replicated once while the cell divides twice, producing four haploid cells, and all three sources of variation act on these pairs during the first division. Crossing over occurs in prophase I: homologous chromosomes pair closely along their length in synapsis, and at points called chiasmata the chromatids break and rejoin so corresponding segments are exchanged, leaving each chromosome with a combination of maternal and paternal alleles that neither parent chromosome carried — new combinations on a single chromosome. Independent assortment occurs in metaphase I: the homologous pairs line up at the equator and the orientation of each pair is random and independent of every other, so with twenty-three pairs there are over eight million possible arrangements and each gamete receives a random mixture of maternally and paternally derived chromosomes. Random segregation occurs in anaphase I and follows from that alignment: the pairs are separated and one member of each is drawn to each pole, so which homologue a daughter cell receives is a matter of chance for every pair independently, converting the random arrangement into four genetically different cells. Together these processes make it effectively certain that every gamete is genetically unique, the millions of chromosome combinations being further multiplied by the countless possible crossover points; combined with the random fusion of gametes at fertilisation they are the principal source of genetic variation in a sexually reproducing population, and so the raw material on which natural selection acts.
Contrast mitosis and meiosis.
Contrast asks for differences only. Unlike compare, similarities earn nothing here, so spending a paragraph on what the two divisions share is spending marks you will not get back.
5 marks. Four or five criteria, each stated for both processes in the same sentence, and nothing else.
Mitosis involves a single nuclear division producing two daughter cells, whereas meiosis involves two successive divisions, meiosis I and meiosis II, following a single replication of the DNA, and produces four daughter cells.
Mitosis is a conservative division: each daughter cell receives the same chromosome number as the parent, so a diploid cell produces diploid cells. Meiosis is a reduction division: the homologous pairs are separated at meiosis I, so each of the four daughter cells receives half the parent’s chromosome number and a diploid cell produces haploid cells.
The daughter cells of mitosis are genetically identical to the parent cell and to each other, since no shuffling occurs, whereas the daughter cells of meiosis are genetically different from the parent and from one another, because crossing over in prophase I and independent assortment in metaphase I recombine the alleles before the cells separate.
In mitosis the homologous chromosomes behave independently, never pairing, and it is the sister chromatids that are separated, at anaphase. In meiosis the homologues pair in prophase I and are separated from one another at anaphase I, with the sister chromatids separated only later, at anaphase II.
Mitosis occurs in somatic cells throughout the body and functions in growth, tissue repair, cell replacement and asexual reproduction, whereas meiosis occurs only in the reproductive organs, in the cells that give rise to gametes, and functions solely in producing gametes for sexual reproduction.
Mitosis involves a single nuclear division producing two daughter cells, whereas meiosis involves two successive divisions after one replication of the DNA and produces four. Mitosis is a conservative division in which each daughter receives the parent’s chromosome number, so a diploid cell yields diploid cells, while meiosis is a reduction division in which homologous pairs are separated at meiosis I, so each of the four daughters receives half the parent’s number and a diploid cell yields haploid cells. The products of mitosis are genetically identical to the parent and to each other, no shuffling occurring, whereas those of meiosis differ from the parent and from one another because crossing over in prophase I and independent assortment in metaphase I recombine the alleles before separation. In mitosis the homologous chromosomes never pair and it is the sister chromatids that separate at anaphase, whereas in meiosis the homologues pair in prophase I and separate from each other at anaphase I, the sister chromatids separating only later at anaphase II. Finally, mitosis occurs in somatic cells throughout the body and serves growth, tissue repair, cell replacement and asexual reproduction, whereas meiosis occurs only in the reproductive organs, in the cells giving rise to gametes, and serves solely the production of gametes for sexual reproduction.
Practise independently
Answer in your own words first. The model answers are for checking, not for copying.
Multiple Choice
+5 XPA fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
Short Answer, 13 marks
+5 XPUnderstandBand 5(6 marks) 1. Describe the processes of crossing over, independent assortment and random segregation during meiosis, and explain how each contributes to genetic variation.
2 marks each process: correctly described AND linked to how it creates variation
AnalyseBand 5(3 marks) 2. Explain why meiosis halves the chromosome number even though the DNA is replicated only once, and state why this halving is necessary.
1 mark: one replication, two divisions · 1 mark: shows the arithmetic across meiosis I and II · 1 mark: links halving to restoring the number at fertilisation
EvaluateBand 5–6(4 marks) 3. Two siblings from the same parents are genetically different. Use crossing over, independent assortment and random segregation to explain why, and assess which of the three you would expect to contribute most to their difference.
3 marks: each process correctly applied to the siblings' difference · 1 mark: a judgement with a reason, either answer credited if justified
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Short Answer Model Responses
Q1 (6 marks): Crossing over occurs in prophase I, when homologous chromosomes pair up and exchange equivalent segments with each other. This produces new combinations of alleles on each chromosome, creating variation by mixing maternal and paternal genes within a single chromosome. Independent assortment occurs in metaphase I, when each pair of homologous chromosomes lines up at the equator independently of the other pairs. Because either chromosome of a pair can face either pole, each gamete receives a random mix of maternal and paternal chromosomes, greatly increasing the number of possible combinations. Random segregation is the random separation of the homologous chromosomes (anaphase I) and then the sister chromatids (anaphase II) into different gametes, so which chromosomes and chromatids end up together is left to chance. Together these three processes ensure each of the four gametes is genetically unique, providing the variation that is the raw material for natural selection.
Q2 (3 marks): The DNA is replicated once, in the interphase before meiosis I, but the cell then divides twice, in meiosis I and meiosis II [1]. One replication followed by two divisions means the original chromosome content is shared out among four cells rather than two, so each daughter cell finishes with half the parent's chromosome number and is haploid [1]. The halving is necessary because two gametes fuse at fertilisation; if each carried a full set the number would double every generation, whereas halving first means fertilisation restores exactly the species' normal diploid number [1].
Q3 (4 marks): Crossing over in prophase I means the chromosomes each parent passes on are not the ones they inherited: homologous chromosomes exchange equivalent segments, so a single chromosome carries a new mix of alleles [1]. Independent assortment in metaphase I means each of the 23 homologous pairs lines up and separates independently of the others, so each gamete gets its own random mixture of maternal and paternal chromosomes [1]. Random segregation means which chromosome, and later which chromatid, ends up in a given gamete is left to chance, so no two gametes from the same parent are alike, and each sibling started from a different sperm and a different egg [1]. Independent assortment alone gives over 8 million combinations from 23 pairs, so it accounts for an enormous share; but crossing over acts on top of that by creating chromosomes that never existed before, so a defensible judgement is that crossing over contributes most because it multiplies an already vast number rather than simply reshuffling existing chromosomes [1].
Retrieve what stuck
Work the spaced-review set first. It mixes this lesson with earlier ones on purpose.
Check what actually stuck
A short retrieval set drawn from this lesson and the ones before it, so the earlier work keeps coming back rather than fading.
Lock it in
bossYou can now describe crossing over, independent assortment and random segregation, and you have modelled the named phases of meiosis. The final lesson of this focus area evaluates why DNA replication, mitosis, meiosis and cytokinesis are all so important.
↩ Back to focus-area hubCompare it with where you started
Read your opening prediction again, then mark the lesson complete.
How did your thinking change?
At the start you worked out the chromosome numbers for a sperm and egg. Each gamete must carry 23 chromosomes (half of 46), so meiosis must halve the chromosome number, otherwise fertilisation would double it every generation. If your first answer kept the number at 46, revisit Card 1: meiosis copies the DNA once but divides twice, which is what halves the count and lets fertilisation restore the full set.