What are chromosomes? Explain how in sexually reproducing organisms the number of chromosomes in the progeny is maintained.

How do Organisms Reproduce?

Sexual Reproduction - DNA Copying and Chromosome Maintenance

Official Answer

Chromosomes are thread-like structures present in the nucleus of a cell that contain information for the inheritance of features from parents to the next generation in the form of DNA (Deoxyribo Nucleic Acid) molecules.

In sexually reproducing organisms, the number of chromosomes in the progeny is maintained through the following mechanism:

Meiosis (Reduction Division): Organisms have special lineages of cells in specialized reproductive organs. These cells undergo a specific type of cell division called meiosis, which halves the number of chromosomes and the amount of DNA compared to non-reproductive body cells. This results in the formation of haploid ( $n$ ) germ-cells (gametes).
Fertilization: During sexual reproduction, a male germ-cell ( $n$ ) fuses with a female germ-cell ( $n$ ) to form a zygote. This fusion re-establishes the original diploid ( $2n$ ) number of chromosomes and the DNA content in the new generation (progeny).

chromosomesDNAmeiosisgerm-cellsgametesfertilizationzygotediploidhaploid

Marking Scheme

1Definition of chromosomes as DNA-containing structures in the nucleus carrying hereditary information (1 mark)
2Explanation of meiosis reducing the chromosome number to half ( $n$ ) in germ-cells (1 mark)
3Explanation of fertilization restoring the diploid ( $2n$ ) chromosome number in the zygote (1 mark)

Hint

Recall the role of meiosis in halving the chromosome number in germ-cells, and how fertilization restores the original diploid count.

Quick Oral Answer

Chromosomes are DNA-carrying structures in the nucleus. In sexual reproduction, germ-cells are produced by meiosis, which halves the chromosome number. When these gametes fuse during fertilization, the original chromosome number is restored in the zygote.

Analysis & Explanation

The cell nucleus contains chromosomes made of DNA, which acts as the information source for making proteins and determining body design. If reproducing cells simply combined their normal body cells during reproduction, the progeny would end up with twice the amount of DNA and chromosomes, disrupting cellular control. To prevent this, multicellular organisms set aside specialized germ cells via meiosis. Meiosis reduces the chromosome number to half ( $n$ ). When fertilization occurs, the fusion of two haploid gametes ( $n + n$ ) restores the original diploid chromosome number ( $2n$ ) characteristic of the species.

Common Mistakes

1Confusing mitosis with meiosis.
2Failing to mention that gametes contain only half the amount of DNA/chromosomes.
3Omitting the role of fertilization in restoring the original chromosome number.

Interesting Facts

While humans have 46 chromosomes (23 pairs) in almost all body cells, their sperm and egg cells carry exactly 23 single chromosomes so that their fusion recreates a 46-chromosome zygote.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2020?

This question carries 3 marks in the CBSE Class 10 Science 2020 examination.

Which chapter does this question come from in Science?

This question is from the chapter "How do Organisms Reproduce?" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Sexual Reproduction - DNA Copying and Chromosome Maintenance" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2020 paper?

This is a Short Answer question from Section B in the CBSE Class 10 Science 2020 paper.