Unit 16 Chromosomes and Meiosis
Chromosomes carry genes in a linear sequence that is shared by members of a species.
Alleles segregate during meiosis allowing new combinations to be formed by the fusion of gametes.
Meiosis leads to independent assortment of chromosomes and unique composition of alleles in daughter cells.
IB Assessment Statements and Class Objectives
In a eukaryote species there are different chromosomes that carry different genes.
Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes.
Diploid nuclei have pairs of homologous chromosomes.
Haploid nuclei have one chromosome some of each pair.
The number of chromosomes is a characteristic feature of members of a species.
Comparison of diploid chromosome numbers of Homo sapiens, Pan troglodytes, Canis familiaris, Oryza sativa, Parascaris equuorum.
Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex.
A gene on the Y chromosome causes embryonic gonads to develop as testes and secrete testosterone.
Cairn’s technique for measuring the length of DNA by autoradiography.
Developments in research follow improvements in techniques – autoradiography was used to establish the length of DNA molecules in chromosomes.
Description of methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and the associated risks.
Use karyograms to deduce sex and diagnose Down Syndrome in humans.
Making careful observations- meiosis was discovered by microscope examination of dividing germ-line cells.
The halving of the chromosomes number allows a sexual life cycle with fusion of gametes.
One of diploid nucleus divides by meiosis to produce four haploid nuclei .
Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells.
DNA is replicated before meiosis so that all chromosomes consist of two sister.
Chromosomes replicate in interphase before meiosis.
3.3.U4: The early stages of meiosis involved pairing of homologous chromosomes and crossing over followed by condensation.
Crossing over is the exchange of DNA material between non-sister homologous chromatids.
Chiasmata formation between non-sister chromatids can results in an exchange of alleles.
Drawing diagrams to show chiasmata formed by crossing over.
Orientation of pairs of homologous chromosomes prior to separation is random.
Independent assortment of genes is due to the random orientation of pairs of homologous chromosomes in meiosis 1.
Homologous chromosomes separate in meiosis I .
Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number.
Sister chromatids separate in meiosis II.
The alleles of each gene separates into different haploid daughter nuclei during meiosis.
Unlinked genes segregate independently as a result of meiosis.
Crossing over produces new combinations of alleles on the chromosomes of the haploid cells.
Crossing over and random orientation promotes genetic variation.
Fusion of gametes from different parents promotes genetic variation.
Non-disjunction can cause Down syndrome and other chromosome abnormalities. Studies showing age of parents
influences chances of non-disjunction.
Unit 16 Knowledge Audit
What is a Chromosome Video (Stated Clearly)
Discovery of Human Chromosome Number Article
Sex Determination Article
Sex Determination Video (TED-Ed)
Importance of Autoradiography in Cell Biology Article
Cairns Technique Video
Discovery of Meiosis Reading
Sources of Genetic Variation
Crossing Over Article
Crossing Over of X and Y Chromosomes Article