Topic 3.3

Essential idea: Alleles segregate during meiosis allowing new combinations to be formed by the fusion of gametes.


3.3.U1 One diploid nucleus divides by meiosis to produce four haploid nuclei

Four haploid cells

3.3.U2 The halving of the chromosome number allows a sexual life cycle with fusion of gametes

sexual life cycle

3.3.U3 DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids

Chromosomes or sister chromatids

3.3.U4 The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation

Crossover between non-sister chromatids

3.3.U5 Orientation of pairs of homologous chromosomes prior to the separation is random

Random orientation of homologous chromosomes

3.3.U6 Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number

Meiosis I and halving chromosomes

3.3.U7 Crossing over and random orientation promotes genetic variation

Cross over plus random equals genetic variability


3.3.A1 Non-disjunction can cause down syndrome and other chromosome abnormalities

Can you explain Non-disjunction and Down Syndrome?

Place mouse pointer on the figure to see how non-disjunction leads to Down Syndrome (Trisomy 21)

Non-disjunction explained

3.3.A2 Studies showing age of parents influences chances of non-disjunction

Influence of parent age on non-disjunction explained

3.3.A3 Description of methods used to obtain cells for karyotype analysis, e.g., chorionic villus sampling and amniocentesis                   and the associated risks

Explanation of amniocentesis
Explanation of chorionic villus sampling


3.3.S1 Drawing diagrams to show the stages of meiosis resulting in the formation of for haploid cells

Meiosis is a reduction division of a diploid nucleus to form a haploid nucleus. he process includes pairing of homologous chromosomes and crossing over, followed by two divisions where each division includes prophase, metaphase, anaphase, telophase and cytokinesis. The result is four haploid cells, known as gametes.

meiosis link meiosis animation
Definition homologous chromosomes
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