R E D U C I N G T H E C H R O M O S O M E N U M B E R

• Chromosomes carry the DNA, our genetic code that creates all the characteristics of our body.
• Human’s have 46 chromosomes, goldfish have 94 chromosomes and fruit flies have 8 chromosomes.
• Now if two human’s mate, why do we not have 92 chromosomes in the offspring?
• Well a little thing called Meiosis happens in our reproduction cells – sperm and eggs.

• As we have learned, our body has cells that divide. These cells are called somatic cells. This process is called Mitosis.
• Now, human’s somatic cells have 23 pairs of chromosomes for a total of 46 chromosomes.
• Our sex cells are called gametes.
• Male gametes are called sperm, and female gametes are called eggs.

• Meiosis only happens in cells that produce gametes.
• Each gametes is then able to produce half of the genetic material for the offspring.
• Cells with half of the chromosomes are called haploid and are symbolized with a single n.
• Cells that have a complete set of chromosomes are called diploid symbolized as 2n.

During Meiosis
Diploid cells (2n)  Haploid cells (n)
• If human body cells have 46 chromosomes, then gametes (sperm and egg) each have 23 chromosomes.

• A sperm will have 23 chromosomes and an egg will have 23 chromosomes
• When the sperm and egg meet, the 23 chromosomes will pair up with a corresponding chromosome, this is called homologous chromosomes.
• Example: one sperm chromosome carries the DNA for eye colour, it will pair up with the egg chromosome that has the DNA for eye colour.
• Thus each parent will contribute one half (n) (haploid) of the chromosomes to create a diploid (2n) somatic cell with the chromosome for eye colour.

Recall this image
Plasma
Membran e
Spindle fibers
Centromere
Nuclear membrane
Centrioles
Sister Chromatids

• Recall Chromosomes are what carry our DNA.
• Before Meiosis begins DNA replication occurs in a process called Interphase.
• Each Duplicated Chromosome contains two (2) sister chromatids which are attached at their centromeres.

• There are four (4) phases in Meiosis I:
• Prophase I
• Metaphase I
• Anaphase I
• Telophase I

• Is the longest and most complex stage.
• Chromatids condense and shorten
• Nuclear membrane disintegrates
• During prophase the homologous chromosomes pair up (synapsis) and the non-sister chromatids exchange DNA.

• During crossing over segments of non-sister chromatids break and reattach to the other chromatid.
Non-sister chromatids
Tetra d

• Multiple crossovers create an infinite number of genetic possibilities for just one gamete.
• Variation is important and crossing over creates variation.

spindle fiber
centrioles

• Shortest phase in Meiosis
• Pairs of homologous chromosomes lines up at the equator of the cell
• Independent assortment occurs.

• The process of random segregation and assortment of homologous chromosomes during metaphase resulting in the production of genetically unique gametes.

Equator
OR
Equator

• Homologous chromosomes separate and are pulled to opposite ends (poles) of the cell.
• This ensures that each new cell will receive only one chromosome from each homologous chromosome.
• Unlike in mitosis the sister chromatids remain attached.

• One chromosome from each homologous pair is at each pole of the cell

• The cytoplasm divides via cytokinesis to yield two new daughter cells.

• Meiosis II has four (4) stages as well, and this occurs after Meiosis I:
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II

• There is one chromosome of each homologous pair in each cell.
• Chromosomes relax and condense again

• The chromosomes line up at the equator of the cell
Equator

• Chromosome breaks apart at the centromere and sister chromatids move to opposite poles of the cell.
• Once the chromosomes separate, each sister chromatids is considered to be a chromosome.

• Spindle fibres begin to breakdown, and a nuclear membrane forms around each set of chromosomes
• Cytoplasm begins to divide

• Each sex cell divides into 2 producing 4 haploid daughter cells
• 4 gametes are produced – sperm or eggs

• Page 42 in Workbook
• Textbook page 78 – 1-11, 14 and 17
Note, question 7, do not answer the fruit fly part.