Does the chromosome number get cut in half in meiosis I or meiosis II?

Meiosis I. Anaphase I separates the homologous partners, so each resulting cell has only one chromosome from each pair — that is already haploid. Meiosis II separates sister chromatids, which increases the number of cells but leaves the chromosome number per cell unchanged at n. What meiosis II changes is the number of chromatids per chromosome, from two down to one.

Why doesn't DNA replicate again between meiosis I and meiosis II?

Because the whole point of meiosis is to reduce genetic content. The single S phase before meiosis I supplies the chromatids needed for both divisions: two divisions after one replication gives four cells with half the original chromosome number. The short gap between the divisions, sometimes called interkinesis, involves no DNA synthesis. If replication did occur there, the products would be diploid and fertilization would double the chromosome number every generation.

How are the four cells produced by meiosis different from each other?

Two mechanisms make them differ. Independent assortment in metaphase I means each cell receives a random mix of maternal and paternal homologs. Crossing over in prophase I swaps segments between homologs, so even the two sister chromatids of one chromosome are no longer identical — which is why the two cells produced from a single meiosis II division also differ. In humans this yields millions of possible combinations from assortment alone, and effectively unlimited variation once crossing over is included.

If meiosis makes four cells, why does a human female release only one egg per cycle?

The chromosome behavior is the same in both sexes, but the cytoplasm is divided unequally in oogenesis. One product keeps almost all the cytoplasm and stored nutrients and becomes the egg; the other three become small polar bodies that break down. In spermatogenesis the cytoplasm is divided evenly and all four products mature into sperm.

BIO-4.3

Meiosis & Gamete Formation

Learn how meiosis halves chromosome number and makes four genetically unique haploid gametes — homolog separation, crossing over, and independent assortment explained.

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A voice-first session with the Crimsora tutor on Meiosis & Gamete Formation, then targeted practice and FRQs — with the tutor adapting to where you get stuck.

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What this lesson covers

Every human body cell carries 46 chromosomes, but a sperm or egg carries only 23. If gametes had the full set, fertilization would double the chromosome number every generation — 46, then 92, then 184. Meiosis is the division that prevents that, and it does something even more remarkable along the way: it shuffles the genetic deck so that no two gametes a person makes are exactly alike.

In this lesson you will trace one starting cell through two consecutive divisions. In meiosis I, homologous chromosomes — the matched maternal and paternal partners — are pulled apart, and that is the step that actually halves the chromosome number. In meiosis II, the sister chromatids of each chromosome finally separate. Along the way, crossing over swaps segments between partners and independent assortment randomizes which partner goes where. The payoff is four haploid cells, each genetically different from the others and from the parent cell.

Diploid, Haploid, and Why a Reduction Division Is Necessary

A diploid cell (

2n

) has chromosomes in matched pairs. In humans,

2n = 46

, which is 23 pairs. The two members of a pair are homologous chromosomes: they are the same length, carry the same genes in the same order at the same loci, but they came from different parents and may carry different alleles of those genes. One homolog of chromosome 7 came from the mother, one from the father.

Do not confuse homologous chromosomes with sister chromatids. Sister chromatids are two identical copies of a single chromosome, produced by DNA replication in S phase and joined at the centromere. Homologs are non-identical partners that were never copies of each other.

A haploid cell (

n

) has one chromosome from each pair — 23 in humans. Gametes must be haploid so that fertilization of a haploid egg by a haploid sperm restores the diploid number:

n + n = 2n

, or

23 + 23 = 46

. The resulting zygote gets one complete set from each parent.

Meiosis achieves this with one round of DNA replication followed by two rounds of division. Because replication happens only once, the cell cannot keep pace with two divisions, and the chromosome number per cell drops from

2n

n

. Biologists call meiosis I the reductional division (chromosome number per cell is halved) and meiosis II the equational division (chromosome number stays the same, but each chromosome loses its second chromatid).

Meiosis I: Homologous Partners Are Pulled Apart

Before meiosis begins, the cell goes through interphase and replicates its DNA, so every chromosome enters prophase I as two sister chromatids. In a human cell that is 46 chromosomes and 92 chromatids.

Prophase I is where meiosis becomes unique. Homologous chromosomes find each other and align gene-for-gene in a process called synapsis, forming a four-chromatid structure called a tetrad (or bivalent). While synapsed, non-sister chromatids break and rejoin at points called chiasmata — this is crossing over, and it physically exchanges matching segments of DNA between the maternal and paternal homolog. A chromatid that emerges with a mix of maternal and paternal alleles is called recombinant.

In metaphase I, whole tetrads line up at the cell's midline, with one homolog of each pair facing each pole. Spindle fibers from opposite poles attach to the two homologs — not to the two sister chromatids of one chromosome, which is how mitosis works.

In anaphase I, the homologs are pulled to opposite poles while each chromosome keeps both of its sister chromatids attached at the centromere. This single step is what halves the chromosome number.

After telophase I and cytokinesis there are two cells, each haploid (

n = 23

in humans) but each chromosome still made of two chromatids. Students very commonly say these cells are diploid because they still contain 46 chromatids. Count chromosomes, not chromatids: 23 chromosomes, one from each pair, is haploid. There is no second round of DNA replication between the divisions.

Meiosis II: Sister Chromatids Separate, Yielding Four Cells

Each of the two haploid cells now divides again, and mechanically this division looks like a small mitosis — but the cells entering it are already haploid, and their chromosomes have been reshuffled by crossing over, so the products are not identical.

In prophase II a new spindle forms. In metaphase II the chromosomes line up singly at the midline, spindle fibers attached to opposite sides of each centromere. In anaphase II the centromeres split and sister chromatids separate, each becoming an independent chromosome. Telophase II and cytokinesis produce a total of four haploid cells, each with 23 single-chromatid chromosomes in humans.

Feature	Meiosis I	Meiosis II
What lines up at the midline	Tetrads (paired homologs)	Individual chromosomes
What separates in anaphase	Homologous chromosomes	Sister chromatids
Chromosome number change	$2n \rightarrow n$	$n \rightarrow n$
Chromatids per chromosome after	2	1
Crossing over	Yes, in prophase I	No
Cells produced	2 haploid	4 haploid total

A useful bookkeeping check: DNA content per cell is halved twice (once in each division), but chromosome number is halved only once, in meiosis I. If your diagram shows 46 chromosomes lining up as individuals in metaphase II, something went wrong earlier in the drawing.

Two Engines of Variation: Crossing Over and Independent Assortment

Meiosis does not just divide — it randomizes. Two independent mechanisms are responsible.

Independent assortment happens in metaphase I. For each tetrad, whether the maternal homolog faces the left pole or the right pole is random, and each pair orients independently of every other pair. With 23 pairs, the number of possible chromosome combinations in a gamete is

2^{23} = 8{,}388{,}608

. In general, a cell with

n

pairs can produce

2^n

combinations from assortment alone. Fertilization then multiplies two of these figures together, giving roughly

2^{23} \times 2^{23} \approx 70

trillion possible zygote combinations from one couple.

Crossing over in prophase I raises that number to effectively unlimited, because it creates chromosomes that never existed in either parent. A crossover between two genes on the same chromosome breaks up the allele combination they were inherited in, so alleles on the same chromosome are not permanently locked together.

Two frequent errors are worth naming. First, crossing over occurs between non-sister chromatids of homologous chromosomes, not between sister chromatids — exchange between identical sisters would change nothing. Second, independent assortment refers to how different pairs orient relative to each other, not to the random separation of sister chromatids in meiosis II. Anaphase II separation is not random; the two chromatids go to opposite poles every time. If the sisters were still identical, that step would produce two identical cells — crossing over is what makes those two products differ.

From Meiotic Products to Actual Gametes

Meiosis produces four haploid nuclei, but organisms package them differently. In human males, spermatogenesis converts all four products into functional sperm; each loses most of its cytoplasm and gains a flagellum. In human females, oogenesis divides the cytoplasm unequally: one product keeps nearly all the cytoplasm and nutrients and becomes the egg, while the other three shrink into polar bodies that degenerate. So one meiosis in a female yields one usable egg, not four. The chromosome math is identical; only the cytoplasmic packaging differs.

Because meiosis handles chromosomes twice, errors are possible. Nondisjunction occurs when homologs fail to separate in anaphase I or sister chromatids fail to separate in anaphase II. The result is a gamete with an extra chromosome and another missing one. If such a gamete is fertilized, the zygote has an abnormal chromosome number — trisomy 21 (Down syndrome), for example, arises from a gamete carrying two copies of chromosome 21.

When you are asked to trace chromosome behavior, build the habit of writing three numbers at each stage: number of cells, chromosomes per cell, and chromatids per chromosome. Tracking all three keeps you from the most common mix-ups — calling post-meiosis-I cells diploid, or believing a second DNA replication happens between the divisions. It also connects directly to inheritance patterns you will use later: independent assortment of chromosomes in metaphase I is the physical reason unlinked genes assort independently in genetic crosses.

Key terms

Homologous chromosomes.: A matched pair of chromosomes, one inherited from each parent, carrying the same genes at the same loci but possibly different alleles.
Sister chromatids.: Two identical copies of one chromosome, produced by DNA replication and joined at the centromere; they separate in anaphase II.
Haploid ( $n$ ).: Having one chromosome from each homologous pair; human gametes are haploid with 23 chromosomes.
Synapsis and tetrad.: The pairing of homologous chromosomes in prophase I, creating a four-chromatid structure (tetrad or bivalent) where crossing over occurs.
Crossing over.: The exchange of matching DNA segments between non-sister chromatids of homologous chromosomes at chiasmata during prophase I, producing recombinant chromatids.
Independent assortment.: The random orientation of each tetrad at metaphase I, so maternal and paternal homologs are distributed to poles independently; gives $2^n$ combinations for $n$ pairs.
Reductional division.: Meiosis I, in which homologs separate and chromosome number per cell drops from $2n$ to $n$ .
Nondisjunction.: Failure of homologs (anaphase I) or sister chromatids (anaphase II) to separate properly, producing gametes with an incorrect chromosome number.

Worked example

A fruit fly cell has a diploid number of

2n = 8

. Trace this cell through meiosis: (a) how many chromosomes and chromatids are present at the start of prophase I, (b) how many tetrads form, (c) how many chromosomes are in each cell at the end of telophase I and how many chromatids does each contain, (d) how many chromosomes are in each final gamete, and (e) how many different chromosome combinations can independent assortment alone produce?

Start with the setup. Diploid

2n = 8

means 8 chromosomes arranged in 4 homologous pairs, so

n = 4

.

(a) DNA replicated during interphase before meiosis began, so each of the 8 chromosomes now consists of 2 sister chromatids. Chromosomes: 8. Chromatids:

8 \times 2 = 16

. Note that replication does not change chromosome number — a two-chromatid chromosome still counts as one chromosome.

(b) A tetrad is one synapsed pair of homologs, so the number of tetrads equals the number of pairs: 4 tetrads, each containing 4 chromatids (

4 \times 4 = 16

chromatids, which checks against part a).

(c) Anaphase I pulls homologs apart, sending one member of each pair to each pole. Each telophase I cell therefore receives 4 chromosomes — one from each pair — which is haploid. Sister chromatids have not separated yet, so each of those 4 chromosomes still has 2 chromatids (8 chromatids per cell). These cells are haploid even though they contain 8 chromatids; always count chromosomes, not chromatids.

(d) Meiosis II separates the sister chromatids without any new DNA replication. Each of the two cells divides into two, giving 4 cells with 4 single-chromatid chromosomes each. Each gamete: 4 chromosomes.

(e) Independent assortment gives

2^n

combinations where

n

is the number of pairs:

2^4 = 16

possible chromosome combinations. Crossing over in prophase I would push the actual number of genetically distinct gametes far above 16.

Practice questions

At the end of meiosis I in a human cell, each daughter cell contains:

46 chromosomes, each made of one chromatid
46 chromosomes, each made of two chromatids
23 chromosomes, each made of one chromatid
23 chromosomes, each made of two chromatids

Answer: 23 chromosomes, each made of two chromatids

Anaphase I separates homologous chromosomes, so each cell gets one member of each of the 23 pairs — that is 23 chromosomes, already haploid. But the centromeres have not split yet, so each of those chromosomes is still two sister chromatids joined together. The choice with 23 single-chromatid chromosomes describes cells at the end of meiosis II, and the choices with 46 chromosomes miss the fact that the reduction happens in meiosis I, not meiosis II.

A student claims that crossing over occurs between sister chromatids and that this is what makes the four products of meiosis genetically different. Identify the error and give a correct explanation of the two mechanisms that generate genetic variation during meiosis.

Answer: Crossing over occurs between non-sister chromatids of homologous chromosomes during prophase I, not between sister chromatids. Variation comes from (1) crossing over, which creates recombinant chromosomes carrying new mixes of maternal and paternal alleles, and (2) independent assortment in metaphase I, in which each tetrad orients randomly so maternal and paternal homologs are distributed independently, giving $2^{23}$ combinations in humans.

The key point is that sister chromatids are identical copies of the same chromosome, so exchanging segments between them would swap identical DNA and change nothing. Homologs, by contrast, carry different alleles, so exchange between non-sister chromatids of a tetrad genuinely produces new allele combinations. Independent assortment is a separate mechanism operating at a different moment: it does not alter the chromosomes themselves, only which pole each homolog travels toward. Together, the two mechanisms explain why all four products of a single meiosis can differ from one another.

An organism has

2n = 12

. How many chromosomes are present in one of its gametes, and how many chromosome combinations can independent assortment alone produce?

Answer: 6 chromosomes per gamete; $2^6 = 64$ combinations.

With

2n = 12

there are 6 homologous pairs, so

n = 6

and each haploid gamete receives one chromosome from each pair. For independent assortment, each pair has 2 possible orientations at metaphase I and the pairs behave independently, so multiply 2 by itself once for every pair:

2^6 = 64

. A common wrong answer is

6 \times 2 = 12

, which multiplies the number of pairs by 2 instead of multiplying 2 by itself once per pair.

FAQ

Does the chromosome number get cut in half in meiosis I or meiosis II?: Meiosis I. Anaphase I separates the homologous partners, so each resulting cell has only one chromosome from each pair — that is already haploid. Meiosis II separates sister chromatids, which increases the number of cells but leaves the chromosome number per cell unchanged at $n$ . What meiosis II changes is the number of chromatids per chromosome, from two down to one.
Why doesn't DNA replicate again between meiosis I and meiosis II?: Because the whole point of meiosis is to reduce genetic content. The single S phase before meiosis I supplies the chromatids needed for both divisions: two divisions after one replication gives four cells with half the original chromosome number. The short gap between the divisions, sometimes called interkinesis, involves no DNA synthesis. If replication did occur there, the products would be diploid and fertilization would double the chromosome number every generation.
How are the four cells produced by meiosis different from each other?: Two mechanisms make them differ. Independent assortment in metaphase I means each cell receives a random mix of maternal and paternal homologs. Crossing over in prophase I swaps segments between homologs, so even the two sister chromatids of one chromosome are no longer identical — which is why the two cells produced from a single meiosis II division also differ. In humans this yields millions of possible combinations from assortment alone, and effectively unlimited variation once crossing over is included.
If meiosis makes four cells, why does a human female release only one egg per cycle?: The chromosome behavior is the same in both sexes, but the cytoplasm is divided unequally in oogenesis. One product keeps almost all the cytoplasm and stored nutrients and becomes the egg; the other three become small polar bodies that break down. In spermatogenesis the cytoplasm is divided evenly and all four products mature into sperm.

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