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Meiosis is a type of cell division that results in haploid daughter cells. It is only found in sexually reproducing organisms and is a reduction divisoin with the diploid number (2N) as found in body cells being reduced to the haploid number (N) for the gametes.
Define homologous chromosomes.
Homologous chromosomes are the corresponding pairs that look the same, except for the sex chromosomes X and Y. They have the same length, genes for the same characteristic at corresponding loci, the same centromere position and in sexually reproducing organsims one is inherited from each parent.
Describe crossing over.
Crossing over is the exchange of certain sections of chromosomes when the homologous chromsomes are paired in the tetrad in meiosis. The point at which the chromosomes cross and break is called the chiasma (pl. chiasmata).
State the stages of meiosis and explain what is happening.
- Interphase: Chromosomes replicated forming identical sister chromatids joined at centromere.
- Prophase I: Diploid number of chromosomes appear as long, thing threads, they shorten, thicken and homologous chromosomes pair with each other. Crossing over can occur. Nuclear membrane disappears.
- Metaphase I: Pairs of homologous chromosomes in the form of tetrads line up at the equator with one chromatid of each pair facing each pole.
- Anaphase I: Chromosomes move towards the poles pulled by spindle fibres. Sister chromatids stay attached.
- Telophase I and cytokinesis: Each half of cell has complete haploid set with two sister chromatids. Cytokinesis and division of cytoplasm occurs at the same time. Two haploid daughter cells are formed.
- Phrophase II: Spindle apparatus forms.
- Metaphase II: Sister chromatids line up at equator and attach to spindle fibres.
- Anaphase II: Centromeres separate and sister chromatids are pulled apart and move to opposite poles.
- Telophase II and cytokinesis: Nuclei form, chromosomes begin condensing and cytokinesis occurs leading to four daughter cells.
Non-disjunction is a mistake during cell division when homologous chromosomes fail to separate properly or sister chromatids fail to separate. It can occur in either meiosis or mitosis and leads to one daughter cell missing a chromosome and one daughter cell with an additional chromosome.
Aneuploidy is a genetic condition where there is an abnormal chromosome number, e.g. there are one or more additional copies of chromosomes or there is a deficiency in one or more chromosomes
What is down syndrome?
Down syndrome is a genetic condition caused by the presence of additional genetic information from chromosome 21.
Describe the main cause of Down syndrome.
The main cause of Down syndrome (around 96% of cases) is the presence of a complete extra copy of chromosome 21, called trisomy 21. Trisomy 21 is mainly caused by non-disjunction of chromosomes during meiosis in the formation of the gamete. This means that after fertilisation has occurred the zygote has 47 chromosomes present with 3 copies of chromosome 21. Non-disjunction leading to a gamete with 24 chromosomes is more common in older mothers.
Outline how translocation can lead to Down syndrome.
Translocation occurs when genetic material from one chromosome attaches to another chromosome. In Down syndrome, genetic material from chromosome 21 attached to another chromosome, e.g. from the 13 - 15 group or the 21 - 22 group. The most common form of translocation that cause Down syndrome is 21/14 fusion. Although there are only 46 chromosomes present, the additional genetic information of the translocation chromosome means the individual will display symptoms of Down syndrome.
Explain how non-disjunction can lead to mosaic Down syndrome.
Mosaic Down syndrome refers to a genetic condition where some cells show trisomy 21 while other cells have the normal 46 chromosomes. If non-disjunction occurs after conception when the embryo is undergoing early mitotic divisions, some cells will be affected with 3 copies of chromosome 21, while others will not. Since there is a mixture of cell types, people with mosaic Down syndrome may show reduced symptoms of down syndrome, or even no symptoms.
A karyotype is an arrangement of chromosomes pairs according to size and shape.
Outline how karyotypes are prepared.
A karyotype is prepared by collecting cells, culturing them and then halting cell division at metaphase. The cells are stained and then the chromosomes sorted according to size and structure.
Identify cells likely to be collected for karyotyping.
Cells likely to be collected for karyotyping include embryonic cells collected by amniocentesis or chronic villus sampling to check for abnormalities in the developing child. Blood cells from adults or children are collected for diagnosis of chromosome abnormalities.