Lesson 3 of 12 / Reproduction
Meiosis halves the chromosome number
How is meiosis different from mitosis?
I can explain the reduction in chromosome number and the role of meiosis in gamete formation.
I can explain ploidy and why reduction precedes fertilisation.Syllabus
Syllabus K325, Topic 13(e). Explain ploidy and why reduction precedes fertilisation.
I can explain homologous chromosomes.Syllabus
Syllabus K325, Topic 13(f). Explain homologous chromosomes.
I can state the genetic and chromosome outcomes of meiosis.Syllabus
Syllabus K325, Topic 13(g). State the genetic and chromosome outcomes of meiosis.
I can connect meiosis with gamete formation.Syllabus
Syllabus K325, Topic 13(h). Connect meiosis with gamete formation.
About 5 min
Make a guess. You are not marked.
Two sperm cells come from the same man. Do they carry the same genetic information?
Show the answer
Usually not; meiosis makes each gamete different.
Meiosis sorts each pair of chromosomes differently and swaps genetic material, so gametes vary.
The key idea
Meiosis separates homologous chromosomes and halves the chromosome number, producing genetically dissimilar haploid cells.
Bars stand for chromosomes; DNA copying and division stages are omitted to focus on sets. The meiosis model isolates assortment without crossing over, so some products shown match. It is not a diagram of four equally developed human egg cells.
Explanation
Meiosis begins with a diploid cell and, after DNA copying and two divisions, produces haploid cells. The separation of homologous chromosomes in the first division reduces the number of chromosome sets from two to one.
The chromosome combinations differ among the resulting cells. The sorting of homologous pairs, together with exchange of genetic material, creates variation. Detailed stages and crossing-over mechanisms are not required for this outline.
Meiosis is used in the formation of gametes. Fertilisation then restores the diploid number. In animals, the resulting cells develop into gametes; diagrams showing four cells should not be read as four equally developed human egg cells.
The key contrast is the outcome: mitosis gives matching nuclei with the original chromosome number, while meiosis gives genetically different haploid cells with half that number. Use both chromosome number and genetic similarity in a comparison.
Step by step
- 1
Start with homologous pairs
A diploid starting cell has one member of each pair from each parent.
- 2
Separate the pairs
Each haploid set receives one chromosome from each pair.
- 3
Connect to fertilisation
Combining two haploid nuclei restores the diploid number in the next generation.
Worked example
Explain variation
Two sperm cells from the same individual can contain different inherited information. Explain why.
One way to explain it
Meiosis produces haploid cells with different combinations of genetic information. Each sperm receives one chromosome set, but the alleles within those sets need not be identical.
Why this answer works
- Same parent does not mean identical gametes.
- The number is the same even though the genetic combinations can differ.
Watch out for this
A student says: "Meiosis makes cells with random numbers of chromosomes." What is wrong with this?
Show the answer
Normal meiosis halves the chromosome number consistently. The combinations of inherited information vary, not the required number of sets.
Check your understanding
A diploid nucleus has 12 chromosomes. What is the normal result of meiosis?
Show the answer
Genetically varied haploid cells with 6 chromosomes each
Homologous chromosomes separate, halving the chromosome number.
Exam-style questions
Check your understanding
A dog body cell has 78 chromosomes. How many chromosomes are in a dog sperm cell and in a fertilised egg?
Show the answer
39 and 78
Meiosis halves the number to 39 in gametes; fertilisation restores 78 in the zygote.
Quick recall
How is meiosis different from mitosis?