4.2+Meiosis

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:**
 * Meiosis is called a "reduction division" because:
 * The daughter cells have only half of the number of chromosomes as the original parent cell.

4.2.2 Define //homologous chromosomes//. **Homologous chromosomes:**
 * They are 2 chromosomes - one from each parent that:


 * Look the same


 * Same size


 * Show the same binding pattern in a karyotype


 * Carry the same genes - not necessarily the same alleles

**NOTE:** // Homologous chromosomes will pair up and split during meiosis //

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 cells.

Outline the process of meiosis, including pairing of homologous chromosomes and crossing over, followed by two divisions, which results in four haploid cells. Meiosis can be divided into two stages: Meiosis 1 and Meiosis 2 Both Meiosis 1 and 2 are subdivided into: The pairing of homologous chromosomes is taking place in Prophase 1 and splitting of the homologous in Anaphase 1 Also crossing over is taking place in Meiosis. Crossing over is the exchange of genetic material between two homologous chromosomes which have paired up during Prophase 1. The genetic recombination (exchange of gene) is the result of crossing over. The point where the homologous chromosomes cross over is called chiasmata. Note: limit crossing over to the exchange of genetic material between non-sister chromatids during Prophase 1
 * Prophase
 * Metaphase
 * <span style="color: #0000ff; direction: ltr; font-family: Calibri; font-size: 120%; margin-bottom: 0px; margin-top: 0in; unicode-bidi: embed; vertical-align: middle;">Anaphase
 * <span style="color: #0000ff; direction: ltr; font-family: Calibri; font-size: 120%; margin-bottom: 0px; margin-top: 0in; unicode-bidi: embed; vertical-align: middle;">Telophase

<span style="color: #008000; font-family: Verdana,Geneva,sans-serif; font-size: 12pt;">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)//

<span style="color: #0000ff; font-family: Arial,Helvetica,sans-serif; font-size: 110%; margin: 0in;">__**Synapsis:**__ the pairing up of homologous chromosomes during Prophase 1 __**<span style="color: #0000ff; font-family: Arial,Helvetica,sans-serif; font-size: 110%; margin: 0in;">Tetrad / Bivalent: **__<span style="color: #0000ff; font-family: Arial,Helvetica,sans-serif; font-size: 110%; margin: 0in;"> each resulting pair of homologous chromosomes
 * __<span style="color: #0000ff; font-family: Arial,Helvetica,sans-serif; font-size: 110%; margin: 0in;">Disjunction: __**<span style="color: #0000ff; font-family: Arial,Helvetica,sans-serif; font-size: 110%; margin: 0in;"> the separation of homologous chromosomes during Anaphase 1. Failure to do this can lead to aneuploidy, means having an extra chromosome or missing one chromosome.
 * __<span style="color: #0000ff; font-family: Arial,Helvetica,sans-serif; font-size: 110%; margin: 0in;">Non-disjunction: __**<span style="color: #0000ff; font-family: Arial,Helvetica,sans-serif; font-size: 110%; margin: 0in;"> when certain tetrads fail to separate

<span style="color: #0000ff; font-family: Arial,Helvetica,sans-serif; font-size: 110%; margin: 0in;">__**Results:**__ one gamete receiving two of the same type of chromosome and another gamete receiving no copy. <span style="color: #0000ff; font-family: Arial,Helvetica,sans-serif; font-size: 110%; margin: 0in;">__**Example:**__ is Down's syndrome which results from trisomy of chromosome 21. This means the individual with the syndrome has received three, rather than two, copies of chromosome 21. //<span style="color: #0000ff; direction: ltr; font-family: Arial,Helvetica,sans-serif; font-size: 110%; margin-bottom: 0px; margin-top: 0in; unicode-bidi: embed; vertical-align: middle;">([]) // media type="file" key="Trisomy.m4v" align="center" width="300" height="300"
 * __<span style="color: #0000ff; font-family: Arial,Helvetica,sans-serif; font-size: 110%; margin: 0in;">Total Non-disjunction: __**<span style="color: #0000ff; font-family: Arial,Helvetica,sans-serif; font-size: 110%; margin: 0in;"> when **ALL** tetrads fail to separate

__**<span style="color: #0000ff; font-family: Arial,Helvetica,sans-serif; font-size: 110%;">Down's Syndrome **__ <span style="color: #0000ff; font-family: Arial,Helvetica,sans-serif; font-size: 110%;">
 * <span style="color: #0000ff; font-family: Arial,Helvetica,sans-serif; font-size: 110%;">If non-disjunction occurs in either parent, one of the two gametes will carry two copies of chromosome 21
 * <span style="color: #0000ff; font-family: Arial,Helvetica,sans-serif; font-size: 110%;">When the zygote is formed, it will contain three copies of chromosome 21 and will be aneuploid (also referred to as trisomy 21)
 * <span style="color: #0000ff; font-family: Arial,Helvetica,sans-serif; font-size: 110%;">Resulting symptoms are called Down's Syndrome and are accompanied by disability
 * <span style="color: #0000ff; font-family: Arial,Helvetica,sans-serif; font-size: 110%;">**Karyotype** (see 4.2.5) of a person with Down's Syndrome is shown below:

<span style="color: #0000ff; font-family: Arial,Helvetica,sans-serif; font-size: 110%;">Female gametes develop before birth (unlike the production of sperm cells in the male which begins at puberty). At birth, the future female gametes are present and remain in this stage until ovulation. This means that for a twenty-year-old woman, her egg cells have collected twenty years of damage (chemicals, radiation, etc.). But for a forty year old woman, the cells have collected forty years of damage andthe chance of non-disjunction, resulting in a baby with Down Syndrome, is increased. <span style="color: #0000ff; font-family: Arial,Helvetica,sans-serif; font-size: 110%;">Sperm cells of the male are are produced constantly; so the age of the father seems to have less effect than the age of the mother. Genetic factors can play a role in either parent: generally **if a person has a relative with Down Syndrome, this person may have an increased chance of having a baby with Down Syndrome.**

<span style="color: #008000; font-family: Verdana,Geneva,sans-serif; font-size: 12pt;">4.2.5 State that, in karyotyping, chromosomes are arranged in pairs according to their size and structure.

__**Karyotyping:**__ process of finding the chromosomal characteristics of a cell Chromosomes can be stained to show structure and banding which can be used to arrange the chromosomes in their pairs - this is used in the process of karyotyping. The figure below shows an idealised picture of the human chromosomes.



<span style="color: #008000; font-family: Verdana,Geneva,sans-serif; font-size: 12pt;">4.2.6 State that karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities.

<span style="color: #0000ff; font-family: Arial,Helvetica,sans-serif; font-size: 120%; margin: 0in;">**2 common pre-natal tests:**


 * <span style="color: #0000ff; direction: ltr; font-family: Arial,Helvetica,sans-serif; font-size: 110%; font-weight: bold; margin-bottom: 0px; margin-top: 0in; text-decoration: underline; unicode-bidi: embed; vertical-align: middle;">Chorionic villus sampling
 * <span style="color: #0000ff; direction: ltr; font-family: Arial,Helvetica,sans-serif; font-size: 110%; margin-bottom: 0px; margin-left: 0.375in; margin-top: 0px; unicode-bidi: embed; vertical-align: middle;">Done at 11-12 weeks of pregnancy
 * <span style="color: #0000ff; direction: ltr; font-family: Arial,Helvetica,sans-serif; font-size: 110%; margin-bottom: 0px; margin-left: 0.375in; margin-top: 0px; unicode-bidi: embed; vertical-align: middle;">Involves taking a sample of the chorionic villi to obtan cells from tissue that was originally from the zygote
 * <span style="color: #0000ff; direction: ltr; font-family: Arial,Helvetica,sans-serif; font-size: 110%; margin-bottom: 0px; margin-left: 0.375in; margin-top: 0px; unicode-bidi: embed; vertical-align: middle;">Cells will have the same genetic composition as the unborn baby, so a karyotype can be made.




 * <span style="color: #0000ff; direction: ltr; font-family: Arial,Helvetica,sans-serif; font-size: 110%; font-weight: bold; margin-bottom: 0px; margin-top: 0in; text-decoration: underline; unicode-bidi: embed; vertical-align: middle;">Amniocentesis
 * <span style="color: #0000ff; direction: ltr; font-family: Arial,Helvetica,sans-serif; font-size: 110%; margin-bottom: 0px; margin-left: 0.375in; margin-top: 0px; unicode-bidi: embed; vertical-align: middle;">Done at 16 weeks of pregnancy
 * <span style="color: #0000ff; direction: ltr; font-family: Arial,Helvetica,sans-serif; font-size: 110%; margin-bottom: 0px; margin-left: 0.375in; margin-top: 0px; unicode-bidi: embed; vertical-align: middle;">Sample of amniotic fluid is taken, and a culture is made.
 * <span style="color: #0000ff; direction: ltr; font-family: Arial,Helvetica,sans-serif; font-size: 110%; margin-bottom: 0px; margin-left: 0.375in; margin-top: 0px; unicode-bidi: embed; vertical-align: middle;">When sufficient cells have been obtained, a karyotype can be done to detect chromosomal abnormalities
 * <span style="color: #0000ff; direction: ltr; font-family: Arial,Helvetica,sans-serif; font-size: 110%; margin-bottom: 0px; margin-left: 0.375in; margin-top: 0px; unicode-bidi: embed; vertical-align: middle;">The dividing cells are photographed, and from the photographs the chromosomes are arranged in homologous pairs
 * <span style="color: #0000ff; direction: ltr; font-family: Arial,Helvetica,sans-serif; font-size: 110%; margin-bottom: 0px; margin-left: 0.375in; margin-top: 0px; unicode-bidi: embed; vertical-align: middle;">Some genetic disorders can be detected this way (e.g. down syndrome)