DoAssignment study guide

D3.1 · Explain meiosis, chromosome movement, and crossing over

Learn to explain meiosis, chromosome movement, and crossing over through clear examples and targeted practice.

Ontario Grade 11 Biology

Genetic Processes

How one cell division sequence makes cells with half the chromosome number

In earlier biology, you learned that cells contain chromosomes and that cells divide. Meiosis is a special kind of cell division. It produces reproductive cells with half the chromosome number of the starting cell. To understand how, follow one pair of matching chromosomes through the process. Their movement and exchange of segments help explain why the cells produced are not all genetically identical.

What you will learn

Explain why meiosis reduces the chromosome number in reproductive cells.
Describe how chromosomes move during meiosis I and meiosis II.
Explain how crossing over changes the combination of chromosome segments.

From cells and chromosomes to the question

A chromosome is a structure in a cell that carries genetic information. Before meiosis, a cell copies its chromosomes. Each copied chromosome consists of two identical sister chromatids joined at a region called the centromere. A chromatid is one of the two joined copies.

In many organisms, chromosomes occur in matching pairs. A homologous pair contains one chromosome inherited from each parent. The two chromosomes have the same kinds of genes in corresponding positions, but they may carry different versions of some genes. A gene is a section of chromosome information related to a feature; a version of a gene is called an allele.

The key question is: how can one starting cell make cells with half as many chromosomes, while also producing new combinations of chromosome segments? Meiosis answers this through two rounds of division after one chromosome-copying event.

A homologous pair has one chromosome from each parent.
Sister chromatids are the copied halves of one chromosome.
Chromosome copying occurs before the two meiotic divisions.

The two divisions and chromosome movement

Meiosis has two divisions: meiosis I and meiosis II. The starting cell has pairs of homologous chromosomes. In meiosis I, homologous chromosomes move apart into different cells. This is the division that reduces the chromosome number by half. The sister chromatids of each chromosome remain joined during this first division.

In meiosis II, the sister chromatids separate. They move into different cells, much as copied chromosome halves do during ordinary cell division. There is no chromosome-copying step between meiosis I and meiosis II. At the end, one starting cell has produced four cells. Each has one chromosome from each original homologous pair, rather than a pair.

For a simple model, imagine a cell with one homologous pair. Label the chromosomes maternal and paternal to show which parent each came from. In meiosis I, the maternal chromosome goes to one side and the paternal one to the other. In meiosis II, the sister chromatids of each chromosome separate. Real cells contain more than one pair, but the model makes the movement easier to follow.

The symbols

2n

and

n

can summarize chromosome number. Here,

2n

means two sets of chromosomes and

n

means one set. The change from

2n

n

occurs in meiosis I because homologous chromosomes separate. The second division separates sister chromatids without reducing the number of sets again.

2n \rightarrow n

Meiosis I separates homologous chromosomes.
Meiosis II separates sister chromatids.
The chromosome number is halved in meiosis I.

Crossing over creates new combinations

Before homologous chromosomes separate in meiosis I, they pair closely. During this pairing, non-sister chromatids can exchange corresponding segments. Non-sister chromatids are chromatids belonging to different chromosomes in a homologous pair. This exchange is called crossing over.

Suppose a chromosome from one parent has a segment represented by

A

and a matching chromosome from the other parent has a corresponding segment represented by

a

. These letters stand for different versions in a simple model; they are not a picture of the full chromosome. If the chromatids exchange matching segments, the resulting chromatids can carry combinations of segments that were not together on either original chromatid.

Crossing over does not mean that whole chromosomes swap places. It is an exchange between chromatids in a homologous pair. It changes the combinations of chromosome segments passed into the resulting cells. The exchange happens before homologous chromosomes move apart in meiosis I.

A chromosome diagram or model helps show the sequence, but it is still a simplified representation. It can show which structures pair, exchange segments, and separate. It cannot show every detail of a living cell. The explanation is a model of chromosome behaviour, not a direct view of every event.

Crossing over occurs between non-sister chromatids of homologous chromosomes.
Matching chromosome segments are exchanged.
The exchange creates new combinations of segments.

Use chromosome movement to explain the outcome

To explain a meiosis diagram, first identify what is being shown: a homologous pair, sister chromatids, or the cells after a division. Then ask which structures move apart. Homologous chromosomes moving apart indicates meiosis I. Sister chromatids moving apart indicates meiosis II. This distinction prevents a common mix-up.

A diagram can also be used as evidence for a proposed sequence: paired homologous chromosomes, an exchange between chromatids, separation of homologues, then separation of sister chromatids. The diagram is an explanatory model. It helps connect the visible arrangement of chromosome structures to the process being described.

The final cells have one chromosome from each original pair. Crossing over can make their chromatids carry different combinations of segments. Therefore, the four resulting cells are not necessarily genetically identical. The model explains a general outcome; it does not predict the exact chromosome combinations in a particular cell.

Identify whether homologous chromosomes or sister chromatids are separating.
Use the chromosome arrangement to explain the stage.
Describe crossing over as a source of new segment combinations, not as a guaranteed specific outcome.

Worked example

Track one homologous pair

A model cell begins with one homologous pair. Each chromosome has already been copied. Describe what separates in meiosis I and meiosis II, and state the number of chromosomes in each final cell.

Identify the starting structures
There are two chromosomes in the homologous pair. Each has two sister chromatids because copying happened before meiosis began.
Follow meiosis I
The homologous chromosomes move to different cells. Each cell receives one chromosome from the pair, and its sister chromatids remain joined.
Follow meiosis II
The sister chromatids separate. Each final cell receives one chromatid from the chromosome it inherited in meiosis I. In this one-pair model, each final cell has one chromosome.

Answer: Meiosis I separates the homologous chromosomes. Meiosis II separates sister chromatids. Each of the four final cells has one chromosome from the original pair.

Check: The reduction happens when the homologous pair separates in meiosis I, not when sister chromatids separate in meiosis II.

Worked example

Identify the division from a diagram description

A diagram shows two cells. In each, the two chromatids of a chromosome are moving toward opposite ends of the cell. Which meiotic division is shown, and why?

Name the moving structures
The description says the two chromatids of one chromosome are moving apart. Those are sister chromatids, not the two chromosomes of a homologous pair.
Match movement to division
Sister chromatids separate in meiosis II. Homologous chromosomes separate in meiosis I, so this cannot be the defining movement of meiosis I.

Answer: The diagram shows meiosis II because sister chromatids are moving apart.

Check: The clue is the separation of the two chromatids belonging to one chromosome.

Worked example

Explain a crossing-over model

Two homologous chromosomes carry the segment combinations

AB

and

ab

. In a simplified model, matching chromatids exchange the segment after the first letter. Give the two changed combinations and explain what the model shows.

Separate the segment labels
The first-letter segments are $A$ and $a$ . The segments after them are $B$ and $b$ . The stated exchange swaps the latter segments between the chromatids.
Record the changed combinations
The chromatid that had $A$ now has the exchanged $b$ segment. The chromatid that had $a$ now has the exchanged $B$ segment. Ab,\ aB
State the model's meaning
The exchange creates combinations different from the original $AB$ and $ab$ . The letters are a simplified way to track segments; they do not predict exactly which combinations will occur in an individual cell.

Answer: The changed combinations are

Ab

and

aB

. The model shows how crossing over can create new combinations of chromosome segments.

Check: Only the segments after the first letter were exchanged, as the problem specified.

Common mistakes and how to avoid them

Saying that sister chromatids separate in meiosis I.

Correction: Homologous chromosomes separate in meiosis I. Sister chromatids separate in meiosis II.

Saying that crossing over swaps whole chromosomes.

Correction: Crossing over is an exchange of corresponding segments between non-sister chromatids in a homologous pair.

Saying chromosome copying happens before both divisions.

Correction: Chromosomes are copied before meiosis I. They are not copied again between meiosis I and meiosis II.

Lesson summary

Meiosis consists of two divisions after one chromosome-copying event.
Homologous chromosomes separate in meiosis I, reducing the chromosome number from two sets to one.
Sister chromatids separate in meiosis II.
Crossing over exchanges corresponding segments between non-sister chromatids and can create new combinations.

Check your understanding

Question 1

Which structures separate during meiosis I?

Sister chromatids
Homologous chromosomes
Centromeres from the cell
Matching segments after meiosis II

Show answer and explanation

Homologous chromosomes

Meiosis I separates the two chromosomes of each homologous pair.

Question 2

A model shows an exchange between non-sister chromatids in a homologous pair. What process does it represent?

Chromosome copying
Crossing over
Separation of sister chromatids
The reduction from two sets to one

Show answer and explanation

Crossing over

Crossing over is the exchange of corresponding segments between non-sister chromatids.

Question 3

When do sister chromatids separate in meiosis?

Before chromosome copying
During meiosis I
During meiosis II
Only after the four cells have formed

Show answer and explanation

During meiosis II

Sister chromatids remain joined during meiosis I and separate during meiosis II.

Key terms

Chromosome: A structure in a cell that carries genetic information.
Chromatid: One of the two joined copies of a chromosome after it has been copied.
Centromere: The region where sister chromatids are joined.
Homologous pair: Two matching chromosomes, one inherited from each parent, with the same kinds of genes in corresponding positions.
Meiosis: A sequence of two cell divisions that produces cells with half the chromosome number of the starting cell.
Crossing over: An exchange of corresponding chromosome segments between non-sister chromatids of a homologous pair.
Allele: A version of a gene.

Continue through SBI3U

View the complete SBI3U Ontario Grade 11 Biology curriculum and lessons

About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 11 Biology (SBI3U), expectation D3.1. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

Official curriculum reference

Report a correction or ask a question