4.2.1

• State that meiosis is a reduction division of a diploid nucleus to form haploid nuclei

» Meiosis is a reduction division of a diploid nucleus to form haploid nuclei

4.2.2 Define homologous chromosomes

• Homologous chromosomes: the same genes that contain different alleles; Chromosome pairs of the same length, centromere position, and stainging pattern that possess genes for the same characters at corresponding loci. One homologous chromosome is inherited from the organism’s father, and the other from the mother.

4.2.3 Outline the process of meiosis, including pairing of homologous chromosomes

and crossing over, followed by two divisions, which results in

four haploid cellsLimit crossing over to the exchange of genetic material between

non-sister chromatids during prophase I. Names of the stages are required

• Prophase I: chromosomes condense, and homologous pairs pair up and become tetrads called chromatids. During this time crossing over occurs, when homologous chromosomes pair up and the genes are swapped between each chromosome, this allows for genetic variation. The centromeres begin to form along with the spindle fibers.

• Metaphase I: chromosomes line up in the center of the cell, still in homologous pairs, and spindle fibers attach to each chromosome.

• Anaphase I: the chromosomes are pulled by the spindle fibers toward the poles, centromeres remain in tact.

• Telophase I and Cytokinesis: pairs reach poles, and each pole has two sister chromatids. In animal cells a cleavage furrow develops and in plant cells a cell plate forms. The cytoplasm then divides creating 2 diploid cells

• Prophase II: spindle fibers form and chromosomes move to the center of each cell

• Metaphase II: chromosomes line up in the center (like in metaphase I)

• Anaphase II: centromeres of chromatids separate and individual chromosomes move to opposite poles.

• Telophase II and Cytokinesis: nuclei form at opposite poles of the cell and cytokinesis occurs after cytokinesis there are four daughter cells with haploid numbers of unreplicated chromosomes

(Figure 13.7 The stages of meiotic cell division pgs. 240-241 in Campbell book)

4.2.4 Explain that non-disjunction can lead to

changes in chromosome number, illustrated by

reference to Down syndrome (trisomy 21)

(The characteristics of Down syndrome are not required)

• Sometimes chromosomes that should separate and move to opposite poles during anaphase don’t, and move to the same pole (this can happen during both phase I and II). This is called non-disjunction, the result is that the gametes are produced with either one too many or too few. Gametes with one usually die to quickly whereas those with one too many sometimes survive When fertilized, a zygote is produced with three chromosomes of one type instead of two. This is called trisomy. Down syndrome is usually caused by trisomy of chromosome 21, a non-disjunction during formation of the gametes.

(Figure 15.14 Down Syndrome pg. 281)

4.2.5 State that, in karyotyping, chromosomes

are arranged in pairs according to their size and

structure

• In Karyotyping, chromosomes are arranged in pairs according to their size and structure.

4.2.6 State that Karyotyping is performed using cells

collected by chorionic villus sampling or amniocentesis, for

pre-natal diagnosis of chromosome abnormalities.

• Karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities

Aim 8:There are ethical and social issues associated with karyotyping of unborn fetuses because this procedure allows parents to abort fetuses with a chromosome abnormality. There is also evidence that, in some parts of the world, abortion on the base of gender is carried out.

TOK: Various questions relating to karyotyping could be raised, including balancing the risks of side-effects(for example miscarriage) against the possibility of identifying and aborting a fetus with an abnormality. There are questions about decision-making: who should make the decision about whether to perform karyotyping and allow a subsequent abortion–parents or health-care professionals or both groups? There are also questions about whether or not national governments should interfere with personal freedoms, and whether or not they should be able to ban procedures within the country and possibly also ban citizens traveling to foreign countries where the procedures are permitted.

4.2.7 Analyse a human karyotype to determine gender and whether non-disjunction

has occurred(Figure 13.3 Preparation of a human Karyotype Campball Book pg.

237)karyotyping can be done by using enlarged photographs of

chromosomes.Aim 7: Online simulations of Karyotyping activities are available

Gender

• The bottom square labeled sex chromosomes contains the x and y chromosomes which identify the gender.

Non-disjunction

• This set of chromosomes have a trisomy of the 21 chromosome causing Down Syndrom

• (what gender is this person?)

Vocab:

• Diploid• Haploid• Homologous chromosomes• Meiosis• Crossing over• Non-disjunction• Karyotyping• Chorionic villus sampling• Amniocentesis