Re-Cap OK so we learned that in mitosis, somatic cells go through cell division. In humans, one cell starting with 46 chromosomes will result in 2 cells each with 46 chromosomes. So what type of cells don’t go through mitotic cell division??

Sex Cells! Sex cells, or gametes, do NOT go through mitosis. They do a different division called meiosis. The result from this type of division is 4 cells that have half the number of chromosomes than the original cell. So one human cell that has 46 chromosomes will produce 4 sex cells (gametes) that each have 23 chromosomes.

But… Why? Every organism has a specific chromosome number. Humans = 46 chromosomes Chimpanzees = 48 chromosomes Ferns = 1010 chromosomes This chromosome number is called the organism’s diploid (2n) number. That is because there are 2 of each type of chromosome. One comes from ma and one comes from pa. These chromosomes are called homologous chromosomes because they code for the same traits.

But… Why? continued* In order for a ma and pa to make a baby, they have to join two gametes (egg & sperm) together. When 2 sex cells fuse, or unite, that is call fertilization and the resulting organism is called a zygote. If those cells were both diploid (2n) then that would mean… Not Good… The chromosome number is not the same.

The Solution? To keep the chromosome number constant, one diploid cell with go through a division called meiosis to produce 4 haploid (n) gametes. That way when sperm and egg come together we can have a whole diploid zygote.

Lets Get Started Before meiosis can begin, the chromosomes have to replicate. This occurs during interphase. Cells will grow in size to prepare to divide Cells will synthesize the necessary proteins

Phases of Meiosis Meiosis occurs in 2 parts. First you have Meiosis I, then you have Meiosis II. In meiosis I you start with one diploid cell and you end with 2 haploid cells. In meiosis II you start with the 2 haploid cells from meiosis I and end with 4 haploid cells. Total, there are 2 divisions in the process of meiosis.

Prophase I Chromosomes become visible Nuclear envelope disappears Centrioles head to opposite poles and spindle forms Homologous chromosomes (one pair of sister chromatids from the mother and one from the father) pair up to form a tetrad The tetrad pairs up so tightly that crossing over occurs

Metaphase I Spindle fibers attach to the centromeres Tetrads line up along the equator (or middle of the cell) Note that homologous chromosomes line up together along the equator in Meiosis where in Mitosis, they lined up independently to one another.

Anaphase I Homologous chromosomes separate and head to opposite ends of the cell Centromeres DO NOT split – Sister chromatids will stay together until the next division

Telophase I Spindle is broken down Chromosomes uncoil Cytoplasm divides into two cells

Prophase II Sister Chromatids are visible If the nuclear envelope has reformed from meiosis I, it will break down again Centrioles will head to the poles and spindle fibers form.

Metaphase II Spindle pulls the sister chromatids to the middle of the cell where they line up along the equator in random order (just as they did during Mitosis)

Anaphase II Centromere of each sister chromatid splits and each chromatid heads for an opposite pole

Telophase II and Cytokinesis Nuclei reform (nuclear envelope reappears) Spindle breaks down Chromosomes uncoil Cytoplasm divides into a total of four haploid cells that will become gametes Each cell contains one chromosome from each homologous pair

Variability Meiosis has a large role in maintaining variability in a species. Through sexual reproduction, offspring are not simply replicas of one organism but a genetic combination of two organisms Crossing over during Prophase I insures that a parent organism can pass on different gametes each time it reproduces, creating a variety of offspring.

Chromosomal Mutations Chromosomal mutations can happen when chromosomes break and do not repair correctly.

NON-DISJUNCTION… oops! Errors can also occur during Meiosis. Sometimes the homologous chromosomes do not separate properly – this is called nondisjunction This results in gametes with either an extra copy of a chromosome or no copy at all. Normal: Non-disjunction examples:

Disorders From Non-Disjunction… Remember: In normal fertilization, a zygote would get one copy of a chromosome from each parent resulting in one pair of each type of chromosome (humans: 23 pairs) Monosomy – when the zygote gets a copy of a chromosome from only one parent so it is missing one chromosome Most zygotes with monosomy do not survive One exception is the case of Turner’s Syndrome Females have only one X chromosome instead of two These people will still have female sexual characteristics but they will generally be underdeveloped

Identifying a Chromosomal Disorder To determine whether or not an organism has the proper number of each chromosome, one can look at a karyotype To make a karyotype a photograph is taken of the paired chromosomes during metaphase These pairs are cut out and arranged in a chart according to length and location of centromere Once arranged, it is easy to see if there are any extra or missing chromosomes