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16.1.116 · Inherited Change

Haploidy and Diploidy — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 8 parts Lesson 2 of 18 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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16.1.1 Haploidy and Diploidy — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Define haploid and diploid and explain why meiosis must halve the chromosome number.Definition — a cell with two complete sets of chromosomes (2n).Contents — chromosomes carry the DNA neede…

  • Calculation
  • Fertilisation
  • Correct number
  • First division
  • Reduction division
  • Contents — chromosomes carry the DNA needed for protein synthesis and cell function

Part 1 of 6Diploid cells

Definition
  • Definition a cell with two complete sets of chromosomes (2n).
Contents
  • Contents chromosomes carry the DNA needed for protein synthesis and cell function.
Humans
  • Humans nearly all body cells are diploid, with 23 pairs (46 chromosomes).

Part 2 of 6Haploid cells

Definition
  • Definition a cell with one complete set of chromosomes (n).
Chromosome number
  • Chromosome number half that of diploid cells.
Humans
  • Humans haploid cells have 23 chromosomes with no pairs.
Gametes
  • Gametes haploid cells involved in sexual reproduction, the egg and the sperm.

Part 3 of 6Applying the terms

Species
  • Species the terms apply to cells across species.
Sets
  • Sets they describe the number of sets of chromosomes, not the total number.
Exception
  • Exception red blood cells have no nucleus.
Calculation
  • Calculation the haploid number is half the diploid number.

Part 4 of 6Why reduction division is needed

Fertilisation
  • Fertilisation gamete nuclei fuse to form the zygote nucleus.
Correct number
  • Correct number both gametes need the right number of chromosomes for the zygote to be viable.
Equation
  • Equation n + n = 2n.
Result
  • Result diploid zygotes need haploid gametes.

Part 5 of 6Meiosis and chromosome number

1Meiosis
  • Meiosis produces haploid gametes in sexual reproduction.
2First division
  • First division the first division is a reduction division, reducing the chromosome number.
3Humans
  • 46 (diploid) is reduced to 23 (haploid).

Part 6 of 6More from the specification

1
Prophase I and Homologous Recombination
  • During prophase I of meiosis, replicated maternal and paternal homologous chromosomes pair up in synapsis to form bivalents,
  • where non-sister chromatids cross over at chiasmata to exchange alleles and generate genetic variation.
2
Metaphase I and Independent Assortment
  • In metaphase I, bivalents align randomly at the spindle equator,
  • meaning paternal and maternal chromosomes orient independently before being pulled toward opposite poles during anaphase I.
3
Anaphase II and Chromatid Segregation
  • During anaphase II, centromeres divide and contracting spindle fibres pull sister chromatids to opposite poles,
  • leading to telophase II where four genetically unique haploid gamete nuclei are formed.
4
Haploid vs Diploid Chromosome Complements
  • Diploid cells (2n) contain two complete sets of homologous chromosomes carrying paired alleles at corresponding loci,
  • whereas haploid gametes (n) contain only one chromosome from each homologous pair.
5
Cytoplasm
  • The jelly in which the cell's chemical reactions happen.
6
Photomicrograph
  • A photograph taken through a light microscope.
7
Cytokinesis
  • Division of the cytoplasm.
8
Homologous chromosomes
  • Matching pair, one from each parent
9
Genetic variation
  • Differences in alleles within a population.
10
Homologous pair
  • Two chromosomes with the same genes in the same order.
11
Independent assortment
  • Random sorting of chromosome pairs into gametes.
12
Daughter cell
  • Cell produced by cell division.
13
Spindle
  • Structure of microtubules that attaches to and moves chromosomes.
14
Metaphase
  • Stage in which chromosomes line up on the equator of the spindle.
15
Prophase
  • First stage of mitosischromosomes condense and the nuclear envelope breaks down.
16
Telophase
  • Final stage in which chromosomes decondense and nuclear envelopes reform.
17
Chromatid
  • One of the two identical copies of a replicated chromosome.
18
Variation
  • Differences between individuals of the same species.
19
Anaphase
  • Stage in which centromeres divide and chromatids are pulled to opposite poles.
20
Allele
  • A version of a gene.

Key terms

TermMeaning
DiploidCell with two sets of chromosomes.
HaploidCell with one set of chromosomes.
GameteHaploid sex cell.
ZygoteCell formed by the fusion of two gametes.
Reduction divisionNuclear division that halves the chromosome number.
MeiosisNuclear division producing haploid cells.

Avoid these mistakes

1
Saying haploid means one chromosome — it means one set.
2
Saying gametes are diploid — they are haploid.
3
Saying red blood cells are diploid — they lack a nucleus.
4
Forgetting that n + n = 2n explains fertilisation.

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